Every now and then we have to insulate the inside of a foundation wall. Concrete walls have zero insulation value, according to the building code. In our cold sector of the country, we have to insulate walls to an R-Value of 21. I recently had just such an occasion, to insulate a concrete foundation wall in a home that we were remodeling in Rye, NH.
You see, this foundation was built as part of an addition, some years ago. It seems that whomever built this addition didn’t pay quite enough attention to the little details. There are water pipes running through this utility area, with no insulation on these concrete walls and no heat source. What do you suppose happened to those water pipes this winter? They sure did freeze, unfortunately.
What did we do about it? Well, we decided to make this utility area of this addition warm. We started by determining what the vulnerabilities are, and then convert them into strengths. The rim band of the floor framing (where floor joists terminate over an exterior wall, in this case foundation wall) is a typical vulnerability. This one’s no different. We decided to use standard 2″ blue foam insulation board and fit it into these spaces between the joists. We then seal around it with the spray foam gun.
The next and only other major vulnerability is the concrete foundation. In this case, the foundation walls are exposed entirely to the elements on the other side, no earth for insulation purposes. So, if these concrete walls essentially have no insulation value, let’s rectify that. I chose to walk a nice balance between frugality and functionality. We wanted to keep the cold out and the heat in, but without spending a fortune. We also want to provide a safe finished product. Leaving most foam board products exposed would be unsafe and a code violation. They will easily ignite. I didn’t want to have to sheetrock or put plywood over the foamboard, so I decided to upgrade to Dow Thermax. You’ll have to research whether this can be left exposed in your municipality, but I chose to go with it here.
We start by applying adhesive to insulation pins. The adhesive is made specifically for this purpose and emits rather potent fumes, so take precautions. After letting the pins cure to the concrete overnight or so, it’s now time to cut and fit the insulation board onto the pins. We air seal the perimeter of each sheet with the fire resistant foam from the foam gun, and we’ve just sealed out the cold. We opted to only go with 2″ foam, which now gives our wall an R-Value of approximately 13. Not quite up to code for new construction, but probably about 500% better than it was. Also, we weren’t required to adhere to building code because we didn’t need a permit for a repair of this nature.
Finally, we decided to maximize heat that was already available, instead of install a new heat source. We cut a nice hole through the plywood that had been used to block in an old foundation window dating from before the addition. On the other side of this plywood window, there is a wall mounted on demand boiler hot water heater. The heat transmitted from the existing mechanical room was ample enough to supply the previously cold, adjacent room.
Eldredge Lumber in York, Maine carries the Thermax insulation for around $60 per 4′ x 8′ sheet, and Ricci Lumber in Portsmouth, NH special ordered in the pins and adhesive. The minimum order was 1,000 insulation pins, and we used a quart of adhesive. The total for these 2 items was under $200. We air sealed the edges of all the panels with a fire retardant spray foam from a can. We used 6 sheets of the foam insulation for the concrete walls, and 2 sheets for the rim band where the floor joists meet the exterior wall. Super simple, you can do it!!!