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Keeping The Heat In - Section 5: Roofs and attics

Relatively easy access has made the attic a favourite starting point to insulate for many homeowners, despite the fact that most other areas, such as basements and uninsulated walls, lose more heat than the typical attic. Even if an attic is already insulated, there may still be an opportunity to improve the energy efficiency and soundness of the house through air sealing and ventilation. Air leaks via the chimneys, vents, plumbing pipes and electrical boxes in the attic can account for substantial heat loss and can lead to a variety of moisture-related problems.

The importance of air sealing cannot be overstated. If you are upgrading your attic, read this entire section for helpful tips and other relevant information. Figure 5-1 displays four different types of attics.

Types of attics

Figure 5-1 Types of attics

Text version

Four illustrations, including one-and-a-half storey house with knee walls and small attic space, a truss attic where the framing forms a w-pattern throughout, an accessible attic which is unfinished and more open, and a cathedral/flat roof/mansard attic which has large interior walls and ceilings and little attic space.

5.1 General considerations for all attics

Regardless of the type of attic or ceiling your house has, there are a number of things to examine before beginning work. A thorough inspection of the following features will help you to develop your retrofit strategy.


Most houses with accessible attics have an interior ceiling hatch, although exterior roof or wall-mounted entries are not uncommon. A hatch should be large enough to allow you to bring in materials. If not, or if there is no access, you may be able to cut a hole in the ceiling in an inconspicuous place such as a closet. Exterior entries, for example in a gable end represent one less opening that will have to be air sealed, which offers ready access to firefighters in the event of a fire.

Check the roof space for obstructions and ease of movement. Vertical clearances of less than 1 m (39 in.) will not allow you to move freely. Attics or roofs without a working space are covered later in this chapter, in Part 5.4, Cramped attics, cathedral ceilings and flat roofs. Figure 5-2 illustrates a more worker-friendly attic space.

Placing boards on the joints creates a working platform that evenly distributes the weight

Figure 5-2 Placing boards on the joints creates a working platform that evenly distributes the weight

Text version

Illustration of a person working in an accessible, unfinished attic, wearing proper safety equipment including gloves, overalls, boots, a dust mask, goggles and a hard hat. The person is kneeling on boards laid across the joists, while holding an unopened bale of batt insulation, and has tools such as a hammer, staple gun, tape measure, and utility knife laid out on the side.


In addition to checking for structural problems, check the condition of the roof framing, sheathing, finish and the soffit and fascia for signs of moisture problems such as leaks, stains, mould, flaking or rot. Uncorrected moisture problems will reduce the effectiveness of insulation and can lead to structural damage such as wood rot or split rafters. Mould or rot are sure signs of advanced condensation problems. Identify the cause and correct it before adding insulation.

Moisture can come from the outside due to failure of the roof or flashing. Typical problem areas include poor flashing at a hip, valley or the chimney and ice dam leaks. Pay particular attention to water marks on the underside of the sheathing or along rafters.

Moisture can also come from inside the house, carried into the attic as water vapour by air leakage. Typical problem areas occur around bathroom and kitchen vents that penetrate the ceiling, around plumbing stacks and chimney chases and at wiring penetrations and pot lights. A tell-tale sign of air leakage is the discolouration of insulation, so do not hesitate to look under existing insulation to locate air leaks.

Check the attic during or just after a cold snap for condensation buildup, which will appear as frost in cold climates. Some frost buildup is to be expected, but if it is particularly heavy buildup (10 mm [3/8 in.] or more), look to make sure ventilation is present and not blocked with insulation. Checking the attic during or just after a rainstorm will help determine whether moisture problems are generated by interior or exterior sources.

Examine the existing insulation for type, condition (dry, wet, compact, etc.), average depth and coverage. If it has been damaged or contaminated by mould or vermin, remove it. However, do not disturb vermiculite insulation (see Part 1.4, Health and safety considerations).

If the insulation is wet, do not cover it until the source of moisture is removed and the insulation is dry. If the insulation is dry, it will probably be all right to leave it in place. Generally, there is no problem in using two types of insulation. Check the depth of the insulation to determine its insulating value. Compare this with the recommended insulation values in Part 2.2, Control of heat flow.

Make sure that the insulation is distributed evenly and that there is full depth coverage, especially around the perimeter of the attic above the wall plates. Uninsulated areas will cause a cold spot where the wall and ceiling meet, which can lead to moisture and mould problems.

Existing air and vapour barriers

Houses should have a vapour barrier on the warm side of the insulation. In older homes, the vapour barrier might have been provided by wax paper, kraft paper-backed batts or layers of paint. Newer houses usually have a polyethylene sheet vapour barrier, but overall, very few houses have an effective air barrier.

If there is an air barrier, locate it and determine its condition. Remember, an air barrier must be continuous; holes or tears must be repaired, and penetrations sealed. Do not hesitate to pull back existing insulation where leakage sites might be found and seal these areas. However, do not disturb vermiculite insulation as previously warned.

Increased insulation means a colder attic, which in turn means that any vapour escaping into the attic can condense before it can be vented. It is essential to air seal the attic to prevent moist indoor air from getting in.

If there is no air and vapour barrier, concentrate on comprehensive air sealing. Create an effective air barrier by using caulking, gaskets and weatherstripping to seal the joints between building components. Do not rely on batt, blown-in or poured insulation to reduce the need for proper air sealing.

Structural considerations

When undertaking attic or roof renovations, consider upgrading the roof structure for future photovoltaics or solar hot water panels. Keep in mind that adding an additional layer of shingles to the existing layer will result in an additional load on the roof structure. It is typically recommended to remove existing shingles when installing new ones.

