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Q&A Spotlight

What’s the Best Way to Insulate a Basement Slab?

For new construction in the northeast, a homeowner considers XPS, EPS, and spray polyurethane foam

In this scenario, the foundation wall and the footing are completely outside the thermal boundary. The basement slab is completely inside the boundary. Illustration courtesy of Building Science Corp.

Dave Feldman is building a new house near Boston, Massachusetts, that will have a walkout basement with a tile floor. His goal is to have the floor comfortably warm in this Climate Zone 5 locale even in the dead of winter.

Feldman plans to insulate the basement walls with closed-cell spray polyurethane foam. For insulation below the slab, Feldman is considering two types of rigid foam insulation—expanded polystyrene (EPS) and extruded polystyrene (XPS)—as well as closed-cell spray polyurethane foam.

He’s uncertain whether to insulate the slab with 2 inches of foam (which he estimates at R-10 to R-14) or invest in 4 inches of foam and double the R-value.

“I don’t want to waste materials if there’s not a useful impact,” he writes in a post in the Q&A forum, “but [I] also don’t want to skimp on something that’s pretty much impossible to change once the house is up.”

His request for feedback is where we begin this Q&A Spotlight.

Please, skip the spray foam

The spray foam that Feldman has in mind is Demilec’s Heatlok HFO High Lift, which uses a hydrofluoro-olefin (HFO) blowing agent developed by Honeywell. HFOs have a much lower global warming potential than conventional blowing agents, but in Dana Dorsett’s view, spray foam is still a poor insulation choice.

“Why?” he asks. “That is some of the lousiest bang/buck going, and one of the least green methods of insulating a framed wall possible, despite using HFO blowing agents. Expensive (financially & environmentally) closed cell foam between framing is a waste.”

Dorsett directs Feldman to this article from Fine Homebuilding magazine explaining the basis for his conclusions while suggesting that dense-packed cellulose is the best choice for cavity insulation.

“Save the foam budget for continuous exterior insulating sheathing where its performance isn’t being robbed by the low-R wood,” he says.

As for the best type of insulation below the slab, Dorsett recommends EPS. Although it has a lower R-value than XPS, its blowing agent is more environmentally benign.

“XPS is the least green insulation in common use today, and pound for pound, inch for inch eventually falls to the performance of EPS of equal density, as the HFC blowing agents that give it its first-decade’s performance boost dissipates,” Dorsett writes. “EPS is blown with a low impact pentane variant, most of which escapes the foam and is recaptured at the manufacturing plant.

“Not all closed cell spray polyurethane does well under slabs,” Dorsett says, “and it’s the most expensive $/R going.”

Two inches of EPS (Type II, Type IX or Type VIII) should be enough, Dorsett says. Citing Building Science Corporation research, Dorsett adds that if Feldman uses reclaimed Type VIII roofing EPS, it may make sense to jump to 4 inches.

Other considerations for sub-slab insulation

Another consideration is how well spray polyurethane foam would perform under a concrete basement slab. Here, Dorsett isn’t sure it’s the best idea.

“Spraying foam directly onto soil or compacted gravel has a somewhat imperfect history,” he writes. “Moisture in the soil can affect the expansion rate/volume, and the temperature of the surface can be quite a bit different from the ambient air temperature, leading to errors in mixture settings, etc.”

The quality of spray foam is always subject to site conditions, but these added variables make the result even more difficult to predict. By contrast, rigid foam insulation is produced in a factory where conditions can be controlled precisely.

Type II EPS (with a density of 1.5 pounds per cubic foot) is “the single most common material” used to insulate concrete forms, Dorsett says, and has long been used for this purpose under airport runways and roads.

Why to avoid spray foam

Matt F suggests that Feldman consider polyisocyanurate, possibly reclaimed or seconds, which would make a lot more sense than spray polyurethane foam for basement walls.

“You can frame right against the foam board,” Matt F says. “Framing against a spray foam wall makes it challenging not to leave air channels as it is not flat.”

Further, Feldman’s stated concerns about indoor air quality suggests that spray foam is a product he should avoid.

“The foam tested may pass, but the crew at your house will be running a complex, highly temperature dependent manufacturing process in likely very different conditions than the lab samples were produced,” Matt F says.

In new construction, he adds, there is no compelling reason to use spray foam at all. Air-sealing is easier, more effective and cheaper when it’s done at the sheathing, he says, so use rigid foam on the outside.

Insulation detail at the footing

With excavation in the near future, Feldman also wonders about where the sub-slab insulation should stop at the perimeter—at the edge of the footing, or go over the top of the footing and stop at the foundation wall.

Jon R’s replies with a link to an article from the Center for Sustainable Building Research at the University of Minnesota. It includes an illustration showing a layer of rigid foam beneath a basement slab that laps up over the footing and ends where the foundation wall begins.

