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Crawl space encapsulation

Chris_King | Posted in Green Building Techniques on

Hello everyone,
First I want to thank everyone on this site for their wealth of knowledge and willingness to help out. I recently purchased a ~2,450 sq. ft. ranch in ocean county NJ about 15 miles from the ocean so while this puts me in zone 4A I am attempting to retrofit my house for a minimum of Zone 5 location. That being said I plan this spring and summer on turning my currently vented crawlspace into an unvented encapsulated crawlspace and have a few questions. I apologize in advance for the possibility of dumb questions as it’s a lot of information on this site and I may have missed it or just not retained the info as I have been reading.

1. I want to add 1 inch of Roxul Drainboard to the exterior of the foundation from the bottom of the siding to wherever the board stops below ground, which should be about 3 feet below grade for a 4×8 board. Is 1 inch enough on the exterior as I am having a hard time finding a supplier who stocks it in thicker pieces and can’t seem to find a concrete answer on if its ok to double up to 1 inch boards together due to the drainage plains on them and being unable to get a tight fit.
a. Where I live, we have a very high clay content in the soil which retains water while I do currently get water in the crawlspace it only comes through a vent that had a downspout right next to it. Temporarily I extended the downspout and added rocks around the vent and have had no issues since so while I don’t think I will have any water issues once the vent is closed I still would like the drainboard on the exterior just as an insurance measure to help push any stray water away to the drainage plane.

2. My father in law who is a mason/carpenter says I should add a vapor barrier such as tar or plastic behind the drainboard that is attached to the exterior block foundation. What are your thoughts on this as I am leery about some of his advice when it comes to energy retrofits as we still have weekly discussions about how houses don’t need to “breath” and how building science has changed since he first started in the business.

3. Currently I have a 2 inch rat slab on the floor of the crawlspace, should I still add a vapor barrier on top of that and if so how far up the block wall should I go with the barrier?

4. For the interior side of the block wall, I was hoping to do 3 inches of Dow Thermax rigid foam but it is only made into 2 inches thick so I have to search for a supplier who can get me that. Would 2 inches be sufficient or should I do a 2 inch board and then another layer of 1 inch on top of that and if so do you feel that extra inch is worthwhile as it relates to the cost and labor involved for my climate zone to add the extra inch.

5. For the sill plate on top of the block I plan on adding cut rigid foam thermax and securing it with spray foam around the edges.
a. I have seen mentioned in various articles about leaving a 3 inch gap for termite inspection, should the 3 inch gap be on the sill plate or the top block of the foundation wall as I have seen arrows pointing to both in articles.

6. Does the vapor barrier that goes up on the wall go directly to the block wall or does the foam go on the block and then attach the vapor barrier to the foam.

7. How would you suggest fastening the foam insulation to the block? I would like to avoid screws at all costs as I hate putting holes in things I don’t have to, can anyone recommend a good adhesive they have had success with?

8. Anything other information that you think I am missing?

I have used the below articles during my research and included the link to the Dow Insulation I am looking at and just want to make sure I am not doing something wrong before I start. Again I thank everyone on this site for their knowledge and contributions.

Rim Joist Insulating

Crawl Space Encapsulation


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  1. GBA Editor
    Martin Holladay | | #1

    Lots of questions. It's usually better to ask one or two questions at a time, and then to come back later with a few more questions.

    Q. "Is 1 inch [of Roxul Drainboard] enough on the exterior?"

    A. If the purpose of your Roxul Drainboard is to provide drainage (rather than to act as insulation), then 1 inch is fine. This approach works best if you can install a perforated drain pipe at the bottom of the Drainboard, with the drain pipe leading to daylight.

  2. GBA Editor
    Martin Holladay | | #2

    Q. "My father-in-law who is a mason/carpenter says I should add a vapor barrier such as tar or plastic behind the drainboard that is attached to the exterior block foundation. What are your thoughts on this?"

    A. This layer is usually called dampproofing. I suggest that you use an asphalt compound for this layer. You can buy asphalt-based dampproofing for foundations at any building supply outlet.

  3. Chris_King | | #3

    Thanks Martin, sorry about that I will keep them short in the future. Could you just provide your opinion on whether the concrete floor of the crawl space is an effective vapor barrier or whether I should add a liner on top of it? That and the exterior Roxul should keep me occupied for two months.

  4. GBA Editor
    Martin Holladay | | #4

    A layer of concrete is not an effective vapor barrier. A layer of polyethylene is an effective vapor barrier, however. Your concrete slab may or may not have a layer of polyethylene under it. You can check whether or not your slab has polyethylene under it by drilling a 1-inch hole in the concrete to see what's under there.

    If the builder who poured the concrete forgot to install a layer of poly, you will have to install the poly on top of the slab. If you need access to the crawl space, you can throw some OBS sheets on top of the poly to protect the poly.

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