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Is it a good idea to use foil-faced rock wool under the basement slab?

alumniu | Posted in Energy Efficiency and Durability on

I will be building a new house in climate zone 5. I’m doing my best to achieve the R10-20-40-60 insulation level goal. I’ve been able to find some discount rock wool board that I would like to use below the concrete slab in the basement. Specifically 1″ thick Thermafiber Firespan 90. This has a 8.0pcf density which would be good for sub slab, however it also has a foil faced vapor retarder. I would like to stack 2 layers of the product to achieve an R8.4 value (close enough to the R10 goal).

The basement slab construction is as follows (from top to bottom):
4″ concrete with wire mesh
6mil vapor barrier
2″ of Rock wool insulation board
Aggregate for capillary break

Is it a good idea to use foil faced rock wool board under the slab?

Is it a good idea to stack 2 layers of the product?

I would think the top layer of foil which is in direct contact with the 6mil vapor barrier would be ok, however, I could see the lower layer being a problem. Thoughts?


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  1. GBA Editor
    Martin Holladay | | #1

    The foil facing won't cause any problems in this application.

    You should be aware, however, that there are a few caveats to remember when using mineral wool under a slab: some local building inspectors may not approve the method, and you can't use mineral wool under footings. For more information, see Sub-Slab Mineral Wool.

  2. GBA Editor
    Martin Holladay | | #2

    I suppose that in rare conditions -- if there is rising groundwater and a simultaneous failure of your footing drains -- the intermediate foil might trap some liquid water.

    If this unlike scenario worries you, you can pierce the foil facing with a wrecking bar -- punching a few holes in each panel of insulation -- as you install it.

  3. alumniu | | #3

    Good suggestions Martin. Thank you once again for your prompt reply. I will plan on puncturing the lower layer of foil, and proceed as planned. Also Ill check with the county building inspector if they allow this product sub slab.

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