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Q&A Spotlight

Insulating a Raised Slab

At least insulate the perimeter of the foundation — but it's safe to skip insulation under the slab in Climate Zone 3

Rigid foam insulation creates a thermal break that protects the finished slab inside. In Climate Zone 3, insulation below the slab is optional, but the strip of vertical insulation on the edge of the slab is a must. [Image credit: Fine Homebuilding magazine]

Planning a new house in Pender County, North Carolina, Jason Dennis finds the aesthetics of a crawlspace appealing, but not the potential problems. Instead, he’s thinking of pouring a slab over compacted fill inside his foundation walls — a raised slab.

The Climate Zone 3 house will be a simple rectangle, 50 feet by 39 feet, with a wraparound porch on two sides.

Dennis wants to know whether the raised slab approach is a good idea, and how such a foundation should be insulated.

“Would insulating the slab be necessary for my climate zone?” Dennis asks in a Q&A post. “If so, how should insulating be done? Rigid foam underneath like with a non-raised slab?”

Those are the questions that get us started on this Q&A Spotlight.

Insulate the slab perimeter

In referring Dennis to on this topic, GBA Editor Martin Holladay says it’s possible to eliminate the horizontal insulation underneath the slab. But in Climate Zone 3, Dennis should not skip the vertical insulation at the slab’s perimeter.

Holladay had written: “While it could be argued that insulation might be useful in Climate Zone 3, it really isn’t needed in warmer climates, where an uninsulated slab helps lower air-conditioning bills compared to an insulated slab.”

How deep should the insulation go?

If Dennis adopts that plan — rigid insulation vertically against the inside of the stem wall but no insulation underneath the slab — the question becomes how far down the insulation should go.

“The insulation should extend down to the footings, or if they end up being very deep, a couple of feet below the exterior grade,” Malcolm Taylor replies.

The foam can be beveled at the top edge so it’s not visible on the inside while bringing the concrete all the way to the edge of the floor to support any flooring that might be installed, as the illustration below shows.

Alternately, a step can be created in the stem wall for the vertical foam, as the illustration at the top of this column shows.

Leave the insulation full thickness

Jon A points out that in the illustration above, the stem wall is the same thickness as the stud wall above it.

“Will this truly be the case?” he asks. “Typically a poured wall has a notch formed into it just like Martin’s detail [the one at the top of this page] shows. I like to leave the vertical slab insulation full width (with no bevel) and sticking out past the wall plate slightly.”

Jon A doesn’t understand how the exposed edge of the foam will affect the installation of flooring; the edge of the foam will certainly be lost under the edge of the drywall and baseboard.

Plus, he adds, if the pieces of foam are cut accurately and set so they are dead level, they make a great gauge to screed the concrete floor to.

The notch in the stem wall would be the same, whether Dennis omits the horizontal insulation or not, Jon A says. “Regardless of whether you fully insulate under the slab you will still need a small strip of insulation where the slab sits on the foundation wall,” he says. “The goal is to thermally isolate your slab from your foundation wall.”

Adding the vapor barrier

A vapor barrier should be installed immediately below the slab so the concrete is in direct contact with the vapor barrier, Holladay says. That’s the case whether or not there is any horizontal insulation below the slab.

But the vapor barrier can end at either the top or the bottom edge of the slab.

“Either way will work — the difference is minor,” Holladay writes. “The polyethylene is a vapor barrier, not an air barrier. In any case, it’s a good idea to seal the perimeter crack (at the perimeter of the slab) with caulk after the concrete is cured, and you don’t want a flap of polyethylene in the way when you apply the caulk.”

Block or poured concrete?

Dennis also wonders whether there are advantages to using poured concrete walls instead of concrete block for his foundation. To Trevor Chadwick, it’s an open-and-shut case.

“The only reason to use block is that sometimes it’s cheaper,” he writes. “Other than that, poured is superior in every single way.”

How so? wonders Aaron Beckworth.

“Every single way meaning what?” he asks. “I would expect block would be less expensive and may be preferred by some builders. Without sound reason, why ask a builder to deviate from what they are comfortable with?”

In addition, block may be less intimidating for a do-it-yourselfer who’s working without a crew.

Cost and local building practices are the reasons to use block, says Taylor, but a solid concrete foundation wall will perform better. Chris Duncan adds this: “A block foundation is harder to build for owner-builders and probably for professionals too,” he says. “And I think a poured foundation is stronger unless you do a lot of extra work with rebar and filling the block openings with concrete. Even then it’s not as strong. All the examples I’ve seen of failed foundation walls were block walls.”

