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Musings of an Energy Nerd

So Many Kinds of Insulated Slabs

Slabs with frostwalls, monolithic slabs, frost-protected slab foundations, and insulated raft slabs

[Image credit: Fine Homebuilding]

Most cold-climate builders know that building codes require slab foundations to be insulated. But the terminology used to describe insulated slabs is quite confusing. In this article, I’ll try to clarify the current muddle over the many types of slab foundations.

First, some definitions

There are at least four kinds of slab foundations:

  1. A thickened-edge slab, also known as a monolithic slab, integrates the footing and the slab in a way that allows all of the concrete to be placed at the same time. The perimeter footing of a thickened-edge slab isn’t very deep.
    [Image credit: Martin Holladay]
  2. A slab with frostwalls has perimeter walls resting on footings installed below the frost line—generally three to four feet below grade. This type of foundation requires concrete to be placed on three different days. The perimeter frostwall footings are usually placed first, followed by the perimeter frostwalls. The last foundation component to be placed is the slab. A slab with frostwalls can be insulated or uninsulated.     
    [Image credit: Martin Holladay]
  3. A frost-protected shallow foundation (FPSF) is a monolithic slab that includes enough insulation (either vertical insulation at the slab perimeter, or horizontal wing insulation outside the footprint of the building, or both) to prevent the soil under the footings from freezing. This type of slab sometimes, but not always, has a continuous layer of horizontal insulation under the entire slab.
  4. An insulated raft slab resembles a frost-protected shallow foundation, with two main differences: an insulated raft slab usually has a uniform thickness rather than a thickened edge, and an insulated raft slab always includes a continuous layer of horizontal insulation under the entire slab.

Slab basics

Regardless of what type of slab you intend to install, remember that site preparation is essential. In most cases, site preparation consists of…

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22 Comments

  1. Expert Member
    Malcolm Taylor | | #1

    I like the idea of monolithic slabs, but every time I try to apply them to a project there are to0 many obstacles (many of which you have touched 0n).
    - They require more site preparation and fill.
    - The depth necessary to provide clearance from grade to the framing above, and down to provide adequate cover for the perimeter drains, means the thickened edge ends much larger than necessary structurally.
    - They preclude stepping down the grade to suit site conditions, meaning they only work on those rare completely flat sites, or necessitate building up the grade round the house which can lead to an awkward relationship with the existing site.
    - They rely on exterior insulation, which here in the PNW means vulnerability to carpenter ants.
    - They don't meet the prescriptive methods accepted by our building code, which means they require the involvement of an engineer - even on projects that wouldn't otherwise need one.

    1. JunglePete | | #8

      Thanks for the PNW info. I am half way up the Oregon coast and would like to build a monolithic slab ("Monolithic turned-down footing" according to the OR IRC). I'm curious how a monolithic slab "does't meet the prescriptive methods" in your area.

      Pete Clusener

      1. Expert Member
        Malcolm Taylor | | #9

        Pete,

        The building code here in BC has a set of requirements for slabs on grade - as long as they aren't load-bearing. If they are they move from those prescriptive solutions in Part Nine to Part Four, which requires the involvement of a structural engineer.

        The second complication is that Part Nine says that footings much bear 0n undisturbed native soil. If you want to build on compacted fill of any type, which monolithic slabs typically require, off you go to Part Four again.

        Both of these are code related rather than practical impediments which may not exist in Oregon.

        1. JunglePete | | #11

          Thank you Malcolm

          The Oregon residential specialty code (a somewhat modified IRC for Oregon) does seem to allow a prescriptive path for load bearing monolithic slabs.

          However, it does seem to require engineering for building over compacted fill... which will likely be necessary on my lot.

  2. Expert Member
    Malcolm Taylor | | #2

    I've found you can sometimes get push-back from inspectors (and engineers) on reducing the thickness of the top of a stem-wall to less than 5". One way around this is to cant the outside top of the wall and start your framed walls above at the mid-point. This reduces the width of the wall to allow insulation between it and the slab without reducing its strength. It also is more forgiving of imperfectly straight foundations.

  3. Andy Kosick | | #3

    This question has been in head a while and I may be building a slab in the near future. If there is no sump and they can't drain to daylight, what is the point of a perimeter drain? Can they be omitted? It appears so from some of the drawings above.

    1. GBA Editor
      Martin Holladay | | #4

      Andy,
      Below-grade foundations require perimeter drains, but above-grade slabs do not. That said, site drainage -- meaning good grading -- is always important. The grade should slope away from a slab in all four directions.

      1. Expert Member
        Malcolm Taylor | | #5

        Martin,

        While practically above grade slab foundations may not require perimeter drains, our building code doesn't directly acknowledge that, and leaves it to the applicant to prove that their presence is unnecessary.

