Attics in Half-Storey Houses
Houses with one-and-a-half or two-and-a-half storeys have attics with several small sections that sometimes make access and insulating difficult. If you cannot get into these spaces to work, 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.
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Air and Vapour Control
Where you have access to the attic space, follow the directions for the control of air and vapour flows. Concentrate on sealing all air-leakage paths into the attic.
One major source of air leakage that cannot be overlooked is through the ceiling joists immediately beneath the knee walls. Prevent air leakage in this area by filling the space with a rigid insulation board, installed flush with the back of the knee-wall finish. Seal the edges of the insulation as thoroughly as possible. Sprayed polyurethane may also be practical for controlling airflow into this space.
Ventilation
Ventilate the attic spaces above and beside the top storey separately by using gable vents. Make sure that the vents prevent wind from blowing through the insulation.
A second option is to use gable vents in the area above the attic ceiling and eave vents in the side areas. This is a good option if the house already has working eaves vents. A ventilation space in the rafter section will permit airflow between the insulated areas. Otherwise, you will have to install air channels in the section along the rafters to ensure adequate ventilation.
These channels must be on the cold side of the space, which then can be filled with insulation.
Insulation
Theoretically, a rigid foam insulation could be used; however, batts are less expensive and easier to handle in confined spaces. If there are any electrical outlets or pipes in the knee wall, make sure that you 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 over the half storey's ceiling (see diagrams) exactly as described previously for standard, unfinished attics.
- The end walls are the full-height walls that are exposed to the exterior. Treat these with blown insulation.
- The knee wall can be treated like an unfinished attic floor, but allow for the fact that it is vertical. Install a combination air and vapour barrier made from polyethylene strips sealed between each stud. The air barrier also can be created by sealing all cracks and penetrations and painting a vapour barrier on the interior surface. Next, install the insulation and secure it in place. This can be done with friction-fit batts or by securing the insulation with building paper, cardboard, olefin sheets or string or wire. One inch of rigid board insulation nailed to the exterior side of the studs can also be used to hold the batt insulation in place. This will increase the thermal resistance of the wall section and reduce thermal bridging.
- The section in 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.
- Be particularly careful when working around electrical outlets.
Other Complications
Wall of Heated Room
Some houses have a wall in the attic that adjoins a heated space. Insulate it as you would a knee wall.
Shared Wall
Semi-detached or row houses that share a concrete-block wall will lose heat into the attic because concrete is a good conductor of heat, and air circulates inside (and through) this wall. Ideally, the shared wall should be plugged at the ceiling level by having a contractor drill holes and inject small amounts of polyurethane foam into the blocks. This is often not possible or economical. If there is a wood-frame party (shared) wall at the top of the block wall, air-seal the junction at the top of the block wall.
The next best alternative is to insulate both sides of the exterior surface of the shared wall in the attic. First, paint the wall with an impermeable concrete paint or cover it with polyethylene. Next, tightly secure a layer of insulation to the wall. The wall normally goes through the roof line and is plugged. However, if there is space at the top of the wall between the concrete blocks and the roof, or if there are any gaps, they should be plugged and covered with insulation material.
Dormer Windows
Many one-and-a-half or two-and-a-half storey houses also have dormer windows. The walls of the dormer may be insulated with batts, as described for the knee wall. Remember that the air and vapour barrier should be on the warm side of the insulation and be sealed at all joints and corners.
The remaining walls and the dormer ceiling are much more difficult to insulate. The easiest solution is to have insulation blown in. Remember to seal any ceiling fixtures or penetrations.
Floorboards
In some houses the attic is floored over, even when it's not used as a living space. You can insulate it by lifting the floorboards and treating it as you would an unfinished attic. Some or all of the boards may have to be replaced to maintain the stiffness of the ceiling.
Alternatively, a contractor can fill the sub-floor space completely by blowing in loose-fill insulation through access holes. Air leaks, such as around plumbing stacks, should be sealed first. To ensure that the space is filled completely and at the right density, you and the contractor should agree on the insulating value to be achieved and the total number of bags of insulation to be used.
Before insulating, either you or the contractor should check for 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 probably not be able to achieve the minimum recommended levels for attic insulation by filling this space alone. It might be worthwhile to add some insulation over the floorboards to keep them warm and reduce thermal bridging through the joists.