Concrete-Free Slab On Grade Foundations with Josh Salinger

Josh Salinger, founder and Principal of Birdsmouth Design-Build, explains how to design and construct a high-performance floating plywood slab on grade assembly successfully. His presentation covers the science behind why this works, how it meets code, and practical methods of incorporating this technique into your projects. Read the transcript below.

Josh Salinger:
I appreciate the opportunity to share this content and be a part of the Accelerator. And I really appreciate the work that everyone at the Accelerator is doing. So thanks for that. With that, I'll share my screen and get this presentation underway. And I'm guessing everyone can see this. Thumbs up. Great. I've just got introduced, so I don't think I have to rehash things too much here, other than to say that our company here is in Portland, Oregon. We're focusing on zero-energy and Passive House buildings and recently have started really paying attention not just to the embodied carbon in the buildings, which we're going to touch on with the concrete-free slab, but also just retrofits and trying to decarbonize the built environment. So those are our main focuses here. I'll do my best for those Canadians in the crowd... If it comes up to speak, and let you know if I'm speaking Fahrenheit or Celsius, those kinds of things. Please ask questions. I'd be happy to answer them at the end of the presentation.

Concrete has huge environmental impacts. Itself, that one material, contributes 8% of the total greenhouse gas emissions on the planet. Just this one material. And also cement, if it was a country, it would be the third-largest emitter in the world, just this one material. It's also the most used material on Earth, behind water. As you can see during the manufacturing process here... This is from McKinsey and Company. You have to go all the way back to the quarry where you're grabbing these raw materials, transporting them to the crusher, to the raw mill. You can see where it's highlighted here in the middle, this is where most of the greenhouse gas emissions are coming from. We're using fossil fuels to create really high temperatures to create this clinker to create the cement that's being used. So this one material really has outsized embodied carbon. And so, obviously, we're trying to use less of this in our buildings.

Another thing that people don't tend to think about... You hear often about these stats about "it's 8%," or "it's got all this high embodied carbon," but the sand that's used in concrete has also got huge impacts. This was a documentary, I believe was in National Geographic, about these sand mafias. It gets into these illegal networks that have resulted in violent conflicts. These people will threaten whistleblowers, they'll bribe local politicians and law enforcement officers. There's been hundreds of murders in the past few years, all around extracting sand. And that's before we even talk about the environmental impacts of extracting the sand too.

The largest extractive industry in the world is sand. This has huge impacts on river deltas, lakes and beaches, and all the destruction that comes along with that. Desert sand doesn't work for concrete. It's too round. We need to have more cubic sand. And this is where you tend to find it, in these river deltas. So embodied carbon, social, all these impacts.

This chart in the middle is one that I always start my presentations off with. Buildings account for about 39% of all the greenhouse gas emissions on the planet. The materials and construction account for 11% of that. So, concrete is also... Besides this impact that it has on the operational embodied carbon, it's a good conductor of energy.

When we're looking at slabs, which is the focus of this talk, most of the losses that are happening from inside that building are happening through that slab edge, because it's just a good conductor. We're losing our energy from that slab, but we're also getting comfort impacts too. If it's cold or hot outside, we're going to reflect that into the building and affect our occupants too. Typically, the concrete in a home's foundation has more embodied carbon than the rest of the house and all the items that you put inside of that house. So we really want to pay attention to reducing the amount of the stuff that we can. It's obviously a very useful product.

One has to ask themselves, "What is this concrete slab doing in our building?" This is kind of a joke here where there's the two types of concrete. There's concrete that's cracked, and then there's the concrete that's not cracked yet. Beyond that, as far as we can tell, the main point of the concrete in slab is for us to stand on and for us to put things on. Typically, for most residential projects, it's non-structural. Certainly, when you get into the larger multifamily and commercial buildings and you have post-tensioned slabs and things like that, it does become a structural element. But in many buildings, it's simply just something to put something on. Put your stuff on or to stand on.