5.1.2 Fire and other hazards in attics

Safety warning:

Certain areas in the attic can pose serious fire and health hazards. Before air sealing and insulating an attic, ensure you or your contractor address the following health and safety concerns.

Masonry chimneys

Check the chimney and surrounding framing for signs of charring, soot deposits, crumbling masonry or mortar, or evidence that the chimney lining is deteriorating. Do not insulate it if the chimney is suspect. Have a chimney specialist repair, seal and insulate it at the same time.

Commonly a gap exists where a masonry chimney passes through the attic. To air seal this area, first install sheet metal flashing to bridge this gap and then seal all the joints with a non-combustible sealant (ensure the product is rated for this application). To prevent insulation, especially blown-in insulation, from coming into contact with the chimney, construct a barrier 75 mm (3 in.) from the face of the masonry chimney and a minimum 75 mm (3 in.) higher than the top of the insulation, as shown in Figure 5-3.

Keep combustibles away from a masonry chimney

Figure 5-3 Keep combustibles away from a masonry chimney

Text version

Cutaway of a masonry chimney as it passes through a ceiling and attic. A 75 millimetre or 3-inch air space is created around the chimney by a non-combustible barrier extending 75 millimetres or 3 inches above the attic insulation. This barrier rests on non-combustible flashing sealed to the chimney masonry and to the top side of the attic joists by high-temperature caulking. An air and vapour barrier covers the floor of the attic space and is sealed to the joists with acoustical sealant.

Metal chimneys

Factory-built metal chimneys should be kept from contacting insulation by a minimum 75 mm (3 in.) unless otherwise specified by manufacturer or local authorities. Install a non-flammable radiation shield that maintains a 75-mm (3-in.) air space between the chimney and insulation. To reduce air leakage, first air seal the fire stop to the air and vapour barrier with non-combustible sealant. The top of the radiation shield should also be a minimum of 75 mm (3 in.) higher than the level of existing or new insulation to be added. To prevent insulation from falling behind the shield, attach a clamp-type storm collar without sealant at the top of the radiation shield, as shown in Figure 5-4.

Keep combustibles away from an insulated metal chimney

Figure 5-4 Keep combustibles away from an insulated metal chimney

Text Version

Cutaway of a metal chimney as it passes through a ceiling and attic. A radiation barrier creates a 75 millimetre or 3-inch air space around the chimney and extends 75 millimetres or 3 inches above the attic insulation, maintaining the distance from combustible materials. The top of the barrier is a storm collar clamped, not sealed, to the chimney. Above the ceiling, the air and vapour barrier is wrapped around the drywall to contact the firestop sealant. From the ceiling side of the floor below the attic, a firestop is installed and sealed with high-temperature caulking. A collar-like ceiling trim plate is attached below the firestop.

Electrical installations

Work safely around electrical wiring (turn off the power). Consult with an electrical contractor if questionable wiring or corroded electrical connections are present.

Recessed lighting

Standard interior recessed lights are difficult to air seal effectively and can become a fire hazard if covered with insulation. Replace with approved airtight recessed fixtures or covers that can be installed safely under insulation. Keep insulation away from a recessed light fixture as shown in Figure 5-5.

Keep insulation away from a recessed light fixture by creating a barrier

Figure 5-5 Keep insulation away from a recessed light fixture by creating a barrier

Text Version

Cutaway of a ceiling. Around the light fixture a 12.5 millimetre or half-inch plywood box lined with a fire-resistant material such as drywall or sheet metal provides airspace around the fixture. All joints and edges of the box are caulked to minimize air leakage. To determine the minimum box volume, allow 800 cubic centimetres per watt of the fixture. For example, a 150-watt the fixture would require a 120,000 cubic centimetre box. Centre the fixture if possible, and keep it a minimum of 12.5 millimetres or half an inch from the sides and top. Above the fixture, leave a minimum 12.5 millimetres or half an inch, but 150 millimetres or 6 inches is preferred.

As an alternative to traditional recessed or pot lights consider light-emitting diode (LED) flush or flat mount fixtures for better energy efficiency. This also offers the opportunity to better seal and insulate the existing cavity. Ensure that fixture installation instructions are followed.

Animal droppings

If you find a significant amount of animal droppings and vermin in the attic, do not disturb them because they can harbour easily airborne moulds, parasite eggs and bacteria that can cause severe illness. See Part 1.4, Health and safety considerations, for more information.

5.1.3 Air sealing

Seal the following areas:

  • Around the plumbing stack and any other pipes. For plastic plumbing stacks that move up and down due to thermal expansion, use a flexible gasket or a polyethylene sleeve sealed to a plywood collar (see Figure 5-6).
  • Around wires or ceiling light fixtures that penetrate the attic floor and partition and exterior wall top plates (see Figure 5-7 and Figure 5-8). See Part 5.1.2, Fire and other hazards in attics, on how to deal with recessed lights.
  • Around ducting that enters the attic from inside the house. Seal duct joints with aluminum duct tape or paint or mastic. Seal the gaps where ducts penetrate the ceiling. The ducts should stay below the insulation or should be wrapped with insulation and be protected from being crushed. Exhaust fans should always discharge to the outside but not directly below the soffit vents.
  • At the junction of the ceiling and interior wall partitions, pull back the insulation to locate and caulk cracks along interior walls (see Figure 4-4).
  • At the top of interior and exterior walls, check to see if all wall cavities are blocked from the attic (usually by a top plate). Block any open spaces in the exposed cavity with a piece of rigid board insulation and caulking. If the top plate is cracked or poorly fitted, use caulking and polyethylene to create a tight seal (see Figure 5-7).
  • Around attic hatches (see Figure 4-7).
  • Around the chimney. Building codes require that air spaces between chimneys and floor or ceiling assemblies through which they pass be sealed with a non-combustible fire stop (see Figure 5-3 and Figure 5-4).
  • Along the edge of shared walls. There is often a gap between the party wall (such as the shared wall between units in duplexes, triplexes and row houses) and the edge of the attic floor. Ensure that this gap is well sealed. If you suspect that air is leaking up and out of a party wall made of concrete blocks, hire a contractor to seal the blocks.
Sealing the plumbing stack

Figure 5-6 Sealing the plumbing stack

Text Version

Illustration of a sealed plumbing stack tube as it passes through a ceiling and attic. A heavy polyethylene strip which is overlapped, caulked with acoustical sealant, and stapled to the space between attic joists, is secured with a pipe clamp around the plumbing stack.

