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Spray foam on stone walls (not basement)

I have a small 150 year old stone house. We're in Ontario, Canada so lots of freezing winter conditions. The walls are 22" thick and sit right on the bedrock, about 2ft down. The floor was rotten - logs split and laid in the dirt with the flat side up, then covered with planking. We dug all the dirt out down to the bedrock, back-filled with packed gravel, insulated -perimeter too- and topped with a 4" heated concrete slab. We have framed with 2x4s about an inch out from the stone wall in preparation for closed-cell spray foam.
Will spraying the foam directly onto the interior of the stone wall cause damage from trapping moisture?
There seems to be some with strong opinions (usually historical purist-type people who believe it it wasn't used when the house was built it shouldn't be used now) that say the walls will be destroyed in short order because moisture migrating into the walls will be trapped by the foam, then freeze.
I understand there could be something to this, but the wall is not a one-way street. So moisture won't be able to dry to the interior anymore...why couldn't it dry to the exterior?
Thoughts on this? And if you have direct knowledge of damage caused be the application of spray foam to a stone wall, I want to hear about it!

Asked by mr157ifhz
Posted Dec 30, 2017 12:29 PM ET


3 Answers

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Q. "Will spraying the foam directly onto the interior of the stone wall cause damage from trapping moisture?"

A. Usually not -- especially if you are talking about a dense stone like granite. The problem you describe can occur in old brick homes, because porous bricks are subject to freeze/thaw damage if they stay wet. (For more information on the problem with brick homes, see this article: Insulating Old Brick Buildings.)

So your plan seems fine to me. Any old brick or stone building will last longer if the building has wide roof overhangs than if it has stingy roof overhangs, because you want to keep rain off the masonry. If there are any flashing defects that allow water to dribble down the stonework, it's essential that your repair these flashing problems to try to keep your walls as dry as possible.

But as I said, stone walls are not as subject to problems as brick walls.

Answered by Martin Holladay
Posted Dec 31, 2017 6:35 AM ET


Be sure no wood is in direct contact with the stone or concrete. You want a capillary brake (foam, rubber, copper or steel) between the masonry and the first piece of wood that needs to be pressure treated.

My guess is the 22 inch thick wall is dry laid and then the inside surface was pointed with mortar. If that is the case I think the wall will be structurally sound without regard to your insulation choices.


Answered by Walter Ahlgrim
Posted Dec 31, 2017 4:13 PM ET


Thank you for the responses. The stone is limestone (not sandstone!) - so I don't think near as porous as brick but not as dense as granite. The walls are mortared throughout, with two outside courses sandwiching a core of smaller stones.

Answered by mr157ifhz
Posted Jan 3, 2018 3:57 PM ET
Edited Jan 4, 2018 11:55 AM ET.

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