Jon R suggests the polyethylene vapor barrier be brought up a little higher so it can be taped to the wall covering. Also, he adds, put a strip of foam between the wall and the edge of the slab.

“We designed our slab/insulation footing detail exactly as Jon R shared,” says Rick Evans. “You may get some funny looks from contractors but it is fine.”

He adds that this “seemingly minor detail” is a requirement for Energy Star certification. After he took photos of the installation at his own home Evans got nearly $6,000 in Energy Star rebates.

Our expert’s opinion

Peter Yost, GBA’s technical director, added these thoughts:

I pulled out three questions from this great Q&A exchange that I’d like to address.

How much rigid insulation should be placed below the concrete basement slab? The key points from the Building Science Corp. resource—High R-Value Enclosures for High Performance Residential Buildings in All Climate Zones (see the link above)—are these:

  • Even though some field research and new modeling does a better job with predicting heat loss from slabs to soil, quantifying savings with different levels of insulation is still very difficult and uncertain. Modeling ground-coupled heat loss to an infinite sink (the earth) is as much art as it is science.
  • While we may not be able to identify the financial “sweet spot” for sub-slab R-value (energy savings), some level of insulation is required for moisture control and thermal comfort. R-7.5 makes sense for slab temperature addressing moisture and comfort, even if there is uncertainty on quantifying energy savings associated with different levels of sub-slab insulation.

What type of rigid insulation to use under the basement concrete slab? Dana Dorsett and others covered this topic well, with the correct type of EPS leading the pack, but I do want to emphasize this from the BSC drawing at the top of this column:

  • Moisture barrier (polyethylene) location, between the concrete slab and rigid insulation, is key.
  • Free-draining gravel between the soil and all layers above is also imperative.

These components are critical in protecting both the rigid insulation and the concrete slab from taking up soil moisture. And, of course, rigid polyiso insulation is not an option because it can absorb much more water than either EPS or XPS.

What is the best approach at the intersection of the slab, the footing, and the below-grade foundation wall? We always want to pull highly conductive elements such as concrete walls, footings, and slabs either completely in or completely out. The BSC illustration with the turned-up slab insulation pushes out the basement wall and footing completely relative to the insulation layer. The entire slab is pulled in relative to the insulation.

-Scott Gibson is a contributing writer at Green Building Advisor and Fine Homebuilding magazine.

10 Comments

  1. user-7451306 | | #1

    Do you have a thermal break detailed under the door sill at the walkout basement entrance?

  2. Steve Grinwis | | #2

    My basement slab is insulated with two inches of foam, and it's actually pretty great. The floor never feels cold even even it's -30 outside.

    Care was taken to ensure no thermal bridging between the slab and the footing. It's completely floating on and between foam.

    1. But Why? | | #4

      When its -30 outside your basement slab is sitting on 40 - 50 degree soil. When its 100+ outside, you basement slab is sitting on 40-50 degree soil. (assuming you are in the Northern portion of the US). Soil temp below the frost line is ALMOST completely agnostic of the outside temperature conditions.

  3. But Why? | | #3

    The best way to insulate a basement slab is before its poured............

  4. Dave Feldman | | #5

    A couple interesting notes i found since the exchange:

    To get the lally columns on the footing directly we’re having to frame the house, place the columns, cut in the insulation around the columns then pour the floor after the house is built. Engineer and architect wanted to ensure the point loads were on the footings instead of the slab in eps.

    This does make it more complicated than an open pour so wanted to mention and share that.

    1. Malcolm Taylor | | #6

      David,

      Why not just put block-outs around there the columns are going on the footing and pour the slab? That's what we do.

      1. Dave Feldman | | #7

        The pads are 3x3 each. Do you block the whole pad and insulate/pour around the columns later? It’s 2” eps and 5” slab.

        1. Malcolm Taylor | | #8

          No we just block out enough space for the base of the column, much as you do for plumbing at tubs. Pour the slab, attach the column in the usual sequence of framing, and fill the void with canned foam.

          1. Tyler Keniston | | #9

            This is a detail I've been wondering about for awhile. I assume you don't bother dropping insulation down and under the footing, do you? The only uninsulated portion being directly beneath the column.

            I've wondered if there's much value to including mid-span footings in the envelope. Many people do that? My gut tells me it's a pretty minimal energy penalty not to, and some engineers /coders may get squimish with the foam there. (I'm talking foam beneath footing, none in between slab and footing, small vertical leg connecting.)

          2. Malcolm Taylor | | #10

            Tyler,

            The equation may be different in cold climates, but here in the PNW I don't worry much about the loss from uninsulated footings. With high compressive strength foam it isn't a structural problem to run it under footngs, but it's something I temperamentally just couldn't bring myself to do.

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