Sealing an electrical box

Figure 5-7 Sealing an electrical box
(Note: See Figure 5-5 for the special case of recessed lighting electrical boxes.)

Text Version

Illustration of a sealed electrical box in an attic space. A heavy polyethylene sheet which is caulked with acoustical sealant and stapled to the ceiling encases the electrical box. An electrical wire leads away from the electrical box and down through a hole into an interior wall. Holes around electrical wires are filled with caulking or foam sealant, and caulking seals gaps along the top of the interior walls.

Sealing a light fixture on the ceiling

Figure 5-8 Sealing a light fixture on the ceiling

Text version

Illustration of a spherical light fixture being attached to a ceiling and sealed with a non-adhesive caulking, or a gasket approved by the Canadian Standards Association.

5.1.4 Ventilation

Attic ventilation serves a number of purposes. It reduces summer heat buildup, prolonging the life of roofing and reduces air conditioner loads. After air sealing, attic ventilation is your second line of defence against the water vapour that may have found its way into the attic. It ensures a colder, well-vented attic space less prone to the formation of ice dams at the eaves.

Make sure that existing attic vents are working properly and not blocked by insulation, debris or other materials. You may have to locate roof or soffit vents from outside if they are not clearly visible from inside the attic.

Houses with peaked roofs and accessible attics are the easiest to vent by using the ratio of 1 to 300. This ratio refers to unobstructed vent area to the insulated ceiling area. The vent area must be increased if vents are covered with screening (to keep out insects, etc.) and with baffles (to keep out rain and snow), which reduce the clear area of the vents. Ideally, locate vents to allow for good cross ventilation from end to end and from top to bottom. However, although important, vents will not prevent condensation and will not solve the problems created by air leakage.

Although an airtight ceiling will significantly reduce the likelihood of moisture in the attic, building codes still require minimum attic ventilation.

Do not automatically increase ventilation. Also, do not use electric exhaust fans for attic ventilation as these can draw more air than can be supplied through the soffit vents. This will actually pull house air into the attic, resulting in greater heat loss and moisture accumulation. They are also prone to failure, noise production and increased energy use. Wind-driven, revolving roof vents do not create suction and are no more effective than other types of capped vents.

The location of vents is as important as their number and type. Often, a mixture of types and locations will work best. The following Parts detail the best approach depending on your attic type.

5.2 Easily accessible attics

After you have inspected the attic and carried out any remedial work, focus first on air and moisture control.

5.2.1 Air and moisture control

There are five options for installing an air barrier system in an unfinished attic:

  • concentrate on air sealing
  • install polyethylene over top of the joists
  • install polyethylene between the joists
  • spray foam between the joists (hire a contractor)
  • spray foam under the roof to create a conditioned attic (hire a contractor)

The first option is the most practical, since installing a polyethylene air and vapour barrier in an existing attic is fraught with obstructions and requires painstaking attention to detail. On the other hand, spray foam offers air sealing and an initial layer of high quality insulation that can be topped up to the desired RSI (R) level.

If the attic retrofit is being completed in conjunction with interior renovations, the easiest approach is to install a new, single air and vapour barrier on the underside of the ceiling joists.

5.2.2 Concentrate on air sealing

It is a good idea to paint the room side of the ceiling with two coats of oil-based paint or apply a single coat of latex vapour barrier paint to inhibit vapour movement. Ordinary latex paint is not a suitable vapour barrier.

See Section 4, “Comprehensive air leakage control,” for detailed information on sealing all air leaks into the attic.

Install polyethylene over top of the joists

If the attic is relatively unobstructed with chimneys, plumbing stacks or structural members, consider installing polyethylene directly over the ceiling joists. This method involves the least number of seams and requires less caulking and stapling than other methods. It also leaves existing insulation in place. However, seal all obvious air leakage paths before laying down the polyethylene. This method is illustrated in Figure 5-9.

Installation of polyethylene sheets over attic joists

Figure 5-9 Installation of polyethylene sheets over attic joists

Text version

Cutaway of the layers in an attic space with polyethylene air barrier over attic joists. An arrow travelling from outdoors through a vent in the roof overhang and into the attic space indicates air flow. There is a layer of rigid foam board on the attic floor at the eaves to maintain ventilation space. A polyethylene sheet is attached to the top of attic joists by caulking and staples. Insulation is both between the attic joists and on top of the polyethylene sheet.

To avoid trapping moisture between the plastic and the ceiling, which might lead to possible wood rot or other moisture-related problems, install a minimum of twice the insulating value over top of the air vapour barrier (such as applying the one-third, two-thirds rule). For example, if the joist height is 89 mm (3 1/2 in.) and contains RSI 2.1 (R-12), install at least RSI 4.2 (R-24) over top of the polyethylene.

