Quote:
Originally Posted by michael
...I've been schooled in the belief that little heat is dissipated downward in such construction, that it's ceilings first, then windows, walls and finally the floor with the stipulation that there be a thermal break around the edge of the slab (when it's not on grade because there one needs lots of insulation below the slab as well) to prevent heat leaking out the sides. That may be old school...
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That's the way I used to think about it, too... as they say, "heat rises".
But it turns out that 'they' were only partially right, and that heat has three modes of transfer:
- convection
- conduction
- radiation
It is only in the case of convection that, "heat rises" is true. It is true that
hot air rises, and it is also true that
hot water rises. It is gravity that makes this so, because hot air and hot water expand, and become less dense and are displaced by colder fluids of greater density.
But with conduction, it is not true. Otherwise, if you were frying bacon, you would be able to lift the skillet and put your hand on the bottom of it, and it would feel cool. An appeal to common sense is probably sufficient, in this case.
Also, radiation is unaffected by "up-ness". My little electric counter-top broiler/toaster works just fine, radiating heat down to my sizzling pork chops. And in the case of the sun, the source of all life, where is up-ness in that case?
In the case of your radiant floor, the heat from the PEX eventually makes its way to the surfaces of the floor via conduction, which works in all directions. So the top will become a heated radiating surface, and the bottom will also become a heated radiating surface, and the sides, too. If the sides of your floor are in direct contact with the anything, heat will flow into whatever material the sides are made of, etc. So you've seen to that with edge insulation.
The heat that makes it to the upper surface of the floor will radiate out. But it will also heat the air that is in contact with the floor, and it will begin some degree of convective pumping. So in truth, a radiant floor is also a conductive floor, and a convective floor. All three modes are going on simultaneously. But since radiation plays a large part (maybe 30%) it is called a radiant floor.
If you crawl under your house and lay in the dirt on your back and look up at your floor on a cold winter night, and put on some infrared glasses, you will see that the floor is hot, and it is heating by conduction, everything that it touches. You will also see that heat is radiating off of the floor, making your stay in the crawl space dirt a little more bearable. You will also see that air that comes in contact with the floor heats up, but gravity will be in your favor, and convection-heated air will not rise away from the floor (in perfectly still air). But any breeze, or puff, or imbalance in air-heat content will strip away the heat from the bottom of your floor.
So you do need insulation under a radiant floor, unless the space underneath the floor is also heated.
And I think that the
minimum R-values still apply...
- roof/vaulted ceiling insulation is R38
- exterior walls is R20
- floors is R30
- windows R2.86
I mean, you wouldn't tolerate money leaking out from the bottom of your bank account would you?
Best,
-AC