Renovation & Design


Renovation & Design

Sprayed-on poly foam OK for stone foundation insulation

Question: I have been trying to do some research on the risks associated with stone foundations that have had spray foam applied on the interior in the crawl space.

We have had an inspection done on a house that had this applied and the inspector raised this issue.


— David Lee

Answer: When relatively new materials or procedures are used in homes, it may be difficult to fully determine their effectiveness or problems for many years.

Spraying polyurethane foam insulation on the inside of a stone foundation should not create any serious issues, but any problems related to moisture may not show up for several years, making speculation about potential pitfalls largely theoretical at this point in time.

Before answering your question, my opinion on insulating most stone or rubble foundations is they should be left uninsulated and uncovered, if possible.

I know this is not practical for several reasons, mainly energy efficiency, but basements built with this type of foundation were not designed as living space — the basement was simply a cellar to store the mechanical systems, and other items, for the home.

It didn’t matter if there was a little seepage through the foundation walls, as there was little to become damaged before this water ran harmlessly to the floor drain.

So, leaving the walls uncovered on the interior allows for periodic inspection and repointing of mortar in between the stones.

This is important to prevent excessive leakage, and to prevent movement in the stones, which can lead to serious structural issues.

Regardless, if you are going to insulate the inside of this type of foundation, sprayed-on high-density polyurethane foam is the best choice.

With many older houses, various dilemmas arose when we started adding large quantities of thermal insulation.

Insulating attics and foundation walls created a good environment for condensation and moisture to develop.

This occurs because most traditional insulating materials do not stop air movement, they only slow it down and trap air to improve their performance.

Because of this, moisture dissolved in the air will cool, and often condense when it hits the cold foundation wall surface, or cold attic space.

Because of this property, moisture damage and mould growth are major concerns.

To counteract this issue, the use of air/vapour retarders has become necessary in conjunction with the insulation.

The most common type is a layer of polyethylene sheathing, often stapled to the framing around the insulation.

While fairly effective in preventing air leakage into the insulation, this plastic can also trap water vapour, which may arise from condensation of air that does get past the barrier.

It is critical to seal this poly around the perimeter, at each framing member, and around any protrusions, to prevent excessive air leakage.

Regardless, it is still nearly impossible to prevent, so problems with using traditional fibreglass batts for foundation walls are common.

What makes it even worse with a stone foundation is the added possibility of wet insulation from seepage.

The reason I have described the traditional basement insulation method is to make the point that even though there may be some issues with installing sprayed-on foam to the inside of a stone foundation, they are much less possible than problems with more traditional batt and poly methods.

The two main concerns often raised in discussions are moisture-related issues and lowering the temperature of the foundation.

Both are somewhat theoretical, as they may not be visible once the entire inside surface of the foundation is covered with spray foam.

Moisture concerns are often related to collection of moisture behind the foam, which may lead to hidden mould growth.

The theory is that any seepage or water vapour that migrates from outside to inside will provide an ideal environment for mould growth.

Since there is dirt and debris on the surface of the stone, and also materials in the mortar which can provide a food source, two of the three major factors for mould are present.

The final component, heat, may not be an issue in the winter, but could certainly be a factor in the warmer months.

While this may be a legitimate concern for many older stone foundations, my opinion again is that it is much less of a concern than with batt and poly applications.

The foam insulation is moisture-resistant, so it may even help prevent some moisture migration due to this property.

Regardless, most mould issues with foundations occur from condensation of warm house air leaking into the insulated wall cavity, not from outside seepage.

Having insulation on the inside that has an excellent, integral air/vapour barrier, like spray foam, will certainly prevent this from occurring.

Also, spray foam conforms beautifully to all surfaces, even uneven stone and mortar walls, so air gaps and voids common with batts and wood studs are virtually non-existent.

Less gaps for air to leak into means less places for mould to grow.

The last concern is that stone foundations were designed to "breathe" on the inside and allow moisture trapped in the mortar to evaporate into the basement.

The concern is that insulating the interior surface tightly, by spraying foam, will prevent warm house air from contacting the stone, allowing it to freeze in the winter.

This could cause the damp mortar to expand and become damaged, without being visible.

While this may occur, it could be years or decades before this theoretical problem occurs and would still be possible with any other type of insulation installed on the inside.

There may be much speculation on this subject available, even by knowledgeable inspectors and building scientists, but only time will show us if they are true, or not.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the past president of the Canadian Association of Home & Property Inspectors — Manitoba ( 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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