The main difficulty with this technique involves sealing the barrier to the wall top plate, especially at the eaves where there is little room to manoeuvre. This area must be well sealed. Spray foam or rigid board insulation can help bridge the gap in this area. Cut rigid board to fit between the ceiling joists and to extend from the exterior wall top plate toward the attic. A second piece of rigid insulation, installed vertically, joins the polyethylene to the horizontal rigid board. Carefully caulk any joints or seams between materials. Expanding two-part spray foam kits (or the service of a spray foam company) are also very good for sealing areas around joists and boards.

Install polyethylene between the joists

Where obstructions, such as a truss roof, make the previous method too difficult, install a polyethylene air barrier or low-permeable foam insulation board between the joists. However, note that this is a lengthy and painstaking process.

Remove existing insulation from the area you are working on and set it to one side. Cut foam boards to fit snugly between the trusses. Caulk all edges, gaps and joints, see Figures 5-10 and 5-13. Obstructions, such as electrical wires, will require cuts in the barrier; seal these carefully to make the barrier continuous. Another option though less effective, is to cut the polyethylene strips about 200 mm (8 in.) wider than the joist spacing. Lay a bead of caulking on the side of the joists along their length and install the polyethylene using staples, installed through the caulk every 75 mm (3 in.).

Foam board laid between the joists and caulked as an air and vapour barrier

Figure 5-10 Foam board laid between the joists and caulked as an air and vapour barrier

Text version

Cutaway of the space between attic joists with a layer of foam board placed between the joists and attached to the attic floor with caulking. A layer of batt insulation is placed on top of the foam board.

Spray foam between the joists

Spray foam contractors can install closed-cell foam between the joists to air seal and add insulation at the same time to the ceiling. All existing insulation and dust must be removed first to allow for a good bond. A minimum of 50 mm (2 in.) is needed; top up with other insulation afterwards.

Spray foam under the roof

This approach involves installing closed-cell spray foam directly under the roof surface, right down to the junction of the ceiling and exterior walls. This eliminates roof venting and creates what is called a hot roof, where the attic space becomes part of the conditioned (heated and cooled) house space. This process may be suitable for some attics, but do not proceed without approval from your building inspector, and then only use a certified spray foam contractor who is familiar with the process.

5.2.3 Installing insulation

Common materials for use in accessible attics are batt or blanket types or loose-fill insulation. If there are obstructions above the joists, such as with a truss roof, it may be easiest to put batt insulation into the joist spaces and then use loose-fill insulation to create a complete blanket of insulation above the joists and around all obstructions. Loose-fill insulation is also good by itself, especially in irregular or obstructed spaces.

Batt or blanket insulation

Fit batt insulation into place between the ceiling joists snugly, without compression. If the joist space is not the standard 406 or 610 mm (16 or 24-in.), cut the batts to fit the space or use loose-fill insulation

Additional installation tips:

  • Butt batt ends together as snugly as possible.
  • Batts should cover the top plate of the exterior wall but not block the venting. To maintain airflow, leave a space of 38 to 50 mm (1 1/2 to 2 in.) between the top of the insulation and the underside of the roof sheathing. Use baffles between each rafter space to prevent it from being blocked (see Figure 5-11).
  • Insulate above and below cross bracing, splitting or cutting the batt to accommodate the cross bracing as illustrated in Figure 5-12. Alternatively, cut one batt into a series of wedges and then fit a wedge under each brace. In either approach, do not compress the insulation.
  • The first layer of batts should be thick enough to completely fill to the top of the joist space. The second layer can then run perpendicular across the joists to block any heat flow through and around the joists (see Figure 5-13). Ensure that there are no gaps between the two layers of insulation.
  • Fill any awkward spaces or gaps with pieces of batt or with loose fill.
  • Apply blanket insulation in the same way as batts. It may be pre-cut with a knife or cut on the spot. Start at one end of the attic and unroll the blanket.
Baffles can be used to maintain airflow through the soffit vents

Figure 5-11 Baffles can be used to maintain airflow through the soffit vents

Text version

Cutaway of baffles between attic roof joists with arrows indicating air movement traveling from the exterior, through a vent in the ceiling overhang, and over the baffles.

Fitting insulation around cross bracing

Figure 5-12 Fitting insulation around
cross bracing

Text version

Cutaway of cut batt insulation inserted between the cross bracing in the attic joists.

The top layer of insulation runs perpendicular to the bottom layer

Figure 5-13 The top layer of insulation runs perpendicular to the bottom layer

Text version

Illustration of layers of batts of insulation between and above attic joists. Batts on top of joists are perpendicular to batts between joists. A polyethylene sheet is laid under both layers of insulation and attached to the top of attic joists by caulking and staples.

Loose-fill insulation

  • Loose-fill insulation can be poured by hand or blown in on top of the air and vapour barrier to a depth greater than the height of the joists. This task can be done by the homeowner using rented equipment or by a qualified contractor.
  • Before adding insulation, nail enough depth indicators (for example, a piece of wood nailed perpendicular to the joist) or mark highly visible lines on truss members so you can easily see and gauge the depth of the insulation. Maintain an even depth throughout the attic by levelling the insulation with a board or garden rake (see Figure 5-14).
  • Fill all nooks and crannies.
  • At the eaves, do not block the ventilation. Prevent insulation from disappearing onto the vented soffits of the eave space by installing a piece of rigid board insulation or a wood baffle before the work begins. Building-supply stores sell cardboard or foam plastic baffles that can be stapled between the rafters (see Figure 5-11). Be sure that the insulation extends out far enough to cover the top of the exterior wall.
  • If the loose fill is deeper than the joists, build insulation framework (a crib) around the attic hatch so that it can be filled to the edge (see Figure 4-7).
  • The bags of insulation material will list how many square metres (or square feet) each bag should cover to provide the required RSI value. Calculate the attic area to help determine the number of bags needed.
  • If you are having a contractor do the work, calculate the RSI value that you want and check the bags of insulation to be used. They should indicate the area that one bag will cover at the selected insulating value. You and the contractor should then agree on the total number of bags to be used, the expected insulating value and the minimum settled depth of insulation throughout the attic, based on a specific density.
  • Remember to seal the attic hatch (see Figure 4-7).
  • Seal and insulate up to party walls in attached housing (see Part 5.1, General considerations for all attics).
  • Do not cover recessed light fixtures unless they are installed in approved covers.
Installing loose-fill insulation

Figure 5-14 Installing loose-fill insulation.

