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Q&A Spotlight

The Best Way to Insulate a Foundation

A builder in Idaho works out the details for adding foam insulation to both sides of his basement walls

Image 1 of 3
Insulating a concrete foundation. A builder in Idaho wonders how to waterproof a foundation wall that will be insulated on the exterior with 2 inches of rigid foam insulation, and whether there is any benefit to fully encapsulating the footing with foam.
Image Credit: Nethaniel Ealy

Nethaniel Ealy, a builder in Idaho who’s about to pour a concrete basement foundation, is trying to come up with insulation and waterproofing details that will be effective and within the budget.

The current plan is to place 2 inches of extruded polystyrene (XPS) on the outside of the foundation walls. At some point in the future, the homeowners would place another 2 inches of foam on the inside of the foundation walls between 2×2 studs, and then apply drywall over the studs.

When it comes to waterproofing, Ealy has a couple of choices. One is a water-based sealer applied directly to the concrete. The other is an elastomeric membrane called that’s typically used over insulating concrete forms. The peel-and-stick membrane would be applied over the foam, not on the concrete.

The Colphene will add another $1,000 to construction costs, Ealy adds in this Q&A post at Lakesideca Advisor. Is it worth it?

He also wonders about the overall approach to insulation.

“Since the exterior XPS just terminates on top of the footing in this design and does not encapsulate the footing, how much of a benefit does it actually pass on in terms of insulation?” he asks. “Would it be better to just install 4 inches of XPS (or comparable) on the interior and forgo the external XPS layer? If so, I would also plan to skip the Colphene and just seal my foundation with a water based coating. Any flag going that route?”

Those are the issues for this Q&A Spotlight.

The plan for interior insulation is not the best

An obvious problem with the plan to add 2×2 studs and 2 inch foam on the interior of the foundation is that the foam is thicker than the studs by 1/2 inch, Dana Dorsett points out. But even if the foam and studs were of the same thickness, this approach still has a big weakness: thermal bridging.

“It would be a waste of foam anyway,” he writes, “since its performance is severely undercut by the thermal bridging of the framing.”

A better plan would be to add 1 1/4-inch-thick sheets of polyiso foam whose seams are sealed with foil tape between 1 1/2 inch and 2 inches wide. The foam would be held in pace by 1×4 furring strips, attached with concrete screws driven into the foundation wall.

“That way you’d have an unbridged layer of R-7 between the furring and the concrete, keeping the furring warm and dry (no need for pressure-treated), and the air gap between the foil and gypsum adds another ~R-1 of thermal performance, from the radiation barrier effect and trapped air films.”

Polyiso 1 1/4 inch thick should be available from a number of vendors, he adds, even if it’s not typically available in big box retail stores. Foil-faced polyiso also is much safer to use on the interior than either XPS or expanded polystyrene (EPS) in the event of a fire.

The other issue to consider, Dorsett says, is the potential for the wall to wick up water from the concrete. A 2×2 applied directly to the foundation would be able to absorb water from the wall and transfer it to the drywall on the interior. That problem goes away when the insulation is held against the concrete with furring strips and concrete screws. “The amount of moisture you’d wick with TapCons is near zero,” he says.

Does the footing have to be insulated, too?

One of Ealy’s questions is whether the lack of foam insulation around the footing makes a significant heat-loss difference in the design. Dorsett thinks not.

“The XPS stopping at the top of the footing does leave a thermal bridge, but it’s between the ground temperature and interior temperature, not the outdoor extremes,” Dorsett says. “Extending it down the side of the footing doesn’t change it much. But the full foam under the slab that turns the corner isolating the slab from the footing is a significant thermal break, and if that’s continuous with continuous interior wall foam it’s pretty good.”

One area not to overlook is the strip of concrete between the edge of the mud sill and the inside of the foundation wall. Any rigid insulation on the interior should go up and over the top of this ledge, while the inside of the band joist also needs insulation.

Applying foam to the foundation exterior

Ealy writes that he has been able to pick up enough 2 1/2-inch graphite-coated EPS, a product called Insulfoam Platinum GPS, to do the entire house. But getting the insulation to stick to the concrete is proving to be a problem.

