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Slab insulation – do I understand the code requirements?

T_T | Posted in Building Code Questions on

Question about slab insulation. The current slab has zero insulation on each side. That was no issue until now because it always an unheated garage. The slab is just that, a slab. No footing, no drainage.

Living in zone 5A and using the table Martin gave me in another thread I conclude I need:
– R-10 on the edge. –> 2.38″ EPS or XPS*
– R-30 on top of the floor indoor. –> 7.14″ EPS or XPS*

*= I use R4.2 for both EPS and XPS because I will use reclaimed foam. From what’s I’ve understood XPS outgasses and it’s R-value drops to that of EPS over time.

Polyiso can’t be used under grade. The floor is likely to cold to put polyiso on top of it. Because it’s old reclaimed material I use an R-value of 5.

I’m aware the exact R-value are a big debate. Often a flame war it seem. I’m not looking for that. I just want to know I really need those values. Do I understand the code correctly?
Do I really need R-10 slab edge AND R-30 floor?
Or is it a bit similar to wall where I need R-20 in the stud bays OR R-13 in the stud bays and R5 continues sheeting?

When reading various sources, including this forum, there seems to be agreement that the edge insulation gives most bang for a buck. And some advise R15 for my zone – 5A.

My problems are:
– R-30 would be a very thick insulation, but I think it’s effective.
– The slab may be hard to insulate?

Attached the dimensions of my slab. Red is concrete. Blue is grade. Ground doesn’t, and can’t slope.

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  1. GBA Editor
    Martin Holladay | | #1

    The language in building codes describing requirements for slab-on-grade Insulation are some of the most poorly written sections of the residential building code.

    Briefly, there are no requirements in the code for horizontal insulation under or over a slab on grade. The only requirements refer to insulation at the slab perimeter, which in most cases will be vertical insulation.

    Just because the code doesn't require horizontal insulation under a slab, doesn't mean that such insulation isn't a good idea. It usually is, especially in cold climates -- and especially if the slab will include embedded PEX tubing for hydronic heating.

    Here are links to two useful documents with more information on this topic:

    Insulating a slab on grade

    Slab-on-Grade Insulation - Code Compliance Brief

    Table 402.1.1

    Here are some relevant quotes:

    From "Insulating a Slab on Grade":
    "In climate zones 4 and 5, the code requires certain slabs (below-grade slabs that are less than 12 in. below grade) to have R-10 vertical insulation at the slab perimeter, extending downward from the top of the slab to a depth of at least 2 ft. If the slab has hydronic tubing, the minimum R-value of this insulation increases to R-15. The same R-value requirements apply in climate zones 6, 7, and 8, but the insulation must extend to a depth of at least 4 ft.

    "There are no requirements for horizontal insulation under slabs, but the requirements for perimeter insulation can be met partially with horizontal insulation. This compliance option is explained this way: “Insulation located below grade shall be extended the distance provided in Table N1102.1.1 [namely, 2 ft. in climates zones 4 and 5, and 4 ft. in climate zones 6, 7, and 8] by any combination of vertical insulation, insulation extending under the slab, or insulation extending out from the building.” "

    From "Slab-on-Grade Insulation - Code Compliance Brief" (see also the image reproduced below):
    "2015 IECC/IRC Section R402.2.10/N1102.2.10 Slab-On-Grade Floors. Slab-on-grade floors with a floor surface less than 12 inches below grade should be insulated in accordance with information provided in the Insulation and Fenestration Requirements by Component Table, which is reproduced following this paragraph. Slab-on-grade insulation should extend downward from the top of the slab on either the outside or inside of the foundation wall. When a slab is independent from a perimeter foundation wall, insulation may be installed either on the exterior of the foundation wall or between the foundation wall and the slab. Insulation located below grade, as required in Climate Zones 4 through 8, should extend the length specified in the table by any combination of vertical insulation, insulation extending under the slab, or insulation extending out perpendicular to the building Insulation extending perpendicular away from the building should be protected by pavement or by not less than 10 inches of soil. It is permissible by the codes that the top edge of insulation installed between the exterior wall and the edge of the interior slab be permitted to be cut at a 45-degree angle away from the exterior wall. Slab edge insulation is not required in jurisdictions designated by the code official as having a heavy termite infestation."

