More Q&A Spotlight
Trevor’s plans for building a new workshop have run into a snag: too much to do, and not enough time to get it all done.
“I’ve fallen far enough behind in my workshop project that I’m doubtful I can get the building closed in and heated before winter,” he writes in this recent post in the Q&A forum.
The workshop will be built on a frost-protected shallow foundation (FPSF), an alternative to a conventional foundation in cold climates that requires less concrete and less work. A monolithic concrete slab, sitting well above the frost line, is protected from cold-induced ground movement by rigid insulation set vertically at the perimeter and heat-trapping wings of insulation that extend outward horizontally.
Trevor says he’d still like to get the foundation in before winter. but he’s concerned that it may be damaged by freezing temperatures if it sits unprotected all winter without the warmth that a heated building would provide.
“My understanding is that this kind of slab requires some heat from the building for its frost protection,” he says. “Would leaving it barren over the winter be bad? Is there some way to mitigate it? What about burying some heating mats under the slab near the perimeter and connecting them to a thermostat set to just above freezing?”
Will this foundation safely see the warmth of spring? That’s the question for this Q&A Spotlight.
Save the foundation for spring
Michael Maines, a residential designer and builder in Maine, says recommended details for a FPSF assume that the building is enclosed and heated to 64°F.
“In those buildings, the earth’s heat from below and the building’s interior heat are both working against cold air trying to freeze the soil,” Maines says. “When that cold…
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10 Comments
Some people seem to dislike exterior and especially subterranean foam because critters can wreck it. Is that a regional thing or overblown? I'd hate to think that after a decade or two my foundation becomes susceptible to frost because of ants.
pjpfeiff,
https://www.greenbuildingadvisor.com/article/if-ants-like-rigid-foam-should-we-stop-using-it
How important is the drain rock under the footing?
It's important for the soil to be drained; if it's saturated with water, it can freeze and expand. It's also important for the soil to be able to support the structure, of course. I don't think there's a code requirement that the material is crushed stone; I want to use Glavel or Aeroaggregate on my next project, but crushed stone is typical these days.
But only if the bottom of footing is above the frost line for your zone? Otherwise there is no danger of freezing right?
Yes, the article is about frost-protected shallow foundations so that's what I was referring to. Though I also don't like placing footings where I know they will be wet; frost actually penetrates deeper than the frost line, it just doesn't usually cause damage. But if the soil were saturated, it could.
When I posted my question, I failed to mention some details that I assumed were standard. One is that the mitigation strategy of "wing" or "skirt" insulation was already in the plan. We're going to have at least R16 under the whole slab and extending 4 feet outside the footprint of the building. So I'm no longer too worried about frost heave over the one winter.
Regarding this concluding statement:
"Forming and placing a concrete FPSF is pretty fast—usually less than a week —so there may be little time-saving advantage to installing the foundation and slab before winter"
I think this overlooks a lot of logistics. Maybe if I owned my own excavating and concrete companies, this could be done in a week. Realistically, getting both of those coordinated around potential weather delays and executed is probably more like a month. Tthen you need to wait weeks for the concrete to cure, not to mention the fact you could start framing a lot earlier in the spring than you could start excavating or pouring concrete. We're talking at least a 2-3 month head start in a 7 month construction season. When you're a 1 or 2 person crew, that's very significant.
I'm a builder in NB with similar climate. Regardless of what you do above the slab, you need to be either frost protect with rigid foam underneath and around the perimeter or have a frost walls going below the frost line (5' in our region) and rigid foam under the slab and on the interior wall of the frost wall 4' deep. Afterwards, the only difference between year one and year two is that the first year you may have more humidity in your slab if it didn't have time to cure before the cold hits. Regardless, if you frost protect your slab, it won't frost heave. If you are concerned about the fresh slab being exposed to the elements during year one, we had good luck laying a foot of straw on top of concrete for the winter. Even with rain and snow, it seemed to keep everything above freezing point! In the spring, shovel the straw aside and spread it around your property for fertilizer.
With a FPSF in an unheated building, can the sub-slab insulation be directly under the slab in lieu of the "non-frost susceptible fill"?
h_mott,
Yes. The insulation usually follows the underside of the slab. If you want to lay it flat (as in the illustration) then the fill above needs to be non-frost susceptible.
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