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Green Architects' Lounge

Foundations — Part 2

Do you really need to have a basement? Fine — then we had better talk about the right way to build one

High density XPS is used at the bottom of the ICF wall as a thermal break.
Image Credit: Briburn
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High density XPS is used at the bottom of the ICF wall as a thermal break.
Image Credit:
Briburn This is the detail of the Edgwaterhaus basement. It features ICF construction with a double-stud wall above. It also features an exterior stone wainscot.
Image Credit:
Briburn This basement is insulated to the inside using a specialized system. The same can be achieved with a number of similar systems or with XPS insulation.
Image Credit:
Briburn This foundation is insulated to both the exterior and interior with substantial thermal isolation of the slab.
Image Credit:
Kaplan Thompson I mentioned in the podcast that I had a photo of this product. This is a THREE sided tape. It's hard to tell in the photo, but the third tab of the tape is already stuck to the top of the concrete with the peel and stick sill seal/capillary break over it. htese other two tabs are ready to be adhered to the white poly below (that goes under the slab, and to the WRB.
Image Credit:

Phil and I have returned to continue our discussion on foundations. In Part One, we covered slabs and frost walls, and in this part we cover basements and crawl spaces.

The Highlights:

Do you really need a basement? If there’s no programmatic need for a basement (like the need for a workshop), then perhaps you can do without one.

Insulation: Inside or outside? There are many reasons to insulate on either side. We weigh the pros and cons.

ICFs. Insulated Concrete Forms are sometimes but not always cost-effective. After discussing the cost-effectiveness of ICFs, I share a neat thermal break detail for the bottom of ICF walls.

Capillary breaks. Your concrete is like a sponge soaking up moisture — moisture out of the ground, and moisture originating as rain. You want to isolate your framing from this moisture. Also, I share the history of the termite shield.

Sill seals. We talk about a couple of our favorites, and, as promised, we provide links. We’ve recently been using a couple of Protecto Wrap products, the Triple Guard Energy Sealer and the Premium Energy Sill Sealer. Phil’s office has been using gaskets from Conservation Technology

Thermomass. We have a discussion about this “inverted ICF” product. (Concrete on the outside, insulation in the middle.)

Crawl spaces. Just don’t do them. If you do do them, treat them like mini-basements.

As always, Phil treats us to a song that we should all have on the studio playlist. This time it’s the song “Content to Reform” by Diane Cluck.

Thanks for tuning in. Have a great summer everyone. Cheers!


Chris: Hey, we’re back. How are you doing, Phil?

Phil: I’m doing great, Chris.

Chris: Great, great. Nice drinks!

Phil: Let’s pretend they’re not all done. We might need another to get us through the rest of the “foundations” topic. Foundations can be pretty dry stuff – if you do it right!

Chris: Nice one. We did slabs in Part One. So, Part Two: here we go! Let’s talk about full-on basements. Phil, where does the insulation go: inside or outside?

Phil: That’s a really great question! And you know what? We still do it both ways.

Chris: Do you? Yeah, we do it too. If you’re doing out-sulation (you know, when you’re wrapping the building in foam), because your wall has projected out in front of that foundation, it’s very convenient and easy to keep that foam insulation going (or the rock wool or mineral wool types of insulation).

Phil: Right. So, these are reasons why you’d choose insulation to the outside of your foundation wall.

Chris: Right. Maybe you want to make that thermal mass work for you, you know? If it’s on the inside, it’s a warmer thing.

Phil: Right. And sometimes that works. But you also have to heat up that whole mass.

Chris: Exactly. Good point! Yes.

Phil: Right. It’s a good question. The way it would work well is if you wanted to dampen the effect of the temperature swings. That would slow things down, so that’s one reason to do it.

Chris: Martin wrote a great article on this. In fact, we’ll make sure that there’s a nice link off to the left of the page in the “Related Content” box.

Phil: He goes through a few other things which I think are always on our minds. Another reason to insulate to the exterior is that it gives you more room in your basement.

Chris: Oh, yeah.

Phil: You don’t have to fill the studs in. You just stick it to the outside. You don’t subtract your square footage.

Chris: That’s right.

Phil: Duh, that’s a pretty good one. You also run into problems with stairs, if you have a stair on the outside wall. That whole thing goes away if you put exterior insulation on.

Chris: You know, I think twice in my career, I’ve been brutally burned by that.

Phil: Twice!

Chris: Twice! And that’s why it sticks with me.

Phil: Shame on you, Chris.

Chris: Shame on me. Kid… intern… nice job… oh, yeah! And, what I’m talking about, people, is when your wall is not that thick – which, of course, shame on you there. (But this was years ago, Phil, years ago!) And you’re planning your stairs, and the first floor looked great, and you say, “Make sure the stair is down.” But then your foundation bumps in. And now your 3-foot-wide stair is 2-foot-6, and… Oops!

Phil: And you try to insulate it – there’s no way!

Chris: Oh, yeah. And now you’re done. Stupid architect!

Phil: That’s how I learned!

Chris: Yeah? Failure’s a great thing. It teaches you.

Phil: Why else, Chris? Why else do we put it on the outside? One is: you could argue that it’s harder to insulate the rim joist – and air-seal the rim joist – on the inside. So, if you’ve got the insulation on the outside, that whole thing is gone. Sometimes we do that anyway, because you get a little extra insulation.

