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Question about finish over exterior foundation insulation.

wdpoole | Posted in Energy Efficiency and Durability on

I want to insulate the outside of the foundation on a house currently in the design phase, but I just can’t get excited about options for covering the insulation in the space between the top of grade and the bottom of the siding… has anybody completed a project with this detail and come up with a material that is both practical and, for lack of a better term, “good looking”?

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  1. GBA Editor
    Martin Holladay | | #1

    Here are your options for protecting the above-grade portions of exterior foundation wall insulation:
    1. Pressure-treated plywood.
    2. Fiber-cement panel siding.
    3. Cementitious coating (stucco) -- either reinforced with fiber, or installed over fiberglass mesh, or installed over metal lath.
    4. Insul-Cap vinyl covering from Wisconsin Poured Wall Products ( Muskego, WI).
    5. Ground Breaker fiberglass covering from Nudo Products (
    6. Insul-Guard 2 fiberglass covering from Diversified Composites (
    7. Surface-bonding cement.
    8. Perma-Bond Complete (foam plus factory adhered cementitious coating) from
    9. FP Ultra Lite panels (factory coated foam panels) from Styro Industries (
    10. Protecto Bond peel-and-stick membrane (

  2. Allison A. Bailes III | | #2

    Martin, I've heard that recommendation for using fiber-cement siding in this application before, and I always worry about that. I've seen scrap fiber-cement siding lying on the ground and delaminating after getting wet. Yes, if the water is managed correctly, it should never be wet for very long, but still, it's outside with the bottom in the dirt. Do you know something about this that makes you feel OK about recommending it?

  3. Michael Chandler | | #3

    We used Hardi-backer tile backer board on this house run 5' vertically and 3' horiz with fiber mesh tape embedded in surface bonding cement at the joints and covered with sponge finish stucco and elastomeric paint. We didn't have any issues with de-lamination of the Hardi-backer but we had to go in within a year and cauld all the vertical cracks and re-paint. Since then we've been fine but on other projects where we stucccoed over mechanically fastened Spider Lathe synthetic stucco lathe we've not had as much cracking.

  4. homedesign | | #4

    What is your insulation material?
    Are you in termite territory?

  5. Michael Chandler | | #5

    Generally it's Insulated Concrete Forms, the 9" waffle core by American Polysteel with the termite treatment integrated into the foam with Premis soil treatment under the slab and in the soil around the house as well as a galvanized termite shield between the sill seal and the sole plate. In the Goldmine house we put 2" of foam under the slab and around the perimeter with a 4"x8"x16" solid block on edge and folded the termite shield down so it was in direct contact with the back of the hardi where it lapped over onto the block.

    In general we leave the termite shield exposed 1" inside and outside the foundation but our local code allows no termite shield when soil poisoning is used. I can't imagine leaving it out though.

  6. GBA Editor
    Martin Holladay | | #6

    Good questions. This is what I wrote in the subject in an article in the March 2003 issues of Energy Design Update:

    "Some builders have used fiber-cement panel siding to protect foundation foam. Only one fiber-cement siding manufacturer (MaxiTile of Carson, California; 800-338-8453; permits the installation of their panel product (Maxipanel) in contact with the soil. Maxipanel siding is available in 4’x8’, 4’x9’, and 4’x10’ panels. A James Hardie technical representative stated that the use of their fiber-cement panel siding (Hardipanel) to cover foundation insulation is “unwarranted.” Yet the James Hardie Web site recommends the use of Hardipanel as mobile-home skirting. Other fiber-cement siding manufacturers, including Cemplank (manufacturer of Cempanel siding) and CertainTeed (manufacturer of FiberTect panel siding), categorically advise that their products must not be installed in contact with soil."

  7. Allison A. Bailes III | | #7

    Thanks, Martin. That confirms my experience with Hardiplank. I'll have to check into MaxiTile.

  8. | | #8

    This is a weak point. I recommend Solarcrete energy efficient building system to use. Wall begin below the frost line! - No energy gap. Same at top, roof line.

  9. NH Green Builder | | #9

    Try a spray on bedliner application - some guys are set up to do it mobil, although it is fairly expensive it looks great. There are also some industrial paint coatings that are now specified for it that can be sprayed or rolled on -- check with you painter. Or for low cost use colored coil stock.

  10. iMarc | | #10

    Prefinished fiber-cement panel and board products and cement backer board may be used where they're in contact with the earth (moisture) in mild climates but the manufacturer's don't recommend them for colder climates where they will be subjected to freeze-thaw cycles. They will begin to deteriorate after 100-150 freeze-thaw cycles. This from the James Hardie engineering department when I called to ask.

