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Q&A Spotlight

Foundation Plan for a Snowy Climate

When building on a slab, how much distance should separate wood framing and grade to prevent rot?

Image 1 of 4
Nathan Scaglione's foundation is to be constructed of concrete block with stem walls extending some 2 feet above grade. His concern is that snow piled against exterior walls will eventually cause rot.
Image Credit: Nathan Scaglione
Nathan Scaglione's foundation is to be constructed of concrete block with stem walls extending some 2 feet above grade. His concern is that snow piled against exterior walls will eventually cause rot.
Image Credit: Nathan Scaglione
Steel protects the wall. Both the metal wainscoting and the rigid foam insulation extend below grade.
Image Credit: Steve Baczek
Corrugated steel wainscoting keeps the wall protected from rain and snow. Because of its shape, the wainscoting introduces an air gap into the assembly. This is snow country. The detail is more than adequate to protect the lower reaches of these walls even when homeowners can expect a winter like this.

Nathan Scaglione’s central New York State building site gets plenty of snow and cold weather during the winter, and that’s proving to be a sticking point in his plans for a new house.

He’d prefer a slab-on-grade foundation rather than a basement, even though a full basement would be a more typical choice in this part of the country. The foundation would consist of concrete-block stem walls extending to a footing below frost line. Exterior walls would be framed on top of the block walls, roughly 24 inches above grade. Inside the block walls, Scaglione will pour a concrete slab floor.

As he explains in a Q&A post at Green Building Advisor, Scaglione has seen this detail in garage construction, not necessarily for a house.

“I would like to build on a slab with radiant floor heat,” he writes. “But I really can’t get behind the idea of melting snow sitting up against the stick frame and gradually rotting the walls. For everything except a [frost-protected shallow foundation], code says 16 inches above grade. Ideally I’d like to have the stem walls be 2 feet above grade. With a platform framed house, now we’re talking about the floor being close to 3 feet off the ground. From a design standpoint, I really don’t like being way up off the ground. I’d rather one step down and I’m walking on the grass.”

The drawing Scaglione has provided (above right) includes the basics. The slab would be about 6 inches above grade, and doors would be “about one step off the ground.”

Are there structural problems here that Scaglione is missing? Does this type of foundation have a track record? Those are the questions for the his Q&A Spotlight.

Is there really a rot problem here?

Malcolm Taylor replies that the stipulated 16-inch separation between ground and untreated lumber that’s mentioned in building codes refers to the structural components, such as deck beams outside the structure, not to exterior wall framing.

“What governs there,” he says, “is a separation of 8 inches from grade to the top of the concrete foundation wall. Seeing as this is the case, and almost all houses in cold climates are built with wood framing which is covered by snow during the winter, is there really enough evidence of widespread rot to make you need to deviate from the how everyone else builds?”

Stephen Sheehy takes a similar line. In Maine, where Sheehy lives, last winter saw more than 6 feet of snow piled against the north wall of the house for three months. “So long as the melted snow or rain drains away from the house, the sills are going to be OK,” he says. It’s drainage, not simply the exterior walls’ distance above grade, that really matters here.”

Although Kevin Zorski also believes that the 8-inch-above-grade guideline is plenty adequate even for a snowy climate, he suggests that Scaglione could incorporate some pressure-treated material into the house near the ground to ease his worries about rot. If he does go ahead with the plan he’s already outlined, Scaglione might consider using an insulated concrete form (ICF) foundation, which combines rigid foam insulation and a structural concrete core.

“These should probably go below grade as we’ll, as there is nothing in your drawing preventing frost from getting under your slab,” Zorski says. “Or you could extend foam down the inside of the stem walls to protect the soil under the slab.”

Wall rot actually is a common problem

Scaglione isn’t convinced that an 8-inch gap between ground level and the bottom of a wood-framed wall is enough. “I have seen sill plates rotting and water damage all over the place where I live,” he says. “I would say it is a widespread problem on houses that are less than 1 foot above grade. I think you’re right that the code doesn’t say 16 inches. The old houses that are in good shape around here all have at least an 18-inch-stem wall.”

