Is radiant barrier necessary for staple-up radiant heat?
I took a local builder's advice and put R-19 batts under the radiant PEX, rather than a radiant barrier like the foil bubble foil, but after reading more I'm worried I made the wrong move. The heated floor is above a half crawl/half basement space. Thanks to this forum I air sealed and insulated the band joist so the crawl/basement is about low 50's when it is single digits outside
Now I'm worried that I got bad advice but I hate to think about pulling out all the insulation to put foil on top (particularly given we just installed it). is it going to kill my efficiency e.g., will all the heat just sit in the insulation? I thought about puting the barrier under the batts but that would seem to be a waste.
Any advice is much appreciated.
Asked by Michael schlee
Posted Sun, 01/22/2012 - 22:30
Edited Mon, 01/23/2012 - 09:10
Posted Sun, 01/22/2012 - 22:30
Edited Mon, 01/23/2012 - 09:10
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Michael,
You're right that R-19 batts weren't the best choice under your PEX tubing. And you're right that the R-value is low.
However, I wouldn't bother to pull out all of the insulation that is already installed. If you want more R-value and better performance, I recommend that you install a continuous layer of rigid polyisocyanurate foam under the joists. Seal the seams carefully with high quality tape.
Posted Mon, 01/23/2012 - 06:12
Thanks Martin (as noted, I took your advice on an earlier post and spray foamed the band joist and air sealed the floor (there were lots of penetrations to seal).
Your thought on rigid foam is exactly what i was thinking. Funny thing is that the designer I'm working with suggested rolling a continuous radiant barrier under the joists. His thought is that it will catch any radiant heat that gets through the insulation but more importantly create an air seal that improves the batt performance. I doubt the radiant side would work at all since it will be up against the insulation (maybe a slight gap in places, but if the primary benefit is in the air seal it would be a lot cheaper than rigid foam board.
If i would have put the radiant barrier on top of the insulation would that have been all I needed (i feel some remorse here)
Thanks again for all your help - your articles and info on this blog are some of the best I've found.
Posted Mon, 01/23/2012 - 09:14
Michael,
Q. "If I would have put the radiant barrier on top of the insulation would that have been all I needed?"
A. No. A radiant barrier alone, even with an air gap facing the radiant barrier, would not have been all you needed. According to ASHRAE Fundamentals, the R-value of an air space faced with a low-e material (that is, a radiant barrier), with heat flowing downward, varies from R-2.5 to R-4. That is much less than the R-19 fiberglass you installed.
Posted Mon, 01/23/2012 - 09:23
As always thanks Martin.
Posted Mon, 01/23/2012 - 10:07
As a side note, the placement of the radiant barrier is actually irrelevant to functionality in terms of which side of the conductive insulation it is placed on. It will work equally effectively located on either the interior or exterior surface, although as you note, placement can sometimes be used to gain other benefits as well. This is somewhat counterintuitive but quite true.
Remember that what a radiant barrier does is reflect radiated heat - something which conductive insulation does not do very well at all. Placing on the exterior reflects it back through the material, just as easily as it passed through on it's way out. While some absorbtion does take place, the overall heat loss to the exterior is the same.
Posted Mon, 01/23/2012 - 14:12
Patrick,
Your explanation is not quite accurate.
First of all, a negligible amount of radiant energy makes it through an R-19 batt. Radiant energy travels well through a vacuum or through air, but not through solid objects.
Second, it's not quite true that "what a radiant barrier does is reflect radiated heat." It's more accurate to say that, when placed near an air space, a radiant barrier can (slightly) raise the R-value of the air space it faces.
Posted Mon, 01/23/2012 - 14:35
Martin, I assume that the by increasing the insulation R value with say the rigid foam under the joists, the radiant heat would migrate to the lower R value area i.e., the subfloor and into the building envelope. So in the end probably better off really insulating to a high R value that messing with the radiant barrier foil/bubble stuff?
Posted Tue, 01/24/2012 - 10:02
Michael,
You're asking the wrong question. You're asking about which direction "the radiant heat" will migrate. But there isn't a special category of heat called "radiant heat." Heat is heat. The idea is that heat flows via three heat flow mechanisms -- but it's still just ordinary heat, not a special kind of heat.
The PEX tubing is warm, and if the tubing is warmer than adjacent objects, heat will flow from the tubing toward cooler objects nearby. Insulation slows heat flow, so if you have a lot of insulation under the PEX tubing, then the flow of heat downward will occur very slowly. If you have less (or no) insulation above, then the heat will flow more rapidly toward the subfloor above, and the subfloor will get warmer.
Posted Tue, 01/24/2012 - 10:14
Sorry, yes that is what I meant - the heat from the radiant system rather than the heat radiation. Thanks for that.
I'm going the rigid polyisocyanurate route and create as much R value as possible.
Thanks again for all your valuable insight and patience with people on the forms who have little knowledge but lots of curiosity.
Posted Tue, 01/24/2012 - 11:38