Sweating Summer Slab Mitigation.
Hi folks,
I’m a carpenter here in Maine. I have a client whose lakeside camp (summer cottage) is built on a concrete slab. No flooring, just carpet on the concrete. He complains that in the summer, water condenses on the slab making his carpet wet, and probably causing damage to the sill plate.
It looks to me like this slab was poured without any stem walls, either directly on the dirt, or more likely, a combination of earth and shale. What’s the best way to mitigate the problem at this point? I believe the moisture is coming from interior condensation since this occurs in the summer (the cool thermal mass of the slab collecting moisture). That said, I can’t be 100% certain since there appears to be no vapor barrier.
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Your choices are either lowering the dew point of the interior or raising the temperature of the floor.
Lowering the dewpoint means either running a dehumidifier or air conditioning. This probably won't be popular with the clients because the reason they want a camp in Maine is to be some place where they don't have to sit in air conditioning in the summer.
Raising the temperature of the floor means insulating it, which probably means building it up a few inches. In finished space this can be really disruptive.
Yes exactly, thank you. I suspect the "right" way to do it is jacking up the house and adding rigid foam, two layers of Advantech sub floor, and finally finish flooring on top of the slab. Obviously a major undertaking.
I'm temped to do all that but don't have full confidence in the existing slab. Temped to jack up the house, jackhammer the old slab and pour a proper a frost protected shallow foundation with rigid foam on the perimeter and under the slab. The issue there is the shale, which is why they probably didn't properly prepare the ground for the original pour. Any thoughts or alternative approaches would be greatly appreciated from you men of knowledge.
Get rid of the carpet and see how it performs for a summer, noting when it sweats, if it does without the carpet.
It kind of depends on how the house is constructed and finished, but jacking the house up is a pretty extreme step. Usually if you want to add to the floor you just reduce the floor-to-ceiling height -- although that plays havoc with door heights and the stairs if you have multiple stories. I guess with simple construction jacking up one side at a time isn't that big of a deal.
How long has the house been there? If it's been there a while that's evidence that the foundation is solid. If the shale is bedrock it's not going anywhere.
Yes, there is a second story. I can't see laying four inches of rigid foam without raising the house. It would look crazy.
I designed a renovation for a house built like that near me, in Freedom, ME. The soil was actually washing out, leaving one corner suspended in air. I recommended excavating the exterior enough to lay 2" foam at an angle below grade and vertically above grade. There was not enough headroom to do much on the interior but I recommended a dimple mat, 3/4" sheathing and new flooring. They decided that the cost wasn't worth it.
Thank you Mr. Maines! Just to be clear so I can explain to the client--the purpose of the rigid foam is what exactly in this case? My understanding of rigid foam on the exterior of a slab (as in the case of a FPSF) is to keep the ground around/underneath the slab from freezing. Since this is a summer cottage, it won't be heated in the winter. Does it still make sense to add rigid foam wings/walls? Will that reduce slab sweat in the summer?
If the building is unoccupied in the winter the ground underneath it is going to get quite cold. In the summer, particularly in early summer, that slab is going to be quite cold as well. Condensation forms when air hits a surface below the dew point of the air. Air at 72F and 50% relative humidity is going to have a dewpoint of 52F, which could easily be warmer than the slab temperature at the beginning of the summer. Where you are it wouldn't surprise me if you regularly get dewpoints in the 60's in the summer.
The thing is you don't need much insulation, a half inch would probably be adequate. I could see doing a half inch of insulation, a half inch subfloor and 3/8" for a laminate floor. Can you find 1-3/8" that you can take away?
If you are going to lift the whole building, six inches is as easy as one, I'd put in a framed subfloor. This makes it easier if they ever want to winterize in the future. Although in Maine I'd imagine an on-grade house would be inaccessible in winter due to the height of the snow. So maybe you lift the whole house three feet and put in a crawlspace.
Have they priced tearing down the whole thing and starting over to build the house they really want?
Interesting. If's it's really as simple as adding a dimple mat or 1/2" rigid foam, maybe they could get away with not lifting the house. DC, do you have an opinion about whether exterior rigid foam wings/walls (as MM suggested) would make a difference in this case? Would it potentially keep the slab at 50° in the winter months?
Lol have not priced tearing the whole thing down and rebuilding. This is just a little camp with sentimental value. Owner recently purchased a retirement home on the ocean, and that's been his main focus.
Unless the house is heated through the winter it's going to get wicked cold inside. Insulation isn't going to keep the slab warm through the winter.
Dimple mat isn't the right product, it doesn't have any insulation value.
Here's the calculation: you want the surface of the floor to stay above the dew point. Let's say the slab is at 50F, the dew point is 60F, the inside temperature is 75F, I think that's typical summer conditions. So you want to keep the floor above 60F. Any surface in a house has a layer of air that is about R-0.5. If you put a half inch of foam that's probably R2, so that plus the air layer is R-2.5. The difference between the slab and the air is 25F, there's going to be a temperature gradient through the assembly of 1F per 0.1 of R-value (25F over R-2.5). So the surface will be 5F below the room temperature, also 20F above the slab temperature, or 70F. With a dew point of 60F that 70F is safely above the dew point.
Amazing. Thank you for that thorough analysis, DC!
Here is a very high rvalue product that might be useful if you can get around the compressive strength of only 23
https://www.kingspan.com/us/en/products/rigid-insulation/insulation-board/optim-r-panels/
They have a fanfold underlayment product in thicknesses of 0.2" (R1) and 0.3" (R3). That might be enough insulation to prevent condensation.
Might.