Another thing about it too is that it's difficult to install flooring over concrete. As it's curing, it releases a lot of moisture. If you're going to do it successfully, you want to make sure you have a good vapor barrier below it. And then you want to make sure that you're ideally coating it with some sort of liquid applied like epoxy or something, a vapor barrier on top of it before you put on your battens and your finished flooring. So it's a complicated thing to put flooring on top of.

Slabs are rarely level or plane, if you really get down to it. Concrete has specialized equipment and subs. It's fairly expensive, especially in today's market. And if you need repairs, it's really difficult to cut concrete out and remove it and then replace it. So we started asking ourselves, "Well, what's the point of all this?"

What do all successful slab on grades have? They've got five basic layers. First of all, there's the native, undisturbed soils. We want to put our houses on good stable soils. Two is the gravel that gets laid down and this is a capillary break to stop that bulk water from being wicked up into our buildings. It's also a soil gas depressurization field. This is what would collect all the radon gases and stuff like that, so they can be safely exited from the building.

On top of that, typically you've got an insulation layer. You want to again, make it comfortable. Then you have your vapor barrier. We want to be able to stop that vapor drive from the ground, which is typically at 100% relative humidity no matter where you are on the planet, from our buildings, which is typically less than 100%, or hopefully less than 100% relative humidity inside your building. And then on top of that, you've got your slab. Your standing surface. This is the part that has that potential for some impact, is to replace that concrete.

Introducing the concrete-free slab on grade. This is just a cross-section of a project that we've done a couple of now where we've used ICFs as our stem wall. I'm happy to share these slides too. I can put a link into the accelerator to get these out. Happy to open source these things if people are interested. But as you can see, all five layers are present. There's really not much difference in the first four layers. And then above that, for the slab itself, we've simply got two cross-laminated pieces of three-quarter-inch plywood. And then on top of that her finished flooring. So that's the main difference here.

This is a great illustration that was done in Fine Homebuilding Magazine about this particular ICF concrete-free slab on grade that we did. I did give this link to Shannon and she could put that up on the chat to check out this article in Fine Homebuilding Magazine. But again, here you can see where we've got the native soils, our gravel, our insulation layers, our vapor barrier, and then the plywood, and then the finished flooring over the top of that. So, it's a fairly simple thing.

I thought what I do is take you through step by step with some photos of an actual project that we completed. We've done I think three, and we're currently working on our fourth concrete-free slab on grade project.

The first one's pretty straightforward here. There's no real difference here. You excavate down, throw down your gravel, compact it and get it ready for your footings and your stem walls.

The next step here is you're going to set your forms, pour your foundation walls, and then backfill. Again, fairly typical. In this case, what we're showing here is some ICFs that we used. Then what we do is we fill the interior part of this foundation with our three-quarter-inch gravel. You want to use gravel with no fines in it. Again, you want this to be something that will not get clogged up and wick water up. So you want to make it a capillary break. And that's it. There's really not much difference here compared to a regular stem wall and footing so far.

Here's where it gets a little bit different. On a typical slab, you get to the point where you'd have your gravel in there, then you would set your insulation, your vapor barrier, and pour your concrete. Now, with the concrete-free slab on grade, we can't really put our vapor barrier down and then start framing because what if it rains? We're going to end up with a giant pool of water. That's bad. Whereas if it was a concrete slab, it would just run right off. It's a moot point. So we have to do things in slightly different order.

So we end up framing our walls and our roofs, and we dry the building out before we install our slab, or before we even install anything, any of the insulation or vapor barriers or anything like that. So it's a little bit out of order, but what it does mean is when you do get this thing stood up and insulated, or dried out rather, when you're completing the rest of the slab, you're working indoors. Which in Portland, Oregon, it's quite rainy here. Winters are cold. Summers are hot. It's fairly pleasant to be working indoors when you're working on that slab.

The next step is to install your sub slab plumbing and all that groundwater plumbing and stuff like that. What we did on this particular project is we had that three-quarter-inch gravel and then we threw some quarter minus gravel on top of that. This was due to timing. In the future, I would hold off on that quarter-inch gravel because when the plumber came through, he started digging his trenches for his plumbing and he had to make piles. You can kind of see on the right-hand photo in the back, we piled up the quarter minus gravel to keep it separate from the three-quarter gravel.

