My husband and I read your column in the homes section of the Free Press every Sunday, and often compare what we have and have done to our house with your advice. My question relates to home insulation, specifically basement insulation.
We live in a 750 square foot bungalow in Old St. Vital that is 57 years old. We had three of our four basement walls excavated because of extensive cracking and leaks. The fourth wall had been repaired with the addition of a knee-wall about 25 years ago and seems fine. During the excavation and repair process we had 3/4″ rigid insulation added to grade to the outside of the three basement walls. We have also stripped the stucco off our exterior walls and added 3/4″ rigid foam insulation under new vinyl siding. The contractor that did the foundation would only insulate to grade, however, and the siding company brought their insulation down to about 6 inches below the floor joists, which sit inside the concrete foundation wall. Therefore, there is about a 2-foot area on the outside of the foundation that currently has no insulation. There is no insulation on the inside of the basement walls at this time.
My concern include heat loss through the portion of the basement wall that currently has no insulation, between grade and where the siding begins, and if that situation will adversely affect our floor joists which sit in the foundation walls. My husband has diligently caulked all the cracks and gaps around the floor joists and all penetrations through the foundation wall such as pipes, plugs and fresh air intake. We have also painted walls and refinished our hardwood floors upstairs, so the gaps the baseboards and floors on the outside walls have been caulked as well. I am also worried about frost, efflorescence or other problems in the region that doesn’t have any insulation. From your experience is this a potential problem and, if so, how would it best be handled?
You provide a valuable service to the public and we thank you for this.
The area between the floor joists, where they are inlaid into the concrete foundation walls, is a difficult area to deal with when insulating, even in new homes. I commonly see poorly sealed polyethylene air-vapour barrier stapled loosely over fibreglass batts in this area. When this poly and insulation are pulled back there are often signs of moisture or condensation on the foundation wall. You are correct in your concern for moisture problems developing in these “pockets”.
The one misconception you may have is about the insulation value of the ¾ inch rigid foam insulation installed outside the foundation walls, and the house walls. Even with higher density extruded foam, the R-value of this insulation may be R3 to R4 at best. This will give you minimal benefit, as far as preventing heat loss is concerned through your foundation walls. This insulation is often installed as a protective covering for the damp-proofing on the foundation and to help prevent frost heaving of the footing under the foundation wall. Without insulation and a well-sealed air barrier on the interior of the foundation walls, you may still have significant heat loss through this area.
The thin foam insulation on the upper house walls may provide a larger benefit in regards to heat loss even though it’s R-value is low. This is because it may provide a reasonable air-barrier beneath the new siding. Hopefully, this insulation is expanded foam that will allow some moisture to pass through but may reduce “wind-washing”, or air movement into the old wall cavities. This, along with the caulking and air sealing, that you have recently done inside the home, may have the biggest effect on heat loss. Due to your husband’s diligence in caulking around the joists and penetrations in the basement, there should be minimal air intrusion through the foundation, which will minimize frost. There still may be condensation due to the cool temperature of the concrete or thermal bridging from the floor joists. This is the reason that insulating this area is recommended, although the remainder of your foundation walls may still have the same issues. It would be ideal to install insulation and an air-vapour barrier over the entire interior of the foundation walls to minimize heat loss and prevent condensation.
The area between the floor joists is ideally insulated with either foam-in-place or rigid foam insulation, but the later is less costly and may be installed by yourself. The benefit of these foam insulations is that they are highly moisture resistant and provide a reasonable air-vapour barrier. The rigid foam may be adhered to the concrete with construction adhesive designed specifically for this material. The edges of the insulation should be caulked to the floor joists, to provide as tight a seal as possible. If extruded foam insulation, such as Styrofoam SM, is cut to tightly fit between the joists it may provide an adequate vapour barrier, on its own. This can later be covered with polyethylene sheathing if the lower parts of the wall are insulated traditionally with studs, fibreglass and an air-vapour barrier.