Chris: Yeah, but… it’s going to be easier for me to argue why not to. For example: sure, you can run that insulation up and insulate the rim joist. But now, I’ve got to protect that insulation – I have to see that insulation from the outside – so, something’s going over it, whether it’s a stucco or it’s a metal. I’ve been doing this a lot; I don’t know if you have…

Phil: Yeah.

Chris: But if you’ve just got that 8-inch gap between your siding and the ground, with this level-ish building…

Phil: What do you do with that? What a drag!

Chris: I’ll tell you what I’d do: I’d get some coil stock.

Phil: That’s it! That’s the best way to do it.

Chris: Yeah. I’d get some coil stock metal with a matte finish (prefinished, whatever…) and you just run that around the perimeter of the building. It’s protected, and basically, it vanishes. And Bob’s your uncle. Done!

Phil: Martin and his article has a whole list of strategies to handle that – metal flashings, fiberglass panels. (People are making fiberglass panels for that.) Another is EIFS. But to me, EIFS is kind of a crappy solution. It can get beat up; you can get your weed-whacker up against it.

Chris: Oh, yeah. Or a kid with a baseball bat. Yeah. It’s tough stuff. But hey, it’s a solution.

Phil: Cement board – I’ve seen that happen a few times. (It is an issue. You have to deal with that.)

Chris: Right. And one way to deal with it: insulate on the interior, especially if you can spare that square footage on your building. Insulate on the interior. Basically, what you’re doing, thermally, is you’re removing the foundation wall from your whole assembly.

Phil: That’s right. If you insulate on the exterior, it’s essentially a thermal bridge all the way through, unless you insulate underneath the footing.

Chris: Exactly. And that’s where I was going with that. Underneath the footing – I’d say 98 times out of 100 – there’s no insulation under that footing. And that can be done, people. That can be done. We do it in commercial buildings – you can get higher-density foams. But that takes some know-how. Twenty-five hundred PSI is your presumptive soil pressure that you use and design your concrete for. I can get foam insulation that can take that amount of pressure. So, you can substitute that and put foam in underneath. But that’s another big step and another labor step and…

Phil: We’ll throw you a detail. I would say, “Be bold!” Look into it. Take a simple job and try it.

Chris: We’ll get into ICFs in a little bit – like the Passivhaus that I did in Saco – Roger Normand’s Passivhaus (Edgewaterhaus: I’ll give it a little plug!). I came up with this detail (and I was surprised that I seemed to be original about it).

Phil: I’m not surprised. You’re smarter than you look.

Chris: Well, thank you (Wait!). Where the ICF meets the footing – you pour a footing, business as usual – you put high-density foam right there along the bottom (it’s almost wedged in at the bottom of the ICF), and you pour the ICF wall on that. So, basically, the wall is this thermal mass inside this insulated thing.

Phil: Essentially, it’s sort of a chunk of EPS that sits right in the bottom of the ICFs – just in that middle piece of cavity.

Chris: Exactly. And I ask my engineer, “Why can’t I do this?” and he says, “Well…”

Phil: “Well, I’m an engineer, so I’m going to say ‘No’ before I…”

Chris: No, no. My engineer’s great. Joe Leasure. He’s a great guy; great guy. Give him a little shout-out: L&L Engineering. He’ll handle the creative stuff.

He said, “Well, structures bear on the most rigid thing. So, what will happen is: it will compress (especially as it’s curing) but then, all of your little rebar that’s sticking out – that’s tying the footing to the wall – is going to take all the load.”

Phil: So…?

Chris: Exactly. And I said, “So…?” and he said, “So, you’re probably fine because there’s no way you’re going to crush that one-inch piece of steel, especially if it’s every three feet. But – here’s the thing – that steel is now open to moisture so it could (there’s not a lot of oxygen in there…) hypothetically spall. So I’m going to call for epoxy-coated rebar.” And I said, “are we done; we got it?” and he said, “yeah” and I said, “alright. We’re doing it!”

Phil: That’s cool! Post that, Chris. That’s good.

Chris: Yeah. It’ll be one of the details. ICF details. So, we were back to insulating on the inside… right?

Phil: Right. Other reasons to do it.

Chris: What’s Martin say? What were his reasons?

Phil: One thing he says is it integrates more smoothly with the construction schedule. It happens after the building’s dried in, rather than when the excavation contractor wants to backfill. (Then you’ve added this extra variable to him. He does not like that!)

Like we’ve just referenced, it’s easier to provide an uninterrupted connection between the below-slab insulation and the insulation to the interior. So then, your only connection is through your rim joist and up into the insulation. So, you don’t have to worry about that thermal break through the footing and the foundation wall.

Chris: And people, you can always take that rim joist – by the way – and even build a normal 2×6 dumb house, you know? Just 2×6 and regular pink insulation. (I won’t name names of what kind it is, but you know what I mean: with fiberglass batts.). And you can take that rim joist and just move it in 2 inches – an inch and a half – and put a band of insulation around that. Boom! You’ve just created a thermal break for a good chunk of your house that ordinarily wouldn’t be there – that was cheap and easy. Sure, you’ve got to think a little, but only a little. Only a little, Phil.

Phil: Only a little. It’s not so hard.

Chris: So, we’ll post some details of insulating to the interior – which is, in my mind, easier to do.

Phil: And then you don’t have to deal with that little chunk of foam of 8 inches on the exterior. You save money there. One of the other things, though, is: even though you put foam on the inside, you’ve got to cover it up by code. You can’t leave it exposed. So you have to put sheet rock over top of it, which means you have a stud wall again, with a chunk, or you go with Thermax… I don’t know if you’ve used that before…?

