I am getting ready to have a contractor insulate my crawlspace. The exterior facing walls of my crawlspace are about 2" above grade, with the remaining 2 ft 6 in - 2 ft 10 in below grade.

One of the first things that we did before we moved in was to have some cracks in the crawlspace's foundation wall sealed, and a cement floor installed. We have lived in the home for about two years now and have been through some rough rains, so I am confident that we do not have any water issues in the crawlspace.

I would like to "go all out" and make sure that this space is as insulated as possible. However, I do not want to create an unfavorable moisture barrier in the process that could cause damage or health concerns.

Based on my research, I am seeing that polyisocyanurate (polyiso) foam insulation is the most efficient choice out there. I am thinking of having a contractor install 4" of polyiso on the foundation walls with at least 4" of closed cell spray foam insulation along the rim joist. In front of the polyiso, I am thinking of installing 2x6 framing with R-19 fiberglass batt insulation between each stud. On top of the studs will be drywall.

This is outlined by Building Science Corporation at the following page: http://www.buildingscience.com/documents/enclosures-that-work/high-r-value-wall-assemblies/high-r-foundation-15-pic-2x6-framing-fiberglass-batt?topic=doctypes/enclosures-that-work/etw-high-r-value-enclosure-assemblie

So here are my questions:

  1. BSC's research indicates that I should not have any condensation/moisture problems with this setup. Actually, they point out minor concern with above slab areas facing north... but luckily this portion of the house faces south. Anyway, I know that polyiso is not very vapor-permeable, so I wanted to double check what I am reading on BSC's site - should I be concerned about moisture? The contractor installing the insulation is a specialist in this area, so you can assume that the insulation will be installed properly with no air gaps, fully sealed, etc.

  2. Do I need to fire block (e.g., install 3/4" OSB) between the closed cell foam and the polyiso? I envision that the closed cell foam installation would seal up the area such that there should be no ability for air to travel behind the stud wall and up into the joist space. Normally I would err on the side of safety, but I do not want to create an unnecessary thermal break. I know I need to check with my municipality - but what do you guys think?

  3. Similarly, do I need to break up the polyiso foam every 10 feet to install fire blocking behind the walls (i.e., between the stud and the concrete wall)? See requirement three on this page to see what I mean: http://contractorkurt.com/2012/12/31/how-to-firestop-your-basement/ I would certainly prefer not to do this as it will create a thermal break and potentially introduce a moisture problem. Can I simply have the contractor use a fire retardant caulk every 10 feet to seal any airspace between the polyiso and the stud? Again, I know I need to check with my municipality - but what do you guys think?

Thanks in advance for the help!


  • 1
    I hope you noticed this from the material you provided: This is the highest R-value foundation system in this study and is likely not cost effective unless the rest of the house is super insulated and airtight. – mjohns Jul 2 '15 at 23:50
  • Yes, my intention is to remodel one room at a time and super-insulate whatever I can. I am starting with the crawlspace because it is the least insulated / most leaky and I want to begin using it for storage. – Frank Lesniak Jul 3 '15 at 14:30

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