Foundation for a room addition
I’m working on a room addition in zone 6, 16′ x 16′ in size. I’d like to explain my current plan and seek
input from folks.
I think this would be called a slab on grade, despite the slab being elevated to match the level of
my current first floor. Right now the first floor is 3′ above soil grade via 2′ of concrete wall and
1′ of sill (4″) and floor joists (8″), but I plan on backfilling higher on the wall, still leaving 12″ between
soil and wood. My cement contractor said we could do a 4′ or 5′ frost wall, with an 8′ footing, and the use of rigid foam to make up for not going deeper with the wall. I would prefer not to dig deeper
than necessary. The site is pretty dry, the soil is compacted, and ledge is seldom far below ground.
Inside the frost wall will be filled (I think sand is typical), compacted, and brought to a level that
leaves room for 2″ of rigid foam and the 4″ slab (and 6 mil plastic below slab) to match first floor.
I have a few specific questions:
Is vertical foam placed outside the frost wall or inside? (I’ve seen diagrams both ways, but think
outside is more protective of the wall).
If placed outside, does the vertical foam start above the footing, along side it, or even under it
as I have seen mentioned. I plan on 2″ of foam.
How far below ground does horizontal foam need to be? I plan on 2″ extending outward about 2′,
more at the corners, like 40″.
What do I need to do against the house? (since the slab will rest against sheathing with rim joists
behind that. Is a water and ice type product good? or metal flashing, or PT plywood, etc?
Is the 6 mil plastic beneath the slab cut to fit between the frost wall, overlapped, or something
I’ve done a lot of reading on this but still end up with uncertainties that GBA is a great help with.
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