Chris: I have, yeah.

Phil: So, it’s a polyiso with a covering…

Chris: A coating on the outside. Then you can get either foil or they’ve got a myriad – I used the word “myriad,” Phil…

Phil: Really? I like “plethora,” but myriad’s good.

Chris: They have a plethora of coating options.

Phil: Thermax is expensive, but it’s still kind of a quick problem-solver, I think.

Chris: If you’re not going for aesthetics or anything like that. I mean, maybe we’ll hear from them saying, “Hey, it’s not that bad looking. It looks good.” But basically, there are flame-spread ratings for Thermax.

I think ICF people run into this all the time, where people are thinking, “Well, I will do an ICF foundation; maybe I’ll do sheetrock later or a gypsum wall board,” and so they leave it that way. And then the code-enforcement officer says, “Hey, hey, hey. You can’t leave it like this,” per whatever – I don’t have the citation number of that code, but because it doesn’t meet the flame-spread rating. So little Johnny can walk over with his lighter and start a fire just by holding it to your wall. You can’t have that in a building material.

Phil: That’s right. One of the other things that we should talk about: what kind of insulation do you put on the outside of your house when you use it?

Chris: Well, I’ll tell you what I like. I like using the Roxul mineral wool.

Phil: The drain board.

Chris: The drain board.

Phil: It’s more rigid than the batts. It’s a different sort of thing.

Chris: Oh, yeah. Yeah. People, get the drain board. It’s almost a board product. In fact, that’s what they call it so that you know it’s not batts. It’s hydrophobic, so…

Phil: Water doesn’t stick to it.

Chris: It sheds, it drains.

Phil: Unlike polyiso, which likes to suck it up.

Chris: Right. And what happens when you bring that in? You’re reducing your effective R-value of that material, because it’s now saturated and cold… conductive water. Polyiso is not a good subterranean insulation, generally speaking.

Phil: No. XPS or EPS [are better].

Chris: Those command that subterranean market, and are laced with fire-retardants and things like that. And that’s the reason why we’re trying not to use those.

Phil: And one of the reasons we’re not using Roxul to the interior is that we’ve got some concerns with formaldehyde.

Chris: Yeah. Formaldehyde and heavy metals, too. I don’t think it’s that bad anymore, because basically, a lot of it’s made from slag from steel manufacturing. It’s not as bad for you, but it is this waste product that can have all kinds of other contaminants to it, depending on how it’s separated from the manufacturing process of another product.

Phil: So, if we shook our next drink, say, with a little bit of Roxul drain board… would we drink it?

Chris: I would.

[The guys laugh.]

Phil: The answer is, “Wait! What’s the drink?”

Chris: Yeah. Exactly. What is that drink? One of these! Gosh, these are good! I’m loving this cocktail, this episode. I’m going to give it five stars. Five stars.

Phil: Oh, excellent! I’m glad to hear it. I don’t very often get to host the cocktail.

Chris: Oh, but every time you do, it’s always a special one. And it’s always really good.

Phil: Thanks for saying so, Chris.

Chris: Well done. So, yeah. What were we doing? We were insulating to the inside. You’ve got to protect that rim joist. It’s good if you protect it both inside and outside. Be careful of the vapor sandwich that you could make.

Let’s talk about capillary breaks, Phil. I remember, I don’t know… twelve years ago? A long time ago… Kid. Chris Briley’s in an architectural office, recommends to his superiors (or his upper management or whatever), “We should put a capillary break underneath the sill.” And they said, “A what??!”

Phil: Really?

Chris: I think it was the era. It was the early nineties. “What do you mean, capillary break? We’ve got sill seal.” Well, it’s different. It’s different than that little pink piece of foam.

Phil: Or blue.

Chris: It could be blue. But, what you’re trying to do is – Oh! Oh! I get to tell the termite shield story!

Phil: Alright.

Chris: And this may have come from Joe Lstiburek when he visited our college and spoke, but I’m not sure…

But anyway. Back when… picture the thirties. Door-to-door salesmen, that sort of thing. Just like they’re convincing people to put lightning rods in their barns, they were also convincing builders and people to “protect your house from termites. Put a termite shield between your foundation and your sill plate.” And basically, that was just a bent piece of metal. It was just metal. It was metal flashing. But the theory was that this metal would project out and bend down. And the termites would crawl up your foundation and hit this metal that would bend down and go, “Oh, oh. We can’t climb around it.” And they’d fall. [The guys laugh.] And entomologists were outraged! They said, “That is ridiculous! They negotiate that in nature all the time. Leaf edges – all the time. That does not stop termites.” So they debunked the whole termite shield industry. And then, what should happen, but not maybe a decade later? All these houses that were in termite zones that did not have the termite shield were infested with termites. And those that had the termite shield were fine.

Phil: How about that!

Chris: How about that! And they said, “See! It protects you from termites. You entomologists are stupid.” And it takes another layer of science for them to go, “Well, actually, termites like – what kind of wood? – dying, dead, decaying wood. Just get in there. Moist wood.” So, basically, that metal – what it was doing – was not keeping the termite out, it was keeping the wood dry. So, ta-da! Capillary break; that’s what that is. So, the termite shield came back, and occasionally, you’ll still see it labeled as “termite shield.” Of course, it actually is a termite shield for the termite country because you’re desperate to keep that sill plate dry and clean and free of termites.

