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What is the best way to insulate the perimeter of a radiant slab?
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John: Up here in Kasilof, Alaska, everyone insulates around a radiant slab (or any foundation for that matter) with 2 inches of rigid foam. The difficulty is in how to cover the foam. Most people just leave it exposed, but there is a willingness to be more architectually-challanged up here than in, say, Hillsborough, CA. The debate is about whether or not to insulate UNDER the slab with an increasing tendency to do so. The two camps are: Don't insulate because heat rises and the conductive loses will decrease with time as the ground warms. Or: Insulate underneath because there'll always be some heat loss to the ground below and the system will be more responsive if it is heating up less mass. To which the don't insulate crowd might respond that radiant slabs aren't reponsive in the first place, it's advantage is the steady temperatures it gives and that more thermal mass can be good thing if there is a power failure and you don't want your pipes to freeze.
On our detached in-law place being spec'd now, I'm going to 1) insulate the perimeter with 2-inch foam, 2) not bother covering the top portion of that insulation, and 3)not insulate underneath (but that's a close call). -David
*The issue is not whether or not the size of the mass matters as much as how fast the heated slab will cool down after the heat source is removed.If you have 2" styrofoam under the slab then the slab is less likely to be effected by any influence of the ground mass below it.Simply put the slab being a poor thermal resistor will perform better if you have the insulation below it.Gabe Martel GSC Project [email protected]
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You might check out http://www.radiantec.com
They suggest 2" 4X8 sheets below the perimeter of the slab, 1" 4X8 sheets inside that, and none in the center. Sounds good to me
*On my own house here in Minnesota, I used 2" R-10 polystyrene on exterior of poured wall foundation and kneewalls (installed all the way to the footings), and 1" R-5 T + G polystyrene under the Wirsbo radiant system in my basement. I also insulated interior framing walls, as well.The decision for me was one of cost. In a big house, the stuff is not cheap. And I felt that the 1" polystyrene would provide enough of a thermal break between slab and ground. But we also have 52" frost lines here, too. If you live in a warmer spot, you may be able to get away with less. I also used the 1" poly under the slab to protect the vapor/radon barrier I installed from the cement crew. This worked well. They busted up some of the sheets of insulation, but it protected the vapor barrier from damage. I agree that the key to any radiant system is thermal mass -- so keeping that thermal mass as warm as possible is always a good thing. I would not, however, worry about PEX tubing freezing in a radiant floor -- it would take a long time to freeze the floor enough to be concerned with frozen pipes. If you do worry about this, consider adding some antifreeze to the system. Antifreeze may shorten boiler life, however.
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In Island County, WA perimeter insulation is permitted only on a slab without radiant heat. If radiant heat is used the slab must be fully insulated underneath, with 2" foam.
*Right on Gabe!!
*On our house we did as David has suggested, but we parged the foam that was exposed above grade with a fiber-cementaceous product called block-bond which was easy to apply with a trowl. (My wife did it on a weekend). It looks good and comes in a couple of colors (avoid white). If you don't do this, the stryafoam will degrade in the sunlight, and will become powdery and will lose a 1/2 every couple of years.Of course David is in Alaska where the sun is gone this time of year, and makes a brief, near-horizon appearance during the summer months, so he doesn't have to worry about UV exposure. BTW David, we have been enjoying our wood-fired hot tub, but still can't match the record number of folks you packed in yours with or without clothes....
*On my new radiant heated home, the cost to insulate the entire slab with 2" rigid insulation was $1300 and the cost to insulate the perimeter only (4 feet) was $720. I wanted to insulate the entire slab but the payback on this $580 difference was far too long to justify the added expense. I spent the money adding more attic and wall insulation where the payback is quicker.
*Jim,Is not b payback....the most b universally used, individualistic terminologyto set b argumentative discoursein b artificially createdlogical context?No?Amorphously semi-disagreeing,Jack : )
*I have used 2" or thicker on the exposed part of the foundation and to 2' below grade and then 1'' for 1 to 2 feet out from the foundation and 1/2" under the slab mostly to protect the poly. The deeper you go below grade, the less thickness you need because of the r value contribution of the soil itself so you can taper the thickness (in steps) as you go down.
*Hey Ron "The deeper you go below grade, the less thickness you need because of the r value contribution of the soil itself so you can taper the thickness (in steps) as you go down" What have you been smoking. What pray tell is the r value contribution of the soil at any depth. Once you are below the normal frost level for any geographic area you don't need any insulation around the perimeter. The installation of insulation on the flat, away from the foundation is done when you cannot excavate deep enough to protect the frost wall from heaving or when using a slab on grade installation and no side wall exist.The protection of the insulation around the exposed perimeter is best done any of the new stucco applications with fibre mesh, designed specifically for installation over insulation.Gabe Martel GSCProject Manager
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What is the best way to insulate the perimeter of a radiant slab?