Where Does the Plastic Go in a Radiant Slab Assembly
This video is an excerpt from the Building Science Fundamentals class taught by Dr. Joseph Lstibureck, P. Eng., of buildingscience.com and founder of Building Science Corporation, back in the day. This segment focuses on where the sheet of plastic goes under a slab. Does it go above the rigid XPS foam insulation or below the rigid foam?
TRANSCRIPT:
Hydronic insulation and stapling question
Betsy: Joe, can we go back to the basement for a minute?
Joe: Yeah, ask away.
Betsy: Okay, Charlie asked a good question here. Do you have a section with hydronic and insulation with the poly on top of the insulation? From a building perspective, the radiant guys like to staple the tubes to the insulation, so we’re trying to understand how to execute this.
Joe: That’s really easy. Have them staple through the poly to the foam. The poly doesn’t have to be continuous—it can have a gazillion holes in it, because the air barrier is the concrete.
So, the short answer is—if that was their big issue—hey, staple up the yin-yang wazoo! It’s okay to have staple holes in the goddamn poly.
Whoa—if that was it, that’s what pissed you off? Man, we could fix this!
Poly, air barriers, and vapor diffusion
Charlie: Yeah, I mean, sorry to keep going on this, but from my understanding, the poly—or the barrier—is meant for radon gas and things like that.
Joe: No, it doesn’t work for that. The concrete is the air barrier. The poly was never meant for radon.
I’m not saying you’re full of it—I’m just trying to tell you.
Charlie: Okay, yeah, absolutely.
Joe: There’s a bad joke: concrete is a really good air barrier—just ask Jimmy Hoffa.
What the poly is for
Charlie: So then what the hell’s the poly for?
Joe: Vapor diffusion.
Betsy: Explain why vapor diffusion works even with holes.
Joe: Okay, let me explain why vapor diffusion still works with holes. Thank you, Betsy.
I’ve got a thousand-square-foot concrete slab, and I’ve got a thousand square feet of poly. Let’s say I wear golf shoes and I spend an hour poking holes in the goddamn poly. What’s the surface area of the poked holes compared to the total surface area? Holy—five percent.
Vapor diffusion is a function of surface area, so I’m reducing the effectiveness of the poly by maybe five or ten percent. Who gives a damn?
Now, if I’ve got five percent poked holes in the poly and I’m trying to have it act as an air barrier, I’m doomed—because I’m going to have a thousand times more moisture flow through a hole carried by air pressure differences than by vapor diffusion. That’s the difference.
So, the concrete is the air barrier. The poly, even with rips, holes, tears, and punctures, is the vapor barrier.
Radon control
Remember the drywall example—the idea I took from Mylene Rousseau, the magnificent, genius, wonderful engineer out of the National Research Council of Canada. I took it to Atlanta, so we covered Ottawa and Atlanta.
Anyway, going back, Charlie, you asked what the radon control should be. That’s really easy.
You basically put a perforated tube in the stone, run it below the slab and below the insulation, and then run it internally up through the entire roof structure. You run it internally because you want the heat from the house to create a chimney, or stack effect. That’s how you handle it.