Under-slab foam: How thick? Spreadsheet for review
I just built a spreadsheet to look at the savings and cost of various thicknesses of foam under a slab. In another thread, I had questioned the PassivHaus use of 14” as being excessive, so I thought I’d take a simplified look for myself, ignoring inflation, etc. This is for zone 8, 14,000 HDD, and $60 for a 4’x8’x4” sheet of 15 psi EPS.
Column B has calcs for my house. Column C is how much money I’d save using each thickness instead of 2”. Column D is the cost of the extra foam (beyond 2”). Column E shows what the money for the extra foam would generate if it were instead invested at 6% APR. This indicates that somewhere around 12” I’d be just as well off to invest the money as put it into foam.
Column F is the savings generated by adding foam in increments of 2”. This is the “diminishing returns” column. Column G shows the cost of each additional 2”(always $3,323) divided by the annual savings for that 2”. If I am thinking right, this indicates that for 4” of foam, I will invest and additional $5 to save $1 annually, and for 14” I’d invest an additional $107 to save $1 annually.
In Row 4 I copied the calculation entry from the last entry in each column, just to indicate whence cometh the entries in that column.This spreadsheet is rough, approximate, and based on certain assumptions, but what I garner from it is “stick w/ about 8” under the slab”.
Let me know what you see, or what/ if I screwed up. If you see a different angle from which to look, let us know.
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