Calgary Attic Guide: Insulation, Ventilation, Air Sealing, and Ice Dam Prevention

A Canadian house roof usually consists of three parts: a ceiling assembly, an attic, and a roofing membrane. As with other components of the building envelope, the CHBA Builders’ Manual states that proper insulation values, air leakage control, and moisture control are essential.

A Calgary attic performs well with RSI 7.0 (R-40) to RSI 10.5 (R-60) of insulation. The CHBA Builders’ Manual lists this range as common for ceiling and roof systems, with Net Zero Energy builders on the high end of this range and beyond. Attic spaces are generally accessible and therefore relatively easy to insulate to high levels.

The quantity of insulation alone does not determine its effectiveness. According to the CHBA Builders’ Manual, overall performance of the roof and attic also depends on seven measures:

• Installing a wind barrier at the eaves to allow air movement but prevent wind from blowing through the insulation.
• Sealing ceiling joints and penetrations to restrict air leakage around plumbing stacks, electrical wiring, junction boxes, recessed light fixtures, partition wall top plates, chimneys, flues, and access hatches.
• Providing adequate and balanced attic ventilation.
• Exhausting bathroom fans to the exterior.
• Eliminating gaps in the insulation, particularly at truss webs.
• Maintaining full insulation thickness at the eaves. In no case should the amount of insulation over the top of the exterior walls be less than that of the exterior walls.
• Reducing thermal bridges through structural members.

Air sealing minimizes the possibility of moisture accumulation in the attic space. Effective air and vapour barriers do this work, and the CHBA Builders’ Manual describes one option, a polyethylene air/vapour barrier installed on the warm side, in the following manner:

• Carry the polyethylene across the top plates of partition walls.
• Seal around chimney and ductwork using firestop materials.
• Seal around plumbing stacks and vents.
• Minimize ceiling-mounted electrical fixtures and penetrations in the ceiling. Where they are necessary, use airtight octagon boxes and lighting fixtures approved by the Canadian Standards Association (CSA) for insulated ceiling applications, and seal the units to create a continuous air barrier.
• Weatherstrip and latch attic access hatches, or place them on gable end walls outside the house.
• Seal electrical wiring and plumbing pipes where they penetrate through the ceiling or wall plates.

These measures reduce the passage of water vapour through a ceiling and into a roof structure. To account for leakage through any imperfections in the air or vapour barrier, the next line of defence is attic ventilation capable of removing moisture.

The National Building Code of Canada (NBC) requires the unobstructed vent area of a typical pitched roof to be not less than 1/300 of the insulated ceiling area. Warranties for asphalt shingles are also based on adequate ventilation.

Because the stack effect that moves air through the attic space is affected by the height between the eaves and the ridges, the amount of venting needs to be increased for flatter roofs, and can be decreased for steeper roofs. Screens and louvers obstruct the openings and must be taken into account when calculating the vent area.

The CHBA Builders’ Manual lists three other design considerations for roof ventilation:

• Maximize airflow by locating vents at eaves and soffits and near the peaks or along ridges.
• Increase the vent area on roofs with pitches less than 6/12.
• Balance the areas of vent intakes (low points) and exhausts (high points).

Proper attic ventilation removes water vapour from the attic space before condensation can cause trouble, and it is essential for maximizing the service life of a roof assembly. Other benefits include a cooler attic in the summer and a dryer attic during the winter. Both result in increased energy savings and greater occupant comfort.

During the summer months, radiant heat from the sun can cause high roof-deck temperatures. Gradually, the entire attic space is heated, and, in turn, the whole dwelling feels the effect of a hot roof. Research cited by the CHBA shows that prolonged exposure to high heat levels shortens the service life of asphalt roofing products. Heat build-up can be reduced by properly venting the underside of the roof deck.

Truss uplift can occur in the winter when the upper chord of a truss contains more moisture than the lower chord. This results from differing temperature conditions: the upper chords sit above the insulation, while the lower chords are insulated and thus at a warmer temperature. The upper chord, being at a higher relative humidity compared to the lower chord, expands by lengthening and bowing upward, pulling up the webs and the lower chord and causing the ceiling to move upward.

Truss uplift can damage interior finishes, particularly where the ceiling and interior partition walls join. The CHBA Builders’ Manual lists several ways to minimize the effects:

• Use half-span trusses supported on roof girders or interior load-bearing partitions.
• Use trusses manufactured with kiln-dried lumber and keep the trusses protected from wetting as much as possible during storage and installation.
• Ensure that the attic space is adequately ventilated and the vents provide balanced airflow.
• Ensure that soffit vents are not blocked by insulation.
• Connect the ceiling drywall to the partition walls using drywall clips, and screw the ceiling drywall far enough from the partition wall intersection, floating the corners at least 300 mm (12 in.) to allow for deflection without joint failure.
• Install 2 x 6 pieces to the top of partition walls and fasten the drywall to the lumber and not the trusses.

Ceiling drywall can sag because of four causes: absorption of water-based textured finishes by the drywall; high humidity from drying lumber and interior finishing materials; condensation on the drywall; and water damage caused by wind-driven rain and snow entering the attic or roof space.

