REPRINT FROM THE INTEGRAL NEWSLETTER
Attic spaces perform several functions. They are part of a home’s building envelope that protect both the home and the home’s residents from the elements, through the use of insulation, ventilation and vapour barriers. They also form part of the roof’s structure. Here are some common methods of creating this space:
- TRADITIONAL STICK FRAMING
In constructing a roof this way, the most straightforward method 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. As well as 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 at the center 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, the main drawback over conventional roof trusses is that insulation space is minimized at the soffit areas.
- Raised Heel Truss — 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).
- 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.
- 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.
- Parallel Chord Truss — This truss consists of parallel cords 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 by using ceiling joists of 2×12 as rafters. However 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.
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; 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 can assist truss uplift 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 so that the ceiling drywall can absorb some deflection;
buy and keep trusses dry.
If a home could talk, would it be saying “let me breath”? That just might be what it would say if there was an inadequate amount of ventilation in the attic. Proper ventilation in an attic is needed to reduce the effects of heat and humidity. These two elements can be costly to the structure and energy efficiency of the home.
Heat: In the summer an improperly vented attic can reach temperatures of up to 150 F. These kinds of temperatures can reduce the life expectancy of roofing materials. It will also make the interior temperatures of the home less comfortable and increase the cost of home cooling.
Humidity: Most of the humidity comes from within the home. Every time the shower or bath is used, clothes are dried, cooking is done, water is run, moisture is released into the air that can find its way up into the attic. Every breath drawn by the homeowners also releases moisture. During the colder months this moisture will condensate and seep into the rafters and roof sheathing. Once in the wood it can create mildew and decay in the wood. Delamination can occur in plywood sheathing. Roof shingles are affected as well.
The proper amount of attic ventilation will prevent damage to the home and will allow the roofing materials to last to their design life, reduce energy use and make the home a more comfortable place to live.
It is critical to ensure that the current ventilation is adequate before adding additional ventilation.
– Vents should not be covered in the winter time in the attempt to reduce the heating bill. If the floor of the attic is insulated with six to nine inches of insulation it is considered adequate by today’s standards. Covering these vents will only hold in moisture that will create the problems mentioned above, and the resulting dampness in the insulation will also lessen its effectiveness.
– If there are soffit vents (vents in the overhangs) make sure that the insulation has not blocked them. Often when insulation is added to older homes by a non-professional these vents tend to be overlooked during installation.
– Older cape style houses can have common areas that lessen the value of the ventilation. In most capes there is a small overhead attic space and a side crawl space behind the knee walls in the second floor bed rooms. Air flow is needed from the crawl space area to the upper attic area and out through vents. All too often there will be insulation installed between the rafters from the crawl space into the attic blocking the air flow. The proper way to insulate this area is to insulate the crawl space floor and the back of the knee walls. The only part of the rafters that should be insulated is behind the vaulted part of the interior ceiling that follows the roof line. This is the crucial area. A spacer may be needed to be installed between the insulation and the sheathing to maintain the air flow into the upper attic. This holds true for all vaulted ceilings. Air flow from soffit vents through a spacer behind the insulation and out a ridge vent is the typical method for ventilating a vaulted ceilings. A combination of soffit and ridge vents work like a natural chimney and can be used for open attic spaces as well.
How much ventilation needed for the attic must be determined, then an informed decision can be made about the type of venting to use. A professional installation may be less expensive in the long run than a “do-it-yourself” approach.
TYPICAL CROSS SECTION FOR VENTING OF CAPE ATTICS