directing heat up?

Dave Millet

I have a radiant floor going in above a heated space where there are exposed joists so no possibility of insulating the ceiling below. The floor will be carpeted and I'm wondering if I need to put something (rigid insulation, reflective bubble/foil barrier?) under my 1 1/2 inch lightweight concrete to direct the heat up? Or is this unnecessary as the space below is heated (with radiant heat)?

Weezbo

I think it good to insulate the floor joyst,,

not quite for the same concerne as you have, ceiling heights and intended use of the space below is a large chunk of using insulation...basically insulation doesnt really make heat rise,it directs heat to the cold and interferes with it escaping to cold, along lines of telling it Heat Go To Window.

Carl PE

umm..

What our friend Weezbo is trying to explain here.. Is that the heat is going to radiate in every direction. The proportion of that heat that goes up or down is based on the resistance to heat flow in each direction.

For example, if you have carpet and padding on the floor above (~R2), and R8 insulation below, 80% of the heat will go up, and 20% down.

The calculations aren't quite that simple, but hopefully that helps. It's very easy to overheat a basement if you don't have adequate insulation on a radiant system above.

Larry Weingarten

You say exposed joists..

How about cutting two inch thick rigid insulation to fit snugly between the joists? Then you could cover with sheetrock or some other sheeting. The heat would have to go where you wanted it!

Yours, Larry

Unknown

Well, what are your heat loads?

If you do not insulate, since you do not have a dead air space under your thinslab (standard joist cavity heated primarily from above so convective loops don't occur), you will be heating the floor below appreciably with your higher thinslab. This may or may not be a bad thing, it might be just fine that the first floor works harder than it has to and your lower floor doesn't work as hard. Not strictly ideal, but it might not be a problem, as your lower themostat/sensor can just shut down your lower radiant and let the ceiling do some of the work.

However, you could overtax the higher slab, and/or you could overheat the lower level. This is pretty likely if you have higher heat loads above and lower loads below (living space over finished basement, for example). If you have higher loads below and lower loads above (first floor with more glass, with open space to second floor with a well insulated ceiling and less glass) then you'd probably be ok.

Prudence would suggest some insulation and/or maybe nixing the carpet in favor of tile or stone. The floors are going to be warm (or at least warm-ish), remember. The reflective insulations however, while easy to install in your case, will only give you about an R1. Better than nothing but probably not adequate. 1" of XPS at an R5 would cover you nicely.

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