Insulation under slab with radiant heat
I have seen on this site it’s highly recommended to place the 10-15mil vapor barrier directly under the slab and then the EPS or XPS below the vapor barrier. My contractor has concerns about doing this as they normally staple the radiant heat tubing to the foam which in this format would put many small holes through the vapor barrier. I called both Dorken and Henry technical support as their products are the two were considering for a vapor barrier and both recommended install the XPS or EPS directly under the slab and then the vapor barrier so the vapor barrier was directly over the aggregate. The only difference in their advice is Henry recommended EPS as the insulation and Dorken recommended XPS. So now that I’m utterly confused on which way is best, how do you decide? Do you just go with whatever the builder is more comfortable with (he recommended the way Henry and Dorken recommended). Also, does it matter if they use XPS or EPS if the seams are taped?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
When I did it[20 years ago] I wanted the tube in the floor not underneath it, so it was attached to the wire reinforcement
This has come up before and the experts say that a vapour barrier can have perforations, even a lot of perforations, and it will perform its function adequately. You definitely want the vapour barrier above any aggregate. As for being below the insulation, I think the concern is for the foam floating up during the concrete pour. I don't know how likely that is, but if it was me I'd be putting the vapour barrier above the insulation.
EPS is exponentially less environmentally damaging that most XPS*, and is cheaper. There really isn't any argument for using XPS. Even the R-value ends up being about the same, after the entrapped gases in XPS leak out over time and contribute tons of CO2 equivalent to the atmosphere.
*there are greener versions of XPS, but you would have to specify it, and it still costs more than EPS. By default I think most contractors are going to use the worse stuff, because it probably saves them a couple of bucks and they literally don't care about anything else.
Trevor,
"...the experts say that a vapour barrier can have perforations, even a lot of perforations, and it will perform its function adequately."
The complication comes when it is also expected to be part of a radon mitigation system, where continuity is important.
is the new compliant XPS much more energy friendly and closer to EPS in terms of environmental impact? i know that there is state-by-state rollout depending on differing state law.
https://www.dupont.com/content/dupont/amer/us/en/microsites/beyond-blue/styrofoam-brand-st-100-series-xps.html
No the HFO-based XPS is still worse than EPS, up to 4x as much embodied GHG depending on manufacturer. The old HFC-based XPS was like 100x worse so at least they are making some progress. Polyiso is also an excellent option which we found out in a recent Q&A post that it is acceptable for under-slab usage and doesn't necessarily hold any more water than EPS or XPS.
Edited to Add:
Reference: Dupont XPS = 6.2 kg CO2e/m^2/RSI-1 (https://www.beyondblue.dupont.com/compliance.html)
Atlas EPS = 2.79 kg CO2e/m^2/RSI-1 (https://www.atlasmoldedproducts.com/uploads/resources/Environmental/EPS-IA-Insulation-EPD.pdf)
Atlas Polyiso = 4.29 kg CO2e/m^2/RSI-1 (https://www.atlasrwi.com/resource-folder/wall/General%20Resources/Miscellaneous/PIMA-EPD_Polyiso_Wall_Insulation_Boards.pdf)
John Siegenthaler recommends tubing is to be within 2 inches of the top of the floor-roughly in the middle of the concrete. That allows the concrete to respond quicker- you get more useful heat from the glycol mixture. Some of the new foam products / Neopor have interlock and are rated as vapor barriers. You may to look at Subterra-plus and tape the seams.
Our contractor seems pretty set on using the system they use all the time. I’m guessing it may be because they put it in every home they build and may have a lot pre purchased. He suggested what I wrote below. I’m not sure if this is a good comprimise?
Part of the reason we were avoiding foam on top is that some said the foam could indefinitely trap water below the slab. But the more I’m researching it seems that’s debatable and some think the only really concern is making sure it doesn’t float during the pour. The moisture was our biggest concern.
Thank you everyone for your replies. My contractor said the vapor barrier is to be a part of the radon mitigation system, so if we want a heated floor we can’t staple through it. He said if we are sure we want foam on the bottom layer to keep the vapor barrier from the aggregate below, he suggested adding another layer of foam on top so it’s creates a sandwich (foam, vapor barrier, foam). Would this work?
Also, my understanding is the concrete person who works with them knows this is how they build and puts weight on top of the foam with rebar or wire to weigh it down during the pour so it doesn’t come up. Is that the only concern? It sounds like permanent moisture in the foam leading to moisture issues isn’t really something that would happen?
Do not put foam between the radiant pipes and the slab. It would reduce the heat transfer efficiency of the radiant system. You can just put the foam over the vapor barrier and it will probably be fine. It’s not the ideal way though. What will your finish flooring be? Is using something like Warmboard over the finished slab an option for you?
The foam itself is impervious to moisture. The job of the vapor barrier is to keep the moisture from rising into the living space where it might cause issues. What they're proposing sounds OK.
Concrete doesn't make for a particularly good heated surface. Ideally, you want something that responds quickly to changes in heating load, which means high conductivity and low heat capacity. Concrete has high conductivity but pretty high heat capacity. A product like Warmboard has low heat capacity and gives better comfort.
The problem is cost, Warmboard is really expensive. Concrete is done because it's cheap.