Second floor under floor insulation?

BenWoj

I'm installing infloor radiant heat in my house. I've installed it on the second floor using between sleepers and plates. What I'm wondering is if I should insulate the ceiling of the first floor (under second floor radiants). Insulating under radiant floors directs the heat to where it's needed, up. But I'm also thinking that since it's the second floor, if I do insulate it won't receive any of the first floors heat( heat rises).

Anyone have any input?

Thx

Gordy

Insulate it. If you designed correctly you won't need any help from heat of the first floor. Is first floor radiant also?

By not insulating you risk over heating first floor through back losses from the second floor, and under heating second floor by losing Btus through those same back losses. It's all about control, and getting BTUs to go where they were designed to go.

BenWoj

Yes, second floor is radiant as well. I was thinking the same thing about putting the heat where it's needed, but never thought about over heating the first floor.

Thanks for the reply.

BenWoj

I'm assuming the insulation can be with no vapor barrier since it's a heated house?

SWEI

Heat does not rise, hot air does. Insulate if you want any kind of response and control on the second floor. Has someone done a proper design for this? What were the heat losses for each floor?

Gordy

Remember radiant heat heats objects so the first floor ceiling would turn into a radiant ceiling with out insulation. If the two floors are seperate zones you could be heating first floor when not needed while second floor is calling.

And if they are not seperate zones still same effect.

There is not much convection effect with radiant so the hot air rising is minimal. Air molecules contacting a warm surface will rise but the effect is minimal. Not like forced air or convection emitters.

BenWoj

Ah, perhaps my thinking of similarities between force air heat and radiant are to blame. Thanks for the "rethink". Makes better sense now.

Yes, there was a room by room heat loss calculation for each room in the house. (Geothermal is being put in). I've read a lot about insulating under radiant loops, but I wasn't sure if I could benefit without insulating. Much clearer now.

Gordy

With geothermal you have to take extra care since your supply temps are limited to 115 ish. So make sure you pay close attention to the joist ban areas of the first, and second floors. Those areas can be brutal.

You should not need a vapor barrier.

Edit: what I mean by the joist ban detail is air sealing, and insulation.