Insulation requirements on top of a basement slab for a hydronic radiant floor heat system
I’m retro-fitting a 1968 rambler in Minnesota, Zone 6B (it gets very cold, down to -24F next week, Yay!). The plan includes switching from 2 forced air furnaces to a whole house hydronic in-floor radiant heat system. The house has a finished basement with a concrete slab. I don’t have a ton of inches to spare for increased floor height, preferably the whole floor system would add 2″ or less, including insulation, pex and finished flooring (pex is 5/8th exterior diameter, finish flooring is about 1/2″, which leaves about and inch for insulation).
The question is; Is that possible? and what would be the best type of insulation product to use for this application?
Thanks!
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The bible for hydronic heating system, Modern Hydronic Heating by John Siegenthaler, has this equation:
R = 0.125*(Tin-Tout), where Tin and Tout are your inside and outside design temps.
So if you like 70F inside and have an outdoor design temp something like -14 (Duluth's 99% design temp), that works out to R-10ish, which is pretty standard. 2" of XPS would give you that. Thinner is less ideal but as long as there's something - even just 1" - separating the tubes from the slab it'll be better than nothing at all.
It is possible, but not ideal. The best way to maximize the total insulation value of the floor below the PEX tubing is by utilizing insulating radiant PEX floor panels, as that allows for insulation between the PEX tubing as well. They come in a few thicknesses, here is one supplier as an example:
https://cdn.shopify.com/s/files/1/0824/6585/files/LIT-1058-Radiant-Comfort-Spec-Sheet-021717.pdf?11202719387897575189
If you use the new construction option, I believe you can install your flooring right on top of it, for a total floor thickness of 2.5" and a decent R value (They claim R12, but the real value is probably closer to R8-10, less directly under the PEX). Otherwise you can use the retrofit panel, and maybe eke in an extra 1/2 or 3/4 inch of rigid foam board underneath if you are so inclined.
However, on the off-chance you are using an electric resistance boiler, there is little point to using a hydronic system -- you'd be better of using electric resistance floor panels, which are more responsive, allow for thicker insulation, and don't have a pump load. Even if you were planning on natural gas or some other fuel source, it will probably be much cheaper to use a properly-sized furnace or ductless minisplits for your primary heating load and supplement with electric floor panels in key areas.
Andrew,
A reasonably insulated basement doesn't need much heat, certainly not as much as doing a full radiant slab.
I would figure out where you want the floor hot and install radiant panels there with as much insulation underneath as you can comfortably put.
By not doing a full slab you can save a lot of money and have a more comfortable space (a full size radiant slab will never feel warm in a basement application)
I've used a panel system with 5/16" pex pipes (maybe Upnor) for a basement bathroom that was quite thin. With limited height there was only room for 1/2" of insulation, not ideal but works. The entire assembly including ceramic tiles was around 1.5".
You might consider radiant ceiling panels instead of a radiant floor. You'll still want as much insulation as you can get between the floor surface and the slab. You might consider graphite-infused EPS insulation, which has a little higher R-value than EPS, while avoiding the severe climate impact of XPS. Neopor is one brand name.
Thanks everyone, I'm going to try for 1" -1.5" Neopor.