Text version

Illustration of a person installing loose-fill insulation. The installer is wearing safety equipment, including goggles, a dust mask, a hard hat, overalls, gloves, and work boots. The person placed a marker on the truss uprights to indicate the final depth of the insulation to be achieved.

5.2.4 Ventilation

Ideally, 50 percent of the attic ventilation openings should be continuous soffit vents, and the other 50 percent gable, ridge or roof vents. Figure 5-15 shows various types of vents. Any of them are adequate when used in conjunction with soffit vents. Ridge vents are preferable where practical, but must be equipped with baffles to deflect wind blowing up the roof and to prevent the penetration of water and snow. Occasionally, a house will only have soffit vents. In this case, install gable end, ridge or roof vents to take advantage of cross ventilation.

Roof venting

Figure 5-15 Roof venting

Text version

Illustration of a slanted roof that highlights different types of ventilation, including soffit vents, gable end vent, ridge vent, and a roof vent.

5.3 Houses with half storeys

Houses with 1 ½ or 2 ½ storeys have attics with several small parts that may make access, air sealing and insulating very difficult. If you cannot get into these spaces, you may have to hire a contractor. If the space is accessible, the following section gives some guidelines on how to do the work yourself.

5.3.1 Air and vapour control

Where you have access to the attic space, follow the directions for the control of air and vapour flows as outlined in Part 5.2, Easily accessible attics.

Houses with half storeys have one major source of air leakage that must not be overlooked: through the ceiling joists immediately beneath the knee walls (see Figure 5-16). This critical air leakage area must be sealed flush with the back of the knee wall finish. This can be done by cutting and fitting foam board that will fill that ceiling joist opening and seal it into place with caulking or spray foam.

Rigid insulation can be nailed over the studs of the knee-wall section

Figure 5-16 Rigid insulation can be nailed over the studs of the knee-wall section

Text version

Cutaway of one side of a slanted roof showing the attic rafter vent at the eaves and a vent space halfway up the roof, foam board is installed on the knee walls and to the ceiling and caulked at the top and base.

Another option may be to dense-pack loose-fill insulation between the floorboards and the ceiling below while ensuring that an air barrier is installed.

5.3.2 Ventilation

Ventilate the attic spaces above and beside the top storey separately by using gable vents. Position the vents to prevent wind from blowing through the insulation.

A second option is to use gable vents or a ridge vent for the area above the attic ceiling and soffit vents in the side areas. This is a good option if the house already has working soffit vents (see Figure 5-17). A ventilation space in the rafter section will permit airflow between the insulated areas.

Otherwise, you will have to install rafter vents in the section along the rafters to ensure adequate ventilation (see Figure 5-17 and Figure 5-18). These vents must be on the cold side of the space, while the warm side can be filled with insulation that can be added to the sloping ceiling portion unless other arrangements are made in the renovation process.

Ventilation occurs between sections through attic rafter vents installed in the rafter section.

Figure 5-17 Ventilation occurs between sections through attic rafter vents installed in the rafter section. (Note: See Figure 5-11 for baffle detail.)

Text version

Cutaway of a slanted roof with triangular vents at opposite ends of the peak and at opposite ends of the base of the overhang. Arrows indicating air flow travel through the vents up the interior of the roof along baffles.

Each Part has its own cross ventilation

Figure 5-18 Each section has its own cross ventilation

Text version

Cutaway of a roof with triangular vents at opposite ends of the peak, and at opposite ends of the base of the overhang. Arrows travel through the vents and indicate airflow.

5.3.3 Insulation

Spray foam and rigid foam insulation can be used, but batts are less expensive and easier to handle in confined spaces (see Figure 5-19). If there are any electrical outlets (use care to avoid shocks – and also take this opportunity to upgrade wiring in this area) or pipes in the knee wall, keep them on the warm side of the air and vapour barrier and insulation, and seal the air and vapour barrier around them.

  • Treat the attic floor behind the knee wall and the attic space above the half storey’s ceiling (see Figure 5-16) exactly as described previously for a standard, unfinished attic; in addition, add air sealing under the knee wall.
  • Treat end walls as described in Section 7, “Insulating walls”. End walls are the full-height walls that are exposed to the exterior.
  • The knee wall can be treated like an unfinished attic floor (see Figure 5-9), making allowances for the fact that it is vertical. Install a combination air and vapour barrier made from polyethylene strips sealed between each stud. Sealing all cracks and penetrations and painting a vapour barrier on the interior surface also can create the air barrier. Next, install the insulation and secure it in place with building paper, cardboard, olefin sheets or chicken wire. Better yet, nail 25 mm (1 in.) or more of rigid board insulation to the exterior side of the studs to hold the insulation. This will also increase the thermal resistance of the wall section and reduce thermal bridging. A wall in the attic that adjoins a heated space can also be insulated like a knee wall.
  • The section between the rafters may be filled with insulation if all penetrations through the ceiling are sealed and if this is permitted by local building codes or standards (see hot roof in Part 5.2.2, Concentrate on air sealing, subsection Spray foam under the roof). The hot roof could involve the slope ceiling and then the knee wall space. It is unlikely to be necessary in the small triangular top attic. Having an insulated roof slope allows the knee wall spaces to be used for storage (as they often are).
  • Exposed foam in accessible areas must be fire protected.
  • Due to limited space for insulation and venting on the sloping ceiling, consider adding rigid board directly to the ceiling, and then air seal and cover with drywall. This method will greatly increase comfort levels in summer and winter, though it may reduce headroom.
  • Some contractors specialize in high density blown insulation that will help reduce airflow while improving thermal values.
The wall and floor sections of a half storey are being insulated and secured

Figure 5-19 The wall and floor sections of a half storey are being insulated and secured

Text version

Cutaway of one side of a slanted roof and the exterior showing a half-storey space with insulated walls, a floor with batts of insulation between the joists, and further batts laid perpendicularly on top.

