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Insulation for a slab on grade?

bobhol | Posted in General Questions on

I am in eastern Ontario (Peterborough) and will be building a 2100 sq. ft. on a slab on grade… kinda….I am on bedrock and it is irregular with the low corner to high corner about 30 inches difference in level.My first thoughts were to build a stem wall of ICF and fill the inside with tamped gravel,.I would put down a vapor barrier and 2 inches of foamboard then pour the cement .I cannot insulate the ground around the perimeter of the stem wall …should I put another 2 inches of foamboard against the ICF to compensate…. and is 2 inches of foam enough under the slab?

I am hoping to heat primarily with 2 mini splits ,one on each floor regards,Bob

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  1. Expert Member
    Dana Dorsett | | #1

    In Peterborough you'll probably want 3" of 1.5 lb density (Type-II) EPS under the slab. (EPS is usually white the same stuff most ICFs are made of.) R12 EPS usually cheaper than 2" of the pink/ blue R10 XPS, and has both a stabler R over time and a 99% lower lifecycle greenhouse gas potential due to the differences in blowing agents. In 50 years R10 XPS drops to about R9, and over 100 years it'll be about R8.5, whereas EPS will perform pretty much the same as it's day-1 R over it's entire lifecycle.

    Even with a minimalist ~R16-R20 ICF you'll have sufficient R on the stemwall to meet your energy use goals, depending on what you do with the above grade walls. If stick built with exterior insulating sheathing, lining up the exterior of the wall-foam with that of the exterior EPS of the ICF does wonders for making a continuous thermal break. IIRC code-min in Ontario is R50 attic, w/ 2x6 with R20 rock wool insulation(?), which would have a whole-wall performance of about R14 with the thermal bridging of the framing factored in. Adding 2.5" of polyiso (to match the 2.5" of EPS on the ICF) adds another R14-R16 of average performance, cutting the heat loss through walls in half. Follow that up with R75 in the attic and better-than code windows, and you're there.

    There are other even higher-R options to consider, but a typical tight code-min house in Ontario will usually come in under 15 BTU/hr per square foot of conditioned space for a heat load at your ~-23C 99% outside design temp (see: ), which would mean a design load of ~32,000BTU/hr. If you cut the heat losses from walls, floor & ceiling by a third to a half beyond typical code min, and the window losses by maybe 1/4 you'll usually end up under 20,000BTU/hr @ -23C, which puts you well within the -25C output range of a couple of Mitsubishi -FE12NA or -FE18NA, or a pair of Fujitsu -12RLS2H or -15RLS2H.

    As a sanity check, Peterborough's climate is pretty compable to a US zone-6 climate. See table 2 p.10 of this document:

    Note, those R-values are "whole assembly", with all thermal bridging factored in.

  2. bobhol | | #2

    Thanks ,thats the info I was looking for ...I am going to use 2 by 6 framing with Roxul or blown in cellulose and 4 inches of foamboard on the exterior...regards,Bob

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