But anyways, point being is it's pretty easy for your plumbers to work in this stuff. They can scooch it out of the way pretty easily, set in all that plumbing. Then you replace your three-quarter-inch and then put the quarter minus back over the top. One thing you do want to pay attention to is making sure that you get your plumbing lines far enough down, that they're not going to rise up above this gravel layer. And I'm going to show you a picture of where that happened to one of our projects. And it's one of those, "Let me fail first and take that punch and then show you guys so you can avoid it yourselves."

The next step is you get that quarter minus gravel down and you screed it out. We simply just shot a laser as a reference, measure down, and you screed it out with just a typical two-by-four. You can see Fidel and Evan here on the right going through leveling that out and then throwing down the first layer of foam. On the left, I've got a red arrow here showing where those plumbing pipes could have been a little bit deeper. What that meant was when we got to installing that layer of foam, we had to carve around those things, which did take extra time and labor and stuff like that. So, something to avoid.

On this particular project, we're using a GPS foam, so this is the graphite impregnated EPS foam. We're using EPS on the current project because you can't get GPS now. One could use XPS. Typically, our structural engineers... The foam rating that you're going to get with a type 1 foam is plenty fine for the loads that you're going to see inside of a residential building. So you don't have to do anything crazy with the foam or the density of it or anything like that.

As we go through, we offset our panels. We're running these foam sheets perpendicular to one another and then offset from one another too, so that we can break those lines. We're not getting any thermal bridging up through those gaps between the foam. This is all pretty easy. You can see that we've got our windows installed. The roof is on. It was a pretty rainy time when we were installing this project, so it was nice for the team to be inside when we were doing it. You can see on the right where we've had to carve out some of that foam for that plumbing again.

Next, throw on your first layer of plywood. We have used in the past just regular CDX plywood, and we've also used the tongue and groove version of the CDX plywood. In the end, they both performed well. But we noticed that the tongue and groove had a little more rigidity on the floor. We're going to stick with that in the future. Even though it's a little more difficult to install, it's not that much more difficult. And we think the benefits are worth it. We've done both. I would have no problem doing either-or, but we tend to go for that T&G just because it gives it a little more rigidity with that.

Because it is wood and it's floating... I should back up a little bit. Once we get our foam installed, we then install our vapor barrier and make sure we tape all those seams. What our team does is we take a flap with like 12 inches of that product, typically a 10 mil polyethylene sheet, and we put that below our mudsill. Then on the exterior, that connects to our air barrier, goes below the mudsill. And then on the interior, we just tape it to the field vapor barrier in there. So we have a completely continuous air barrier that travels under that mudsill and then connects to the sheathing, which is our air barrier in this case. So really easy to throw down that 12-inch strip of polyethylene. Rolling out the polyethylene in the field is easy too.

Next, you're going to install your second layer of plywood. What we do here is we get some construction adhesive, lay that down, and then we install the sheet again, perpendicular to the lower layer and offset at those seams to give the most rigidity. And then we screw it together with inch and a quarter screws. So we've got two three-quarter-inch layers of plywood, that's inch and a half, and we throw down those inch and a quarter screws, because we don't want to go through there and puncture our vapor barrier. If there were a few screws that puncture through here and there, I really wouldn't be too concerned. A little bit of vapor coming in from the ground and a few holes really isn't going to be that big of a deal. There's not a lot of air movement going through the ground. But clearly, if you can avoid it, avoid it. And it's pretty easy to do.

This is just a photo in progress here. In this particular case, this is a different project where the windows were delayed due to market reasons, but we went ahead and started installing this anyways, and it really wasn't much of a problem. We didn't get enough rain that it filled up our house and created a bathtub. The arrow I'm showing here is pointing to these little kick plates. As we were installing our second layer of plywood and using kind of like a big sledge to kind of bang it together, it shifted the lower layer of plywood as it was doing that. So what we did is we put down that first layer... second layer, excuse me, of plywood over the first. And then we just took some scrap plywood, two bys, and then nailed it to the mudsill. And then to that first layer... So that when we were putting the subsequent layers down and smacking it in with our sledgehammers, it didn't shift it and move it.