Phil: So, what do you use? We use this little gasket by a company called Conservation Resources that we typically spec.

Chris: Oh really? I don’t know if I knew that gasket. What did we just use…? I want to say it was Tremco. No, it wasn’t Tremco. Oh crap! I’m going to send a link; I even have a photo of it. I’m spacing out. Maybe I’ll edit in right here. [It was Protecto Wrap Triple Guard Energy sill seal.]

Phil: Okay, we’ll both follow up.

Chris: We’ll follow up and we’ll put in our products. It was a T-shaped product, so it capped the wall and then it ran down the wall and then you could put on your sill plate and you could peel off another piece of stickiness and put it back on. So, you just capped off everything and sealed it and…

Phil: Alright. I want to see yours. I’ll show you mine if you show me yours.

Chris: You’ve got it, buddy. That’s what this podcast is all about. He says with a wink.

Phil: I’m glad this is not video.

Chris: Yeah, me too. Thank God it’s radio.

Phil: So, Joe Lstiburek had a six-digit idea.

Chris: A six-digit idea? Did he use those words?

Phil: He didn’t, but our friend Martin Holladay used those words in referencing one of Joe’s ideas.

Chris: Well, he’s welcome to it. So, what did Joe say?

Phil: He suggested that it was time for manufacturers of fiber-cement siding to create siding which is bumpy on the backside. Isn’t that a cool idea?

Chris: Yeah.

Phil: One of the questions I have is: would we gain anything if we had exterior insulation on the outside that was bumpy as well?

Chris: Maybe.

Phil: It would be akin to that dimple-mat.

Chris: It would, but if you’re inviting air in behind the insulation – I mean, that could be cold air, so – it could be defeating your insulation.

Phil: So, we’d have to seal that on the outside.

Chris: Yeah.

Phil: Maybe it’s not such a great idea. Joe was smarter than I was in this case; I was trying to interpolate.

Chris: Well, nice try.

Phil: I tried.

Chris: But, I’m happy to shoot it down, Phil. Lob another one up and I’ll blast it away.

Phil: I’ll let you know if I come up with anything.

Chris: But, no. I love that idea. Why wouldn’t they make it so there are little gaps there?

Phil: Yeah. There’s our rainscreen. We’re doing it all the time.

Chris: Yeah. I’m shocked when I find out someone’s not doing a rainscreen. But, anyway, we’re talking about foundations. What about crawl spaces? Are we going to go to the evil, evil crawl spaces? Or do you want to talk about ICFs?

Phil: Let’s talk about ICFs for a second. I know that ICFs are something that you guys are using.

Chris: Yeah. Have you used ICFs?

Phil: We’ve only managed to get it into a project once or twice. We used to start with it a lot more often, but it always gets value-engineered out. Builders say, “I can do that. Why can’t I just make my own?”

Chris: You see, it’s interesting – because the projects that I’ve done, the builders do it themselves. They don’t hire the form-work guys. They are carpenters and they do the wall. They are happy to learn ICFs, and do the ICFs, and so they’ve adopted that as their trade. So, the concrete guy comes and delivers the concrete and vibrates it there and they do that.

Phil: But, at the same time, proprietary ICF package is more expensive than just going out and buying your own XPS or whatever.

Chris: Oh, without a doubt! You’re going to pay more for the product, but you’ll pay less for the labor. If your builder is not into that labor anyway – if that is not labor money that he is trying to carve out – then he’s not into that product. I think it’s one of those things where – and it doesn’t go off without a hitch. Heck! Just read Roger Normand’s Edgewaterhause blog. (You know, the Passivhaus idea.) Whooo! That was a tough ICF job. They were tall walls, and it was their first time doing it. Man. They had some blowouts and it’s really spooky to see an 8- to 10-foot tall wall full of concrete…

Phil: Blow out?

Chris: Well, not just blow out, but… You watch them true it up. They can shake it, wobble it… and you just think to yourself, “My God, that is heavy. That is a lot of weight that they’re just kind of moving with these braces and all that jazz.” So, it’s not easy. Everyone says it’s like Lego blocks and you fill ‘em – done! Awesome! And it actually takes a little more work than that. You’ve got to wire them up and get the rebar and someone’s got to do some engineering to it. But, we’ve done it successfully many times and it’s been, I think, a really great product.

Phil: Well, I’d love to hear some comments from listeners, even if it’s just anecdotal. Are more people starting to use ICF, or is it going in the other direction?

Chris: I tend to feel like it’s going in the other direction, but that’s personally. I think it’s going that way in our office. It’s been a while since we’ve done ICFs.

Phil: I’ve had an issue in the past with some of the ICF guys – some of the sales folk – who say things like, “It’s equivalent to an R-40.”

Chris: Yeah. In Arizona, punk!

Phil: “It’s an effective R-40.”

Chris: Yeah. “Effective!”

Phil: And I’m counting it, saying, “Wait a second. You’ve got… 2 inches to the outside?!”

Chris: And 2 inches to the inside! Yeah, here’s my pet peeve: that’s not science, buddy.

Phil: That’s R-20!

Chris: I think it was Oak Ridge National Laboratories – they did the whole ICF study – they actually did walls in different climates and tested this so-called “effective R-value” that these guys peddle. And, okay, here’s the thing: R-value is not even a real number; it’s a value.

Phil: I thought ‘R’ stood for real!

