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Community and Q&A

How to detail underslab perlite insulation at doorways/thresholds?

Kelly Lerner | Posted in Energy Efficiency and Durability on

Climate Zone 5B/6B – Spokane, WA (east side of WA – cold winter – 6800 heating degree days, dry – about 16″ rain/year, water table is usually quite low and soils are well drained).

In an effort to get away from the global warming potential of and fire retardants in rigid insulation, I’m beginning to design with compacted perlite as under-slab insulation in strawbale houses.

I’ve found a relatively local source for perlite (Portland) and the costs for R-value are comparable to or less than rigid insulation. I’m installing perlite under the slab and wrapping it up the edge of the slab on the interior of the stem wall – under the 18″ wide strawbale wall, but I’m having trouble detailing “slab edge insulation” at the door ways. Any suggestions?

Here’s more information on perlite:

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  1. GBA Editor
    Martin Holladay | | #1

    If you are trying to avoid the use of foam, then the best product to insulate the edges of your slab is probably Roxul DrainBoard (mineral wool).

  2. Lucas Durand - 7A | | #2

    I've heard of perlite being used to insulate the combustion end of so-called "rocket mass heaters".
    How does the perlite stand up to compaction?
    Have you had any compacted samples tested to see if its "loose-fill" qualities persist?

    I'm having trouble detailing "slab edge insulation" at the door ways.

    How thick is the perlite at the slab edge?

  3. Kelly Lerner | | #3

    Actually, I'm considering using the perlite under a floating slab that is contained within a conventional stemwall (with footing below). I like the idea of using 2-3" of rigid mineral wool between the edge of the slab and the stemwall. I will detail the door to be inset (from the outside edge of the stemwall) so that the threshold can cover the rigid mineral wool. Hope I can get it in Spokane, WA ;). If not, I'll go with EPS.

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