Our expert’s views

Peter Yost, GBA’s technical director, had this to add:

Do we need insulation under the slab in mild climates?

The heating and cooling energy in a mild climate arising from ground-coupled heat transfer is incredibly complex. Being connected to the earth as an infinite heat sink is not a good thing during the heating season but certainly can be a good thing during the cooling season (see by the Florida Solar Energy Center). But we be won’t solving that puzzle in a GBA blog.

What about the tapered edge perimeter insulation detail?

I don’t see the tapered edge perimeter insulation really working. I don’t think it’s feasible to taper the concrete to a thin edge, and I don’t think a thin edge would hold up over time.

Do we need slab perimeter insulation in mild climates?

I don’t think there is much of a debate regarding the importance of slab edge insulation in Climate Zone 3. But locating that insulation gets interesting.

See the illustration below for a solution proposed as part of a Building America project, with annual energy savings estimated at 13% for mild climate locations (Sacramento and Santa Maria, California; Reno, Nevada; Atlanta, Georgia; and Fort Worth, Texas). This comes from the on Energy Efficient Buildings – “The Last Big Leak: Exposed Slab Edges.”

Is there another approach that solves the thermal bridge problem at the slab perimeter?

I got to talking with architect about insulating slabs and whether it makes sense to flip the insulation and concrete slab, per his detail below.

In this assembly, the rigid foam insulation is above the concrete slab. (Image credit: Steve Baczek)

“It works just fine and eliminates the thermal bridge at the perimeter,” says Steve. “I have even done a recent project or two where we eliminated the concrete slab, since it’s not really structural.” See detail below.

With the approach shown above, the non-structural concrete slab is eliminated completely. (Image credit: Steve Baczek)

It certainly makes sense to have the insulation underneath the slab for radiant floor heating. But other than that, just why do we pull the concrete slab into conditioned space rather than pushing it out?

Which is easier: DIY concrete block or cast concrete?

As a remodeler doing quite a few small additions, garages, and utility buildings, we did a lot of flat work because it’s really pretty straightforward — not requiring much skill, frankly. Laying block, on the other hand, takes a lot of skill: buttering the blocks, maintaining courses, keeping everything square and level. For that reason, we rarely did it.

13 Comments

  1. Jason D | | #1

    In the last image without the concrete slab, is there any reason to put the poly under the stone pad as opposed to on top of the stone and under the foam?

    1. Malcolm Taylor | | #2

      Jason,

      I can think of a couple of reasons it would make more sense to move the poly as you suggest.

      - It allows the rock to act as a capillary break below the vapour barrier.
      - It raises the vapour barrier relative to the surrounding grade.

    2. User avater GBA Editor
      Martin Holladay | | #3

      Jason,
      Here's another vote in favor of moving the polyethylene up to a position above the crushed stone. That way, any water droplets in the stone can drain downwards.

      1. Jason D | | #4

        Playing devil's advocate, is there a reason to not put the poly in contact with the foam? Like could trapped water cause mold or smells?

  2. Jon R | | #5

    I'd be careful not to build any of those with low perm flooring (like vinyl or tile). Without drying to the interior, the OSB is likely to rot.

  3. Jason Dennis | | #6

    Jason Dennis here. I must say that I was surprised and happy to see my question chosen for a Q&A Spotlight post, but then even more surprised that another "Jason D" commented on the post. What are the odds of that? :)

    1. Jason D | | #7

      Maybe it's your other personality :)

      1. Jason Dennis | | #10

        Dammit! He promised he was gonna stop posting in forums.

  4. User avater
    Greg Houston | | #8

    I'd like to see some more info on this slabless floor setup.

    1. User avater
      Michael Maines | | #11

      Greg, here's how I did it: .

      1. User avater
        Greg Houston | | #13

        Thanks guys.

  5. Nick DeFabrizio | | #12

    Joe Listurbek says never put the poly below the crushed stone.

    I would like to see more articles on building without basements, slabs or traditional foundations. Too much concrete and too many problems related to the earth( moisture from vapor or liquid water, termites, radon, heat loss, etc.). I am beginning to think that building on pilings or helical piers is the way to go. Beach communities have been doing this for 100 years and the foundations are a fraction of the cost and go in in hours, not days. With the money saved you can buy a lot of mini splits and additional 375 watt pv panels.

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