Chris: [He laughs] It’s supposed to be resistance, but ‘U’ is resistance and ‘U’ is a factor – because you are actually using that in an equation – and ‘R’ is a value so the consumer can understand it. ‘R’ is resistance. There are three ways to transfer heat (you know that): radiation, convection and conduction. And ‘R’ deals with conduction. So when they say, “an effective R-value” – Shut up! An R-value is resistance. By having concrete in there, you’re not increasing the R-value – you’re doing thermal mass – you’re increasing thermal mass. That’s a whole different metric and a whole different ball game. And so, I cannot plug R-40 or R-50 into my energy model. Thank you very much, moron.

Phil: [He laughs] I’m with you. And I asked him, and he said, “A typical wall has thermal bridging everywhere – and we don’t have it – so, we’ve bumped up our numbers to…”

Chris: Right.

Phil: Come on, really??! Eyes wide open.

Chris: Eyes wide open, people. Here’s the thing: if you’re insulating your foundation – the thing is, you can screw sheet rock right to it – have your builder run the numbers in terms of whether it’s cost-effective. And for you, and for that builder, it might get written off and deleted and then you go with something else. And I’m fine with that.

Phil: Bottom line is that it’s not a bad product.

Chris: Not a bad product.

Phil: It’s a good idea. And maybe it’ll come down in price.

Chris: But it’s not the only way to do this.

Phil: It’s not. Have you ever used the Thermomass product, Chris? It’s like the inverse of ICF. You’ve got concrete on the outside and a chunk of insulation on the inside.

Chris: I’m actually using that on a slab. We’re going to do the frost wall with that. I should say what it is, first. You can Google ‘Thermomass’ and you’ll see that it’s the opposite of ICF. They have a little insulated insert in the middle.

Phil: Give us numbers, Chris. Do you know the thickness of the insulation? I remember it being – what is it – 4-2-4 or something like that?

Chris: Yeah. 2-4-8-10… you can keep going. But, here’s the trick: there’s some engineering involved. You have to use proprietary insulation in the middle – it looks like just Dow blue board; it looks like XPS, but – it has this coating on each side of it that they say is important. You have to have this coating.

Phil: It’s a thin layer of magic, and for that, you pay.

Chris: Right. You can’t use some other product. So, you buy this sheet-good – it already has these pre-determined places where the little fiberglass thing goes through and it locks (so it sits in the form nicely and all that jazz). So it’s a great idea, a great product, but I always feel it’s overpriced. We almost did it on a project in New Hampshire, but we ended up just insulating to the interior because it just got cost-engineered out. Value-engineered out. But, we’re about to do it for this little frost wall.

Paul: The cost is coming in okay?

Chris: Yeah, it’s coming in okay. The builder is really fine with it. It’s a little early – it might fall out of the project – we’ll see.

Paul: And you’re doing it because you just want to try it? It seems like a cool idea…?

Chris: I’m doing it because the builder said that’s his preferred method of doing this, and have I ever heard of Thermomass? And I said, “Let’s talk, buddy!” So right now, I’ve got a builder who’s saying he likes the stuff, he’s used it before, he’s very comfortable with it, and he thinks it’s great. I think that it’s one of those things where, optimism is worth – I don’t know – ten to a hundred grand, depending on the size of the project. Attitude is worth something; it’s worth real cash money.

Phil: I like the fact that you just quantified optimism in dollars. That’s awesome!

[They laugh.]

Chris: Yeah, if only I could get paid for my optimism… I’d be rich!

Phil: That’s really interesting about the Thermomass. I’m a little suspect. I remember walking the floor of a building energy conference a couple of years ago with my friend who’s an engineer and he looked at those concrete walls that were thinner – I think that the concrete was four inches – and he said, “I don’t like that. It feels funny to me. I don’t trust it.”

Chris: Here’s the thing: they have their own engineering staff that will help you out and will stamp it. On this project in New Hampshire, we went through the engineering. Those fiberglass links? That’s what’s important about them. He said, “You don’t have a 10-inch wall anymore. You have two 4-inch walls.” Really we’re only bearing on one, so yikes! Engineering is a big piece of that.

Phil: Alright. What else have we got? We’re pretty good!

Chris: Well, except for crawl spaces.

Phil: Alright. Do you want to talk a little bit about crawl spaces, Chris?

Chris: Yeah. Don’t have them! Done!

Phil: A crawl space is the same thing as basements, right? It’s a short, little basement. And if you’re really going to do it right, you’re going to do it the same way you did the basement.

Chris: Exactly. With a slab and a vapor barrier, because… come on! The reason why we hate them and are calling them evil is because, a lot of times, they are open right to the ground. Right? So we have moisture coming up from the ground – and it’s lingering in the space – and then we have people saying, “Oh, then you need to vent the crawl space.” And then you have people saying, “No! For God’s sake, don’t vent the crawl space – now it’s a frigid crawl space with condensing water everywhere and now you’ve got bugs and all that.” They are evil, evil, evil things. Please don’t do them if you can help it. But they’re out there. And that’s the other thing. What about the existing foundation? I don’t know if we have time to get into the existing foundation.

Phil: I don’t think so. I think that would be a deep-energy retrofit. A DER. Have we done a DER podcast?

Chris: I feel like we have, actually. We’ve talked about it while drinking before, Phil.

[They laugh.]

Phil: That’s right. It could have been that.

