Stem Wall Slab Foundations | The HOW and WHY

hey todd usher with addison homes and today we're talking about how we build a high performance stem wall slab foundation [Applause] [Music] [Applause] so i'm out here on one of our job sites where we're going to install a stem wall slab foundation and you'll notice behind me a couple of key ingredients for a high performance stem wall slab foundation foam board is what we use to insulate the perimeter of our stem wall slab and then we've got all of our masonry products behind me and our masons are just getting started today we've poured the footings and now we're laying the block stem wall for that slab i want to show you a little detail on exactly how we make up that stem wall construction let's go over and take a look i've got some block laid out to show you exactly how this is going to work so here i'm on top of our footing that we poured about a week ago concrete is now set and solid and we're starting to lay our stem wall foundation for our slab and two components we're using one is an eight inch concrete block otherwise known as a cmu which stands for concrete masonry unit and a four inch concrete block or cmu and what we're going to do here on this particular stem wall slab is we're gonna first lay the eight inch concrete block and then our masons are gonna lay the four inch block on top of that and this is a particular construction that we use to create a high performance stemwall slab this connection creates a ledge to support our concrete slab around the perimeter of the foundation and also this ledge creates an opportunity for us to install insulation around the perimeter now we're only in climate zone three so we don't need to insulate underneath the entire slab in fact we get a lot of benefit from that stable ground temperature at about 60 degrees year round and it helps us offset our cooling load in the house but the perimeter is a short circuit if you will it's where we can lose heat out of the edge of the slab in the winter months by doing a stem wall slab it allows us to put perimeter insulation around that we're going to backfill of four inches of stone on this but before we put that stone in we're going to lay an 18 inch piece of r5 foam board on top of this ledge and out under the slab then we're going to put another 1 inch piece on the perimeter up to the top and that's going to give us our perimeter insulation around the edge of the slab that's going to prevent extreme heat loss out of the edges of the slab so you might ask why why are we doing a stem wall slab foundation on this particular site and the answer is very simple the site is pretty much flat except for the lower end of the foundation and so when we're that flat it really makes sense to do a stem wall slab if we were going to try to make this a crawl space we'd have to excavate all that dirt out and then we'd have the expense of building the foundation wall up high and then encapsulating the crawl space you've seen our videos on crawl space encapsulation fantastic foundation but it's pretty expensive on our stem wall slabs when we have some grade change we actually might have to lay a block wall up and in this case we're going to lay a block wall up about eight courses of block which is a little over five feet on the lower corner of this foundation we're gonna have to backfill this area so when we get much over four courses of block we do a reinforcing strategy as you can see here the masons have dowelled steel rebar into the concrete footing every four feet and then what they're doing is they're filling each cell that has that steel rebar coming up through it solid with mortar and that's just going to provide some extra support extra strength in that wall to prevent it from pushing over when we backfill the lower side of the slab now we're not back filling this with dirt we're going to backfill this with stone which also reduces the amount of hydrostatic pressure or force on that wall so even if we didn't put this reinforcement in we really wouldn't have a problem but its best practices again you hear me say belt and suspenders approach for how we construct our foundations and how we build our houses as a whole now the other thing you'll see here is the ladder wire this ladder wire goes every other course of block as we build up multiple courses and the ladder wired is again an extra layer of reinforcement and this ties this taller part of our stemwall slab foundation in to be a robust strong wall that we can backfill against with no issues whatsoever now you can kind of follow me down on this footing and you can see this section where we've reinforced the wall and now you can see this foundation come into shape as i mentioned earlier we're we lay this first eight inch block and then a four inch on the outside of it to create this ledge for our concrete this is what we call a stem wall slab foundation we feel this is the easiest slab to insulate to create a high performance foundation from an energy and comfort standpoint the other type of slab that's very commonly used in residential construction is what's called a monolithic slab i just want to explain the differences between the two this stem wall slab gets its name because we're creating we're laying a masonry stem wall that sits on a concrete footing and this stem wall will actually support our slab with this ledge this ledge also gives us a great perimeter to insulate very easily a monolithic slab is done when the concrete is poured and then down into a dug footing all in one monolithic concrete pour so the footing and the concrete slab is poured together it doesn't get a stem wall and it's kind of all in one creates some challenges for insulating because really the only option we have for insulating easily in this climate zone is on the exterior of the slab and there's some challenges with putting exterior insulation on our foundation from a damage and and finish standpoint now we're going to take a look at a couple of other things on this foundation because if you remember the foundation grade here isn't dead flat so we have the luxury of just two courses here on the flatter area but where the grade slopes off behind me we actually had to lay more courses up to adjust for grade and we're going to backfill that all with stone so that we don't have to worry about compaction that stone is self-compacting and then we'll put the foam board in over the stone put an extra four inches of stone on top of the foam and then pour our concrete after we lay the vapor barrier down the first thing we do when we insulate our stemwall slab is we've got our one inch r5 xps foam board and fortunately this foam board comes pre-scored so we've got some pre-scored lines on here that actually work perfectly for the dimensions that we're going to use to insulate our foundation step one is to break this foam board apart and i usually do that with a swift kick of the knee and that breaks it in half and then we're going to take and we've got another scored line that is our perfect 8 inch riser piece and our perfect 16 inch under slab edge piece we're actually going to put this piece on first and that's going to insulate the perimeter of our slab then we're going to put this second piece on and we've got slab edge protection or insulation and then under slab perimeter insulation we're actually going to use an adhesive spray instead of trying to anchor these in with concrete anchors we're going to use a new adhesive spray that's going to stick this foam board directly to the wall [Music] all right so this is our spray glue spray adhesive and we're putting it on the wall as a new simpler way to actually attach our foam board to the wall before we shoot our gravel on top all we do is we stick the board in and get it up against the wall and then just press it on and this stuff grabs like crazy so this is this is a great improvement to this process because before we would use what are called ram set anchors powder actuated fasteners to put this on and the challenge is when the stone thrower which is a truck that actually shoots stone with a conveyor belt comes out it's tough on this foam board so if the foam board's not locked in really well the stone will just knock it off all right so as you can see behind me we have finished insulating the perimeter of this slab so we are ready to go with the next step which is going to happen tomorrow morning where we're going to have our stone thrower truck come out and shoot four inches of stone on top of the entire foundation that we've backfilled here including the perimeter insulation and then we're going to be ready to put the polyethylene film down which is our vapor barrier that goes under the concrete and then we're good to pour our concrete slab foundation so we'll be ready to frame within the week hopefully now you get a better picture of what a insulated stem wall slab foundation in climate zone 3 in the south looks like we've got our perimeter insulated to stop that heat loss in the winter from the edges we're not insulating under the main portion of the slab because we want that cool temperature from the earth coming up to help us offset our cooling loads in the summer if you like what you've seen please subscribe to our youtube channel and follow us for more educational videos still to come thanks for joining us we'll see on the next one soon [Music] you

Channel: Addison Homes

Views: 77,787

Keywords: addison homes, high performance, highperformance, building, construction, addison, homebuilding, homebuilders, healthy, efficient, durable, local, dwell, well, green, home, homes, greenville

Id: -lE3bV-gDRo

Length: 11min 6sec (666 seconds)

Published: Mon May 30 2022