I’m currently designing my new home that I’ll be building here in Maine. It, along with the attached garage, will be on a slab with radiant heat in both. I plan to pour a 12″ x 12″ footing first, then a 6″ slab on top of that. I plan to vertically insulate the inside perimeter of the footing and horizontally insulate under the entire slab with 2″ rigid foam. I’d like to attach the same foam to the outside of the footing and slab, but I’ve never seen rigid foam hold up well above grade, even when its finished with a foundation coating. I expect to have about 8″ showing above grade, and about 10″ below grade. Is it really enough of a heat loss at the edge of a slab to require insulation, and if it is, is there a better way to protect it and make it look good? Thanks for any and all input- John
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I just use the coating made for it.
But you have more study to do first.
for one thing,
You need to keep the frost from getting under that shallow foundation. I don't know if I see a reason for the vertical on the INSIDE of the footing. You want to keep that underlying soild from freezing.
and
Are you sure the 12"x 12" footing is enough? Seems narrow to me for the soils we have here in Maine. It might be OK if part of a monolithic slab over plenty of compacted gravel but not so good as a separate footing.
here's a little bedtime reading for you - no, file's too big. I can zip it and mail you with iot if you send me your email addy through here. It's the Design Guide for FPSF - mention that in your email
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I’ve studied everything I have that I believe is reliable, but I am always eager to read/learn more. Any reading recommendations are gladly welcomed, thank you, I’ll send you that email.
All my design decisions so far have come from 2 FH articles: Energy Efficient From the Ground Up by Al Rossetto, FH #161 pg. 74 and A Slab on Grade Foundation for Cold Climates by George Nash, FH Apr/May 1990, and a Taunton book by Joseph Lstiburek, Builder’s Guide to Cold Climates.
You need to keep the frost from getting under that shallow foundation.
For that, I’m insulating horizontally out from the perimeter with 2” rigid foam as shallow frost protected footing, as shown in those three sources. The width would equal the depth of the frost, although I’m not sure exactly how deep frost can get here.
I don't know if I see a reason for the vertical on the INSIDE of the footing.
George Nash and Joe Lstiburek did this, so I was going to, but it does seem redundant if you have the shallow frost protected footing I mentioned.
Are you sure the 12"x 12" footing is enough? Seems narrow to me for the soils we have here in Maine. It might be OK if part of a monolithic slab over plenty of compacted gravel but not so good as a separate footing. No, I’m not sure. George Nash was the only one that specified size (his was 10” W x 23” D plus a 6” slab), and his was two piece, but was over lots of gravel like you said. What size would you do for a Cape with a full shed dormer on one side?
I just use the coating made for it.
What coating do you use, and do you find it holds up over time? I see lots of blue holes in the stuff after a few years, but I guess that’s repairable and preferable to no insulation there at all.
Thanks for taking the time to respond, I really appreciate all the great advice I get from all of you here at the Forum. Where are you in Maine? I’m in Belfast and building in Union.
John
I'm right out in the Bay on Islesboro. Buytmy stuff from Vikings and occasionally EBS or Rankins. Some kind of shirtail relatives in Union area. Well, actually, they've been here longer so I'm the one who is a shirtail inlaw, I guess.
I see your message in my box so I'll get to it in line. Can't send attachements thru the Prospero system here.
I usually use a 20" - 24" wide footing ten inches deep, but for monolithic slab over compacted gravel 18" deep, I have been OK with a 12" x 12" thickened edge with three #4 rebar in it. When part of a slab it handlels stresses differently than if a single grade beam type footing. The idea of a footing is to spread the weight of the building over a wide area of resistance. Our soils are full of clay here but up towards Union, you may find more firm feet to stand on without sinking in. Ask the locals what they do up there. While you're asking, check frost level. Probably four feet is fine.
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12x12 is jsut too small a footing - at a minimum has to be 24 deep with the monolithic pour for the floor -- you just cannot go cheap on the foundation
For the coating - forgot to mention - Have used several. Amsytone make sa good easy to use one. Comproco does OK too. Their's seems harder finish to me but that can be from curing conditions and admixture used too.
With any of them they have some fibre mixed in and you add either water or their admix and trowel it on.
I also prep the surface by cleaning if muddy. dragging an old wire brush across it, and rolling on a coat of bonding agent. If you see it flaking off, it is from one or both of two reasons. Either a poor bond from applying to a dirty or slick foam surfacce, and/or it was troweled on too thin. Could also be moisture trapped behind it that froze and drove the stuff off.
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