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Regarding spray foam insulation.

I bought an 1853 Cape. On the second floor the ceilings were raised, new colar ties and rafters installed then spray foam insulation. The company rep advised I do the same for the basement/crawl space by spray foaming the stone foundation. There is only a small, 15X15 foot, 5' high basement under the main part of the house. The rest is crawl space, mostly on ledge. I want to have them put plastic under the first floor boards then spray foam under that. A lot of cold air comes up through the floors and I used up almost $5,000 in propane last winter. My carpenter feels the foundation should not be sprayed because it will weaken the foundation and make any future repairs too difficult. Please advise as to the most energy efficient and effective way to better seal up the house.

Asked by Miriam Nesset
Posted Jun 11, 2014 7:13 AM ET


4 Answers

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I don't advise you to install plastic under your subfloor, or to install spray foam on the underside of your floorboards.

The best way to insulate a basement is to insulate the basement walls, not the basement ceiling. The same approach works best for crawl spaces. Here are links to three articles with more information on the topic:

How to Insulate a Basement Wall

Building an Unvented Crawl Space

Fixing a Wet Basement

Apparently your spray foam contractor has advised you to install spray foam on the interior side of your stone foundation walls, while your carpenter has doubts about the plan. I am going to side with your spray foam contractor.

Unless the walls have major structural problems or show signs of collapse, the best possible way to stabilize them is to install closed-cell spray polyurethane foam on the interior of the walls. The spray foam performs several functions: it provides structural support by consolidating the walls and securing loose stones; it provides an air seal; it provides a vapor barrier; and it provides an insulation layer.

Answered by Martin Holladay
Posted Jun 11, 2014 8:45 AM ET
Edited Jun 11, 2014 8:47 AM ET.


Spray foam will not negatively impact stone foundations, not in the slightest!

In unheated buildings where the basement might drop well below freezing there would be some increased risk of frost heave by insulating the foundation, but frost heave propagate in the direction of heat loss, and in a heated building the basement will always be above the temperature of the near-freezing ground on the exterior of the foundation, making the direction of heat loss (and thus frost forces) away from, and not toward the foundation.


Note the conclusions in this document:


"Frost heave due to the interior retrofit insulation of foundations (due to the reduced heat flow to the exterior) in very cold climates (DOE Zone
6 and 7) has not been conclusively proven or disproven as a significant risk. Additional study is warranted, including the effectiveness of
remedial measures such as exterior “wing” insu lation and/or addition of exterior drainage to prevent saturation of exterior soils. "

If the footing is above the frost line and you insulate both the foundation walls and the slab there are some conditions that might induce frost heave in a VERY cold climate, but I'm gonna guess that your 1853 Cape isn't located in the Alaskan interior:


But in shallow foundations with footings too close to (or above) the frost line, using exterior "wing" insulation to limit frost heave potential would be best practices whether you insulate the foundation walls or not.

Answered by Dana Dorsett
Posted Jun 11, 2014 4:48 PM ET


Miriam, If you do not have mechanicals in the crawl space area, insulate the floor. Air seal the floor and then insulate with mineral wool. I disagree with the above comments because you left out to many variables which can compromise the foam during installation and after. Water and insects are just two points. With old rubble stone foundations it allows an easy path for insects to penetrate the foam with ease unless you have a pesticide company on speed dial. Most pesticide companies will not warranty their work against insect/pest infestation when they are aware foam is installed in this scenario.


CUFCA (Canadian Urethane Foam Manufacturer's Association) has a published document they ran to to shrug off the media when one Canadian lady's home was infested with insects in her foam. CUFCA cited this lady did not design the foundation in accordance with modern building code to resist termites and she did not have a pest control contract in place.


Here's a nice education on Ants.. http://www.extension.umn.edu/garden/insects/find/carpenter-ants/

Here's a previous Green Building article discussing the risk of using foam when insects and water are potentially a problem.


FYI... I live next to an elementary school where an artist friend of mine built numerous abstract fish art out of 2 inch blue Dow board. The art is attached to the face of brick masonry up high on the facade of the school. Today the foam is filled with carpenter ants well above grade.
Here's a link to see a glimpse of the foam fish when they were new. Unfortunately the school web site no longer illustrates the fish facade.


I hope this helps.

Best of Luck!

Answered by Richard Beyer
Posted Jun 11, 2014 10:17 PM ET


In general the most efficient way to insulate and air seal you basement/crawlspace is as Martin and Dana have suggested. There is data on this through Build America Reports on the Building Science website.
Like anything in life there is no perfect solution and taking any action brings about some risk (as taken no action does). You should try to mitigate that risk as much as possible. I think Dana has identified the risk of frost heave, very low, depending on your specific situation. Richard has identified another possible issue, insects. If you live in an area with high likelihood of having termites then you’ll want to think about this. As Martin has pointed out in other post (which I couldn’t find to link) if you live in a termite area then you should leave an inspection zone at the top plate for the pest guy to inspect. This means you’re not going to be able to do a great job of air sealing, but that’s what you get for living in a termite area.
On the other hand if you insulate the floors and you have equipment (ductwork, air handler..etc) or storage you run other risk.
My two cents from the little information we have, spray foam the walls. Be sure to get the proper R-value for your climate zone to meet code.

Answered by mark moses
Posted Jun 12, 2014 8:20 AM ET

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