He first read about extending the height of stem walls in a book by Ben Falk, who said one of the most common problems in houses built in cold, humid climates was the interface of the foundation and the frame.

Before buying his lot, Scaglione saw several houses that were rotting in this area. One house he remembers in particular was a timber frame built in the 1970s, with one corner about 8 inches above grade.

“The [pressure-treated] sill was hanging in there, but they also put down a timber sill that was completed rotted away to the point that the house was sagging and there were gaps around the windows,” he says, “I want my house to last longer than 40 years.”

Although replies posted to his original question suggest there are ways to detail a wall so melting snow drains harmlessly away, Scaglione still wonders whether adding two courses of 8-inch block and getting the walls that much farther away from the ground is, in the end, a simpler and more foolproof approach.

“Just about every well built old house in this area has at least 16 inches of clearance above grade, and I don’t think those guys were stacking rubble for fun,” he says.

A full basement would be a better choice

GBA senior editor Martin Holladay sees no structural problems with Scaglione’s plan, and he agrees with Scaglione’s concerns about water damage to the lower parts of exterior walls when they’re only 8 inches or so above grade. Increasing the separation between soil and wood can’t hurt.

Still, Holladay would proceed differently: “I would go for a full basement (with poured concrete walls, not CMUs), and my first floor would be framed with joists rather than using a slab,” he says.

Scaglione can’t disagree with the functional advantages of a full basement built with concrete, rather than block, walls, but there’s a labor issue here as well. Scaglione is working by himself, and he doesn’t have access to concrete forms.

With than in mind, Charlie Sullivan suggests ICFs for the foundation walls, “one of the easier solo building approaches if you don’t mind calling a concrete company to come fill them.” Scaglione could even do the entire first floor with ICFs, Sullivan adds.

Scaglione has considered, and rejected, ICFs after seeing a fellow owner/builder bracing his ICF walls before the concrete was poured with metal forms he had to rent. Second, Scaglione wasn’t embracing the idea of having the expanded polystyrene foam of an ICF exposed on outside walls.

An alternative suggested by James Morgan would protect the base of the framed wall with a peel-and-stick waterproof membrane (in red) and a masonry veneer outer layer. It’s a detail Morgan has used successfully for many years.

Another option: Masonry veneer and a waterproof membrane

To James Morgan, it’s getting a little complicated. A simpler approach, he says, would be to frame the exterior walls from the slab. To guard against water damage, Morgan would apply a peel-and-stick waterproof membrane to the first 2 feet or so of sheathing, and protect all of it with a masonry veneer wall. (See the drawing at left for a sample of this construction detail).

“Backfill against the masonry to your heart’s content,” Morgan says. “Protect exterior door sills with at least 4 feet of gable porch overhang. Exterior walls are insulated right down to the slab and you will have no jamb issues with inward opening doors.”

Morgan says that finish grade can even be higher than the finish floor when necessary. “We’ve used variations of this kind of detail reliably for many years when at-grade entries are either preferred or required, e.g., for wheelchair accessibility,” Morgan adds. He concedes that his experience has been entirely in Climate Zone 4A, bordering a county with an even milder Climate Zone 3A, but there’s no doubt it’s been successful.

Sullivan would be “paranoid” about the peel-and-stick membrane becoming a “wrong-side vapor barrier” and potentially allowing wood wall components to get wet by condensation.

Another possible problem, Holladay adds, is that the brick veneer will draw soil moisture upward via capillary action. “This shouldn’t be a problem if the mason remembers to install through-wall flashing a few inches above grade, along with an adequate number of weep holes to allow drainage and to provide ventilation inlets,” Holladay adds. “Your sketch fails to show the flashing and weep holes, but these details are essential. They are also tricky, because the mason may not know how the excavation contractor plans to backfill and grade the site.”

Says Morgan, “I did not go into into the complementary details which I assumed would be taken for granted in this forum: proper roof water management strategies, good overhangs, proper surface grading, etc. But bottom line: I see a clear advantage in a clean and consistent wall insulation and service run condition all the way to the interior floor level. (Maybe I missed it but nobody seems to have mentioned the problems that the electrician and the plumber would face in the originally proposed configuration).

