Quote:
Originally Posted by AC_Hacker
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.
-AC |
'Heat' does not "rise" in any sense of the word. Heat energy flows from a hotter body to a colder body by any or all of the stated means.
In Convection heat transfer, a hot (lighter) fluid may rise within a body of colder (heaver) fluid because of differing fluid mass densities subject to gravitational forces, if physical conditions allow. Why we use glass insulation
in wall and ceiling in spite of the fact that glass has a much higher thermal conductivity than does air. It physically restricts or eliminates the convective heat transfer process, thus a worthwhile trade off.
AC Quote;
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
The R rating of insulation is a measure of conductive resistance. Inasmuch as in the downward direction convection is a negligible problem, an R zero film is sufficient to prevent air movement within the joist space, air being a better conductive resistance than is any insulating bat material.