Insulating on the outside is best, but it is often necessary to insulate from the inside for economical and practical reasons. Sometimes a combination of approaches is required. Examine the advantages of each approach carefully.

i) Insulating inside

This may involve installing rigid insulation board and drywall, a wood-frame wall and insulation or other combinations of insulation. Your choice of method will depend on a number of factors, including whether there are moisture issues, the need to account for moisture and air/vapour barriers, how you plan to use the space and, finally, cost.

Advantages of inside insulation

• It can be incorporated into a plan to finish the basement.
• The work can be done at any time of the year and can be done one section at a time. Often all or part can be a do-it-yourself job.
• It is often easier and cheaper to insulate the full wall and achieve high insulating values.
• The landscaping and driveway will not be disturbed.

Disadvantages of inside insulation

• Do not insulate a basement with moisture problems from the inside (leaks, dampness, efflorescence and blistering paint). If you must, take corrective measures to eliminate the moisture before adding insulation or your new walls will rot.
• Adding insulation to the inside will make the foundation walls even colder. Any humid air that comes in contact with these cold walls will condense. Interior finishes hide or obscure moisture problems as they develop. This can lead to long-term mould exposure as people are less likely to fix a moisture problem if they have to remove finishing that they have erected.
• Obstructions such as electrical panels, wiring, plumbing, stairs and partition walls make the work more difficult and the insulation and air barrier less effective. If part of the basement wall is already finished, this too may prove troublesome, although wall paneling may be easy to remove and re-install.

TECHNICAL NOTE:

Some authorities have expressed concern about the possibility of frost action and structural damage when foundations are insulated from the inside. The concern is that frost will penetrate deeper down the outside of the foundation wall. Research has found that this is not a problem. Under some circumstances, such as in soils that are particularly frost-susceptible in extreme climates, there could be a problem caused by some construction techniques. Check with your local building authorities or find out if your neighbours have experienced any difficulties with frost action on their foundation.

ii) Insulating outside

This involves excavating around the foundation, waterproofing and installing insulation, as per Figure 6-3. Flashing must be attached to keep water from getting behind the insulation and a protective covering installed on exposed sections of insulation.

Advantages of outside insulation

• The outside wall tends to be more continuous and easier to insulate.
• You can effectively see and correct any moisture or structural problems (efflorescence, cracks, spalling and eroded mortar). Rubble or brick foundations and foundations with water leakage, dampness or other moisture problems should be insulated from the outside. Repairing the foundation, parging, waterproofing and installing a drainage system can be done at the same time.
• There is no disruption in the house, no interior work disturbed and no inside space lost.
• Freeze-thaw stresses are eliminated, and frost is unlikely to penetrate down to the footings.
• The mass of the foundation is within the insulated portion of the house and will tend to even out temperature fluctuations.

Disadvantages of outside insulation

• Digging a trench around the house by hand can be difficult and risky depending on soil type and depth. It is much easier to use machinery but access could be a problem.
• Storing the dirt can be a problem.
• Excavation cannot be done in winter and can be a problem in the spring or throughout the year if the property has a high water table.
• Features such as non-removable steps, paved carports, shrubbery, trees or fences can make the work difficult.
• Rubble or brick foundations could be partially supported by the soil. Get expert advice before starting.
• It is expensive to obtain high insulation levels, and the retrofit may detract from the appearance of the house.