Dormer windows

Many 1 ½ or 2 ½ storey houses have dormer windows. The walls of the dormer may be insulated with batts, as described for the knee wall. Dormers typically include a lot of framing members that limit the amount of insulation that can be added. Besides air sealing, consider dense-pack blown-in insulation and also the addition of rigid board insulation and drywall to reduce thermal bridging.


Some houses will have the attic floored over, even when it is not used as living space. You can insulate it by lifting the floorboards and treating it as you would an unfinished attic, though some or all of the boards may have to be replaced to maintain the ceiling stiffness.

Also, a contractor can fill the subfloor space completely by blowing in dense-pack loose-fill insulation through access holes. Air leaks, such as around plumbing stacks and open eave spaces, should be sealed first. With the contractor, ensure that the space is filled completely and at the right density. Alternatively, spray foam can be used to air seal and insulate this space if accessible.

Safety warning:

Before insulating, check for wiring (replace damaged or frayed wiring), recessed light fixtures or other sources of heat that may be concealed beneath the floorboards. All heat sources must be protected from the insulation or removed entirely.

You will not likely be able to achieve the minimum manufacturer’s recommended levels for attic insulation by filling this space alone. If possible, add some insulation on top of the floorboards to keep them warm and to reduce thermal bridging through the joists and floorboards.

5.4 Cramped attics, cathedral ceilings and flat roofs

This section discusses methods for insulating difficult areas.

5.4.1 Cramped attics

There are three options for insulating an attic that is too cramped to work in:

  • Insulate on top of the existing roof (see Part 5.5, Ice dams).
  • Add rigid board and drywall to the ceiling if there is adequate headroom.
  • Have an experienced contractor blow in dense-pack loose-fill insulation.

The last two options can also be done together. The first and last methods may apply where there is inadequate headroom.

There will likely be no way of installing a new sheet-material air barrier. If one does not already exist, it should still be acceptable to install insulation if there is no evidence of moisture problems, if humidity levels in the house are reasonable and if any air leaks through the ceiling into the attic are sealed. You can achieve added protection by painting the ceiling below the attic with latex vapour barrier paint (ask paint suppliers or manufacturers) or two coats of oil-based paint.

Ensure that the contractor prevents insulation from entering the eaves and blocking the ventilation. If there are any recessed light fixtures, chimneys or other sources of heat in the attic, make sure precautions are taken to avoid creating a fire hazard.

Ventilating cramped attics can be difficult because of the difficulty of creating an adequate airflow. Where the roof extends past the exterior walls, it may be possible to use soffit vents in combination with roof vents or built-up ridge vents.

Many houses with cramped attic spaces lack eaves. In such cases, approach ventilation with caution. The best approach is to carefully seal the ceiling below the attic from inside the house and then insulate without installing additional vents.

If possible, check the roof space for moisture problems during or just after a cold snap in mid-winter. Some frost is to be expected, but if the buildup is especially heavy, you will have to vent the space, thoroughly seal all air leaks and reduce humidity levels in the house.

Check with local building authorities to determine which procedures are permitted in your area.

5.4.2 Cathedral ceilings and flat roofs

A house (or any portion of a house) with a flat roof, cathedral ceiling or some other attic-less construction is difficult and will require the services of a qualified contractor. All renovation options involve some risk of either moisture problems or thermal bridging that can reduce the effectiveness of the insulation. Before proceeding with the work, make sure that any material added will not exceed the load capacity of the existing structure.

The main problem with these roofs is the limited space for insulation and ventilation (see Figure 5-20). If there is already some insulation in the joist space, trying to add more may not be beneficial. However, if you decide it is worth your while to increase insulation levels, there are a number of options. One option is to install rigid board insulation directly under existing ceiling and cover with a new finish such as drywall. Before proceeding discuss with a contractor or building official on how to locate and install a new vapour barrier. A technique involving blown insulation is discussed below.

Flat roof

Figure 5-20 Flat roof

Text version

Cutaway of flat roof showing arrows indicating air flow traveling up through the soffit vent of the roof overhang, over the insulation, and exiting a capped pipe on the roof.

Part 5.6, Renovations and repair, discusses both interior and exterior retrofits, including the addition of a new roof. The existing space between the ceiling and roof can be dense-packed with cellulose or glass fibre loose-fill insulation by a contractor. Densities are typically between 56 to 72 kg/m³ (3 ½ to 4 ½ lb./cu. ft.). The contractor should calculate and confirm the density for each roof cavity.

The dense-packed approach eliminates roof ventilation, so it is generally not recommended (check with local building authorities first). However, the high density of the insulation, combined with comprehensive air sealing, should reduce airflow sufficiently to avoid condensation problems.

Take extra care to make sure that air leaks into the ceiling are sealed from below. This is difficult because wiring and plumbing usually puncture the ceiling in a number of places. Moreover, the partition walls may not be completely blocked off at the top, allowing large amounts of air to flow through the interior walls into the ceiling.