Chris: And again, I should just bring something with me all the time when we’re together – with beverages talking about architecture – because I always come away from these conversations saying, “Wow! We should do a podcast on whatever it was we just talked about.”

Phil: That’s alright. That’s why we got inspired to do this!

Chris: Yeah, exactly. We would actually go to a forum or a seminar, meet with that person who did the seminar, and you and I would be talking (or it’s just you and me in a group of people), and we say – “My God, this is way more interesting than what we just sat and listened to.” The stuff that we’re learning here, and the experiences that we’re all sharing…

Phil: Maybe not quite as polished as the guy.

Chris: Oh, nowhere near as polished. But that’s another attractive feature to it.

Phil: Here it is; case in point.

Chris: Alright. Do you want to wrap it up?

Phil: Yeah.

Chris: That’s “Foundations and Slabs and Stuff,” from the Green Architects’ Lounge.

Phil: That was fun, Chris. Good to see you again.

Chris: Good to see you again. Let’s do this real soon.

Phil: Let’s do it. Cheers!

Chris: Cheers!

Phil: Cheers! Thanks, all!

[The episode closes with a song by Diane Cluck: “Content to Reform.”]


  1. User avater GBA Editor
    Martin Holladay | | #1

    Termite shields
    I enjoyed your termite shield story. These days, of course, everyone know that a termite shield won't stop termites. But in addition to the benefit you mention -- providing a capillary break that helps keep mudsills dry -- a termite shield has another function: it forces termites to build their tunnels (mud tubes) out around the drip-edge of the termite shield, so that their presence (and manner of entry) is more obvious. This makes annual inspections easier.

  2. Steve Young | | #2

    A few questions
    First of all, I love these podcasts.

    ISO seems to have a reputation for not being used below ground due to water saturation issues? Has anyone seen this? Is it documented? If water can get in, surely, the blowing agents can get out ( I recognize that water is a small molecule, but I have to believe that foam insulation is a bubble mass.)

    How do you attach concrete sheets, or any other material over insulation to a concrete foundation/wall?

    What is so evil about the pink or blue rolled foam as a sill sealer? Does it decompose? We use this for our Habitat for Humanity homes south of Houston, TX.

    Is putting foam under footers becoming a somewhat "normal" practice in construction - enough so that when I specify it to my foundation/slab contractor, they won't assume that I am from Mars?

  3. User avater GBA Editor
    Martin Holladay | | #3

    Response to Steve Young
    When you mention ISO, I assume you are talking about polyisocyanurate rigid foam insulation. Polyiso cannot be buried or exposed to soil. It definitely absorbs water, as any roofer who has ever taken apart a failed low-slope roof can tell you. While the blowing agents that are trapped in the polyiso do eventually escape into the atmosphere, that process takes many decades.

    If you want to fasten rigid foam to a concrete wall, you can use foam-compatible construction adhesive or roofing buttons and TapCon screws. For more information on this issue, see How to Insulate a Basement Wall.

    Q. "What is so evil about the pink or blue rolled foam as a sill sealer?"

    A. It isn't evil; it just isn't airtight. You can verify this with a blower door. The easiest way to seal this type of air leak is with a bead of silicone caulk on the interior side of the crack.

    If you specify rigid foam under your footings, your concrete contractor will definitely think you are from Mars. That doesn't mean, however, that it isn't a good idea.

  4. Dan Kolbert | | #4

    Cooling climate
    Steve - as has been discussed elsewhere on the site, "it's different in the south." In a climate where cooling is a much bigger problem than heating, you may not want to thermally isolated your basement from the ground, since it could help cool your house. You still need to worry about condensation issues, but foam under footers in Houston may not make sense.

  5. Jason Hyde, Peterborough 6A | | #5

    Interior drain tile doing double duty
    In the second image, the interior drain tile is serving as both radon mitigation and is tied to the exterior drain tile. I've seen this detail a few times now. Is this an accepted practice?

    After re-reading "All about Radon" and a few other documents, I cannot find any mention of this.

    At first glance, it seems like a great idea. My question is about the drain tile which eventually drains to daylight, with a passive system. Does this interfere with radon escaping through the vertical vent pipe (via stack effect)?

    This post;

    seems to indicate that linking them would defeat, or at least hamper an active system. What about a passive one? Am I missing something?


  6. User avater
    Christopher Briley | | #6

    Is it an accepted method? Yes, in that it has been accepted in the past, but check with your Code enforcement officer first. It is not a prescribed method in the IRC. This is a great technique (In my opinion) for a passive radon system. Especially since radon is a heavy gas. It will theoretically flow out the bottom drain to daylight with the rest of the water. However, you'll want to make sure there is a back flow preventer on that drain tile connection between the interior and exterior that will keep exterior water from infiltrating the sub slab environment via the drain tile. If the house is tested and the passive system turns out to be not good enough, your clever architect will have made it so that you can install a fan to the radon stack (in the attic, or truss space) and the back flow preventer will assist in allowing the fan to depressurize the crushed stone bed under the slab.

  7. Steve Young | | #7

    For Dan K
    Yes, I am stuck in Texas now, but I plan to move to Ohio and build my retirement house in a few years. Sorry for the confusion.

    Now, if I can only convince the folks down here to insulate the edge of the slab......

  8. Flitch Plate | | #8

    Jason … Radon cannot rise by
    Jason … Radon cannot rise by itself nor go uphill passively. It needs help (wind, vacuum, mechanical force). Stack-effect is not strong enough to push it up a vent. Radon is a heavier than air gas. What stack effect does is create a flow of compensating incoming air to fill the stack effect's negative pressures and this in certain seasons and conditions is enough movement to pull in the radon gas though cracks and gaps in the foundation. Then it stays there, on the floor.

