Renovation & Design

Is spray foam insulation suitable for stone foundations?

Question: We live in a 110-year-old home in the Wolseley area of Winnipeg. There is a stone foundation and the basement currently has batt insulation. My husband and I are considering spray foam insulation, but have heard many different opinions about whether that’s the way to go, or not. What would you suggest? Thanks, Julia.

Answer: Insulating the inside of a foundation with high density spray-on polyurethane foam may be the best method, regardless of the composition of the walls. While there is still much debate over possible negative outcomes of this construction procedure on stone foundations, only time will tell if those potential issues will come to pass, or if they are purely theoretical.

Right off the bat, replacing batt insulation that is applied to the inside of a foundation in our climate with any type of moisture-resistant materials will be a huge upgrade. This not only applies to concrete foundations, but also to older limestone or rubble ones common in this area. Fibreglass batts, commonly used for this purpose, are a poor choice for two main reasons. First, they are subject to easy absorption of moisture, whether it is wicking it up from a damp basement floor, from melted frost on the inside walls, or from seepage typical of really old stone foundation mortar. Either way, if that type of insulation gets wet it becomes thermally ineffective and subject to mould growth from dirt and other particles suspended in the fibres.

The second reason not to use fibrous batts is due to the lack of resistance to air movement through the insulation. The fibres only slow down air movement, which can lead to a loss of heat energy, increasing the possibility of condensation and frost development. That is why heavy 6MIL polyethylene sheathing is required to be installed on the warm side. This poly air/vapour barrier may only be effective at stopping warm air intrusion into the wall cavity if it is extremely well sealed, especially at the top and bottom. That can be very difficult to achieve in an older basement. So, replacing your older batts with better insulation should always be an improvement.

Any insulation used for this upgrade should be waterproof, easy to apply, and highly resistant to air penetration to be most effective. While rigid extruded polystyrene, or other similar products, easily fit these criteria they have one major detraction for installation inside your stone foundation. They are flat and rigid and will not be able to conform well to the contours of the individual stones and mortar comprising your foundation walls. If a significant number of gaps are left behind even this high-quality insulation, air will be able to sneak in. That will still be a recipe for condensation and frost, which will likely lead to wet floors during the spring thaw.

This leaves us with a dilemma about whether to use the flexible fibreglass or mineral fibre batts, which may be able to fill most of the gaps between the individual stones in your foundation or rigid foam that can’t serve that purpose. The simple answer is to use neither in your pursuit of a totally covered, well insulated and sealed foundation wall with good moisture resistance. The solution is two-part, high density foam that is mixed onsite and applied by spraying on in a liquid form. This material will fill every nook and cranny and forms a hard, crusty surface after the short curing time. A minimum of two inches is required to achieve good air resistance, but more may be desired for a better thermal seal. That can be done by spraying successive layers, normally no thicker than three inches each time, due to the extensive heat-generating properties of the foam as it cures. Otherwise, two to three inches of foam can be installed in a single application, later to be covered with less expensive conventional insulation, to achieve a higher thermal insulating value.

The main concern I have heard with this method, for older stone foundations, is the performance of the foam over time to potential moisture penetration from the stone wall. Will this become wet on the exterior-facing side, creating an environment for mould growth and mortar deterioration? If the foundation is persistently damp, will the foam even stay adhered once the wall is covered up? My expectation is that applying a thick layer of this plastic foam to the inside of the older walls will not only help the structural integrity, but may also prevent moisture intrusion on the inside. Unless the foam is sprayed on a wet wall, or one where the mortar is loose and deteriorated, it should adhere better than almost any other building material I have seen. This should help partially seal the inside of this structural element from periodic seepage, but will also prevent condensation, by not allowing the warm interior air to touch the cold surface of the foundation in the winter.

There are theories about a stone wall performing better if the mortar is slightly damp, rather than dry, but I think that is just speculation. Also, there are opinions that the inside of the stone wall should be completely uncovered, allowing the inside heat to prevent any moisture in the mortar and stones from freezing in the coldest weather. That may be somewhat true, but is an equal risk with the current fibreglass batts, as well. If the wall is going to be insulated from the warmth of the home, much better to do it with poly foam than other poorer quality substitutes.

In my experience, the best way to insulate the interior of an older stone foundation wall is undoubtedly with spray-applied, high density polyurethane foam. This product has only been in common use for this purpose in our area for a little over a decade, and appears to have performed well. The coming years and decades should tell whether its use is a true success or a failure.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and a Registered Home Inspector (RHI)( Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at


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