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Roofmate XPS below grade?

Ryan_Mount1 | Posted in General Questions on

Hi All,

I’m thinking of insulating the exterior of my foundation with 3″ of recycled Roofmate XPS.  This is to go along with interior insulation as well.  I couldn’t find anything on this application for it from Dow. 

Does anyone see any issues with this or is XPS all the same for this application since I don’t need extra compressive strength here?

Thanks in advance.

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  1. onslow | | #1


    I insulated my entire foundation, the slab, and part of my garage walls with recycled XPS that was said to come off a commercial re-roofing job. I do not know if it was specifically Roofmate from Dow, but it is a light blue in color. It is very resistant to stomping on, so I think it is 25 psi.

    While I obviously cannot offer guarantees of performance, I will say that the material I have is pretty tough stuff. I have left over chunks that have sat outside for four years now (much to my wife's annoyance) and while the surface is sun chalked, the breaking strength seems unimpaired. My foundation insulation is completely sealed in a Grace bitumen based material from top to footing, as I did not want water to be able to get behind it or between the sheets. It was anchored to the concrete with ramset plastic washer things. The name escapes me at the moment. They were about a buck a pop five years ago. I certainly did not see anything funky going on before I set the skirting material over the Grace product to prevent animal attack. I did not apply any insulation inside the foundation, I didn't want the expense of covering it with drywall. My shop is pretty comfortable as is.

    The under slab foam only shows at the column locations where I plan to seal the slab to the columns. (some day) No signs of anything there either. The garage wall got stucco and all is well there too. The extra compressive strength is quite useful in my mind. Certainly under the slab it was nice to not have to worry about all the glomping about on it prior to the pour.

    You haven't indicated how you will handle covering the above grade portion or for that matter how far down the wall you will be going. If this is not a fresh foundation with access all the way to footing, then try to get at least two feet below surface. Further if you are in a CZ 6. If the back fill will be very rocky, it can be helpful to put some super duty poly over it. The stuff I used is 30-40 mil.

    I used 1/2" Hardiboard as skirting material non-stamped side out. It looks a lot like concrete, a bit fussy to keep the edges locked together for smooth runs, I can describe if needed. You probably could parge some EIFS type materials directly, though you will want to mesh tape the seams between sheets and embed some as you go along. I will have to assume that you have worked out the details of offsets between upper walls and foundation.

    In general I would not be afraid to use the same material again for most any use.

  2. Ryan_Mount1 | | #2

    Hi Roger,

    Thanks very much for your detailed response. I'm glad to hear you've had a good experience with it so for.

    To answer your questions, yes I am in CZ 6, and yes I will go right down to the footing. My plan is to cover it above grade just as you mentioned with some parging meant for the job and the mesh. Not the most durable finish out there but easy enough to fix should it get damaged. I have also considered the cement board for extra durability as you did but was thinking of sticking with just the parging and see how it holds up.

    I wasn't going to cover the foam at all below grade. I wanted my waterproofing and dimpleboard directly against the concrete, with the foam exterior to that.

    Thanks again.

  3. onslow | | #3


    Glad to that I can help someone, I have had found lots of advice on this site over the years, so time to return the favor. I am a bit curious about how you intend to attach the foam. Holes in the dimple mat won't matter much, but will your waterproofing choice be as forgiving?

    In my method of insulating, the foundation did not receive the waterproofing as a surface treatment. Instead I used the Xypex additive to render the footings and walls waterproof. The foam sheets being anchored directly did not impact the waterproof quality. A bunch of drilled holes you can't treat with a sealant might be a risk if your water situation is wetter than mine. We only get 11-12" of precipitation a year so the spring melt is really my only wet time.

    I should note the Xypex is waterproof against bulk water. Cracks will self heal up to 1/32 or so. If I was sitting next to a lake the the foundation would still be able to transpire water albeit very slowly. I only get wet toes when the snow melt comes down slope, so I may have gone a bit overboard. However, I do like having a nice dry shop space.

    I couldn't parge the foam since I encapsulated the whole foundation with the Grace product. I did however have to battle flickers and woodpeckers, who like to peck at stucco. I went with a Parex brand base coat that is twice the impact strength of Dryvit. It comes in bags rather than ready mixed. The blue mesh comes in grades or weights as well that improve impact resistance, though with more cost for heavier materials. I certainly would not take a hammer to any of the walls, but the casual bump with a rake handle hasn't caused me to speak in tongues. If you have active children or other impact sources, parging on a tougher coat might save some time and bother later.

    Stucco materials are seldom happy about being buried in ground due to freeze-thaw cycling. Anything you can do to keep them drier, like make sure gutters and runoff don't dump close to the house will help their longevity.

    Gutters where I am are kind of hopeless due to ice build up. All my melt drops right next to the house. The temperatures swing about 30-40 degrees every day, so roof melt and gutters quickly lead to bars of ice that tear the gutters off. To get around this issue, I hung my panels of Hardiboard from the bottom flashing strip. The bottom edges are actually only 1" below the belt of washed rock I have around the house for fire protection.

    The rock drains quickly to underground pipes , which keeps the boards from soaking in water most of the winter. A few places on the north side do stay pretty much embedded in ice for the whole winter, but so far no visible spalling. I suspect that minimizing soggy soil will provide similar improvements that will keep the parging in good shape.

    Hope your project goes smoothly.

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