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Community and Q&A

Foundation for a room addition

Howard Gentler | Posted in General Questions on

I’m working on a room addition in zone 6, 16′ x 16′ in size. I’d like to explain my current plan and seek
input from folks.

I think this would be called a slab on grade, despite the slab being elevated to match the level of
my current first floor. Right now the first floor is 3′ above soil grade via 2′ of concrete wall and
1′ of sill (4″) and floor joists (8″), but I plan on backfilling higher on the wall, still leaving 12″ between
soil and wood. My cement contractor said we could do a 4′ or 5′ frost wall, with an 8′ footing, and the use of rigid foam to make up for not going deeper with the wall. I would prefer not to dig deeper
than necessary. The site is pretty dry, the soil is compacted, and ledge is seldom far below ground.

Inside the frost wall will be filled (I think sand is typical), compacted, and brought to a level that
leaves room for 2″ of rigid foam and the 4″ slab (and 6 mil plastic below slab) to match first floor.

I have a few specific questions:
Is vertical foam placed outside the frost wall or inside? (I’ve seen diagrams both ways, but think
outside is more protective of the wall).
If placed outside, does the vertical foam start above the footing, along side it, or even under it
as I have seen mentioned. I plan on 2″ of foam.
How far below ground does horizontal foam need to be? I plan on 2″ extending outward about 2′,
more at the corners, like 40″.
What do I need to do against the house? (since the slab will rest against sheathing with rim joists
behind that. Is a water and ice type product good? or metal flashing, or PT plywood, etc?
Is the 6 mil plastic beneath the slab cut to fit between the frost wall, overlapped, or something

I’ve done a lot of reading on this but still end up with uncertainties that GBA is a great help with.

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  1. GBA Editor
    Martin Holladay | | #1

    You are describing a typical frostwall-and-slab foundation.

    Here is a link to an article about insulating this type of foundation: "Insulating a Slab on Grade."

    By far the most common way to insulate this type of foundation is to install vertical rigid foam on the interior side of the stem walls. In your climate zone, the rigid foam should begin at the footing and extend all the way to the top of the slab. The top of this insulation is often beveled, as shown in the illustration accompanying the article.

    The insulation needs to have a minimum R-value of R-10, unless you have hydronic tubing for heat embedded in the slab, in which case the minimum R-value is R-15. Thicker insulation is better, of course.

    Finally, it's also a good idea to install horizontal insulation directly under the slab. As noted in the article, "In climate zones 4 and higher, it’s also a good idea to install some horizontal insulation under a slab on grade. If you want to save money, you can install a 4-ft.-wide band of horizontal R-10 (or higher) insulation at the perimeter of the slab in a picture-frame configuration. Builders with a bigger budget, especially those in cold climates, should consider installing continuous horizontal insulation under the entire slab. A continuous layer of horizontal insulation reduces heat loss in the winter, and it reduces condensation of humid air on the slab during the summer."

    If you choose to install vertical rigid foam on the exterior side (rather than the interior side) of the frostwalls, you can. It's rarely done, because exterior rigid foam requires physical protection from impact and sunlight. For more information on exterior foundation insulation, see "How to Insulate a Basement Wall."

  2. Howard Gentler | | #2

    Martin, thanks so much. I had read your article, learned a lot, and just reread it. I like the preferred mode of insulating the stem wall inside. As you know, this avoids the nuisance of protecting the above grade rigid foam.

    Could you weigh in on my other questions, specifically, how far below soil surface (eventual grade) the horizontal insulation should be placed. I'm thinking it should not be too deep or it would not be affording the wall much frost protection from the outside ground. And the question about what to do with the house wall that is wood and would have the slab and some fill against it. Any best practice on how to install the 6 mil plastic?


  3. Expert Member
    Peter Engle | | #3


    You haven't mentioned what climate zone you are in. It makes a difference.

    From your post, it sounds like your foundation guy is suggesting something of a frost-protected foundation. With this type of foundation, horizontal insulation extends out from the foundation to protect it from losing heat to the outdoors and freezing under the footings. There are prescriptive requirements for the insulation in the IRC (see attachment). Using a frost-protected foundation allows you to avoid digging a deep trench to place the footings below the frost line in your region.

    However: There are quite a few specific rules, including: The building must be kept heated in winter (no garages, sheds, etc.). If you have an attached garage, it must have a deep footing, and this makes the horizontal insulation for the frost-protected section difficult. other rules apply when adjacent to heated structures. It gets complicated.

    Also: It seems that these foundations work by using heat rising from the ground that becomes trapped under the horizontal insulation. But they also work by using heat from the building to heat the ground under the building. If you insulate the slab, the math for the frost protection changes, and the prescriptive methods may no longer work.

    If you follow the prescriptive methods as shown, it will result in a relatively low-cost and low-energy foundation. However, if you are shooting for a very low-energy insulated slab, a frost-protected foundation would require engineering specific to your site and building.

  4. GBA Editor
    Martin Holladay | | #4

    Peter Engle,
    You wrote that Howard didn't mention which climate zone he is in.

    In fact, he did -- in the very first sentence of his post.

  5. Howard Gentler | | #5

    Peter, thanks for your response. Not everyone knows or uses the term (including my cement guy), but I would say this is a frost protected shallow foundation. The building will be heated in winter. I'm considering radiant in slab heating for this room. Sounds like it may make the FPSF (due to the foam under the slab) more vulnerable, perhaps due to reducing heat escape to the ground. But, don't almost all slabs, radiant or not, have rigid foam beneath them? I wouldn't think the prescriptive requirements suggest skipping the under slab foam.