  4. Deleted | | #6

    “[Deleted]”

    1. GBA Editor
      Martin Holladay | | #7

      Andrew,
      You should never finish a basement unless you are sure that there are no water entry problems. Start with the interior French drain, and monitor it for a few years to see if you've solved the problem.

      Some sump pumps end up running continuously for weeks. (That happens if your site has a high water table in the spring.) If that's the case at your house, you'll be discharging a lot of water into your septic tank -- possible overloading the system. Use common sense. If you sump pump rarely comes on, the septic tank may be able to handle the discharge.

      Do you know you have a radon problem, or are you just guessing? Radon fans use electricity, and you don't want to run a radon fan continuously if you don't have a radon problem.

  5. JunglePete | | #10

    Seismic concerns

    I'm planning on building a house with slab on grade in a somewhat seismically active area halfway up the Oregon coast (D2 in the IRC) (Zone 4C). Hard to find information on the internet for the best slab foundation approach in earth-quake zones.

    I found a hint at the end of a GBA article titled: "Installing a Concrete Slab the Right Way"....
    ""King responded, “Here in the land of the dancing earth, we engineers tend to tie everything together, like slab edges to perimeter footings. But there’s nothing sacred about that. ...""

    My fear with the stem wall approach is that the interior slab is not tied together with the stem walls and the thermally isolated slab within the stem walls could, in theory, slide around on the polyethylene and rigid insulation like a hockey puck and slam up against the stem walls during an earthquake....
    Well, not as fast as a hockey puck, but any velocity on a 24'x34', 30 ton slab seems like a lot of energy for the stem walls to take on. Working with slide hammers (or watching slow speed vehicle fender bender's) has given me a lot of respect for mass at speed.
    I can imagine the hockey puck phenomenon with a raft foundation as well, but in that situation perhaps it would be almost therapeutic, as the building floats above the shaking ground.… Although it would wreak havoc on the subterranean sewer and power service..

    So I'm thinking I'd like to go with monolithic (thickened edge) with outside edge insulation and horizontal interior insulation under the slab.... accepting the loss from the thermal break in the footing area between the two insulated areas... A thermal compromise for a more seismically resilient structure?

    At the same time, I'm considering hydronic heating in the concrete slab, so a fully thermally isolated interior slab within stem walls seem like it would be a superior assembly for that. Maybe my seismic concerns are exaggerated?

    Thanks

    Peter Clusener

  6. Expert Member
    Malcolm Taylor | | #12

    Pete,

    There are various levels of seismic resistance which can be designed into a building. I did an addition to a fire hall which was " Post disaster" - meaning it designed to be there after any anticipated seismic event. But like all other safety features we put in code compliant residential structures (fire resistance, wind-loading, railing heights, etc.) the results leave you in a grey area. That is, they are supposed to go a reasonable distance in controlling the anticipated threat, but there are no guarantees. How far you want to exceed code mandated seismic requirements, and how much more that will help if the Big One hits is a judgement call each individual home builder has to make.

  7. JunglePete | | #13

    Malcom,

    Yes, gray area... I hear ya.
    Looks like there are some impressive earth quake technologies out there. I was reading about a woman living in a house in Japan, set on rubber piers. During an earthquake she watched, through a window, her husband falling over the moving ground outside, while the house she was in virtually stood still. And there are some convincing videos online of model houses on shaker tables to back it up.

    However, I want to keep it simple.
    I accept that a slab may not be the best technology out there for earth quakes, but I want to make it as robust as possible (for a slab).... And I'm not sure if the assumptions I presented in post #10 are heading in the right direction? Sometimes when delving into something new, my intuitive ideas are dead wrong. Like in the 90s, when I though that if installing polyethylene on an inside wall was good, than installing it on both, inside and outside walls, was even better!... I was wrong. Thanks to experts who share information (like GBA), I don't do that anymore.

    Basically I don't want to make my slab foundation worse by trying to make it better, because I'm running with flawed ideas and missing the big picture.

    Thanks

    1. GBA Editor
      Martin Holladay | | #14

      Jungle Pete,
      It sounds to me like you need to consult a seismic engineer. They do this for a living.

      1. JunglePete | | #15

        Various slab foundations seem to be within the scope of the Oregon residential code prescriptive path for my seismic zone (virtually the entire west coast of the country is within this D2 zone or higher), so I don't think I will need an engineer for the foundation. I just want to arm myself with sensible information about seismic design, so that I can build the best (most sensible) slab foundation I can.

        I will need some engineering on a planned 2nd story deck, so I can probably discuss the slab design with them as well. The problem is that there is a building boom here with a limited amount of local engineers and my small project doesn't seem to interest them.... nor does my thirst for information on seismic design... the Engineers that I have talked to are swamped right now (probably with multi million dollar projects). I feel that if I get as informed as possible, before I call on them, I will sound like less of a dope and that may help me get my foot in the door and on to their project list?

