Where headroom is sufficient, constructing a dropped ceiling to hold insulation is an excellent way of thermally upgrading a cathedral ceiling or flat roof, especially when planned as part of a renovation. The following are several options:

• Construct a new ceiling immediately below the existing ceiling. If the roof has exposed joists or beams (usually for decorative purposes), it may be possible to close the space, which creates a new ceiling. Install batts or rigid insulation in the space followed by a continuous air and vapour barrier and a new ceiling. In all cases, you must prevent warm air from getting into the new cavity space and bypassing the insulation. Seal the perimeter of the new ceiling and any possible air leakage paths through partition walls.
• With the ceiling removed, extend existing rafters or trusses to accommodate additional insulation. Cross strap the existing rafters or extend the rafter cavity with 2 x 4 lumber and plywood gussets (see Figure 5-24).

Although it is not necessary to remove the interior finish and expose the rafters and tops of the walls when constructing a new ceiling, it is a good idea. This will allow you to perform air sealing, check the state of the insulation and verify if there is a vapour barrier.

• Mechanically fasten rigid board insulation directly to the structure of an existing ceiling. Although this technique avoids the mess and time required to tear down the ceiling, it will make access to the space between the ceiling and the attic floor more difficult (e.g. to install recessed light fixtures). If rigid board insulation doubles as the air barrier, fit the boards tightly and seal the seams well. Extend electrical fixtures to accommodate the depth of the new ceiling.