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How problematic is XPS foam board applied against asphalt-based foundation coating, on exterior?

Anita Brosius-Scott | Posted in GBA Pro Help on

As we move forward with energy retrofit insulation work on our house, we finally decided, due to our moist clay soils and northern climate (Maine), to insulate the exterior of our foundation rather than the inside walls in the basement. Some fun.
At this point the plan is to keep the interior of the concrete wall exposed to the inside air.
We’re laying 2″ XPS foam boards against the foundation walls (Obviously that means we’ll have to deal with a protective covering above grade).

Today’s question has to do with foundation coating on the walls, vs. the XPS foam boards laid against it.

After rolling on a good coat of the asphalt-based (black tar-like, clean-up with solvent) foundation coating on one full facade of (old, well-cured) concrete , we learned that the XPS foam board would be expected to react with / be eaten by the coating. Oh great.
Given we’re laying against it foam boards that are a full 2″ thick, and the coating is dry to the touch, not still gooey, is the foundation coating really going to corrode that much of the XPS, or too little to be concerned about? I just can’t believe that it would really compromise the foam board to any appreciable degree.
If the former, then we need to figure out how to protect the XPS foam boards (not yet installed) from the tar coating that we already laid onto that one facade.

How about laying down a thin physical barrier between the foam board and the tar-coated foundation, of:
a) a sheet of weed cloth from the garden center
b) a layer of Typar house wrap?

Meanwhile, if it is a genuine problem, then we’re underway sourcing a foundation moisture coating that will work touching the XPS foam, for the rest of the job. If you have a favorite, would appreciate hearing about them. Some of those rubber coatings and self-adhering sheets are way expensive and not really necessary where we don’t have through-wall seepage; am thinking XPS boards plus foundation coating should be enough for practical purposes.

Your thoughts will be, as ever, much appreciated.

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  1. John Klingel | | #1

    Glad to hear you are upgrading. Have you contacted the manufacturer about barriers? And, while you are doing all this, have you inquired locally about the wisdom of adding a sheet of XPS horizontally at the footer?

  2. Anita Brosius-Scott | | #2

    What do you mean by "wisdom of adding a sheet of XPS horizontally at the footer"?

  3. GBA Editor
    Martin Holladay | | #3

    You should be fine, as long as you wait until the asphalt dampproofing has cured and is no longer tacky. (That allows most of the solvents to evaporate).

    Here is information from an Owens Corning Web site about your question. (Foamular is a type of XPS):

    Q: "Can FOAMULARĀ® be used over foundation dampproofing?"

    A: "Yes. FOAMULARĀ® provides excellent water resistance and R-value retention in below grade use. It also protects foundation waterproofing and dampproofing from damage during backfill. If a solvent based foundation treatment is used, allow the coating to fully cured, and solvents to dissipate before placing FOAMULARĀ®. Polystyrene may be damaged by solvent base materials. This caution is not necessary with water based emulsions."

    I called the tech hot line at Dow, and they said that Dow Styrofoam can be used against asphalt dampproofing, as long as it isn't tacky.

    If you have further questions, you should contact the manufacturer of the asphaltic dampproofing.

  4. Anita Brosius-Scott | | #4

    Thank you so much, Martin. Bet you heard my sigh of relief from there!

  5. Riversong | | #5


    If you've gone to the trouble of excavating the entire foundation, then it's worth while to use a true waterproofing rather than the "damp-proofing" offered by asphaltic emulsions. I haven't used "tar" foundation coatings for 20 years.

    I use exclusively UGL Drylok latex masonry sealer. It's a bit pricey because it requires a lot for two-coat coverage, but it's available in 5-gallon buckets, is applied with a coarse 4" or wider brush, dries quickly, is cleanable with water, and provides waterproofing even against a 20' hydraulic head of water. Any cracks or holes in the foundation must be first filled with hydraulic cement, which sets in minutes.

    And the Drylock is 100% compatible with all rigid foam boards. To be safe, i would hang a sheet of 6 mil poly between the tarred wall you already coated and the foam board. This will also provide additional water-proofing. Neither weed cloth nor Typar will do, as neither will prevent solvent diffusion or keep water off the foundation.

    Be sure to install a perimeter drain, bedded in crushed stone and all wrapped like a burrito in heavy duty road-bed filter fabric, and drained to daylight or to a sump to be pumped to daylight.

