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Insulation in vented crawl with fieldstone foundation

aherrick | Posted in Energy Efficiency and Durability on

Just another first time home owner/DIY’er looking for some guidance and/or confirmation to ensure my work doesn’t end up creating a different set of issues. I live on the border of climate zones 4A & 5A in western NJ.

The crawl space floor is dirt and there is no vapor barrier on the ground. However, it is surprisingly dry. I see no evidence of moisture or significant water intrusion outside of the drooping fiberglass batts that were installed 30 years ago. The floor joists above have no visible mold or rot. There is only one vent which is only closed in the winter.

There is a ton of information out there on crawl spaces, but most of it assumes the reader is dealing with a poured or concrete block foundation. With an irregular fieldstone foundation, encapsulation is possible (, but not without some pretty costly and invasive tasks that are best left to experienced contractors. I am hoping to take a different path to solve my problem which is the temps of the floor and space in the room above.

My plan instead is to follow the recommendations outlined in Figure 7 in the following article:

However, instead of fiberglass, I will be using Roxul/mineral wool batts (R23) with 2″ Dow Thermax polyiso sealed with tape & foam. This approach was also recommended by Martin Holladay in the following post: One slight difference is that the building science article recommends airspace between the subfloor and batt insulation instead of the batt insulation being pushed up against the subfloor. The joists are 1×12 so they will be only partially filled. Not something I am overly concerned about but it is an interesting difference.

What I am concerned about is trapping moisture between the polyiso and the floor above which is tile and plywood. It is my understanding that a tile floor can be semi-permeable or non-permeable depending on the underlayment between the tile and plywood. The floor was put in many years ago so I am really not sure what was used. If the underlayment is indeed non-permeable material, would this assembly cause mold & rot in the joists over time?

Any guidance would be greatly appreciated.

– Andy

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  1. aherrick | | #1

    One more question to add - If I move forward with the mineral wool & polyiso, is a vapor barrier on the ground still recommended or would that introduce another moisture trapping situation? Seems redundant as the polyiso would isolate the space above.

  2. GBA Editor
    Martin Holladay | | #2

    1. You definitely need polyethylene over the dirt. The fact that the soil feels dry tells you nothing -- after all, the soil has been evaporating continuously for decades.

    2. Ideally, you will consider turning your vented crawl space into a sealed crawl space. Installing closed-cell spray foam on your crawl space walls would provide many benefits. For more information, see this article: Building an Unvented Crawl Space.

    3. Joe Lstiburek likes tweaking authority, and he's proud of his system that leaves an air gap between the insulation and the subfloor. He's right that his approach works -- but only if the rim joist is impeccably air sealed. If there are any leaks at the rim joist, you've got cold outdoor air circulating between your insulation and your subfloor. That's no good. Since, in the real world, many rim joists leak, I favor filling the joist spaces entirely with insulation. For more information, see this article: How to Insulate a Cold Floor.

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