Where the interior walls are completely open to the ceiling, there is no easy solution unless you are prepared to have your interior walls blown full of insulation and sealed along all trim, outlets and other penetrations.

5.5 Ice dams

Ice dams are large masses of ice that collect typically on the lower edge of a roof and in the gutters (see Figure 5-21). As melting snow (or rain) runs down the roof, it meets this ice mass and backs up, sometimes under the shingles and then leaking into the attic.

Ice dams

Figure 5-21 Ice dams

Text version

Cutaway of a slanted roof covered in snow with an ice dam at the base. There is a thin ice slab beneath the snow and water is running down the underside of the sheathing, trapping water below. This forms ice on the interior of the eaves and icicles on the exterior.

Ice damming usually occurs when there is a significant amount of snow on the roof. The snow itself, especially when deep, can act as an insulator. If the attic temperature is above freezing, it warms the roof sheathing, which melts the layer of snow that is in direct contact with the roof. Melt water runs down the roof until it meets the overhang.

If the air and the overhang are below freezing, the water freezes on the roof and starts the ice dam. In addition, southerly exposed dark wall siding heated by the sun can cause warm convection currents to enter the eave and roof area above it, causing ice damming.

5.5.1 Roof type and ice dams

Uninsulated attics usually do not have ice dams because the heat coming through the attic tends to melt snow as it lands and prevents much accumulation. A well-sealed and insulated attic results in a cool roof and generally will also not have ice dams. Complicated roofs with many valleys, dormers or large overhangs are more prone to ice damming as they tend to have more thermal bridging and air leakage.

5.5.2 Locating areas prone to ice dams

One way to find areas prone to ice dams is to look at the roof after the first heavy frost in fall or light snow. Note where the snow melts first and find out what is under that spot. One common sight in such conditions is a horizontal melt line across the roof of a 1 ½ storey house, where the short knee wall meets the ceiling. Other places would be beneath a roof-ducted exhaust fan, plumbing vent, under a skylight and above a leaky attic access hatch. The best prevention for ice damming is to seal all attic air leaks and insulate thoroughly.

Dark wall finishes can absorb solar heat and cause warm convection wall currents that may be to blame for ice dams. Sealing the associated soffit vents may help reduce the problem. However, other venting may still be required to ensure that the roof space is properly vented.

5.5.3 Dealing with restricted attic space

Many attics, including those under low-sloped roofs, do not have enough space for adequate insulation at the edge of the attic floor. If a baffle is required at the edges of the attic floor to keep a ventilation opening between the insulation and the roof sheathing above, there may only be about 100 mm (4 in.) of space left for insulation. Since it is not possible to put as much insulation there, this may result in the snow melting just above the overhang and promoting ice damming.

Try to put the best insulation possible at that edge to reduce heat loss. Spray foam is ideal as it air seals as well as insulates. Cut pieces of extruded polystyrene will help as well. Install a piece of extruded polystyrene 25 mm (1 in.) from the sheathing to maintain the ventilation air space and then fill between this foam board and the attic floor with insulation (see Figure 5-22).

Creating ventilation space with rigid insulation

Figure 5-22 Creating ventilation space with rigid insulation

Text version

Illustration showing how wood blocks act as spacers between the rigid insulation and the sheathing of the slanted roof. The insulation is flush with the rafters and extends down to bottom of the attic joists.

5.5.4 Cathedral ceilings

Ice dams on roofs with cathedral ceilings are more difficult to resolve as they are harder to access. The same principles apply to preventing ice dams – stopping house air leaks, upgrading insulation, possibly improving roof ventilation. Ice dam issues on cathedral-type roofs are usually best dealt with by contracting an insulator with dense-pack insulation experience or when re-roofing.

5.5.5 Installing a new roof

When re-roofing, remove the roof finish and the sheathing, air seal and fill the cavities with insulation and then install the new roofing materials, including an ice-shield membrane. A well-sealed roof will need little to no ventilation. If you are uncertain about whether or not the sealing can be done effectively, leave a ventilation channel under the sheathing from the soffit to the peak. Sometimes insulation can be added to the ceiling inside, although this approach will not stop the air leakage.

An alternative measure to prevent ice dams is to make the roof impermeable by using a self-sealing membrane under the shingles. Building codes require such membranes on the lower part of the roof in new houses. These membranes do not stop ice dams, but they prevent the water from leaking through the roof sheathing and into the house. Ice damming can still occur and possibly damage shingles and gutters.

5.5.6 Quick fixes and their limitations

There are many popular, quicker solutions but they have drawbacks:

  • Attaching electric heating cables will melt channels in the ice, sometimes alleviating a problem, but they use a significant amount of electrical energy while being an eyesore on most roofs.
  • Removing gutters will keep them from becoming ice traps, but gutters keep roof water away from the foundation.
  • Attacking ice dams every winter with an axe or ice pick will damage the roof surface.
  • Although removing snow from the roof will also help, removing ice dams and snow is not always easy or safe to do.

For certain older houses with complicated roofs, it may be impossible to completely eliminate ice dams without resorting to several different techniques.

The preferred solution for most houses is to keep house heat out of the attic by air sealing and insulating.

5.6 Renovations and repair

Renovations or repairs provide an opportunity to ensure a well-insulated attic ceiling and walls with an effective air and vapour barrier.

