Attic spaces perform several functions. It’s part of a home’s building envelope that protects you and your house from the elements through the use of insulation, ventilation and vapour barriers. Here’s are some common methods of creating this space:
TRADITIONAL STICK FRAMING — In constructing a roof this way, the most straightforward is a gable roof. Here, all roof rafters are cut to the same length. At the ridge, the proper angle is determined according to the pitch/slope of the roof. At the eaves/soffit area, the rafters are notched (called bird mouths) to sit on the exterior wall or rafter plate. Dormers and skylights can be added. Aside from the simple gable style are others such as a hip roof.
Ceiling joists are used to support the ceiling finish and act as ties between exterior walls. In some cases they may also be used as a tie for opposing rafters. In a pitched roof, the ceiling joists are nailed to the side of the rafter to prevent them from moving outward. The ceiling joists overlap over the centre load-bearing wall and are nailed together providing a continuous tie across between opposing rafters.
In a rafter-framed attic, where the pitch/slope of the roof is less than 4/12, (considered a low slope roof), the loads of the roof are carried by a beam(s) as opposed to a truss. Here, the roof rafters and the ceiling joists are designed to carry loads from the weight of the roof with snow etc. These loads are then carried to interior partitions by supports angled greater than 45°, knee walls, and ceiling joists. In doing so, the outward force is reduced on the roof and continuous ties between the lower ends of opposing rafters are not necessary.
Advantages — provides for more insulation at the edge of the ceiling.
Disadvantages — generally, they are more costly to construct than trusses.
ROOF TRUSSES — Today, builders often use prefabricated roof trusses. Although convenient and less time consuming than traditional framing, its main drawback over conventional roof trusses is that insulation space is minimized at the soffit areas.
a) Raised Heel Trusses — generally, this type of truss will resolve this problem. Although costs associated with a raised heel truss (manufacturing cost, extra siding, extra insulation,) may be greater, energy savings and the prevention of surface condensation (reducing the likelihood of ice damning) can justify the investment.
Advantages — provides for full insulation depth in all areas above the ceiling; a clear span; and, a continuous air-vapour barrier.
Disadvantages — more costly than conventional truss (e.g. more soffit siding will be required).
b) Dropped Chord Truss — this truss consists of a conventional truss with a second lower chord below it.
Advantages — may reduce truss uplift; provides full depth insulation up to the perimeter walls; allows for a clear ceiling span and continuous air-vapour barrier.
Disadvantages — requires taller studs; more siding is required; blocking required at the ceiling and wall junction for air-vapour barrier attachment.
c) Scissor Truss — the lower chords of the truss are sloped, rather than horizontal, allowing for the construction of a house with cathedral ceilings. In doing so, there is not the need for a bearing beam or wall. These trusses can also be modified to accept more insulation when used in conjunction with a raised heel.
Advantages — often easier to get more insulation in than with other types of cathedral ceilings.
Disadvantages — may be more costly than other methods of construction; may be more difficult to insulate between chords if not using blown insulation.
d) Parallel Chord Trusses — These trusses consist of parallel chords of wood that are joined by an open web of wood, or steel braces, or a solid web of plywood. This type of truss permits high levels of insulation in cathedral ceilings.
Advantages — allows for large amounts of insulation in cathedral ceilings and also can provide ventilation without purlins. Can provide large, clear spans and allow for application of a continuous air-vapour barrier.
Disadvantages — higher cost than dimensional lumber. With a web of steel braces, heat losses due to thermal bridging can be high. Difficult to insulate between chords, but blown insulation may help with this problem.
CATHEDRAL CEILING FRAMING — One method of framing a cathedral ceiling is ceiling joists of 2×12 are used as rafters but if the two layers of R-11 batt insulation are desired, the entire rafter space will be filled with insulation. To allow for the proper ventilation space above the insulation, 2x2s are first nailed to the top of each rafter and parallel to them, to give at least 11/2 inches of air space above the insulation. Then a second layer of 2×2 (2x3s or 1x4s can also be used) are nailed to the rafters and perpendicular to them to allow for attachment of sheathing and roofing. This alternative assures a cold, ventilated roof with adequate ventilation parallel to each rafter space when tied to appropriate eaves and ridge ventilation, and still allows a cathedral ceiling design.
Advantages — In some areas, lower cost than parallel chord trusses.
Disadvantages — Limited to a maximum of R-40. Reduced insulation values at the ceiling joists.
TRUSS UPLIFT — In some houses with trusses, an upward movement of the ceiling occurs resulting in damage to interior finishes, particularly to interior ceiling and walls. This can result from: different moisture contents in the lower and upper wood chords of the truss; lumber expanding at different rates than others e.g. varying upper and lower chords; and, insulation covers lower cords resulting in differing expansion and moisture than that of non-insulated cords.
Proper grading and drying of lumber can minimize truss uplift. In addition, the builder assist by providing: adequate attic ventilation; don’t block the soffit vents with insulation; use drywall clips to connect the ceiling drywall to the partition-wall top plates; fasten the ceiling drywall far enough away from the partition wall such that the ceiling drywall can absorb some deflection; and, buy and keep trusses dry.