what to do with an insulated slab over the winter
I’ve given up on getting any structure up on my workshop slab, so now I need to decide what, if anything I should do to protect it over the winter. I’m in zone 5, close to zone 6. The slab is 36×50, 6″ deep with 12″ thickened edge. There’s R22 of insulation underneath, and R33 of insulation on the edge. Eventually there will be skirt insulation around it, but I have to grade a slope around the slab first. The concrete is 32MPa, 4-5% air entrained.
My primary concern is frost heaving, so getting the skirt insulation done is certainly a good idea, but is a lot of work. My house actually has no skirt insulation, just 12″ of insulation on the slab edge, and it’s been fine for 7 winters. So I’m wondering if I can get away with leaving the area around the slab uninsulated for one winter.
The secondary concern is freeze-thaw cycling on the top of the slab. I think the air entrainment will help with this. Should I even worry about it at all? If the risk is only surface level, aesthetic damage then I’ll just take my chances. I’m planning on putting a floor coating on it, and filling in some areas of spalling and whatnot won’t be the end of the world. The only mitigation method I can think of is buying more foam insulation to cover it, which would cost me about $1000, and I’d have no use for it afterwards. I looked into straw bales, but that’s more expensive than foam. Should I cover it with plastic, to keep it dry? I could put an ice rink on the plastic.
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I don't see any need to protect the slab from freezing. If you have good drainage underneath with a proper layer of stone, you shouldn't have any issues with frost heave. You shouldn't have issues with spalling, either, that usually takes a while to develop over many freeze/thaw cycles. The only thing I might consider doing is putting a tarp down to make it easier to clean off in the spring (just pull the tarp off and the slab is magically pristine instead of having to get bits of leaves and insect/bird debris cleaned off), but that's a little something extra -- it's not a requirement.
Bill
I agree with Bill as long as you placed your insulation and vapor barrier directly under slab. If you placed rock on top of foam I would cover to reduce risk of water entrapment. Also try to get a bit of slope away from footings. I wouldn't worry about frost heaving unless you have bad drainage and a lot of deep freeze/thaw cycles
This question was first asked a year ago and was the subject of a Q&A Spotlight:
https://www.greenbuildingadvisor.com/question/insulated-shallow-frost-protected-slab-with-no-building-on-it
https://www.greenbuildingadvisor.com/article/protecting-a-concrete-slab-through-a-long-winter
I had forgotten I'd been at the same stage a year before. How depressing.
I found this statement especially painful:
"Forming and placing a concrete FPSF is pretty fast—usually less than a week —so there may be little time-saving advantage to installing the foundation and slab before winter"
My contractor took four and a half months getting it ready to pour, and still wasn't done. I ended up finishing his job for him.
Trevor,
I don't understand what has happened. The typical time-frame from excavation to getting an occupancy permit for the houses I built was four to five months. Now as you say it seems to be a year or more. Everyone works at a good pace when they are on site, but then disappear for weeks on end. I'm baffled.
Trevor,
I don't recall if you're self-performing most of the work on your build or not, but if you are, a friend recently shared with me his method of estimating how long work on his own personal building projects will take...
It's similar to estimating cost, where you figure out how much it's going to cost in materials, double it, and that's probably in the right ballpark.
For estimating time, you estimate how long it's going to take, say one week.
Then you double it, so you're at two weeks.
And then - and this part is the key - you double the unit of measure.
So, one week becomes two weeks becomes two months.
That math seems to check out pretty well for all the work I've done on my own house.
I am doing most of the work (perhaps all the remaining work). I did most of the work on my house, so I have a decent idea on how long things take me. I'm pretty sure I could have done the work this contractor did in less time than he did, just using a shovel and a wheel barrow.
Two follow-up questions:
1. What is the minimum amount of skirt insulation I can get away with for my climate? The plan called for 4 feet wide. I think that's overkill. The materials I have are 21.5 x 63.5 inch panels. I have enough on hand to cut them in half and have a single row of panels just short of 32" wide (plus the 6" of slab edge insulation). This is my preferred option, not only because it means not buying more materials but also because it means one less seam I have to join (if I did it in two rows along the long edge of the panels).
2. This one's pretty far off topic for the site, but is there any reason I can't use clear 6 mil poly for an ice rink? The recommendation from the rink building community is to avoid it, but the reasons given are that it (and dark tarps) kill the grass and absorb IR radiation from the sun. Being on concrete negates the first argument completely, and concrete is only slightly darker than a white tarp. Will the clear poly degrade under UV exposure? I only need it to last one winter.
Yes, clear poly will degrade from UV exposure. I don't think you'll have any problems with it lasting through a single winter though. When used as a cheap greenhouse film, you can usually get a few (maybe 2-3) seasons out of the stuff. It doest get a little "frosty looking" after a while, and eventually it will start to tear more easily, but you aren't going to see that to any significant amount over a single winter.
Regarding your question #1, I think 32ish inches out (I'm assuming you're going out semi-horizontally here) is probably going to get you most of the benefit you'd have seen with more, but I don't have any numbers or studies to back that up. I have usually wanted to see the skirt go out about the same distance as the frost depth is deep.
Bill
Just make sure they are to code, r403.3 has a table with horizontal r value and distance required.
I'm having a hard time figuring out which air-freezing index to use. I can't find an equivalent table for Canada. I found a site that shows the "freezing degree days", which is supposedly what the air-freezing index is based on. However, when I look at the map, the numbers I'm seeing don't really seem to make sense. My location has a 100 year average in the ballpark of 500C (930F). That's well below the lowest column on the chart, which covers places like Alabama and California.
If I use the nearest US state, Michigan, and pick an inland county at the same lattitude as I am, it's an index of 2000, which indicates no horizontal insulation is required at all. Most areas of the US appear to require no horizontal insulation, according to the charts, which strikes me as odd.
Even in the worst case, 24" is the widest listed, so I guess I should be fine using 32"
Ya the highest is 36” at the corners. Here is a link to a paper that has an air freezing index map for Canada see fig 2. It should get you in the ballpark, it works like a topomap, pick you location and see what lines you are between/on.
https://onlinepubs.trb.org/Onlinepubs/hrr/1963/33/33-008.pdf
That aligns with the other data I saw, being <1000. It's also worth noting that the number of freezing degree days has been declining precipitously with global warning, and the data for all these charts is pretty old. The index for my location over the next 30 years is projected to be 660F. Based on this, any amount of horizontal insulation is overkill. I think I'll just put one row lengthwise, so 21.5".
Ya I’m not sure where you are but only a very small section in Canada is 1000. If you are in Ontario, (near Michigan) most of Ontario is over 3000
According to the map you linked, only the part north of Lake Superior is over 3000. The horizontal line that meets the eastern edge of Lake Huron is the 1500 line. I'm in line with the northern tip of Lake St. Clair, which is below the 1000 line. Area-wise, I guess more than half of Ontario is above the 3000 line. Population-wise, over 90% is below the 1500 line.
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