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Zone 6 foundation insulation

Informed_for_Trouble | Posted in Energy Efficiency and Durability on

Zone 6 Foundation Insulation

Existing Conditions:

1 75 foot long ranch log home built in 1984 in southern Vermont (zone 6).

2 1/3 of foundation is a basement garage.

3 2/3 is unvented crawl space of 3-7 feet high with a “Clean Space” basement system with drains and sump pump.

4 Historical volumes of mountain water runoff have been mitigated with many hand dug drains 10’-20’ away from house.  After one year through snowmelt, no water detected in sump pump basin leading me to believe that the surrounding soil is quite dry.

5 Now I have to insulate walls and am worried about moisture drive into sill plate (no capillary break).

Proposed Insulation Assembly:

1 3” XPS seems to be too much vapor resistance and would cause a drive upwards into sill plate. One possible saving grace might be the ability for the exterior of the concrete (above grade) 1-5’ around the perimeter to release moisture.

2 Would it be smart instead to limit the XPS to 1 1/2” to control the dew point and cover with Rigid Rockwool for the rest of the assembly allowing a higher perm rating?

3 This assembly would be duplicated in the garage with the addition of furring strips to support sheetrock.

I have read every post you have had regarding this subject and understand the stated assembly of #1 matches your recommendation.  Am I overthinking vapor permeability and sill plate problems?

In one article Martin mentioned the possibility of a dimple mat behind the insulation. Could you expand on that assembly because I cannot find any additional information?

Thank you for all the information. Quite helpful and fascinating.


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  1. Expert Member
    Dana Dorsett | | #1

    Limit the XPS to 0". It's very UN-green (HFC blowing agents), and too vapor tight.

    Insulate the exterior with 1.5-2" of EPS (pentane blowing agent, very low impact) from the top of the foundation down to a couple feet below grade, and use 3" rock wool sound abatement batts (or R15 studwall batts, if cheaper) on the interior. That way the assembly dries primarily toward the interior, and meets the IRC's prescriptive R15 continuous insulation minimum.

    Figure out how to install flashing to direct water coming down the wall to the outside of the foam, away from the foundation, and use a cemeticious protective coating on the exterior foam- some people just use stucco scratch coat mix applied directly onto roughened EPS, but there are application specific materials as well (eg Quikrete Foam Coating).

    You could just do the R15 batts (no studs) on the interior strapped to the wall with chicken wire, and no EPS but without the exterior EPS there would be more ground moisture making it into the foundation wall and a slower drying rate.

    With any approach a ground vapor barrier is necessary in the crawl space, sealed to the foundation wall.

    1. Informed_for_Trouble | | #3

      XPS is reclaimed can’t do anything about the purchase order 20yrs ago but I’d rather use it now then see it in a landfill. If I’m wrong about that please educate me.

      Thank you for the suggestion unfortunately due to large ledge rock exterior would be problematic, not impossible but I’ve done enough feather and wedge through boulders for a lifetime.
      I’m more concerned about losing the drying potential of the exterior concrete considering it’s predominantly in the sun and most of the water is during the warm mud season.
      I would just jack up the house and slip in a cap break but the logs make the weight and access difficult.
      There probably isn’t an answer but thanks anyway

      1. exeric | | #4

        RK, XPS use is absolutely proper in that circumstance as it isn't new and can be considered reclaimed. We were all a lot more ignorant 20 years ago. My lashing came from my seeing so many people "today" who don't have any excuse for their ignorance. I apologize for chastising you for something you didn't deserve.

  2. exeric | | #2

    I'm very grateful to you, as I'm sure many others are, that you continue to let people know about the damaging effects to the planet of the manufacture and use of XPS insulation. But I have to say that I can only marvel at your patience with people who come here for advice. I sincerely wish I had it too. How many times does it need to be said before people GET IT about XPS. You have the patience of Job and are doing God's ( or whatever your belief system supports) work.

    Perhaps some administrative tool can be built into the Q&A structure such that individuals will be encouraged not to use XPS even before they submit their question? For me, and I can't speak for others, it is a constant source of irritation that such remedial understanding has to be added on to answers to more substantive questions.

    1. charlie_sullivan | | #5

      In two more years, regulations pending in California, NY, Connecticut and Maryland will ban HFCs in XPS, which will force US manufacturers to at least offer greener XPS, if not switch over entirely. It's too bad that consumer pressure isn't sufficient to switch before then, but at least there's light at the end of the tunnel.

  3. Expert Member
    Peter Yost | | #6

    Hi R K -

    IF you have managed surface and ground water around your foundation such that your CURRENT moisture content of concrete and sill plates are not elevated, then my approach would be: proceed with interior or exterior insulation plan and simply MONITOR moisture content of sill plates over time; that is what I have been doing in my own home for almost 20 years: And:

    Since my footing does not see the ground water table and I have managed surface water around my 100-year-old plus foundation, my framing in contact with foundation wall has never been elevated (I check moisture content of first floor framing--which is let into the foundation--each spring and always fine.

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