        Anyway, thank you for the article, and your response. And thank you for this great web site. I really appreciate all the great information here and how it is presented.

        Pete Clusener

        1. Expert Member
          Malcolm Taylor | | #16

          Pete,

          I ran into the same thing this summer with two workshop I was building. I simply couldn't get an engineer to look at a small project.

          I'm sure you can come up with an adequately sound slab design, and if you do manage to ask about seismic events and perimeter insulation between the slab and stem-walls I'd be interested in hearing what they say.

        2. James Murray | | #17

          Hi Pete,

          As someone familiar with seismic design, this is an interesting challenge! You are likely right that a monolithic slab would perform better in an earthquake.

          If unrestrained (tied back to any slab/floor structure), the top of the stem walls may slide or rotate outwards in an earthquake, potentially destabilizing the structure above (this is really worst case scenario, not trying to be alarmist!). Without actually putting numbers to anything, you could improve the seismic performance of the stem walls by adding rebar dowels through your thermal break between the slab on grade and the side of the stem wall to tie them together. Exactly how this is built... depends on geometry and sequencing during construction.

          All that said... some perspective on earthquakes: 99.9% of the buildings built in the US and Canada (even Malcolm's post-disaster fire hall, if we're being honest) will not be usable nor functional after their "design earthquake" (i.e. the big one). We simply try to design them to not collapse on people, so people can run out and say "WOW that was crazy!" and try to post on instagram, except the cell towers are down. Base-isolated buildings (such as you describe in Japan, where it is slightly more common) is an exception but very specialized ($$$).

          Fortunately, 2 things:

          1) earthquakes are exceedingly rare, even on the west coast! and,
          2) Wood frame buildings with osb or plywood sheathing have typically performed well in past earthquakes at "not collapsing". Tying your stem wall to your slab on grade may not hurt, but there are likely much bigger considerations (like if one side of your house is all windows/doors...).

          1. Expert Member
            Malcolm Taylor | | #18

            James,

            I agree. My "post-disaster" Fire hall is simply better than the usual stuff. I'm under no illusions it will be there after any significant quake. I'm pretty sure the design engineers w0uld admit that too if you got enough drink into them.

          2. JunglePete | | #19

            Thanks James.

            Great response. I understand that there are some things out of our control, especially within common building practices. But I appreciate hearing the broader consensus on what are the "better" performance possibilities within those common building practices.

            And I identify with you comical, yet accurate "WOW that was crazy!" response after some events. I built a second story on top a of a shed out back and not long after it got a direct hit from lighting. Splitting the wall in half and sending 2x4 pieces across the room... and from there getting into the #8 romex, blasing the insulation off around random bends, before blasting through the sub slab underground conduit into the ground. The inconvenience of repairing the "shed" was more surreal than upsetting... Then a couple years later the main colvert for the neighborhood got clogged before the water could reach the lagoon... during a hurricane. The house had 6" of water while the shed had 24". "Wow this is crazy". But after the water receded the shed slab started to slowly sink to one side. A surreal event, but also a lesson.... concrete foundations are hard to repair... better try and build it right the 1st time.

  8. JunglePete | | #20

    Polyethylene edge detail?
    I was wondering about the variations in the polyethylene details along the edge of thickened edge slabs.
    One detail shows the polyethylene wrapping around and terminating all the way at the top of the slab. While the other (more commonly seen) detail shows the poly terminating as a small lip a couple inches above the footing base. I assume that the lower terminating poly is just above a possible drainage plane, so the footing is not sitting in direct contact with a puddle of water that a drainage system would not pick up (if drain was level with the footing floor). I am also wondering if the poly is terminating low down because the poly "origami".gets overwhelming... or there are other reason? In this lower terminating poly senario, are additional water proofing slab edge treatments recommended?

    Some times I see the poly terminating at grade level. Trying to figure out what makes the most sense.

    I've added an attachment and I am curious about other edge details I have seen. Specifically wondering about:
    - How the poly is terminated, if it is brought all the way around to the top of the slab?
    - Does an additional piece of poly make sense?

    Thanks,
    Peter Clusener

    1. GBA Editor
      Martin Holladay | | #21

      Peter,
      With a floor slab assembly, the concrete is the air barrier. The polyethylene is the vapor barrier.

      While air barriers need to be free of holes, vapor barriers don't. A vapor barrier that consists of 95% intact material and 5% holes is 95% effective at stopping the flow of vapor.

      My advice: Don't overthink this issue. Terminate the polyethylene wherever you want, and don't worry about it.

      1. JunglePete | | #22

        Okay. So, I'll have some flexibility in this area. Thank you!

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