A popular renovation activity is to convert an existing unfinished attic into a new living space. Depending on the depth of the rafter space, it may be difficult to install as much insulation as is recommended. If you choose to finish your attic, check for local code requirements and then consider the following steps:

  • Install collar ties between every pair of rafters (see Figure 5-23). These ties will provide structural support for both the roof and the new ceiling.
  • Ensure that the insulation installed in the rafter spaces fits snugly against the top of the walls to form a continuous thermal envelope. There must be no gaps around the perimeter of the attic floor, otherwise heat will escape and ice damming may occur near the eaves. In most cases, it should be possible to insulate each rafter space all the way down to the eaves and over the top of the exterior wall. Leave a clear space between the top of the insulation and the underside of the roof sheathing to allow for roof ventilation. Seal any roof vents into the parts of the attic that are not heated.
  • Staple a continuous polyethylene air and vapour barrier to the rafters and end-wall studs; then carefully seal the edges and seams with acoustical sealant.
  • If your attic is not too cramped, consider adding strapping onto the rafter bottoms and wall studs to allow for more insulation (up to the recommended minimum level for attic insulation, if possible). After installing insulation in the existing cavities, nail or screw strapping (2 x 2 or 2 x 3 on-edge) perpendicular to the rafters and studs. Space the strapping to suit the width of the insulation you will use. However, the strapping should be no more than 610 mm (24 in.) apart – measured on centre – or you will have difficulty attaching the interior finish. Fit insulation snugly between the strapping, covering all the rafters and studs. Finally, staple a continuous, sealed polyethylene air and vapour barrier to the strapping. Alternatively, secure rigid board insulation 25 to 50 mm (1 to 2 in.) to the rafters and studs, seal or install a vapour barrier and then cover with an approved finish (for example, drywall).
  • Insulation between the collar ties is applied from below in much the same way, with a continuous polyethylene air and vapour barrier applied last. If the collar ties have already been insulated and if there is access to the upper portion, then more insulation may be added as in a normal attic.
Collar beams or collar ties provide additional support

Figure 5-23 Collar beams or collar ties provide additional support

Text version

Illustration showing two joists meeting at an angle with a third nailed to each, forming a triangle.

5.6.1 Dropped ceiling

Extending the rafters provides space for insulation and ventilation

Figure 5-24 Extending the rafters provides space for insulation and ventilation

Text version

Cutaway of ceiling rafters which have been extended with wood underneath to create a deeper cavity filled with thick insulation.

Where headroom is sufficient, constructing a dropped ceiling to hold insulation is an excellent way of thermally upgrading a cathedral ceiling or flat roof, especially when planned as part of a renovation. The following are several options:

  • Construct a new ceiling immediately below the existing ceiling. If the roof has exposed joists or beams (usually for decorative purposes), it may be possible to close the space, which creates a new ceiling. Install batts or rigid insulation in the space followed by a continuous air and vapour barrier and a new ceiling. In all cases, you must prevent warm air from getting into the new cavity space and bypassing the insulation. Seal the perimeter of the new ceiling and any possible air leakage paths through partition walls.
  • With the ceiling removed, extend existing rafters or trusses to accommodate additional insulation. Cross strap the existing rafters or extend the rafter cavity with 2 x 4 lumber and plywood gussets (see Figure 5-24).

Although it is not necessary to remove the interior finish and expose the rafters and tops of the walls when constructing a new ceiling, it is a good idea. This will allow you to perform air sealing, check the state of the insulation and verify if there is a vapour barrier.

  • Mechanically fasten rigid board insulation directly to the structure of an existing ceiling. Although this technique avoids the mess and time required to tear down the ceiling, it will make access to the space between the ceiling and the attic floor more difficult (for example, to install recessed light fixtures). If rigid board insulation doubles as the air barrier, fit the boards tightly and seal the seams well. Extend electrical fixtures to accommodate the depth of the new ceiling.

5.6.2 Adding a new roof

Insulation can also be added on top of an existing ceiling or roof. This option is most suitable for cathedral ceilings and flat roofs when major exterior alterations, such as a new roof, are required. Air sealing, installing new insulation, sheathing, roofing surface and replacing eavestroughs, soffits and fascia plus disposal will cost several thousand dollars. You will almost certainly need the services of a qualified contractor.

One method involves installing rigid board insulation on top of the existing roof (see Figure 5-25). The higher RSI value of rigid insulation means a smaller increase in the roof thickness, although several layers of insulation may be needed to meet the desired RSI level.

A new insulated roof can be built over the old roof

Figure 5-25 A new insulated roof can be built over the old roof

Text version

Cutaway of a slanted roof and the top of an exterior wall. Blocking insulation is shown at the joint between the wall and the original roof and is sealed with caulking on the inside face of the exterior wall. Two layers of rigid insulation are placed over the existing roof finish. Boards are placed across the new insulation, perpendicular to the rafters, and the new roof sheathing and finish are attached.

Sheet polyethylene or an elastomer membrane is first placed on the existing roof to provide the air and vapour barrier, following the 1/3 – 2/3 rule. Alternatively, a layer of closed-cell spray foam or low-permeability rigid board can be installed with the joints taped to prevent air leakage. This step is not necessary if an air barrier is already in place.

It is important to insulate (such as, block) and air seal the spaces between the joists along the perimeter of the roof. This prevents heated air from escaping around the insulation. Line up the edges of the blocking material with the inside finish of the exterior wall and thoroughly caulk all seams. Another approach is to hire a spray foam contractor familiar with air sealing to block these critical areas.

Another method is to build up the existing roof or to frame a new roof and fill it with batt insulation or loose-fill insulation, which is blown in. Ensure that the air and vapour barrier is tightly sealed. The new space or roof frame may add to the structural loading of the entire assembly, so this must be done following local code requirements.

This approach is best done with new trusses or by installing cross members (called purlins) that span the length of the roof. Once the purlins are in place, the new rafters are installed, followed by the proper installation of the insulation to avoid air movement and thermal bridging. Leave enough clear space above the insulation and above the new roof rafter to allow for ventilation.

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