It looks like he’s sticking with the plan to use the Colphene membrane over the insulation, so the issue is getting the EPS to adhere to the foundation wall long enough to allow the application of the Colphene and then backfill — a matter of a couple of days.

He’s tried a water-based adhesive; it took 1 1/2 tubes of the stuff to get a bond, and that would be too expensive to be used on the entire foundation. He’s now considering large plastic washers helped in place with concrete screws, although it’s more work than he’s like to invest.

GBA senior editor Martin Holladay suggests a specialty fastener, such as insulation anchors or .

Choosing rigid foam insulation

A consideration of several types of rigid insulation — polyiso, EPS and XPS — inevitably brings up a discussion of blowing agents they’re manufactured with. EPS and polyiso both are made with pentane, a hydrocarbon, while XPS is made with a hydrofluorocarbon (HFC).

“XPS is one of the least-green insulating materials out there due to its HFC-soup blowing agents, the predominant component of which is HFC134a (automotive AC refrigerant), which has a global warming potential about 1,400 [times that of] CO2,” Dorsett says. “EPS and polyiso are blown with pentane, at 7 [times] CO2.”

Manufacturers’ claims their insulation won’t damage the atmosphere’s ozone layer are another, unrelated topic, Holladay says. Ozone-damaging blowing agents have been been illegal for years.

Not interested in a discussion of blowing agents, Ealy says, just the problem at hand.

But the topic is germane, Dorsett replies, because blowing agents affect performance of the insulation over time.

“The blowing agent is a long term-performance issue, not just an environmental discussion,” Dorsett says.

XPS loses its blowing agent over a span of 40 or 50 years, he says, so that its performance gradually drops to that of EPS of the same density. On the other hand, foams blown with pentane lose all of their blowing agent in a matter of weeks, and they’re labeled at the “full depleted R-value, which is stable over time.” The foil facing of polyiso slows the process somewhat.

“EPS in thicknesses of a few inches will be fully depleted in the first year after manufacture, maybe even the first month,” Dorsett says. “Thick blocks of EPS manufactured at a few feet of thickness are something of a fire hazard in the first months, since they are still outgassing significant quantities of pentane. Type-II (1.5-lb. density) EPS is usually cheaper per R than 1.5-lb. XPS, too, and in applications where you don’t need the lower vapor retardancy or thinner profile (such as under slabs) it’s perfectly reasonable to substitute EPS where XPS was specified.”

Polyiso, however, should not be used under slabs or in contact with soil because it will absorb groundwater over time, resulting in lower performance.

Our expert’s opinion

Here’s what GBA technical director Peter Yost thinks:

A number of interesting issues have been raised here, and I’ll address them one at a time.

1. Placing insulation on both sides of the foundation basement wall. If you place insulation on both sides of a concrete foundation wall (as in insulated concrete forms), the interior insulation dampens the thermal mass impact of concrete.

In general, I like to pull any structural wall to the inside, including concrete, to take advantage of the thermal mass but also because continuity of the air and thermal barriers is always easier from the exterior. But, of course, exterior foundation insulation introduces two other issues: dealing with the planar difference with the above-grade exterior wall and how much more susceptible the exterior insulation is to damage, both during construction and occupancy.

There is really nothing wrong with insulation on both sides; it just irks me when proponents of this approach claim benefits from the thermal mass.

2. Foundation walls and the difference between the above-grade and below-grade portions of the wall. There is considerable debate, particularly in the Passivhaus community, about just what level of insulation should be achieved for below-grade assemblies, including down at the basement floor, generally at least 6 feet below grade.

There is no doubt that the portion of the concrete wall that is above grade should be insulated to the same levels as any above-grade wall. But there is a changing temperature profile as you drop from grade, even in the winter, which means that by the time you are 6 feet under, the temperature difference between the inside (the home) and the outside (the soil) is about 15 F° at most (see the photos below).

The real reason to insulate the total depth of the concrete exterior wall and underneath the basement slab is comfort and interior air quality. Elevating the surface temperature of the lower walls and floor reduces opportunity for biological growth (primarily mold and dust mites) and increases thermal comfort (due to the higher mean radiant temperature and higher floor surface temperature).

Insulating the footing just does not prove to be worth it for energy efficiency, thermal comfort, or air quality.

