Hey Folks, Going to be pouring a thickened edge concrete slab this summer to turn our covered porch into an enclosed part of the house. Space is approximately 11x32. I'm figuring about 4" thick for the main part of the pad - about 12" thick border for the thickened edge. How much insulation do I need under the 4" part of the pad? Also - since I have no hydronics in the house - I'm looking at simply putting electric radiant over top of the concrete to provide heat for the room. Good idea or bad idea - anything better to use? Would it be cheaper to use a small dedicated electric boiler over electric radiant? Room will be pretty much all windows - so a relatively high heat loss, but it doesn't need to be at a balmy 20 degrees either. Sean.

You obviously need a lot more foam under the slab if your subsoil is permafrost than it if it is 20C sand. Location matters- care to share?

An insulated stem-wall (poured in an ICF) on a footing and a floating slab with foam under it, thermally broken from the stem wall is less likely to crack than your grade-beam approach.

To come up with the appropriate heating solution for the ~350' room requires doing a heat load calculation on it. An electric boiler/hydronic solution might be overkill. A half-ton mini-split or PTHP might be a better solution, again, depending on your location, heat load, and 99% outside design temperature , etc.

Ugh! That was in my head to type, but never made it to my fingers... You can take the location as Ottawa, Ontario, Canada. Sorry!

Sean.

At Ottawa's subsoil and 99% outside design temps R15 under any heated room, and R20+ under a heated slab makes lifecycle economic sense, (especially when the heat is from an expensive source such as electricity.) You can't put the foam under a grade beam or footing without doing the engineering on it, but under a residential slab that isn't supporting the weight of the exterior walls + roof it doesn't need advanced engineering.

Using reclaimed foam from building demolition & commercial re-roofing takes the sting out of the wallet for that much foam. DON'T use polyisocyanurate under slabs- it can become saturated and lose performance, but EPS or used XPS is fine. Derate any XPS to R4.2/inch, which is it's performance point after it loses it's HFC blowing agents. Type VIII (1.25lbs per cubic foot) EPS runs about R4.1/inch and is commonly used in commercial roofing applications. Type-II EPS (1.5lbs nominal density) runs R4.2/inch, just like fully-depleted XPS. While it's theoretically OK to use 1lb density Type-I EPS (R3.9/inch) under slabs it's not really a great solution. The sheets are easy to break in handling, and the intersitial spaces between the macroscopic beads are large, causing it to under-perform in damper soil conditions. Any slab should have at least 4" of clean gravel under it for drainage,, which should be protective of even Type-I EPS, but I'm personally a bit skeptical of using it under slabs.

It may take some investigation to find foam reclaimers in your area. They sometimes advertise on craigslist and other low-cost venues, eg:

https://montreal.craigslist.ca/search/sss?query=rigid+insulation

Have you attempted to calculate the actual heat loss of that lossy room? Are the windows Low-E + argon, clear glass double-panes, leaky wood sash double hungs with leaky clear glass storm windows, or... ???

Thanks for the info. The windows will be brand new - PVC frames, double pane, low-e, argon filled with triple weather stripping. I've been slowly replacing the whole house with these.

Excellent suggestion on the reclaimed insulation. Found a listing in Ottawa for 4" - R20 sheets (pink) 24"x96" for $25 each on Kijiji.

I have not calculated the actual heat loss for the room at this point. It's ok if the floor isn't enough to fully heat the room. If it takes the chill out of the floor and partially warms the room, I don't mind topping off with small portable heat pump that would serve to cool it in the summer as well.

Sean.

The 4 "pink sheets should be derated to R17 at full blowing agent depletion.

At $1.56 per square foot it's still a bit pricey, even if you took it at full R20 face value it's 7.5-8 cents per R-foot. That's about the same price as virgin stock Type-II EPS. If the price is negotiable try not to spend more than $15 for 2' x 8' x 4" for used or factory-seconds XPS ($10 would be more like it.)

Wow. Prices must be massively different in the US than Canada... I can't even get the 4" stuff at our local Home Depot. The 2" stuff is already $33 for a 2x8 sheet. That's where I figured $25 for a 4" sheet was actually pretty good.

Appreciate the perspective. I'll keep looking around (new and reclaimed) and see what I can find.

Thanks!

Sean.

Home Depot in the US doesn't even sell Type-II EPS, but has a few overpriced Type-I EPS options. $33 for a 4'x8' sheet is $1.03 per square foot, divided by R7.8 Type works out to 13 cents per R-foot, which is quite a bit higher than what Type-II EPS cost at distributors catering to commercial builders or direct from the EPS blower. That isn't too different from US pricing at that chain.

A 2" x 4' x 8' sheet of crummy Type-I EPS at the US-midwestern box store Menards is ~$13:

https://www.menards.com/main/building-materials/insulation/insulation-panels/expanded-polystyrene-foam-insulation-2-x-4-x-8-r-8/p-1444435971902.htm

Assuming it's really R7.8 (not rounded up to R8) that's ~41 cents per square foot, ~5.3 cents per R-foot, which isn't the same retail gouge for Type-I EPS seen at some other box stores.

Being 1.5x denser than Type-I, Type-II EPS is more expensive than Type-I, but usually not 1.5x more. Even multiplying the Menards box store pricing for Type-I by 1.5x would only be 8 cents per R-foot. Distributor pricing would be a bit less than that.

If you can't find a foam recalimer with the right stuff at the right price, see if there are any EPS blowers manufacturing sheet goods near you. You can probably order up some 4" Type-II from companies like this one in the Toronto area(but after shipping it wouldn't necessarily make sense in small quantities):

https://www.foamconcept.ca/en/products/insulation

There are almost certainly building supply companies usually catering to commercial construction that carries Type-VIII (1.25lb density roofing EPS, which is fine) and Type-II goods at more competitive prices than HD. Commercial roofers might even have some reclaimed goods in the back shed they'd part with at a decent price.