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insulation for house on piers

Matt | Posted in Energy Efficiency and Durability on

Hi:  My question is about insulating a raised / exposed floor assembly for a cabin built on piers on bedrock in climate zone 7 (very cold).  The cabin was built a long time ago but is being renovated.  As such, we have to work with what is there. It is about 1200 sq. ft.  The main floor assembly is 3/4″ plywood subfloor over 2×6 joists, 16″oc with 2×6 ROCKWOOL batts.  Under this is 2×4 strapping 16″oc oriented 90 degrees to the 2x6s, filled with ROCKWOOL batts.  Under this is 1/4″ hardware mesh.  There is a section that is very difficult to access (~ 200 sq ft.) to add further exterior insulation because the cabin is built on bedrock which results in variable height of the assembly above the surface of the rock (the rock isn’t level – good for drainage, bad for access)!  I will likely be able to install a layer of exterior plywood as an air barrier under that section but it won’t be an enjoyable experience.  I am wondering about adding a 1.5″ layer of taped foam board insulation on the interior above the existing subfloor and then putting another layer of 3/4″ plywood over the foam.  I am thinking that this foam will act as an air barrier and reduce thermal bridging and also discourage condensation on the exterior surface of the subfloor which would otherwise be cooled from cool indoor air during the summer months.  I am thinking that the floor assembly can also dry both to the interior and the exterior if needed.  The only potential issue I see to drying to the interior is the type of flooring.  The wife wants luxury vinyl click together planks but this may not be compatible with the assembly.  Thanks for any advice you can offer.

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  1. Expert Member
    BILL WICHERS | | #1

    You should be OK if you stay within the compression limits of the rigid foam you select (which will have a PSI rating). The "new" 3/4" subfloor above the foam will help to distribute the load, and the "old" 3/4" subfloor under the foam will help distribute that load back to the floor joists so that that the rigid foam insulation will not see much in the way of point loads which is where compression (denting/squishing) can be a problem. I would probably use EPS here, in which case 1.5" EPS would get you about R6.3 of additional continuous insulation over your floor assembly, and would still be a little bit vapor open, which shouldn't really matter if the underside/exterior of the floor assembly is just wire mesh.

    Mineral wool isn't as big of an issue with a missing air barrier on one side compared to fiberglass, and walls are where this issue is the biggest problem anyway. If you can't get an air barrier under the floor in some areas, it's not a show stopper, although that plywood air barrier would be helpful.


  2. Matt | | #2

    Bill: Thanks for your thoughts on this. A couple of follow up questions: What if we went with 1.5" of Comfortboard 110 here in between the subfloor plywood layers and then put the click together vinyl plank flooring on top which I was told is essentially waterproof and therefore must be considered a vapour barrier?

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