An irregular basement is usually made of stone or rubble and is rarely waterproofed on the outside (see Figure 6-2). While an exterior retrofit is always recommended, it may be possible to insulate on the inside if there are no water or moisture problems.

First, cover the interior wall with cement-based parging to smooth the surface and to protect the existing mortar. Then build a frame wall, add a maximum of RSI 2.1 (R-12) batt insulation and finish as previously described in Frame wall with single or double layer of batt insulation.

In general, do not go with higher insulation values as there is a risk that the wall may be subjected to damaging freeze-thaw cycles. Check with your local building authorities for more information on how to deal with this potential problem.

Closed-cell spray polyurethane foam has been used successfully to reduce dampness problems for rubble walls while offering some thermal protection value. Do not exceed RSI 2.1 (R-12). This product must be installed by a certified installer and be covered by an appropriate fire-resistant material.

For very wet basements prone to flooding and high moisture problems, it may be best not to insulate the basement walls. Instead, consult your building authorities and an experienced general contractor about the potential to treat the floor joist space as an exposed and insulated floor.

Additional modifications will be needed to take care of the plumbing and heating system.