  6. Raff | | #6

    We just went through the same thing on our retrofit. Here's what I did:
    fourth line of defense:
    -blue seal waterproofing on block wall
    third line of defense:
    -Delta MS waterproofing membrane
    second line of defense:
    -4" R20 XPS
    first line of defense:
    -3/4" washed stone backfill to grade with Getextile fabric separating it from soil

    plus the weeping tile around the footing drained to daylight....

    I also added 2" XPS to interior walls and existing slab for an additional layer of COZY

  7. John Klingel | | #7

    Anita: What do you mean by "wisdom of adding a sheet of XPS horizontally at the footer"? In some places, a piece of 2", or so, of XPS is laid horizontally on top of the footer and butts against the XPS on the foundation wall. Look here for an example picture. ( Then click on the Frost Protected Shallow Foundation). The practice helps, I believe, "moved the frost line" away from the foundation. Whether or not it is of merit in your area, I don't know, but I thought I'd mention the practice since now is the time, if you feel it helpful. I will be doing it here, for sure. john

  8. Riversong | | #8


    Most foundations in New England are full basements, with the footing well below the frost line, so no footing insulation is required unless the slab is thermally coupled to the footing.

    The only advantage to wing insulation at below-frost depth is if the basement slab is radiantly heated, as the additional insulation will reduce heat loss to the ground by maintaining a wider "bubble" of warmed earth.

  9. Anita Brosius-Scott | | #9

    Horizontal insulation at footer depth is a very interesting wrinkle to add to the insulation equation! It gives me pause....
    Consider the walk-out section, where 2" XPS will be attached to the (air-exposed) vertical exterior walls. The footer, which would lie not far from the earth at this point, and particularly the section below the door sill, are fully exposed to minimum winter temperatures, and thus provide a serious cooling factor to the foundation via thermal coupling.
    Now, I have never heard of this, but...
    The suggestion of horizontal frost protection for a shallow foundation would surely apply to the foundation around the walk-out door, right? So it would stand to reason that one should excavate, say, 4' out beyond the door at footer depth and lay a horizontal XPS sheet, then cover again with fill to the door sill.
    Walk-out basements are not rare installations. But I've never heard of insulating the foundation in such a way in order to minimize the thermal impact of the below-grade foundation at this portion, by using horizontal insulation.
    OK, Panel of Experts, what's the practice here?

  10. GBA Editor
    Martin Holladay | | #10

    You're right that foundations under doors in walk-out basements are often poorly insulated. But vertical insulation works fine in this location. You can install horizontal insulation too, if you want. It's all good.

  11. Riversong | | #11


    The foundation insulation strategy depends on how the foundation was built. A walkout foundation is required by code (and common sense) to be fully frost-protected on all sides. This typically means that there is a deeper footing and frost wall below the walkout side, which is stepped up to the high side.

    The alternative to a stepped footing and frostwall is to use rigid foam (should be XPS) to create a shallow frost-protected foundation at the low (walkout) side. The thickness, size and location of the foam depends on your local air-freezing index (HDD base 32 and depth of footing.

    For a cold New England climate, vertical R-10 extending 12" below-grade and 12" of the same horizontally out from the bottom of the vertical board is sufficient. We used to think that 4' of foam board was required, but that's not the case, since the house warms the ground beneath it.

    Most heat loss from a slab is at the edges exposed to ambient air temperatures, which means the walkout edge in this case. So it's critical to insulate that edge as well as possible in order to maintain a comfortable floor and avoid major heat loss.

  12. Anita Brosius-Scott | | #12

    Wow, the info re. R-10 only to 12" below grade, plus to 12" horizontal, is mind-bending!
    We'll also look at insulation at the door - now probably dig down at the door sill and out, to put some R10 foam board down, and out 12" below the door. Thanks for that input!
    Your contributions will help us plan the layout of our digging and foam board. We'll combine those thoughts with consideration to places where drainage at depth also needs to be taken into consideration.

  13. Riversong | | #13


    The horizontal (or slightly sloped for drainage) wing insulation must be at least 12" below grade, as in the diagram on page 3 of this Revised Builder's Guide to Frost Protected Shallow Foundations: And it should be sealed to the vertical insulation with canned foam or foam-compatible adhesive.

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