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Q&A Spotlight

How to Insulate a Foundation

Choosing the right material and the right technique for a house with a walkout basement

Image 1 of 4
Expanded polystyrene is one of several options for insulating the slab and foundation walls of a house. A GBA reader who is about to start work on his new house wonders which is the best choice.
Image Credit: Scott Gibson
Expanded polystyrene is one of several options for insulating the slab and foundation walls of a house. A GBA reader who is about to start work on his new house wonders which is the best choice.
Image Credit: Scott Gibson
Photo 2: The basement of this home undergoing renovation is insulated with R-19 kraft-faced batts, some of which have been removed.
Image Credit: Peter Yost
Photo 3: The uninsulated wall is pretty cold — 26 degrees in the crosshairs, an area right above finished grade. The image was made with an infrared camera.
Image Credit: Peter Yost
Photo 4: The temperature difference between insulated and uninsulated sections of wall fades significantly as the depth below grade increases.
Image Credit: Peter Yost

About to start a new house in Climate Zone 5, Nicholas C is working out the details of how to insulate the basement slab and foundation walls. There is more than one type of rigid foam insulation he could use, and it could be applied on either the inside or outside of the foundation.

For a couple of reasons, he’s planning on 2 inches of extruded polystyrene (XPS) beneath the slab rather than expanded polystyrene (EPS). The XPS would perform better in a wet environment, Nicholas says, and Owens-Corning, one insulation manufacturer, claims it no longer uses a “bad” blowing agent.

He’s not had much luck in tracking down a local supplier of Type II EPS, while XPS seems much more available.

“If I have the option to get EPS Type II, I want to know if I should use it or XPS for my foundation walls,” Nicholas explains in a Q&A post at GreenBuildingAdvisor. “This is a walkout basement, meaning one wall is completely below grade, one wall is half-and-half, one wall is completely exposed, and the other wall is conveniently below grade because the garage is on that side, so it has dirt beneath it but also less exposure to elements since it is also the garage stem wall. “

He also wonders whether he should hold off on installing insulation on the outside of the foundation walls now and tackle that project from the inside as a retrofit a few years down the road.

Those related questions are the start of this Q&A Spotlight.

Waiting is not a legal option

Whether Nicholas chooses XPS or a dense form of EPS, he has to choose something, replies GBA senior editor Martin Holladay.

“I’m not sure about code enforcement in your area, but in Climate Zone 5, basement wall insulation is required, not optional,” Holladay says. “Leaving any walls uninsulated (whether above-grade walls or below-grade walls) is an example of builder error, and in most jurisdictions it is illegal. So my advice is: you can install basement wall insulation on the interior or the exterior, but you have to install it somewhere. Omitting it is not an option.”

From a performance standpoint, he adds, either type of polystyrene would work.

Assuming that foam, and not fiberglass batts, are used, the 2012 International Energy Conservation Code requires foundation walls to have R-15 of continuous insulation on either the inside or the outside. But Nicholas doubts that the code is enforced where he lives, either because building officials don’t know about it, or because the code isn’t up-to-date.

Further, he’d rather not apply the 3 inches of insulation he would need to reach R-15 because it would require more than a single layer of foam. “I was only planning on 2 inches of EPS or XPS on the exterior,” he says. “Three inches to achieve R-15 will be troublesome because it means I need to buy two layers to achieve 3 inches. I know it will be backfilled on some of the sides but the side it is not covered at all, I would like to fasten without any of the anchors you mention. Is adhesive just not a good option on the exterior side?”

“I strongly urge you to at least meet minimum code requirements,” Holladay replies. “How you fasten the foam on the exterior depends in part on your choice of materials to protect the above-grade portions of the insulation.”

Comparing XPS and EPS

EPS does absorb some water, writes Dana Dorsett, which can lead to an R-value reduction of about 15 percent. But, he adds, “that performance is restored when the tide goes out. It dries as quickly as it takes on water.”

XPS bleeds some of the blowing agents used to manufacture it over time, he adds, also resulting in a 15 percent loss of insulating value after a few decades. “But that performance hit is permanent,” Dorsett says.

Dorsett also takes issue with claims from Owens-Corning about the blowing agent it uses.

“Owens Corning still uses HFC blowing agents for XPS, (the predominant component of which is still HFC134a) and have not moved over to HFO1234 blowing agents with low to very low global-warming potential,” he says. “The ‘70% less Global Warming Potential than the blowing agents used before our blowing agent conversion in 2009’ claim means that instead of having 200 times more climate damage than EPS, they’re ‘only’ 60 times more damaging. That’s 6,000 percent more climate damage than EPS.”

Dorsett’s formula for insulating the slab and foundation walls would go like this:

Start with 2 to 3 inches of Type II EPS under the slab, with 2 inches of EPS between the slab edges and the foundation wall. “You can then hit R-15 whole-wall R performance using 1 1/2 inch of EPS or 1 inch of foil-faced polyiso trapped to the foundation wall by a 2×4 stud wall insulated with unfaced (or kraft-faced, but not foil-faced) R-13 or R-15 batts.”

If Nicholas is unable to find local sources for the EPS, he could try ordering used-once, surplus, or factory seconds ordered from Nationwide Foam or Green Insulation Group, where prices are as much as 75 percent lower than virgin stock goods.

