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 Colphene ICF 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 Green Building 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.
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.