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How much of a 2 x 6 plate has to be on the stem wall?

Mychief | Posted in GBA Pro Help on

The real question is whether to use 6 in or 8 in blocks for my stem wall. Stem wall is 24 in high, this is slab construction.

The wall is metal, membrane, 1.5 in ISO, 5/8 OSB, 6 in plate, 5/8 drywall, trim.

If I use a 6 in block and line the outside of the ISO with the outside of the block, that puts only about 3.5 in of the plate on the block. Will this work? The good part of this system is that the 3 in of styrene on the inside of the stem wall can be let untouched and thereby give me a good thermal break between the floor and the block.

If I use an 8 inch block and line the same, it puts all of the plate on the block, but I have to cut the styrene back so it is not showing in the finished room. Obviously, the puts the cold block almost in the house.

I hesitate to put an inch of styrene on the outside of the stem wall because the critters dig in and destroy it. But, if that is the best, then tell me what to do. Thanks ahead of time. It has been a trip the past few months working on the house.


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  1. GBA Editor
    Martin Holladay | | #1

    I'm not an engineer, but here in Vermont, builders used to install 2x6 plates with a 2 inch exterior overhang all the time back in the 1970s and 1980s, so that the plate aligned with the 2-inch-thick rigid foam installed on the exterior of the concrete foundation. The logic was, "We used to frame with 2x4s, and they only had 3.5 inches on the foundation."

  2. Expert Member
    Dana Dorsett | | #2

    If the 2x6 studs are 16" o.c. cantilevering the framing off the stem wall by 2" isn't very different from a 2x4 wall aligned with the edge of the stem wall. But if the studs are 24" o.c. it's a subtly different dynamic- not sure if that would fly. In seismic zones there may be special restrictions on how this is done too.

    That said, it's pretty common to put polystyrene foam on both sides of a stem wall, applying a critter & UV resistant protective finish such as Quikrete Foam Coating, which is a hard cementicious material. Aligning the exterior stem-wall foam with the exterior studwall foam gives it a very good thermal break from the exterior, since there is no path from the framing through stemwall to the exterior that doesn't pass through foam. Without the exterior foam the stud plate only has CMU block between it and the great outdoors.

    If you're not insulating under the slab, you're better off putting ALL the stemwall foam on the exterior side, from a thermal performance point of view. But if you're putting an inch or two of foam under the slab it's worth insulating the slab edge down to the footing, even if you have foam on the exterior side of the stemwall.

  3. Mychief | | #3

    Thanks. We are insulating the slab also. The studs are on 16 in oc. So either would work, but the best would be to insulate both sides of the stem wall. Would metal valley flashing work to protect the outside insulation?

    I originally thought this was a "snobby" yankee site with out a touch of reality..... but I have changed my mind.... My wife is a yankee also, but of the good variety. (She cooks good Cajun food now, tho she still refuses to eat crawfish. Again, thanks.

  4. GBA Editor
    Martin Holladay | | #4

    Not all Yankees are snobs. Some of us are regular rural Yankees who make do with this and that, whatever is on hand, and don't trust city folk.

  5. Expert Member
    Dana Dorsett | | #5

    I thought the cagey-owley mistrustful rural folk in the backwoods of VT were called "woodchucks" by those snotty up-town folks in Burlington (?). :-) VT is still half Cajun anyway- the don't call it "ver mont" for nothing. Louisiana Cajuns were originally refugees/emigrants out of the Canadian maritimes, non?

    I've heard Maine (and it's economy) described as " a bit like Alabama, only a heluva lot colder". Not sure that the analogy works on all aspects, but some.

    With any stemwall foam (on either side) there is a potential issue with ants & termites tunneling through it. Backfilling with compacted sand sometimes works against termites, but encapsulating the foam with a hard cement material would probably be more permanent.

    Metal flashing extending from the interior side foam under the studwall plate and out over the exterior side foam makes an adequate termite block, albeit at the cost of some thermal bridging, since the metal is very thermally conductive compared to wood or concrete block. There are copper-clad plastic sill gaskets for stopping ants & termites, but I don't know how well they work.

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