A note of caution about external foam

From a thermal standpoint, foam can be installed on either the inside or the outside of the foundation walls, but James Kreyling offers this footnote:

“A note of experience with exterior-applied XPS foam,” he writes. “Back in 1987 I built a home in New Hampshire — climate zone 5 — and based on the common wisdom of the time, insulated my foundation on the exterior with XPS foam, two layers of 1 1/2 inch each.”

Then one New Year’s Day, he discovered the south wall of his kitchen was “disintegrating” after being nearly consumed by termites and carpenter ants.

“Yup,” he adds, “they found their way in through the darkness of space between the XPS and the concrete foundation. I will never apply exterior foam on the foundation in contact with the soil again. Creating an unseen highway for the critters to move in and feast on my house.”

The answer to that problem, says Dorsett, is copper Z-flashing installed over the foam and behind the water-resistive barrier (WRB) along with a copper-clad plastic or sheet-metal capillary break between the concrete and the foundation sill.

“The 1980s practices did not usually have capillary breaks at both the footing and between the foundation and sill, which leads to higher moisture content in the foundation sill (even if it had a foamy or fiberglass sill gasket),” Dorsett continues. “A capillary break at the footing as well as under the foundation sill reduces the humidity in the basement, and keeps direct wicking into the foundation sill well controlled.”

Also, he says, applying a cementitious finishing material over the entire length of foam, not just the portion that’s above grade, will reduce insect access to the foam.

Or, consider a non-concrete foundation option

Jerry Liebler has another suggestion: a foundation built with pressure-treated lumber and insulated with mineral wool batts, rather than a conventional concrete foundation and footing.

“It is truly unfortunate that PWF (‘permanent wood foundation’) basements are not considered,” he writes. “The greenhouse gas emissions due to making the cement make concrete foundations far from green!”

Not only would a PWF basement cost less, a 2×8 wood-framed foundation sheathed in plywood and insulated with mineral wool batts would come in at R-25 at a thickness of 8 1/2 inches and it would not require concrete footings.

“Before you dismiss PWF, there are over a million in North America. None has termite problems and they come with a 75-year guarantee,” he adds.

Our expert’s opinion

Here’s what GBA technical director Peter Yost has to add:

Let me just say that it is pretty hard to improve upon Martin’s “treatise” on basement insulation.

Confused basements: More than a few of us building professionals — and certainly many of our clients — have a gut feeling or sense that below-grade spaces are inherently damp or prone to moisture problems. This comes from so many below-grade spaces that have not been designed and detailed as truly interior spaces — confused below-grade spaces not really sure if they are “in” or “outside” the living or conditioned space of the building. These spaces often lack continuous management of heat and moisture. Since it is much more difficult and expensive to properly manage heat and moisture down the road, decide now if the basement is inside or outside your home, and manage heat and moisture accordingly.

Exterior or interior: From a hygrothermal perspective, I have always liked pulling any part of the building enclosure into conditioned space, particularly high-mass walls such as concrete; this allows the mass to be a thermal “flywheel.”

But I have just seen too much damage to exterior basement insulation, hidden damage by insects and exposed damage from weed-whackers and the like. Covering or “protecting” the exterior rigid insulation is often done with just a simple parge coat, and it’s just not robust enough. Cementitious cover boards work well but are pretty expensive protection.

Concerns that below-grade concrete walls must dry to the interior — limiting choices for interior insulation systems — have proven to be unfounded.

The need for basement insulation/air sealing: I just wanted to share a short series of photos that place basement thermal performance in perspective, especially in terms of relative thermal impact.

Image #2, below, is of an existing home, under renovation: a concrete basement with a stud wall insulated with R-19 kraft-faced batts. It’s about 5°F outside — an early morning in January — and it’s about 50°-55°F in the basement. Note that some stud wall bays to the left are insulated; others around the basement window had the insulation removed some days prior.

The next photo (Image #3, below) is an IR close-up of the window and surrounding wall. Note how “cold” the uninsulated wall is (the cross hairs at 26°F are just a bit above finished grade on the exterior) and how “warm” the insulated basement wall is to the left.

The last photo (Image #4, below) is an IR photo showing quite a bit of the basement wall below grade. Note how the temperature difference between the insulated and uninsulated wall fades significantly as the depth below grade increases.

Now, certainly the delta-T and R-value of the insulation for this particular situation are driving the specific temperature results, but what this tells me is that regardless of the delta-T and R-value of the basement insulation, the most important parts of the basement walls to get detailed right are the portions above and just below grade, in terms of both thermal insulation and air sealing (note the really dark black spot in the first IR photo in the lower left hand corner of the window).

As you near relatively stable, year-round ground temperatures in the 50s, the performance of the wall is more about thermal comfort of occupants and IAQ/moisture management than it is overall thermal efficiency.

So, in any basement insulation system, focus on the thermal and air barriers near and above grade, particularly at the transition from below-grade to above-grade wall system — and certainly on the completely above-grade, walkout wall.


  1. George Hawirko | | #1

    How to Insulate a Foundation
    How do you writers find this disinformation, and then never check it out for accuracy before puublishing? People who have even half a brain will start to not trust any information on this website. Education on the proper use of construction grade foams needs to be addressed by the educational and scientific people before we have every idiot following these unresearched misleading articles.

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to George Hawirko
    Here's my advice: if you are going to accuse GBA of publishing disinformation that has not been checked out for accuracy, it would be appropriate for you to mention at least one statement in the article that you deem to be inaccurate.

    Absent that, it's hard to assess your opinionated posting.

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