So, easy enough to do. Quick little tip. You can see too that we've given a little bit of a gap between the edge of the plywood and the wall itself of the insulation. Because it's wood, it will expand and contract. So we give it at least a half-inch, ideally three quarters of an inch from the edge so that it can move and expand and contract without buckling or anything like that.

Once that's installed, it's time to frame your interior walls. It's again a little bit out of order. None of the walls in this particular project were load-bearing. If there were, we'd have to run probably a stem wall and a footing through the interior of the building and then detail around that. But in this case, there were no structural interior walls, so we just go ahead and start framing them up afterwards. On that bottom plate, when we're nailing that in, we're using two and a half-inch nails. Again, you've got an inch and a half for that bottom plate and an inch and a half of built-up plywood. So you wouldn't want to use a three-inch nail or a three-and-a-half-inch nail because you would go all the way through. So just shorten up your nails there, not too big of a deal.

Lastly, start installing your finishes. Really, it's no different at this point than installing something on just a typical wood frame floor. So not too big of a deal.

One thing that you do want to pay attention to is because it is a wood floor and it's on-grid, you want to make sure you're not creating some sort of vapor sandwich. We want to be able to make sure that we're paying attention to the permeability of the products that we're putting on top of our wood slab. And so, we want to make sure that they are vapor permeable so that if you do get the occasional spill or overflow or something like that, that that water won't get trapped in there and create havoc in that assembly. And so, we're looking for a permeable flooring.

And interestingly, originally, we found this bamboo engineered flooring and it turned out that once we kind of started... Well, first of all, no flooring manufacturer has any idea what the permeability of their product is, or at least none that we reached out to. It's not something that they typically declare on their products. So it is hard to find permeability of specific flooring products. So it can be tricky, but some flooring manufacturers are willing to work with you. We did find a bamboo that looked great. It was a green-certified eco-product whatever it was, but it turned out it had like a real thin layer of plastic sheeting going across one of the layers. Luckily, we discovered this before we laid it down and we were able to switch it out with the clients and use something really similar.

So a dimensional, natural wood, an engineered wood or something like that, a cork, a Marmoleum, all these things are good. They're all permeable, but you're definitely going to want to pay attention on some of these products that can get a little bit tricky. Again, you wouldn't want to trap moisture, more so than that plywood has an ability to dry.

This is the finished product here. No different than a regular floor. It's great to install. If we go back here on the bottom right, you can see the gang is just using your typical flooring install, nail guns and all that stuff. They didn't really know the difference. Just nailed it in. And when you're walking on these floors... It has a give to it. When you stand on a concrete slab, it's hard. That'll get people's knees over time. It's not super pleasant to be on for a long time. These... They don't squish. It's a similar experience to like a wood-framed floor, maybe a little bit softer, which our clients who in this case were... gosh, they were probably in their 60s, really enjoyed and have really enjoyed since. It's just got a little bit more of a give to the floor because it is a floating assembly.

Things to pay attention to. Bathrooms and wet rooms. People will choose tile typically in these locations. Oftentimes, that tile is not vapor permeable. And in this particular case, we had a client decide to do a threshold-free shower after we had started the assembly. And then we were really left scratching our heads, "Well, how do we do a threshold-free floating plywood and get the tile set and all this stuff?" So we actually had to retrofit it where we cut it out and it's a lot easier to cut out plywood than four inches of concrete. So grab the saws, cut it out, removed a layer of foam, dropped it down a little bit, reinstalled our vapor barrier, and poured a mud-set pan over the top of that. So it worked out pretty well.

One thing you're going to want to do is use a decoupling membrane if you're putting tile over this because the floor is floating, it does have a little bit of give, and so you're going to want to make sure that you're decoupling that movement from the floor on the tile so that you don't get broken grout lines and stuff like that. So, definitely want to throw down that decoupling membrane, which by the way, is not vapor permeable. This is just a plastic product. But as long as there's adjacent areas for that wood to dry too should it get wet, we're not really too concerned. There's the storage capacity of materials. We have to be able to allow our materials, if they get wet, the ability to retain moisture without damage and be able to dry.

