Insulation in vented crawl with fieldstone foundation
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 (https://buildingscience.com/documents/insights/bsi-041-rubble-foundations), 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: https://buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces.
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: https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/19583/keeping-crawlspace-vented-insulation-options. 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.
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