  11. Max York | | #11

    For insulation, I would say go with Demilec Soy spray foam. The idea to cover it with a spray-on bedliner is really interesting... has anyone else tried that? It doesn't sound great for the environment though.

  12. Riversong | | #12


    For 20 years, I've used surface-bonding cement (fiberglass-reinforced, acrylic-modified cement for dry-stacked CMUs) with excellent success. I add extra acrylic mortar modifier in the mix water for greater plasticity, water-resistance and adhesion. I believe this mix would stick to anything (it's hard to clean your tools), but for a belt-and-suspenders approach, I attach ½" hardware cloth to the sill and extend it down a few inches below grade before troweling the blockbond.

    It leaves a highly water-resistant and very tough "stucco" coating that looks great and has excellent tensile strength and crack-resistance because of the fibers.

    Another alternative, to avoid the problems of external foam board, is to build on a ThermoMass foundation, which places the XPS midline in the concrete wall, with the inner and outer whythe connected 12" oc with fiberglass ties that have the tensile strength of ½" rebar. They offer up to 4" of XPS, for a foundation that is not only very well insulated, but also with an integral capillary break that is out of reach of insects, UV and physical damage, and almost the same dynamic thermal mass advantage of an exterior-insulated 8" concrete wall.

    1. cmylks | | #25

      Hello Riversong,
      We are considering to use the 'belt and suspenders' approach you've described above (surface-bonding cement over metal lathe / 1/2" hardware cloth) to cover the exposed, above grade portion of exterior insulation. The insulation product we've used is the semi-rigid Rockwool Comfortboard 80 not rigid foam. Our local contractors supply guys have cautioned about the cementitious coating over Rockwool due to moisture issues - they fear that there could be efflorescence over time on the cement coating.

      I wonder if you've ever used this approach over Comfortboard, or always over foam?

      Thanks so much!

  13. 50_Pascals | | #13

    Robert, I love the use of the SBC for covering the foam. I need to do this to my foundation before winter. At the risk of naming names. Quickrete makes a SBC and they have "Concrete Acrylic Fortifier." Is this the type of acrylic additive you refer to? Or could you share a recipe? I've not liked the performance of other masonry coatings I've seen thus far.

    Have you an estimate of the permeability of these coatings? I am also looking to parge coat the existing rubble stone foundation. The old lime mortar is in good shape, but want to brighten up the old to match the new.


  14. Riversong | | #14

    Robert S,

    Yes, I use any of the acrylic fortifiers sold at masonry supply outlets. SBC should be quite vapor permeable, even with acrylic additive which is similar to latex paint.

    To waterproof it, and keep it vapor permeable, I use UGL Drylok latex masonry sealer which comes in concrete gray or white.

  15. Lisa the Engineer | | #15

    Question for Robert Riversong, please....

    Who makes and/or where could I find those fiberglas ties that you mention that will tie inner and outer wyeths.... and do I understand correctly that these are for use in a CIP (cast-in-place) foundation?

    Thanks much,

  16. Riversong | | #16


    The ties are manufactured by ThermMass as part of their proprietary foundation system for both cast in place and tilt-up foundations.

  17. Riversong | | #17

    Correction: that's ThermoMass.

  18. user-716970 | | #18

    I made an inquiry with a ThermoMass rep and she said they would be willing to sell the ties separately if one wanted to do the form work and insulation themselves and do their own concrete pour.

  19. Susan | | #19

    Where would a DIYer purchase the SBC in retail quantities? I'm in the south, and foundation insulation is not common. And the acrylic mortor modifier?

  20. GBA Editor
    Martin Holladay | | #20

    As Robert said, you should be able to order acrylic modifiers at a masonry supply outlet. Any good lumber yard should be able to order surface bonding cement for you, even if it's not in stock.

  21. Allen | | #21

    The acrylic mortar modifier - is that the same as bonding agent?

  22. Jeremy | | #22

    What about bending aluminum and using that to cover to grade? Perhaps in a color that matches the window cladding.

  23. GBA Editor
    Martin Holladay | | #23

    I've seen people do it. It dents easily, though.

  24. Riversong | | #24

    "The acrylic mortar modifier - is that the same as bonding agent?"

    No, bonding agent is coated onto old concrete before applying new to improve the poor bond of a "cold joint".

    Modifier, such as the common Silpro C-21, does that and much more:

    • Increased adhesive bond strength
    • Improved flexural and tensile strengths
    • Improved abrasion resistance
    • Reduced shrinkage
    • Improved crack and chip resistance
    • Improved water resistance
    • Can be totally submerged after curing
    • Resists staining and discoloration
    • Resists lifting, spalling or crumbling after repeated freeze-thaw cycles
    • Does not yellow or break down in sunlight
    • Does not require damp curing

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