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Q&A Spotlight

How to Insulate a Wood Foundation

A homeowner wonders whether exterior insulation is his best choice, and if so how it could be detailed on a log home

Insulating a wood foundation: The American Wood Council's handbook on permanent wood foundations includes this detail showing cavity insulation on the interior and a vapor barrier that extends a foot below grade. The vapor barrier is folded into the wall and attached to the exterior plywood wall. Note the 2-inch gap at the bottom of the wall.
Image Credit: American Wood Council

Jeepasaurus, a GBA reader from Eau Claire, Wisconsin, recently bought a log house sitting atop a permanent wood foundation (PWF). Although initially reluctant to buy the house because of this detail, Jeep did enough research to convince him there’s nothing inherently wrong with a wood foundation. The problem is how to insulate it.

In a Q&A post, Jeep explains that he has gutted the basement to the 2×6 studs, removing the wood paneling and the underlying gypsum drywall that had covered the walls. The previous owner had installed a vapor barrier behind the drywall and insulated the stud cavities with fiberglass batts. The insulation was damp in a few spots near the bottom of the wall, but there was no sign of mold.

The basement is generally dry. The only time Jeep has seen evidence of water there was after an early spring rainstorm when the ground was still frozen. In that instance, some water seeped through the wood sheathing covering the outside of the foundation wall. The plan is to turn the basement into two bedrooms for his children, and for that reason Jeep wants the space to be well insulated and comfortable.

Spray foam insulation is too expensive. Jeep has gone through Joe Lstiburek’s book on building in a cold climate and noted the recommendation that the foundation wall be insulated on the exterior with rigid insulation.

“I like the idea of the rigid insulation on the exterior,” Jeep writes, “but I’m curious if I would have to install it all the way down to the footing, or if it could be installed from the top plate to 16-18 inches below grade? Then on top of that … how could I make the transition from the logs to the rigid insulation as the rigid insulation will now be sticking out 3 inches? As I stated in the beginning, it’s a log home, with full logs. So, I can’t carry the rigid insulation all the way up to the roof line.”

An alternative, as described in the American Wood Council’s guide to PWFs, shows insulated stud cavities, but no external rigid insulation, and a vapor barrier on the upper most part of the wall on the inside.

“I’m leaning toward the AWC guidelines because super sealing and insulating the basement isn’t going to ‘make-up’ for the insulating values of the logs,” Jeep says. “Also, the previous batt insulation was there for 36 years and there wasn’t mold… so, it works. If I go this route, I will just have to spend some time insulating the rim joists with rigid insulation and Great Stuff spray foam.

“I’m not afraid of digging in and doing the work,” he adds. “I just only want to do the work once.”

Take care of bulk water first

The first order of business, writes GBA editor Martin Holladay, is to make sure water is not getting into the basement. Creating basement bedrooms is not a good idea until this problem has been solved.

“This might require re-grading, waterproofing, fixing the footing drains, or changing the backfill to free-draining material,” Holladay says. “Whatever you do, though, you need to solve the water entry problem before you put bedrooms down there.”

Trevor Chadwick agrees. “I don’t see how it can be livable until you stop the water problem,” he says. “The only real way to do that is to excavate the perimeter to the footer, and once you are at that point, why wouldn’t you insulate the outside?”

Jeep emphasizes that he’s seen water in the basement only once in the year and a half he’s owned the house: “With that said, I completely understand that any water infiltration is really bad,” he says. “And it will be addressed in the next year. I don’t subscribe to the notion that it must be dug down to the footings as it really depends on the soil. Again, we are on sandy soil. Water drainage is fantastic … except when the frost hasn’t let out of the ground. When I described the water coming in and ‘pooling’ in the stud cavities; volume-wise, we are talking a couple of tablespoons.”

