Retrofitting rigid foam insulation to a slab foundation?
Hey, gang,
I’m hoping to improve efficiency in my friend’s small-footprint (~850 sq.ft.) slab-on-grade one-story house. I’ve done various energy efficiency remediations in it elsewhere, but began lately to wonder about insulating the exposed slab around the perimeter. The slab is contained to the house footprint (no patio extensions or whatnot).
If I were to consider installing a stiff foam (board) insulation, what suggestions might you have about details? I assume I’d have to dig out enough to reach the bottom of the perimeter edge of the slab. I doubt I’d be game to take on digging underneath the slab at all. What sort of foam board would be the best choice? Most durable? Polyiso seems too fragile; blue or pink board seems better, but isn’t impervious to damage. What kind of protection should I consider for its exposed surface? Solar UV seems dangerous, as does mechanical/impact damage.
Thoughts?
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Replies
Benergy,
First, it is important to know what climate zone your are working in. It is also very important to know what the other upgrades you have done are. Better windows? Upgraded attic insulation? If you are in CZ 4 or lower the foundation losses are minimal and you might be building in a termite problem for your friend.
If the house has under slab insulation in place then a case might be made for doing a short wall insulation wrap combined with wing insulation ala' a frost protected shallow foundation. A prime article exists to explain:
https://www.greenbuildingadvisor.com/article/frost-protected-shallow-foundations
As a post construction effort there will be many details that may have you pulling your hair out, so think carefully before committing. The greatest losses for a slab occur below the sill plate and above the finished grade. Wrapping the exposed vertical parts will help, but you don't need to go all the way to the footer if using the wing insulation technique to temper the ground under the slab and outward beyond the stem wall limits. Your climate will drive the depth and total R for useful gains. Code will also need to be followed for your locality. Your downspouts and general drainage patterns will also need to reviewed.
Polyiso is not suitable for ground contact. Blue or pink XPS is better than EPS in my opinion. Much arguing is done about this. Insect resistant EPS is available, not sure about XPS. Recycled foam can be an affordable way to not carry on the GWP penalty created by foam. New lower GWP foams do exist and frankly the national obsession with SUVs and giant pickups probably do far more damage than all foams.
Protecting the foam from physical impact is a problem that has numerous answers, most tedious and labor intensive. Raw XPS typically fairs better than EPS when exposed to sunlight. Aggregate coated panels for the vertical (generally EPS) might save a lot of fuss. The problem with all added foundation foam will be the belt line junction with the house's cladding. Unless you have exterior insulation on the house now, there is likely to be a significant mis-match between the house cladding and the final protected foam face. Flashing and insect protection will be key here. Critter control is a wild card that varies with whatever critters you have.
Hope this gives you some food for thought. You are planning a big task that might not have sufficient reward.
Thanks, this is very helpful.
The house is in CZ4, inland-coastal OR.
It was built in 1979, and is part of a working-class neighborhood of not-very-well-made housing. So I very much doubt there is under-slab insulation.
I'm guessing you're right, it may not be worth the trouble to take on this as a project.
The house has vinyl siding (with 1/2" R3 foam boards underneath), and the slab rises above grade and is easily visible in most locations around the perimeter. So access to cover it would be relatively easy, though I'd still have to dig a bit to reach the bottom of the slab. I would definitely not dig underneath!
Protection of the foam seems key. The site stays relatively dry (no pools/puddles in rain that I know of), but that doesn't mean the foam could not be saturated by some rain event in future. Worth the trouble? Hmmmm......
You can't use polyiso underground like this, so it's out. You DO NOT want to try to EXCAVATE UNDER THE SLAB. Trying to do so will probably cause failure of the slab, or at least a lot of cracking.
I would use XPS on the exterior. XPS typical holds up best when exposed to water long term. It will need physical protection above grade. I recently saw a textured cement board product made for this purpose, which gives you a sort of facade-like exterior cladding. There are other ways to do it, but you do have to provide protection from both sunlight (UV), and critters that would try to chew it. Rigid foam isn't very durable by itself.
Ideally you want to insulate down to the frost line, but you can put a "skirt" going out away from the foundation if you don't want to dig as deep. I'm not sure if that saves you any effort though overall. My preference would be to dig down to the footing, and insulate the entirety of the exterior of the foundation wall if you want to insulate on the exterior. If you have a thickened slab type of arrangement and not a seperate footing (not sure what you're working with here), then I'd dig down to the bottom of the thick part of the slab and use a skirt. DO NOT try to dig UNDER the footing. You do NOT want to introduce settling issues with the footing.
I would use EPS above the slab with a plywood or OSB subfloor over the top. That's really your only practical option to insulate the slab.
How much R value you use depends on your climate zone (use at least code minimum for your CZ), and your budget -- sometimes adding more than code is a good thing.
Bill
Thanks, Bill.
I'm glad to read these thoughts and details.
It's in CZ4 inland/coastal OR, so frost is not a big issue here.
I'm not sure about the slab type or dimensions. 1979 low-cost construction in a tract of similar, smaller houses. I can see the slab pretty much all the way around the perimeter, as it rises well above grade; but I don't know how far down it goes, nor to what type of footing.
I'd prefer XPS for durability, but apparently stone-clad foam boards are only available in EPS? Not sure about that.
I'll keeping considering options.
Cheers!
Ben
There are different grades of EPS based on density. Higher density material is more durable, but more expensive, and also a bit harder to find to buy in some cases. One advantage for 10 and 25 PSI XPS is that those are both commonly stocked at many box stores, so that stuff is easy for the typical DIYer to buy.
EPS can work underground, but I prefer XPS there based primarily on a study by the Alaska DOT that showed XPS to take on less water over time, and thus preserve it's R value a bit better.
Bill
Benergy,
I looked up inland coastal Oregon on a state climate map and found a region that includes Tilamook and Hebo which have basically the same yearly range of temp and precipitation. If you fall in this very appealing climate zone then the gain for effort is not going to be worth a whole lot. I see that the yearly range is roughly 70F to 36F and at that the December range is 50F to 36F. A bit rainy but lovely would be my assessment. If the higher summer range you have experienced in recent years becomes a feature and not an aberration then the number of days with a delta T over 30F (relative to 70F interior) do not appear to dominate.
Short version: I would look to upgrading the heating equipment if not already done.
TL/DR version: The walls are likely to be 2x4 with R-13 batts and effectively R-13 or less despite the "R-3" under the siding. With approx 1000 sq ft of wall area the heat loss would be roughly 2800 BTU/hr at a delta T of 30F. (I am ignoring windows) The foundation edge exposed to air would generate about 1800 BTU/hr assuming a one foot exposure, R-2 for the concrete and same delta T. Remember wrapping the slab edge with foam will mediate the exterior range high and low, but not be of much value on the ground side where the temp will average in the mid to high 50's. The best you can get there will be a constant delta T of 20F which is a loss in winter and a marginal gain for summer cooling though you seem to qualify for no A/C to be comfy.
The interior 850 sq ft of concrete floor is a constant drain to ground without the under slab insulation. Even with some long term soil temp rise it would be very unlikely that the delta T would drop below 10F. Rugs or carpeting would help make the floor more pleasing underfoot because they won't feel cold like bare concrete or vinyl over concrete. I am retracting the window upgrade suggestion simply because the temperature range you appear to be in can't support an ROI. Unless you have single glass units and not double pane the gains aren't enough.
If you do live where it is cold longer you can use the above assessment to judge how much more thermal value would be provided. Still, you might find that an ASHP upgrade to be a good choice. The major names are offering units now that easily survive the Tillamook climate. Maybe there are rebates at state or federal level that you can access to reduce costs.