  2. STEPHEN SHEEHY | | #2

    Tony: It's not clear what your plan is. Are you finishing the garage and making it a conditioned living space? Is there no footing at all? Are you concerned about frost? Are you sort of retrofitting a frost protected slab? Are you contemplating putting R-30 on top of the slab?

  3. GBA Editor
    Martin Holladay | | #3

    In Zone 5, the minimum R-value for your perimeter insulation is R-10. So you can't use R-5 vertical and R-5 horizontal. The section of the code you quoted (about "any combination") simply refers to the distance that the rigid foam must extend.

    I told you that you code is poorly written, didn't I?

  4. T_T | | #4

    Hi stephan,

    Currently an unconditioned garage is on the slab. The garage will replaced by a totally new conditioned building. Only the slab remains.
    The slab has no inslution on top, underneath or on the sides. Neither does it have any footing.

    "Are you contemplating putting R-30 on top of the slab?"
    Not really because such a thick layer of foam would be very unpractical. But if ccode dictates it I will do so.
    But after reading Martin's reply it looks like I misunderstood code. So likely it will be between R8 and R10.

  5. T_T | | #5


    So if I put R-10 at the perimeter (2ft deep) and nothing else I meet minimum code?

    And even a fraction less breaks the code; even when combined with something insane like R-100 insulation on top of the floor?

  6. STEPHEN SHEEHY | | #6

    Tony: saving the slab may not be a good idea. No one would even think of building in zone 5 without any foundation. If you need to comply with a building code, I'd speak to the code enforcement officer to see what, if anything, you'd need to do to be code- compliant. Putting a house on top of an uninsulated slab with no foundation or footing seems risky to me.

  7. T_T | | #7

    Hi Martin,

    " any combination of vertical insulation, insulation extending under the slab, or insulation extending out from the building.”

    Ok, I understand that; a combination. But what's the 'formula' for that combination? R5 vertical and R5 horizontal is code minimum?
    Not that I'm planning something like that because I'll at least put in 2" on top of the slab to fill out that 2" in the drawing.
    The vertical insulation will be 2" or more (R8 or more)

    I would like some advise on how I apply the insulation. And then especially the corners.
    The vertical insulation end at the top of the slab, 3" above grade.
    Indoors the 2" will be filled with foam boards.
    That leaves the very top of the slab. The wall is put on that.
    I think that will leaves gaps. The vertical insulation reaches 2ft down but at the very top of it cold just creeps in.
    Just put the wall on that ledge and seal the cracks with canned foam?

    "Slab edge insulation is not required in jurisdictions designated by the code official as having a heavy termite infestation."
    Can I look up that info somewhere? There are termites for sure. And carpenter ants. And some wood bees. :-(
    Will putting some quickcrete on the vertical foam stop the termites?
    The termites on my yard seem very selective. They ate 24 window frames in the house but almost didn't touch anything else in the house (all wooden floors)
    The very old garage that's currently on the slab is 100% termite free. So what's heavy?
    Regardless of your answer I want good insulation. Bug proof insulation :-)

  8. GBA Editor
    Martin Holladay | | #8

    Q. " 'Slab edge insulation is not required in jurisdictions designated by the code official as having a heavy termite infestation.' Can I look up that info somewhere?"

    A. The best approach is to use a telephone. Call up your local code official and ask.

  9. T_T | | #9


    The slab is in very good condition. Still level and uncracked after many years.
    The planned building is just one floor and not much heavier than the current garage.
    It won't be a house but a meeting area. No beds, shower, etc.

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