3. Waterproof the most durable component of the wall — the concrete. Keeping the foundation wall dry, given that you are burying it for the life of the building, means waterproofing — not dampproofing — the concrete foundation wall. Waterproofing materials — defined by ASTM standard as elastic enough to span cracks and keep water out under a head of water equal to 8 feet — is more expensive than dampproofing, but this is not the place for reduced performance driven by reduced cost.

4. EPS and pentane as a blowing agent. I checked in with the technical team at Atlas Insulation on this one: just about all of the pentane used as the blowing agent in EPS insulation leaves the insulation during manufacture and is recaptured in the factory for use as a fossil fuel for space heating or other needs.

15 Comments

  1. JAMES KREYLING | | #1

    Termite Highway
    As I have written about in previous blogs about exterior foundation insulation- make sure you do not create a highway for the termites to migrate to preferred feeding grounds- namely, your house. Personal experience discovered on a New Year's day in New Hampshire attests to their voracious appetiete for your yummy framing and even the paper backing of your sheetrock walls...

  2. User avater
    Dana Dorsett | | #2

    Depletion of pentane in EPS
    Several years ago I read a document from an EPS manufacturer selling large slabs (a few feet thick) for use under runways & roadways discussing taking care storage & handling of freshly blown EPS for the first several days/ weeks due to the potential fire hazard. I remember being surprised to read that (even in large slabs) there would be enough pentane left to be even a remote safety issue in the field. (I'll look for it online when time allows.) To be sure, the pentane has no effect on it performance over any significant time period.

    It's likely that manufacturers cutting it up into board stock of a few inches would let it outgas prior to cutting, since it would present a hazard during processing. If fabricated in thinner sheets the outgassing would be quite a bit faster.

    I'd be curious to know how foil facers on polyiso affects outgassing rates. (I suspect that polyiso too is largely depleted of blowing agents prior to leaving the factory.)

  3. David Hicks | | #3

    retrofitting foundation insulation
    Is there a GBA article on adding insulation to an existing slab? Not underneath, obviously, but vertically and along the edge to get some of the benefits of a FPSF.

  4. User avater GBA Editor
    Martin Holladay | | #4

    Response to David Hicks
    David,
    As far as I know, GBA has not published an article on that topic. (If any GBA reader remembers such an article, I'm be happy if you provided a link.)

    Here's a brief outline of the needed work:
    1. Excavate a trench around the perimeter of your house. In a warm climate, you might go 1 foot below grade; in a very cold climate, perhaps 2 feet below grade.

    2. Install rigid foam rated for soil contact (Type II EPS, Type IX EPS, or XPS) vertically up against the concrete. I would use at least 2 inches in a warm climate, and 3 or 4 inches in a cold climate.

    3. Protect the above-grade portion of the insulation from physical abuse and sunlight with one of these materials:
    A cementitious coating or cementitious stucco (for example, ), with or without metal lath
    A cementitious coating that includes chopped fiberglass (for example, or surface-bonding cement)
    An acrylic coating like or
    EIFS (synthetic stucco)
    Cement backerboard, with or without a layer of stucco
    Pressure-treated plywood
    Metal flashing
    A fiberglass panel like from Nudo Products
    (XPS coated with mineral granules adhered to one side)
    (a flexible peel-and-stick membrane with a textured, gritty coating)

    If necessary, fasten the protective material to the concrete with TapCon fasteners. (In some cases, the backfill is all you need to keep the rigid foam and protective cover in place.)

    4. Install Z-flashing to protect the top of the rigid foam. The top leg of the Z-flashing should be integrated with the wall's WRB or siding.

    .

  5. David Hicks | | #5

    Thanks Martin!
    Thanks Martin!

  6. Tim Johnson | | #6

    basement walls
    Check out Composite Panel Systems of Eagle River WI. These are wall panels made of composites filled with foam. No wicking, no thermal bridging, built in service cavity, strong, and come in panels up to 9x20. Put up the entire basement in an afternoon.

  7. Gred Gross | | #7

    below grade walls
    No one has addressed the problem I had in my house.
    My ground floor has two below grade walls, insulated on the outside. For 4-5 months a year, I would have a regular problem with damp outside air coming in and condensing on the walls. The walls weren't cold to us, but enough below the dew point to cause a problem. I ended up insulating the inside and covering it with sheetrock. Problem solved.
    Admittedly I do live in a humid environment, the southern Appalachians, but many places have this situation in the summer months.
    And I would beg to differ with Peter Yost: there is still a thermal mass benefit with insulation on both sides.