Now, I probably wouldn't put a decoupling membrane or an impermeable flooring over the entirety of the whole slab, but we feel comfortable doing it just in a wet room or something like that, knowing that there's adjacent areas next to where that can dry too.

I want to be realistic here. There are challenges. One of the things that the carpenters learned real quick was you can't just throw down your framing lumber and frame off the ground anymore because you're working off of gravel. And so, we ended up basically having to build... Got these little sawhorses and you can see on the right where we built the walls off of before we stood them up. Took a little extra time, little extra hassle to do, but not the end of the world. And the team was actually able to be pretty efficient on these builds.

Another thing we learned... Current project we're working on, it's got a moderate slope. And so it dropped maybe two and a half, three feet from the highest point of the house to the lowest. And as we excavated that and set our forms and did the concrete, we then had to fill that back up with gravel. And it did use a lot of gravel. We had to truck in quite a bit. And that was a pretty good expense for that. And the team was like, "Well, is this really that much better than if we were to hang a wood-framed wall off of that?"

But in the end, we think it still made more sense from a building science and thermal perspective than some of the origami that you'd have to do with a wood-framed wall, like a conditioned crawl or something like that. Not to mention the cool thing about slab on grade is that they're really accessible. This particular house we're building is for an older couple and one of their sisters is in a wheelchair. And so, this is all ADA. And the nice thing about slab on grade is they're not raised up like a typical crawl would be. So there are certainly benefits to it, but we did spend some money on that gravel and it's a little bit different.

Also, as we're raising up walls, you have to set up your wall jacks to keep everything plum. And we ended up just kind of driving stakes into the gravel. Typically, this would just get nailed off on your wood deck, but not the end of the world, just something to pay attention to.

So the project I showed was an ICF stem wall. The current project that we're working on, we kind of had an epiphany was, "Well hey, why not... If we can skip the ICFs and not have that plastic in the ground too, wouldn't that be all the better?" And so, what we did is we did a typical footing and stem wall. We did a knockout with just a two by six on the top of the stem wall and then filled that with our foam in there. Then attached our sub-slab foam to that, and then had exterior insulation on the wall, the framed wall, and then dense-packed cellulose on the framed wall.

This actually works, so that we can avoid exterior insulation on the outside of the stem wall, which is a headache really. I mean, do you put it in while you're pouring the concrete? Do you put it on after? How do you hold it in there? What's the embodied carbon of that particular material? What kind of protection board are you putting over the top of it? All those expenses are really avoided.

We ended up doing a thermal model on this. It works, as you can see here. If you look at the temperature gradient, we're staying at 66.6, kind of evil degrees on the inside here. But in Portland, Oregon on a design degree day, I want to say it was 14 degrees, I think is what they punched in on this one. It's still staying warm on the interior and there's no risk of condensation or mold at that critical junction right there. So it works. We were able to enter the psi value into our WUFI Passive model and it barely moved the needle. So really cool way to avoid that extra embodied carbon, the extra labor and steps there.

Here's a picture of that exact assembly here. Again, we took that little 12-inch rip of polyethylene under our bottom plate. We still put the Solseal on there. And then we attach our mudsill to that. On the exterior, we actually set our mudsill in a half-inch into the building so that when we put on our sheathing, it's in the same plane as the concrete. What that allows us to do is to take a piece of tape, like a Siga Fentrim or something like that, attach that to the concrete, it bonds to that green vapor barrier, air barrier, and to the plywood, which is our primary air barrier there. So really secure, good air barrier connection there, and easy to do really.

Some of the fringe benefits of the concrete-free slab on grade. It removes delays due to the busy trades. So, if you're you're waiting on your concrete sub or you're waiting for the concrete truck to show up, or any of those things, suddenly you don't have to worry about that. This is all under your purview as a contractor. You can schedule your team to work, start framing as soon as you're ready and not have to rely on others. The fact that it's done in-house, here's a little picture of our team, means that we actually have a higher margin on work that's performed in-house than is subbed out. So financially for us, we're either able to lower the cost to our clients, or we're able to make more money, or use that as a buffer for other parts of that project. So it's just more cost-effective.