    That posting you refer to has a flawed assumption if it assumes vent stacks disperse radon or a mechancial radon mitigation system disperses it via a roof stack. If someone is doing that, they are crazy. One definitely does not want to entrain radon gas (put into the air) out a roof stack or anywhere near people and animals. It will cascade down the building exterior and into the bedrooms, playgounds, wells, feedlots;etc. Radon's parent (radium) and daughter (polonium) are even more dangerous than the radon.

    Why radon is a problem is a special problem is its weight: it lies on the floor in a building and does not easily disperse or reduce in concentration like other radioactive particles.

    Thank Eldorado, Cameco, GE and the Manhattan Project for much of the radon problem in Peterborough.

  9. Kye Ford | | #9

    ICF's are da bomb
    The house I'm building now is the second I've done with ICF's. This house had more bumps and corners than the rather simple boxy duplex we poured last fall but no issues complete breeze. With ICF's I have been able to get rid of the concrete subcontractor, we pour our own footings and do our own flat work. There are no time delays waiting for a sub and we can control the quality...make it perfect.
    As to the r value debate of ICF's , you can always add more to the assembly.
    As to costs, I have found them to be competitive or less than the cheapest concrete contractor in town.
    To me it's all about being able to do another portion of home construction myself and it's kind of fun working with the giant Lego blocks.

  10. Jason Hyde, Peterborough 6A | | #10

    Fitch, I am a little confused
    Fitch, thanks for the response. Am I missing something?

    I've been using these articles as reference material for my own home's design.

    They both suggest that in a passive system, the vent stack should pass through the thermal envelope and exit via the roof. What exactly are you saying, that the passive system outlined is dangerous, ineffective, or both? What would be a better system?

    Is there some other reference material you might suggest I read? Or am I missing some key point here?


  11. Flitch Plate | | #11

    Jason … You pointed out a
    Jason … You pointed out a feature of the design noted in the illustration in this article and how it might interfere with radon mitigation by stack effect.

    Radon, being heavier that air, should not be removed from any building by forcing it upwards (passively or actively) through a stack (or by any means) that terminates by dumping it into the air above living beings; where it will, by the nature of its molecular weight, not disperse, but instead, fall downwards, go into the building through windows, and enter the breathing space of anyone below.

    I was addressing what seemed to be confusion and error in some postings about cellar ventilation stacks. They must not be used for radon mitigation. Radon is mitigated by preventing its entry and if it does enter, by removing it to a neutral location where it decays (in about 7 half-lives – 25 days) into radionuclides that will disperse more effectively (safely) than the radon. Cellar ventilation and radon mitigation are separate design features.

  12. Jason Hyde, Peterborough 6A | | #12

    Ok, I think I understand now

    It would appear that you're saying EPA guidelines, for both passive and active systems, both which expel radon entrained in air via a stack at the roof line are just plain bad ideas. Moreover, the idea that radon is passively vented via the stack effect is optimistic at best. Finally, that if the sub slab environment is drained to daylight, even an active system would have problems depressurizing enough to vent the gas vertically.

    This appears to fly in the face of what I have found online. The guidelines for radon mitigation techniques here in Canada mirror those listed by the EPA.

    Your initial response has caused me to take a second, more detailed look at radon in general. Given what you are saying, which to me now makes sense, it seems strange that regulatory agencies recommend we vent at the roofline (or in some cases at a sidewall) where it will invariably come into contact with living things above grade. While it might be interesting to explore that train of thought, lets assume for now that they just got it wrong, and it's a bad idea. What now?

    I believe I now have a decent understanding of how to prevent radon from entering, or rather, how to minimize its entry. Free draining material as a base, a vapour barrier such as polyethylene under the slab and careful air sealing of slab penetrations. That makes sense, and most GBA readers are probably planning to do that already, for other reasons. It is also tempting to consider a scenario where the basement or slab on grade floor is slightly pressurized. This may be problematic for some homes, however.

    The idea of connecting an interior drain with an exterior one, which will eventually drain to daylight via gravity now seems to make more sense. Provided we omit the stack. It does raise the question of where the radon goes, but at least it is not into the home. In theory, if such a system was slightly pressurized, it would further facilitate radon being removed from the sub slab environment.

    As for removing radon that does make it inside the foundation (home), the only option I can think of is ventilation of some kind. I can see a few problems with this, as any ventilation system in a typical home would just dump the radon outside, or worse, recycle and distribute it. A quick search turned up this study, which suggests that radon cannot be filtered from air in an effective, or practical, way.

    You describe an ideal scenario where the majority of radon is mitigated at the slab, and the remainder is removed to a neutral location. Could you please elaborate? What would you define as a neutral location? A dry well of some kind?

    Fitch, please understand I am asking because I am interested, not to be adversarial. At risk of stating the obvious, you are suggesting that conventional radon mitigation techniques involving a vent pipe/stack are dangerous and/or ineffective. Furthermore, that we should be more careful where we do vent, or dispose, of radon to avoid contact with both living things, and ground water.

    I've never seen a detail that gives a second thought to where radon is vented or drained to, nor have I seen a system that did not use a vent/stack to the home exterior.


