Storey and a half attics
This type of attic can be difficult to retrofit. Insulation is applied to the
outer attic floor, walls and ceiling of the attic, as shown in Figure 16.
Outer attic floor and attic ceiling
• To prevent warm air from bypassing the insulation, block and seal the joist
spaces under the knee wall with an impermeable rigid insulation and
caulking (see Figure 17).
• Seal and insulate the outer attic floor and attic ceiling
• Seal any obvious air leaks and then place batt insulation between the knee
wall studs (see Figure 17).
• If the studs are 89 mm (31/2 inches) deep, use a single layer of RSI-2.1
(R-12) insulation vertically between the studs and another layer horizontally
behind the studs. Support the second layer of insulation with 19 x 64 mm
(1 x 3 inches) framing nailed to each rafter.
To increase the effectiveness of the insulation, place a housewrap material
(Tyvek, Typar or similar) on the back side of the insulation. The housewrap
should be secured to the rafters and held in place by the 1 x 3 framing.
• The end walls can be insulated in several different ways; another publication in
in this series, entitled Booklet #4: Wall insulation, provides details.
• Sloped sections of the ceiling can be strapped, insulated and covered with a
new air-vapour barrier as shown in Figure 18. Two layers of strapping at
right angles can be used to attain a higher level of insulation and to reduce
“thermal bridges” through the wood strapping (wood has a lower resistance
to heat flow than insulation).
With this method, the existing air-vapour barrier should be punctured two
or three times between each rafter as a precautionary measure. This is
necessary because the existing air-vapour barrier will now be on the cold
side (in winter) of the new insulation.
• This method can also be used on the knee walls and flat portion of the attic
ceiling, if access to these areas is difficult.
• Remember that most rigid insulation is flammable and must be covered with
a minimum of 13 mm (1/2 inch) gypsum wallboard.
• As an alternative, it may be possible to cut pieces of rigid insulation and
then slide them into the rafter spaces (remember to leave at least a 25 mm
(1 inch) space above the insulation to allow for ventilation). Since the rafters
are often only 89 mm (31/2 inches) in depth, it is difficult to achieve much
more than RSI-2.1 (R-12) without plugging the rafter spaces completely.
This is below the recommended value for a ceiling but may be acceptable
from a cost-effective standpoint