Rigid insulation to the exterior is best

When it comes to an insulation strategy, Holladay is convinced the best path is to install rigid foam insulation on the outside of the foundation wall. The next best choice is to spray closed-cell foam on the inside of the wall. Ideally, the insulation should extend all the way from the top plate to the footing, but if that’s not possible due to cost, the upper part of the wall should be the first priority.

If foam is applied on the outside of the wall, the top edge should be protected with metal “Z” flashing, installed as close as possible to the bottom log of the house. There also will be some additional details, such as moving or replacing any frostproof sillcocks on exterior walls, or adding exterior jamb extensions and new sills for windows.

As to Jeep’s inclination to follow the AWC guidelines on insulation, Holladay adds this: “The choice is yours. The AWC guidelines aren’t based on building science. These guidelines are a compromise. The guidelines were developed in hopes of promoting wood foundations without incurring high expense. As you noted, ‘When removing the batt insulation we could feel it was a little damp near the bottom in a few spots.’ This type of dampness may not matter for an unfinished basement, but it’s not a good idea for a bedroom.”

No, exterior foam is not the best idea

Jerry Liebler agrees that solving the water problem is certainly a top priority, but on the question of insulation, Holladay’s recommendations fall short. “I respectfully disagree with Martin, who is openly hostile to PWF!” Liebler writes. “How can he justify the assertions about the origin of the guidelines and their lack of basis in building science? Was he involved in their generation, has he even talked to anyone who was? The best way to insulate your basement is not exterior foam!”

The best way to insulate the wall is to follow the AWC’s recommendations, he says, which means fiberglass or, alternately, mineral wool in the stud bays. Jeep also should take care to leave the 2-inch gap at the bottom of the wall, as described in the guidelines to prevent water from wicking into interior finish materials.

“The performance of the insulation will be degraded unless you achieve a good air barrier with whatever interior finish material you use,” Liebler adds. “In other words, the drywall should be airtight. Even without an air barrier, moisture in the insulation or condensation are not issues. The design is based on wet wood which will dry out if there is proper drainage. Don’t waste good money on spray foam!”

If the guidelines on PWFs are followed exactly, Jeep will end up with a warm, dry, energy-efficient basement far superior to what he’d get with a concrete foundation, Liebler says.

Advocating the user of exterior foam on a PWF is “nothing more than a blatant attempt to eliminate the clear advantage (low-cost insulation) of PWF over concrete,” he says, adding, “A PWF differs from interior wood walls in a concrete tub in that drainage is available for any condensate which is not the case in a concrete tub.”

Looking for value with investment

An important consideration for Jeep is whether spending a lot of money to insulate the foundation makes much sense when the walls of his log house will never perform as well. Suppose, he says, that after solving any bulk water problems he sprays 3 inches of foam into the stud cavities, for a total R-value of 21 on the basement walls. Upstairs, the exterior log walls will have an estimated R-value of 10 at best.

“Are we really gaining anything by spray foaming?” Jeep asks. “Or even stepping it up and doing the ultimate of rigid insulation on the exterior of the PWF? Again, do the ends justify the means? I understand that it’s a great way to insulate … and there is building science behind it to support it. But is it money well spent? Is the return on investment within my lifetime?”

With the foundation wall stripped last winter and no insulation whatsoever, Jeep says, the family was comfortable and the electric bills weren’t any higher than those of his neighbors, who also have ground-source heat but live in a house only three years old.

Concrete vs. wood foundation

Jeep also pokes at the notion that concrete foundations are infallible while there’s something second-rate about a PWF.

“When we first heard that the foundation was wood, we were hesitant to pursue purchasing,” Jeep says. “But, after doing some research, PWFs have been around for longer than I’ve been alive. And guess what? They are still standing! Everyone that I have talked to that has actually lived in a PWF said it was extremely comfortable (of course, if it was installed correctly).”

He points to an article published by The New York Times last year about problems a number of Connecticut homeowners are having with their concrete foundations.

“I’m not saying PWF is better than concrete or vice-versa,” Jeep says. “I’m just pointing out that both work perfectly well as a foundation, if they are installed correctly.”

