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Community and Q&A

Insulate a slab in a mild climate

nrj | Posted in Green Building Techniques on

We’re building a 250 square foot in-law unit on a monolithic slab in zone 3c (bay area, calif.). The slab will be 6.5 inches above grade to meet code. The slab will also double as the finished floor. It will NOT have radiant floor heating.

Do you think it’s a wise investment to add rigid insulation around the exterior of the slab and under the slab? If so, what r-value should we shoot for?

We’ll be aiming for about r-20 walls and r-40 roof and we’ll be building a tight envelope (I hope). We’ll ask some local builders what they think too, but I’d like the opinions of the gurus on this forum too.

Thanks for any suggestions.

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  1. GBA Editor
    Martin Holladay | | #1

    As long as you follow local advice concerning termites -- (a very local issue about which it is hard to generalize) -- you definitely want at least R-10 of vertical rigid insulation around the perimeter of your slab. I'd be very surprised if Title 24 requirements in California allow you to omit it.

  2. nrj | | #2


    As crazy as it sounds, California doesn't require slab insulation unless the slab is heated. For our area the minimum for a heated slab is R-5 on the exterior and underneath.

  3. homedesign | | #3

    If you are in Termite Country....
    Check out "Slab Happy" Figure 3 & Figure 4

  4. nrj | | #4

    Yep, we're definitely in termite country here. Thanks for the link. In the article it shows how to insect and rodent proof the insulation up at the top of the foundation, but what about the insulation under ground? Is there a standard material used to protect the foam boards, sort of like an exterior cladding?

  5. homedesign | | #5

    I don't know of a termite proof way to insulate the face of the foundation.
    the problem with a "facing" or cladding.... the termites might just tunnel up thru the foam behind the cladding....undetected
    The BSC above the slab option is the best proposal I have seen......
    course I have never seen it done ...and I am not sure it has a track record.

  6. wjrobinson | | #6

    Foam and termites not good. Forget foam anywhere outside the concrete perimeter. Under and at the edge of a slab if the slab is poured inside an exterior concrete footer and short wall or full wall (termite stop). Otherwise not needed where you live. An inch or two under the slab would be fine but check those where you live as to do with termites with underslab foam. They know, I live without the critters.

    As to saving E. A split air for HVAC should be very affordable to install and to pay for monthly no matter how much you pay for E. I am not sure Splits are made small enough even for the load your little addition will have. And you have excellant PV prices. Go PV if you have the exposure and when you can afford it if you want to do anything toward being less tied to fossil fuel. Also, I would not go to tight with a need for ventilation unless your air is polluted. IF it is polluted where you live, it is better to move to where it is not. Why would anyone live in a polluted enviroment. Two visits to LA and I still shake my head at the thought of millions not knowing enough to not live in a cloud of smog.

    You live in a mild climate. Do not encourage your termites just because you discovered this site.

    Lastly, you need a contractor that can build and has built what you are asking them to build. Key.

  7. homedesign | | #7

    The Bay Area is not the same climate as LA
    Probably more HDD than Dallas
    In Dallas we are not currently employing slab insulation...however, Building America and many of my Energy Geek friends believe it would be beneficial in Dallas

  8. nrj | | #8

    The HDD in my town is about 2300 with a base temp of 65 degrees. The CDD is about 2500 with a 55 degree base temp. Although using PV and mini splits and other high tech stuff to heat and cool the house would work, we just don't do it enough to justify the cost. Our annual gas and electric bills total about $250.

    Our low budget strategy for heating and cooling the in law unit is to use a 1500W space heater in the winter and to open windows in the evening in the summer. I'm a little worried the slab will be a thermal bridge without exterior insulation, but with more research I'm getting the feeling that it might not be worth the cost to add it.

  9. wjrobinson | | #9

    $250! Just being attached to gas and electric with zero use, costs close to $500 here. Time to move.

  10. nrj | | #10

    Well, $250 is definitely on the low end around here too. We enjoy being frugal with our energy use :)

  11. Expert Member
    Dana Dorsett | | #11

    If slab-edge foam is completely encapsulated in concrete below grade, and has copper Z-flashing at the top the termite-tunneling issue goes away. Pouring the stemwall with insulating concrete forms and using stucco or fiber-cement approach over the foam can get you there, with some attention to detail.

    SCIPs are essentially reinforced concrete/shotcrete encapsulated EPS foam structures, and do very well in high-termite risk areas.

  12. homedesign | | #12

    Dana, do you have a link or details where vertical insulation was "completely encapsulated" ?
    how did they deal with thermal bridging and reinforcing steel?

    the attached snipit is from BSC Designs that work .....

  13. user-433254 | | #13

    Has any one looked at Robert Riversong's solution to this problem. I think he calls it a thermomass foundation. Any thoughts? is the link go to bottom of page.

  14. pacificsash | | #14

    Nick, what did you decide to use? I am building a similar structure here in the peninsula (san carlos) and need make a move in one direction or the other.

  15. sjnick | | #15

    Hi Branden,

    We used a little over R-5 worth of the pink rigid foam under the main body of the slab and no insulation along the exterior. Without any insulation along the exterior, I doubt it's worth putting insulation under the slab too. But the more experienced folks on here may refute that.

    We did however decide to put some laminate flooring over the slab, as we got the flooring used off Craigslist for a song. It would have been far costlier to hire a crew to come in and polish and seal the slab as a finished floor. The laminate floor feels warmer to the touch and has a wee bit of cushion compared to the slab.

    We tried really hard to create a robust air barrier, using techniques learned from reading this website. The wall and ceiling insulation is also above code, but nowhere near Passiv House standards. Despite this, the temperature in the in-law unit has remained very comfortable during the couple of heat waves we've already had this year.

    If you want to compare notes outside of this forum, you're welcome to reach me at [biznick at gmail dot com]


  16. Expert Member
    Dana Dorsett | | #16

    "We used a little over R-5 worth of the pink rigid foam under the main body of the slab and no insulation along the exterior. Without any insulation along the exterior, I doubt it's worth putting insulation under the slab too."

    The center-slab insulation is still "worth it", in a zone 3 location, but slab edge insulation would save more energy and provide higher comfort. The energy use reduction provided by insulation is a function of square feet x temperature difference/R-value . The peak temperature differences are much greater at the slab edge, but it's fewer square feet. By letting the slab edges run cold/hot seasonally, it undercuts some of the performance of the sub-slab insulation nearest the edges, so it's better to have both.

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