Compared to a concrete slab, it's more comfortable. And that's what your clients are really going to let you know if it's not comfortable. Having that plywood underfoot underneath all that installation. Even if you have concrete, that's at 70 degrees, it still feels cold to the touch. And having the wood floors is a nicer experience.

I get lots of people that think this is crazy and they come up with all sorts of reasons why it won't work. "But what if you get a flood? What if your water heater explodes or your kid leaves the faucet on overnight?" or who knows what. But what I'll say to that is, "Well, what if you had a concrete slab on grade and you had your wood floors over it and you had a flood, well that's a real hassle to deal with, with the slab on grade anyways, right? I mean, floods are bad anyway, even with concrete. You still have to replace all the sheetrock that got damaged and all that."

And I'll make an argument that to replace... The concrete is hard, but if you have to replace the plywood, let's say there's a leak, or you have to access a broken plumbing line below this thing, you just grab your saw, you cut out a square, and then you cut out a smaller square and you can access the thing, put your smaller square back, glue it, put your bigger square back and walk away. You can do that in less than an hour with typical tools versus having to break out concrete and cut it and all that kind of stuff. The other question I get too, is like, "That can't possibly support all the loads inside of a house." This is the Trans-Alaskan highway, where they actually are supporting a road. Lots of airports are built with this geofoam. There's really no issue from a structural perspective with using foam when it comes to loads.

Another common thing I get is, "Well, what about bugs, critters, all that kind of stuff?" I just took a screenshot of a typical termite-resistant assembly. No reason you couldn't do the same thing. Keep your wood siding eight inches above grade, make a metal termite shield at that mudsill, use your gutters and downspouts, keep it dry, treat your soil for termites, remove all the flora and stuff from around your house to keep the bugs away from it. You can do all this to keep the bugs out. And typically, bugs aren't super interested in dry wood. They like moist wood. And if you're doing it right, there's no reason that plywood should ever be moist.

And maybe this isn't for everything. I mean, if you're really in termite land and there's nothing you can do about it, maybe the concrete-free slab on grade isn't the best foundation for you. Maybe you should be using piers and setting it up off the ground. Maybe you should do a full basement. Maybe you should do a regular crawl. So I'm not going to say it works for every single scenario, but we're certainly not super concerned about the bugs and vermin and stuff like that.

We touched on this a little bit, the floors are warm, they're forgiving, they're comfortable. Doors and thresholds, we're trying to make those thermally broken from the outside. It's actually really nice and easy with this, because you can just connect your thermal layer really well. And there's just less thermal bridges with this particular thing. So there's less opportunity for mold growth and poor indoor quality and that kind of stuff.

Then I'll say, "Don't just stop there." I mean, we got rid of all that concrete that was in that slab, but we still have concrete where we have our stem wall and footing. This is our current project. We're working with the Oregon DEQ on a new slab blend. We're using some slagging here and we're able to knock down the embodied carbon in that concrete by 50%. The foam that we're looking at there, it's XPS but it's the newer formulation, the NGX I think it's called. Where it has a much lower... It's more similar to like an EPS foam, roughly. So, pay attention to the materials that you are using. We got rid of that concrete slab, the concrete we had to use we knocked the embodied carbon back by 50% on top of that.

Steel, lots of embodied carbon. We use fiberglass rebar. It's also less conductive. That's a great thing to do. Also, design. Try to engineer or work with your structural engineers so that you have less interior footings and point loads. Maybe there's a way to use a beam above instead of a grade beam. Maybe you can switch to a TJI or a parallel chord truss to avoid interior footings. So all those interior footings are going to require more foam and more concrete. So let's just not use them in the first place through good design.

The other thing to point out is, with the wood that you're using, if you can, use the Forest Stewardship Council certified wood that comes from local sources. That's what we're doing on our current build. That's hugely impactful too. And I took this picture on the right because I was just remarking on how nice this framing lumber was. It was actually created as a number one, there's one in the front here. That's some really great-looking lumber that was harvested just locally here in the Portland, Oregon area. And with that, thanks.