These problems can be reduced or eliminated by:

• Using thicker drywall on ceilings (15.9 mm, or 5/8 in.) and controlled-density or sag-resistant drywall in place of regular gypsum board, or reduced spacing of supports, for example by strapping at 400 mm (16 in.).
• Preparing the surface with a compatible primer before applying heavy water-based textured finishes.
• Providing ample ventilation during construction.
• Preventing wind-driven rain and snow from entering the space.
• Insulating the ceiling immediately after installing the drywall and before heating commences.
• Ensuring installation of drywall sheets perpendicular to the strapping or framing to which they are attached.

Engineered trusses that lift or drop a chord create room for full-depth insulation at the eaves, where standard trusses pinch the insulation thin. The CHBA Builders’ Manual notes there may be a small cost premium for these trusses compared to standard trusses, but they provide several roof performance advantages.

• Raised-heel truss: has a short, vertical chord member over the top plate bearing point that provides additional room for insulation over the wall/ceiling intersection. The higher the heel, the more space there is to add insulating value at the roof/wall junction. Advantage: a uniform thickness of insulation can usually be installed over the entire attic, including above the top plates. Disadvantages: the cost is higher than for standard trusses, and the design increases the height of the exterior stud wall, so the siding area increases.
• Cantilever truss: has a short, inclined chord member over the top plate bearing point, and the bottom chord projects to the eave. Whether there is adequate room for full-depth insulation over the wall/ceiling intersection depends on the length of the cantilever and the length of the chord.
• Dropped-chord truss: has an additional bottom chord. The space between the structural bottom chord of the truss and the dropped chord is filled with insulation.
• Scissor truss: has a sloped bottom chord and provides a cathedral ceiling without a supporting beam or wall at mid span. Advantages: free-spanning cathedral ceiling, higher insulation levels than a rafter cathedral ceiling, and full-depth insulation up to the perimeter walls. Disadvantages: less access room for installing insulation, baffles may be required to prevent settling of loose-fill insulation, and roof pitches are limited to 5:12 or more depending on snow loads.
• Parallel-chord truss: consists of parallel wood chords and steel or wood webs. Advantages: full-depth insulation in cathedral ceilings, and ventilation without purlins (transverse framing). Disadvantages: steel-web versions have higher heat loss due to thermal bridging, and the restricted height between chords may make insulating difficult.

Yes. Insulation, ventilation, and moisture vapour control are equally important for a flat roof as for pitched roofs, according to the CHBA Builders’ Manual. Flat roofs are used frequently in commercial construction and occasionally in residential construction. Built-up and flexible membrane roofing can be used, as can garden roofs that employ vegetation to reduce interior heat uptake and water runoff. Vinyl membranes suitable for residential flat roofs are also available.

Flat roof advantages: auxiliary use of roof space, rainwater harvesting, a variety of membrane options, and garden roofs. Disadvantages: limited space for insulation (at least some rigid insulation is required to provide R-values), at least twice as much roof vent area required compared to steeper roofs with attics, risks of moisture problems or thermal bridging, and higher structural loads if garden roofs are present.

A superinsulated framed attic uses standard dimension lumber, with the ceiling joist extending beyond the top plate of the wall at the eaves to produce a triangular structure that can accommodate higher levels of insulation at the perimeter wall. The advantage: higher insulation levels above the top plate. The disadvantage: the superinsulated framed attic is more expensive to construct than a truss roof in most cases.

A framed cathedral ceiling uses dimension lumber with 38 x 286 mm (2 x 12 in.) rafters and 38 x 38 mm (2 x 2 in.) purlins. Two layers of RSI 3.5 (R-20) batts between the joists allow for adequate ventilation of the attic space. Where a cathedral ceiling creates living space in the attic, all attic spaces need to be ventilated. Costs can be lower than parallel-chord truss construction, but clear span is limited by the maximum size of dimensional lumber, insulation is limited to a maximum of RSI 7.0 (R-40) unless additional strapping and batt insulation or boardstock insulation is used, thermal bridging occurs through the rafter, and material and labour costs may increase.

Interior partition construction can greatly affect the energy efficiency of a ceiling, because a considerable quantity of air can move through the ceiling/partition wall junction. The CHBA Builders’ Manual describes two approaches:

• Sealed polyethylene approach (SPA): a 600 mm (24 in.) wide strip of 0.15 mm (6 mil) polyethylene is placed between the partition wall top plates. The polyethylene flange is sealed to the ceiling air barrier on either side of each partition before the ceiling drywall is installed.
• Airtight drywall approach (ADA): standard pre-cut wood or steel studs can be used for interior partitions if a 19 x 89 mm (1 x 4 in.) plate is placed on top of the exterior wall top plate. This raises the trusses enough to accommodate 16 mm (5/8 in.) drywall and allows the interior partition walls to be raised into a vertical position. The drywall can be taped before the interior partitions are raised. Cove moldings may be required to conceal truss uplift, and the method involves coordinating trades.

The four diagrams below explain the chapter’s core concepts at a glance.

An attic inspection finds the frost, the blocked baffles, and the unsealed penetrations before they become ceiling stains. Vertex Roofing & Exteriors has served Calgary, Airdrie, and Okotoks since 2011, is WCB covered and fully insured, and backs its work with a 10-year workmanship warranty. Contact Vertex to schedule an inspection.