  8. User avater GBA Editor
    Martin Holladay | | #8

    Response to Gred Gross
    Gred,
    You didn't describe the type of insulation used to insulate the exterior of your two below-grade walls, or how thick the insulation was. Nor did you tell us whether your basement slab was insulated. Nor did you tell us whether the two other walls are framed or poured concrete, nor how these two other walls are insulated.

    So there are a lot of details we don't yet know. I'll say this, however: if you had a condensation problem on your below-grade walls, either (a) the exterior insulation wasn't thick enough, or (b) there was thermal bridging at the corners of your house.

    It's possible to insulate below-grade walls with enough insulation to overcome the condensation problem.

  9. Charlie Sullivan | | #9

    Condensation
    Exterior insulation can't solve all possible condensation problems. For example, if you open the windows for 24 hours because the outside temperature is pleasant, but it's cool at night and warmer and humid during the day, you could get condensation just because the night temperature cooled the thermal mass to below following day's dew point.

  10. User avater GBA Editor
    Martin Holladay | | #10

    Response to Charlie Sullivan
    Charlie,
    In theory, the situation you describe could also happen with a granite countertop in your kitchen. However, actual summer temperatures don't get cold enough for that to happen, even if you leave your kitchen windows open all night long.

    The only way you get condensation on concrete surfaces in your basement during the summer is if the builder forgot to install an adequate thickness of rigid foam to separate the concrete from the cold soil.

  11. Charlie Sullivan | | #11

    Where would we be without granite countertops?
    As Martin said in 2011, "Where would we be without granite countertops? They’re such handy devices for making almost any argument…"

    https://lakesideca.info/blogs/dept/musings/payback-calculations-energy-efficiency-improvements

    With more details from Gred on the insulation levels and locations and particular climate, we could figure out better what happens in his case. There was a day in Sept. here in New England when the daytime dew point was more than 20 F above the preceding overnight temperature, but that was cold enough (40 F) that even Vermonters would close the windows. Further south, there might be similar fluctuations in a temperature range where open windows would be more appealing.

  12. Gred Gross | | #12

    condensation
    I have 2" of blueboard outside, and 2 " under the slab. The other walls are wood framed, with south- (and west) facing window and doors.
    Yes, we often have days and nights with 100% humidity in the summer times, and when we would leave the doors open at night, the walls, which kept the rooms comfortable in the daytime, would collect dew on them. No granite counter-tops to test here, but I still feel that the thermal mass effect is still there, even with insulation, hence the slower change in temperature which led to the condensation.

  13. Paul J. Boniface | | #13

    Insulated Concrete Form (ICF)
    From what I have read, it seems to me that Insulated Concrete Forms are the way to go when installing a new basement. Kills two birds with one stone.

  14. Quint David | | #14

    Gred's Appalachian Condensation
    Gred,

    Dew point calculations only work when you assume the interior temperature is constant and also at a constant humidity. If you are leaving the doors and windows open, and the block wall is in contact with the ground (or not), it will always sweat any time it is colder than the dew point, just like rocks sitting outside on foggy mornings.

    Ambient temperatures can move your wall temp colder than the dew point overnight, and then the dew will collect there as the humidity rises as the sun comes up, just like on the ground on many beautiful mornings.

    Your options are to close the windows or insulate the wall on the inside. The exterior foam is not protecting the wall from cooler ambient temperatures you are letting in through the windows.

    Hope this helps! There is also a handy program called the 'climate consultant' from the folks at UCLA and the California Energy Commission that can put together a psychrometric chart, and display ground temperatures as well. Ours swings above 50 in july and below 50 in december. Asheville ground temperatures nearby are nearly 10 degrees warmer every month and swing up in may instead of july. That gives you plenty of months for condensation potential with the windows open when the relative humidity never goes below 60%.

    -Q

  15. George Hawirko | | #15

    ICFs are wasteful as much as
    ICFs are wasteful as much as this idea, don't waste
    your time and $

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