With both wood frame and masonry walls, you can build a new insulated wall onto the existing one. The new frame wall can be installed any distance from the old wall as long as all cavities are filled with insulation (see Figure 7-6). If you ensure that a new, well-sealed vapour barrier is installed, there is no need to remove or damage any existing vapour barrier.

In both options noted above, follow the relevant part of the instructions for insulating a basement from the inside. This includes sealing air leaks, framing the new wall, framing around window and door openings, insulating (ideally in two layers so there are no gaps), installing an air and vapour barrier, and applying the new finish. For more details, see Insulating the basement from the inside.

Additional points

• When installing the air and vapour barrier, cover the entire wall area, including window and door openings (see Figure 7-5). (Later, this will be cut with a large X - from corner to corner - and the flaps sealed to the frame.) Make sure the air and vapour barrier is well sealed at all joints, openings and penetrations.
• Though it may be difficult, plan for the insulation to extend behind any obstructions (e.g. pipes, electrical boxes and heating ducts), so that these are on the warm side of the insulation. New and existing electrical boxes need to be moved into the new wall and installed in poly pans that are sealed to the new air and vapour barrier. Plumbing pipes should be moved into the new wall or carefully insulated only from behind to prevent freezing and bursting. Any air duct joints must be air sealed with approved sealants.
• If you are rebuilding the existing wall, seal all cracks around door or window frames with non-expanding polyurethane foam sealant or stuff the gap with insulation or backer rod and caulk. If installing a new wall, it is usually not necessary to install extra framing around windows and doors, as the new wall is non-load bearing.
• Consider extending the insulation past junctions at partition walls. To do this, remove the drywall on the partition wall back one stud space and cut back the partition wall enough to extend the insulation and seal the air and vapour barrier through the partition wall.
• As a fast option to increase the wall RSI value while reducing thermal bridging, install at least RSI 0.88 to 1.76 (R-5 to 10) rigid board insulation directly to the wall finish and cover it with drywall. You will still need to ensure the vapour barrier is sealed and will have to reposition such things as electrical switches and plugs. Seriously consider having insulation blown or injected into the wall cavities at the same time.