    Do you know where on the shallow wall the horizontal foam should go (how deep)? And are you aware of
    good sources of explanation for this foundation, -places where I can get more detail? I have done a good bit of research on this and it is difficult to find really clear and definitive information.

  6. GBA Editor
    Martin Holladay | | #6

    Peter and Howard,
    It sounds like Howard described a frostwall-and-slab foundation, not a frost-protected shallow foundation (FPSF).

    For more information on FPSFs, see my article on the topic, "Frost-Protected Shallow Foundations."

    Peter, you may be interested in this quote from my article: "These shallow foundations don’t depend on leaking building heat to keep the soil warm. Instead, horizontal wing insulation extending from the bottom edge of the slab helps to retain the natural warmth of the earth."

  7. Expert Member
    Peter Engle | | #7


    I think Howard himself is not entirely clear, and like you said in your article, the separation between the two is becoming a bit blurred by subslab insulation requirements.

    I think the biggest difference between the two is where the insulation goes. With a FPSF, the insulation has to be on the exterior of the foundation, with horizontal wings extending out. For a frost wall foundation in cold climates, the more common treatment is to install insulation on the inside of the foundation, thermally separating it from the slab.

    Martin, as you note in your article, if the required slab edge insulation is installed on the outside of the foundation, it can do double duty as the required insulation for a FPSF. The IRC even allows the 2' extension of the slab insulation to go horizontally rather than vertically, again allowing for a FPSF configuration.

    Howard did acknowledge in post 5 that he agrees he is considering a FPSF. In answer to his question, the IRC requires the horizontal wings to be at least 12" below grade. In your area, the bottom of the FPSF probably has to be 16" below grade. I do see now that you are in Zone 6 (thanks for pointing that out, Martin). The IRC defines the Air Freezing Index differently than climate zone, so the actual town would help there.

    But looking back at the OP, Howard discusses a "4' or 5' frost wall with and 8' footing." I don't completely understand this. With a 5' frost wall and 12" of it above grade, the footing is already 4' below grade. That's got to be close to the frost line in your location, no? The whole point of a FPSF is that you only need to go down 12"-16" below grade, saving on excavation and concrete costs. It still sounds like you're not completely clear on which style of foundation you are considering.

    Finally, Martin, I disagree with the statement in your article that heat from the house is not a requirement. If that were true, the IRC would allow FPSF in unheated buildings. Clearly, heat from the building is a factor in the protection of a FPSF, and subslab insulation is going to decrease that heat flow. I don't know whether this is a big or little factor, but it is not insignificant.

    Howard, I'm sorry i don't have time right now to find any other resources for you. Now that you have the code terms for a FPSF, you may want to search using them. Let us know what you decide.

  8. GBA Editor
    Martin Holladay | | #8

    Here is a quote from "Revised Builder’s Guide to Frost Protected Shallow Foundations":

    "Frost protection of an insulated foundation also works for an unheated building by conserving ground geothermal heat beneath the building. Unheated areas of homes, such as garages, may be constructed in this manner."

  9. Howard Gentler | | #9

    Thanks gentlemen. I was thinking FPSF but hesitated to use the term. The 4 or 5 foot wall I mentioned is really 3' above current grade, so about 20" (a foot of wall and 8" of footing) is below. I am able to backfill another foot or so, -therefore the foundation wall will start about 32" below ground. This probably represents a FPSF that is a bit deeper than it needs to be minimally, which can only be a plus.

    Sounds like I should be doing rigid foam on exterior of wall, now that we know it is a FPSF, correct?

    I also came across, in my reading, that a FPSF could be used on an unheated building, but some of the details were enhanced. I will probably use vertical foam outside the wall, and some horizontal as well. Perhaps this qualifies as what Martin calls a "belt and suspenders" approach!

  10. Expert Member
    Peter Engle | | #10

    Martin, Thanks for posting that manual. That is exactly the sort of resource Howard was looking for. For even more detailed engineering design data, the manual refers to ASCE 32. That would be the source materials for any sort of hybrid or mixed system.

    The manual also shows specifically what I was talking about above. The IRC prescriptive design applies only to heated buildings, because heat from the building is a part of the frost protection. This is shown graphically in Figure 3 of the manual. The "simplified" design section of the manual was used to develop the prescriptive requirements of the IRC. The manual then extends the concepts to designs for other applications, with both heated and unheated buildings.

    On another currently active thread about insulated foundations in unheated garages, the discussion is about using geothermal heat to keep an unheated garage at slightly above freezing, even in zone 6. This manual takes a different approach, and runs the insulation under the slab so that the slab can freeze but the soils under it cannot. I hadn't thought of that, but it makes sense.

    So I guess we're both right. The IRC method applies only to heated buildings, where heat from the building contributes to the frost protection. I would not use this method if there is going to be substantial subslab insulation. The manual you posted, and ASCE 32 provide extended design information that can be used to design a FPSF for the conditions where there is a mixture of exterior and subslab insulation, heated slab, or an entirely unheated building.

    For Howard's case, he can probably save some $$ by doing a FPSF. While the IRC method may not apply, the detailed design method for heated buildings looks like it would handle Howard's needs, and he could tweak the combinations of slab insulation, vertical and horizontal insulation and footing depth to his heart's content.

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