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Community and Q&A

Encapsulating a Crawlspace Using Poly and Rigid Foam

4khz | Posted in General Questions on

Hello GBA community. I am a homeowner acting as a GC.  I am ready to encapsulate my crawl space and make it a conditioned space in Zone 5A.  I’ve researched here and several other places on how to insulate the interior (CMU) crawl space walls. Some call for Poly sheeting on the wall and then polyiso rigid foam board. Others called for just rigid foam board attached directly to the wall.

What might be the reasoning to add both ? Advantages and disadvantages ?

It seems to me that the easiest and cost effective way to go is to directly attach the rigid foam (2″ Atlas energy shield) to the wall using an adhesive and foregoing the poly sheeting between the rigid foam and wall.

Also – I plan to add 2″ of  rockwool comfortboard to the exterior down to the footing. Will these two types of insulation on either side be compatible for the crawl space wall / rim joist?

Michael R. Smith

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  1. Andrew_C | | #1

    I think the standard approach is to run the vapor barrier that you have on the floor up the wall a bit (say, 6 to 12 inches), then put the rigid foam board on the wall and run it down to the ground over the vapor barrier.

    1. 4khz | | #3

      Andrew- thanks for pointing to the standard approach... It wasn't so apparent after reading thirty + articles specifically about crawl space encapsulation.

  2. Expert Member


    Your plan will work fine. Lap the poly as Andrew suggested, and make sure to tape and seal the foam to prevent moist air from making its way to the concrete.

    installing both exterior and interior insulation is a lot of work. Have you th0ught of using more in one location and skipping the other?

    1. 4khz | | #4

      Malcolm - First off, this community / website has been incredibly valuable. I've learned so much in the last year or so. GBA is where I go first.

      I'm hoping that my response below isn't too incoherent. This is just how it came out :)

      On my existing (newly purchased) raised ranch with 2x4 walls (and CMU knee wall), my plan is to add 2" of rockwool comfortboard on the exterior all the way down to the footing. The non vented & non insulated (except for floor joists) crawl space, at the time, seemed fine .. pretty dry.

      Then I started renovation and now the crawl space is/was wet. Here in the Hudson Valley, NY, we got prolonged humid weather, a lot of rain, and and the gut renovation at the same time. I suspect that the removal of the batt insulation in the floor joists along with the crawl space door being left open for a few weeks is the culprit. But not totally sure. I installed a dehumidifier a few days ago ( and made the directive to keep the door/hatch closed) and the condition has greatly improved.

      My strategy to isolating the problem (without - for now - adding a drain mat /vapor barrier on top of the rat slab) is to insulate the interior walls and keep the dehumidifier going. I can't really go further out (aesthetically speaking imo) with the exterior insulation on the lower wall as it would then protrude further out then the upper wall.

      I will excavate down for waterproofing, insulation, perimeter drains, etc) once I begin the addition (which i put off until lumber prices come down somewhat).

      Oh, and I have a ducted ASHP mini-split going down there.

      Thank you and any input would be greatly appreciated


      1. Expert Member
        MALCOLM TAYLOR | | #8


        You should end up with a much better performing house once all that is done.

        If you are excavating down to the footings to deal with water issues, adding the exterior insulation makes sense. If the only reason you were excavating was to add it, I'd thicken the interior crawlspace insulation instead, and only add the rock-wool to the framed walls.

        1. 4khz | | #10


          Thanks and that's my goal - to have a good performing house. The first floor is partially below grade on three sides and has three bedrooms. I figured that with everything I'm doing (gut reno), that this should be done to ensure a more comfortable and secure environment.


  3. walta100 | | #5

    “Encapsulating a Crawlspace” I have a problem with the words people read the word encapsulate and think the crawlspace is somehow isolated from their home and they don’t need to spend money condition the space and when they don’t it turns into a moldy mess and they are somehow shocked.

    Your plan on how to connect your crawlspace to the house is just as important as your plan to isolate the crawlspace from the exterior.


    1. 4khz | | #6

      Walter - Can you elaborate on that ? As far as I see it, to "encapsulate" a crawl space is the majority of the cost to bring it to a "conditioned" space. And isn't sealing/ insulating the floors / walls of a crawl space one of the primary strategies to properly connect the crawl space to the house ?

  4. Expert Member
    NICK KEENAN | | #7

    Two reasons for the poly:

    First is if you need the air barrier to conform to an irregular surface, you can cover it with poly and tape it to the foam.

    Second is polyiso foam shouldn't be exposed to liquid water. If there is a danger of liquid water entering the poly sheeting protects it.

    1. Expert Member
      BILL WICHERS | | #9

      If you're using foil-faced polyiso, it's only the edge of the sheet that needs protection from liquid water -- the facer provides protection for the face of the sheet. This means you don't really need to run poly up the foundation walls, since the foil faced polyiso can already provide a vapor barrier. You just have to be sure to tape everything properly.

      Personally, I like to run the poly all the way up the wall and then insulate over it. I think it makes the sealing step a little simpler, since the poly is the only barrier everywhere, and is the only thing needing to be detailed as such. Once the poly is up, you can insulate over it without worrying about an air seal since that step was already done. It's a tradeoff of a bit more materials (more poly), for a simpler construction operation (I find it easier to detail the poly than the polyiso).


      1. 4khz | | #11


        Thanks for the clarification and recommendation. The material and labor costs wouldn't be much more, so I may just go this route.


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