The widely reported problems with foundations in Connecticut were apparently caused by a mineral in the aggregate, but to Dana Dorsett the rare failure is “the exception that proves the rule.”

“Barring that sort of problem, concrete may still not be forever, but long enough from a human or U.S. home lifecycle point of view,” Dorsett says.

Our expert’s opinion

Here’s what GBA technical director Peter Yost had to say:

One of the NAHB Research Center homes — in which my wife and I lived for nearly a year when I started there in 1993 — had a pressure-treated wood foundation. Given this was a demonstration home, the water management details (that were visible) were superior. The basement was never wet but the above-grade portion of the foundation was not air sealed and leaked like a sieve.

Like Martin, I prefer exterior rigid insulation because it warms the building assembly to its interior — in this case a wood one.

In homes where exterior foundation insulation is installed, the only way I have come up with to deal with the planar difference between the below- and above-grade assemblies is a water table. Some details to consider may be found here. A water table of 3+ inches on a log home probably just won’t work; it will just look too “kludgy.”

The starting point for me with this wood foundation would be to meticulously air seal the above-grade portion, including the rim joist area (not knowing how floors are framed in your log home…).

You want to air seal and insulate the finished basement so it can be used for bedrooms. For energy efficiency, your instincts that the biggest bang for the buck is where the foundation is exposed above grade is a good one. But there are two other considerations:

(1) Thermal comfort: If your uninsulated below-grade walls are pretty much reflecting soil temperatures, let’s say around 55°F, that is too cold for comfort. Are R-19 batts good enough below-grade? I would say yes, so long as the assemblies are air sealed. And since people stand, sit, and sometimes even lie down on floors, it’s really important to insulate the basement floor; again, not so much for energy efficiency, but for thermal comfort.

(2) Soil gas/radon: Just about all of Wisconsin is Zone 1 for radon. Air sealing your entire basement exterior surfaces will have a big impact on measured radon levels. I wish I could tell you that perfect basement air sealing would lower radon levels in the basement without active mitigation, but what little research there is (none really published…) suggests that we don’t understand the relationship between basement air sealing and radon levels. Sometimes radon levels go up, sometimes they go down, and sometimes they don’t change after weatherization, including the basement.

But you can be sure that air sealing the basement will increase the effectiveness of whatever radon mitigation system you might need to install (if, after your basement renovation project you end up with basement levels equal to or greater than 4.0 picocuries per liter).


  1. Greg Philliips | | #1

    Egress ?
    Is there the proper egress for two "legal" bedrooms? Include this in your plans.

  2. Michael Kerfoot | | #2

    WeatIfher Sealing
    I have designed/built a few score pwf's over the past few decades. They are our preferred option due to fewer basement problems, lower embodied energy, ease and lower cost of insulating with non-foam products, reduced thermal break issues, fewer trades and ease of scheduling, easier finishing,.... Why would we consider concrete? But of course proper water proofing strategies are integral. The walls are placed on a minimum of 5" of clean crush. A sump is required by code. Exterior ply wall joints are caulked with butyl, 2 coats of water based foundation coating are applied and a dimpled foundation mat with exterior geotextile covers the wall. The pwf walls under consideration here should be excavated and assure proper water proofing treatment is applied. The sandy soil indicated as well draining is suspicious since there appears to in fact be water present (otherwise the ground would not have been frozen in the spring as indicated). If the walls are excavated and the soil at the bottom is not well draining (no gravel strip below fdn.plate) replace a few inches below and to the side of the bottom plates with clean gravel. Cover the gravel with some landscape fabric before backfilling. Use a good interior air barrier with variable vapour permeability such as Membrain. You will have a warm, dry home with a basement far outperforming the logs above.
    In my experience achieving high energy performance with more responsible materials can be met at lower cost to the project and the environment with pwf's. I live and work in Alberta.

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