“Stepping out to grade seems to be Nathan’s core performance requirement,” Morgan continues. “I stand by a construction strategy which with appropriate local detailing offers a way to achieve both objectives without making a dog’s breakfast of the exterior wall construction.”

Our expert’s opinion

Here’s what Peter Yost, GBA’s technical director, adds to the mix:

Any foundation and above-grade wall combination system, and with any cladding in any climate, works for me, so long as both bulk and capillary water are well managed.

If you don’t manage both well, there is no combination of assemblies and claddings that will be robust and durable.

In cold climates, you do see a lot of failures in walls where there is no capillary break in the transition from the below-grade portion of the wall to the above-grade portion, and in walls where the barrier to bulk water entry is not continuous. But these failures are more about the detailing than they are about the type of assemblies and claddings.

Just to be sure, I checked in with GBA architect Steve Baczek, who does a lot of high-performance designs for cold climates. Here is what Steve had to say:

“This is a water management problem. More specifically, a bulk water management problem. Snow against a wall doesn’t really bother me; what bothers me is what happens when the snow or ice melts. Where does the water go?

“First, there is the question of the proximity of wood to the water. Code requires a distance of 8 inches between grade and a wood sill. Start with that. In addition, you should provide a relatively steeply sloped grade away from the foundation to make sure water doesn’t pool around the house. As snow melts, it will typically stay below the 8-inch mark. Nathan could increase that distance to 10, 12 or even 18 inches, but I don’t think that’s necessary.

“The problem with Nathan’s wall as drawn is that’s it’s a single-wythe barrier. Because of that, I question how one manages any water or moisture in that system. On a rainy afternoon in late fall, for example, that block wall could get saturated with water, and then become susceptible to freezing through the night. Eventually, repeated freeze-thaw cycling will take its toll.

“I think it’s important to build in a protective layer. If Nathan sticks with a lower wall made of block, he might build it with two layers of masonry and leave a gap between the two. This is similar to the suggestion from James Morgan. Alternately, Nathan could lower the wood-framed wall so it starts just one course of block above grade, but add a ventilated rain screen to create a capillary break. That’s better, but it still leaves a course of block at the base of the wall not as well protected as I’d like. Let’s not forget block is not as robust as a poured concrete foundation would be.

“There’s one more potential detail he could consider — one I have used on a house in snow country: a layer of corrugated steel protects the bottom of the wall, with a layer of rigid foam insulation between the steel and the wood framing behind it. (See Images #2 through #4, below.)

“In summary, keeping wood framing above the soil will certainly help, but true success lies in proper management of the water that the wall is bound to see.”

3 Comments

  1. User avater
    Ethan ; Climate Zone 5A ; ~6000HDD | | #1

    Ants!
    When taking into account the possibility that global warming will bring more pests up north, and the conversation about ants in foam (https://www.greenbuildingadvisor.com/blogs/dept/qa-spotlight/if-ants-rigid-foam-should-we-stop-using-it), I've become increasingly worried about relying on foam, particularly in hidden locations like foundations. I'm not sure what to do with this newfound concern, except to build with AAC, rockwool, and foamglass.

  2. Charlie Sullivan | | #2

    Pet anteaters
    Adapting to climate change requires considering new approaches.
    http://www.livingwithanteaters.com

  3. User avater
    Ethan ; Climate Zone 5A ; ~6000HDD | | #3

    They are feeding that anteater ants imported from China
    ... What could possibly go wrong?

    https://farm3.static.flickr.com/2041/2216480360_b5a8efc3df_m.jpg

    The anteater cost between 5,000 and 8,000 and just eats ants as a treat. They import ants for it "in bulk once a year from China."

    In addition, the anteater eats raw ground beef, wheat bran, spinach or thyme and and some other things. They "REALLY love beef and ... blue cheese.

    Source: http://www.livingwithanteaters.com/p/anteater-faq.html

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