  13. Dan Kolbert | | #13

    Radon code
    I don't have the ability to weigh in on the science, but I can say that, at least locally, we don't have to install a radon mitigation system in new construction, but if we do, it has to comply with ASTM E-1465, which is the source for EPA's guidelines.

  14. User avater GBA Editor
    Martin Holladay | | #14

    Response to Flitch Plate
    You wrote that conventional radon mitigation systems are based on "a flawed assumption... [that] assumes vent stacks disperse radon or a mechanical radon mitigation system disperses it via a roof stack. If someone is doing that, they are crazy. One definitely does not want to entrain radon gas (put into the air) out a roof stack or anywhere near people and animals. It will cascade down the building exterior and into the bedrooms, playgrounds, wells, feedlots; etc."

    There is absolutely no evidence to support your theory. First of all, every radon mitigation system that has ever been installed in the U.S. includes a roof stack that penetrates the roof or runs alongside an outside wall. There is no evidence that radon from these stacks is "cascading down" in such a way as to cause risks to people in bedrooms or playgrounds.

    In fact, radon is a naturally occurring gas that can be found in outdoor air. It is present in such low concentrations outdoors, however, that it is considered harmless. The idea that a vent connected to subslab stone layer might contain radon gas in high enough concentrations that, even when it mixes with outdoor air, can result in harm to human health, is preposterous.

  15. Flitch Plate | | #15

    Confusing regulators with science
    Martin ... I am not going to argue with you on this point. You’re wrong. My field is radiation protection and nuclear contamination assessments. Radon is heavier than air, and it should not be entrained into the human breathing zone. Period. Bad EPA designs are bad EPA designs. Keep radon out with bullet proof foundation materials. Anyone who designs a solution that puts it above their children’s bedrooms is absolutely idiotic. Anyone who follows this advice is at risk. If you need a study to tell you radon is dangerous when it’s inhaled, and that it should not be entrained above human living space, then you've lost your common sense.

    Jason, your analysis/conclusion is correct on this. We are misled by the authorities. Mitigate by using prophylactic designs. Bleed off the contaminant into a well or outside-envelope, buried drain tile. It decays in 21 days. DO NOT use a roof stack to expel radon by stack effect or by mechanical device.

    By the way, Peterborough has its own special radionuclide problems as you know (tritium, and earlier days of Cameco/Eldorado/Zircatec waste, etc.)

  16. User avater GBA Editor
    Martin Holladay | | #16

    Response to Roger Berry
    Thanks very much for your comments.

  17. Roger Berry | | #17

    Radon Properties
    Having recently invested in radon mitigation, I feel it important to amplify upon Mr. Holladay's position regarding Mr. Plates radon contentions. Air dispersal of the radon gas as directed by EPA recommendations is completely logical and correct. The removal of radon gas through proper ventilation of mines has been standard practise for some time and ventilating to remove the "miasma" in the mines, was perceived as a solution well before modern science could even identify the elements behind the mysterious debilitations affecting miners. While it is true that the radon gas is heavier than air, it is equally true of dust, water vapor and many molecules and particles that can be air-borne. Clouds weigh tons and yet they float overhead freely.

    The daughter products of radon are all particulates, not gases, so the time to remove the offending element (radon) is in its gaseous state, which it retains for a statistical 3.8 days more or less. Once decayed it precipitates out onto surfaces and if that surface is a dust particle, it can then either accumulate undisturbed or become air borne and a bonus hazard to your lungs if inhaled. Smoking, which is inherently bad for your tissues due to chemical insult, becomes even more damaging with the addition of radioactive energy discharge provided by the radon daughter product hitching a ride. Smoking while cleaning out a 30 year old un-mittigated basement of dust could possibly be a great way to speed up one's exit from this earth.

    Regarding the swipes at various corporate malfeasance inflicted all over the United States, it is best to be very certain of the radionuclides being addressed. Radon is diffusely omnipresent and if an increased prevalence is found, typically it is related to uranium mining/processing or geologic hot spots. I moved from an area where nearby farmland had very "hot" wells. Iowa is noted for being a high radon region due to ancient geologic processes. Mischaracterizing the source will undermine any attempts to make corporations address the very real problems they cause. Being wrong on the science gives them the immediate advantage in dismissing your concerns.

    Lastly, the negative pressure method for mitigation, whether under slab or under a plastic barrier should make perfect sense. This site is devoted to the difficulty of sealing houses to manage air flow, so it should be a small leap to see why proactively controlling the subtle, but real air flow from below, would be the correct course of action. Mr. Holladay might agree as he has often mentioned the "magic arrows" in discussions of air management. Magical thinking based on a misunderstood property of radon would be a very bad choice. If you choose to direct your radon ventilation exit point further from your homes windows, then do so. It will only mean a more complicated system than needed with more back pressure. But please do not advise that anyone wait for the radon to slide out from under their home by gravity.

    A partial correction on pooling of heavier gases is due lest anyone under estimate the dangers of sewer gas or propane's ability to collect in low areas. Radon is present in very tiny amounts relative to the pool of other gases it encounters and is subject to molecular motion effects that would be different if it became the dominant gas present. On a global scale, gravity and pressure variables may well account for the shift in oxygen concentrations seen at the top of Mt. Everest and other high points. I appear to be guilty of the lack of science charge as I simply don't know for certain if that is the mechanism. So with a chastened addenda, I would still advise following established recommendations for radon mitigation. A typical home has a very active air system stirring things up with the furnace blower and duct work.

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