Common materials for use in accessible attics are batt or blanket types or loose-fill insulation. If there are obstructions above the joists, such as with a truss roof, it may be easiest to put batt insulation into the joist spaces and then use loose-fill insulation to create a complete blanket of insulation above the joists and around all obstructions. Loose-fill insulation is also good by itself, especially in irregular or obstructed spaces.

Batt or blanket insulation

Fit batt insulation into place between the ceiling joists snugly, without compression. If the joist space is not the standard 16 or 24 in. (400 or 600 mm), cut the batts to fit the space or use loose-fill insulation.

Additional installation tips:

• Butt batt ends together as snugly as possible.
• Batts should cover the top plate of the exterior wall but not block the venting. To maintain airflow, leave a space of 38 to 50 mm (1 1/2 to 2 in.) between the top of the insulation and the underside of the roof sheathing. Use baffles between each rafter space to prevent it from being blocked (see Figure 5-11).
• Insulate above and below cross bracing, splitting or cutting the batt to accommodate the cross bracing as illustrated in Figure 5-12. Alternatively, cut one batt into a series of wedges and then fit a wedge under each brace. In either approach, do not compress the insulation.
• The first layer of batts should be thick enough to completely fill to the top of the joist space. The second layer can then run perpendicular across the joists to block any heat flow through and around the joists (see Figure 5-13). Ensure that there are no gaps between the two layers of insulation.
• Fill any awkward spaces or gaps with pieces of batt or with loose fill.
• Apply blanket insulation in the same way as batts. It may be pre-cut with a knife or cut on the spot. Start at one end of the attic and unroll the blanket

Loose-fill insulation

• Loose-fill insulation can be poured by hand or blown in on top of the air and vapour barrier to a depth greater than the height of the joists. This task can be done by the homeowner using rented equipment or by a qualified contractor.
• Before adding insulation, nail enough depth indicators (e.g. a piece of wood nailed perpendicular to the joist) or mark highly visible lines on truss members so you can easily see and gauge the depth of the insulation. Maintain an even depth throughout the attic by levelling the insulation with a board or garden rake (see Figure 5-14).
• Fill all nooks and crannies.
• At the eaves, do not block the ventilation. Prevent insulation from disappearing into the eave space by installing a piece of rigid board insulation or a wood baffle before the work begins. Building-supply stores sell cardboard or foam plastic baffles that can be stapled between the rafters (see Figure 5-11). Be sure that the insulation extends out far enough to cover the top of the exterior wall.
• If the loose fill is deeper than the joists, build insulation framework (a crib) around the attic hatch so that it can be filled to the edge (see Figure 4-7).
• The bags of insulation material will list how many square metres (or square feet) each bag should cover to provide the required RSI value. Calculate the attic area to help determine the number of bags needed.
• If you are having a contractor do the work, calculate the RSI value that you want and check the bags of insulation to be used. They should indicate the area that one bag will cover at the selected insulating value. You and the contractor should then agree on the total number of bags to be used, the expected insulating value and the minimum settled depth of insulation throughout the attic, based on a specific density.
• Remember to seal the attic hatch (see Figure 4-7).
• Seal and insulate up to party walls in attached housing (see General considerations for all attics).
• Do not cover recessed light fixtures unless they are installed in approved covers.