Frost-Protected Shallow Foundation for Addition
Hello everyone, I have a few questions/ideas to bounce of this great community here at GBA. I am adding on to a 30’x40′ building. This existing building has a typical spread footing below frost line with a stemwall/frost wall and a floating slab floor. The original building is a gable end roof with ridge running north to south, with the east and west walls (40′) being the load bearing walls. The addition will be a 20′ x 30′ gable end roof running east to west with the load bearing walls being the north and south walls (30′). The gable of the addition will hit just about the middle of the existing roof.
All of that was for visual. I would like to do a heated building fpsf for the addition’s foundation system. Does anyone see an issue with this? Does the slab need a thickened edge where it meets the existing building, as it will NOT be load bearing? Should I pin/dowel my rebar into existing foundation? Should I worry too much about the foundation seeing some 30 degree weather for a bit before I can insulate and heat it?
The building will be in East Central Missouri, with an AFI of 1000 or less and a 30″ frost line.
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I think that system should work well for you. I would pin the new slab to the existing foundation to prevent uneven settling, which requires at least a 5" slab with #4 rebar. Be sure to drain the slab perimeter well; they are at more risk of frost movement than traditional foundations. If you are insulating beyond code-minimum standards, either go farther below grade, add frost wings or both since you won't be getting the benefit of heat loss through the slab. 30° air for short periods won't cause frost heaving but it can be hard on the concrete itself if freshly poured.
I'm no expert but I can say I poured a very similar foundation this year. 10x12 heated PFSF addition. I just put door and windows in it yesterday. I made it with 3 thickened edges and no thickened edge on the house side. It is doweled to the ~ 5 ft deep house foundation every 24" with #4. It's a bit unusual as it's a 34" tall monothlithic foundation. 12" "footer" at the base tapering up at 60 degrees to 5" slab which is the finished floor. Did the forms, pour, finishing myself. It's on top of 4" of crushed stone. Center mound out of compacted 1/2" "plant mix" on top of 25 psi R15 XPS foam. All insulation values, other parameters per HUD FPSF design guidelines. I have been stressing about getting it heated before it becomes real winter here in Vermont, but it seem like I still have a few weeks until it's below freezing 24-7. I haven't been worrying about nightly frosts, and since it's backfilled now, I reckon I have a few days, at least, of sub freezing temperatures before I'd have to heat it.