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Insulating under in-floor radiant heat

I have in-floor heat. The tubes are in metal attached to the undersides of the subfloor.

I put 3-inch fiberglas insulation between the joists and then stapled plastic to cover it. I was told the the fiberglass is absorbing the heat instead of reflecting it back up. Should I get the foil bubble type so it reflects the heat back up?

I want it to be as efficient as it can. The house is about 3000 sq. ft. It's a log home.

Asked by Rodney Coryer
Posted Jun 19, 2014 10:20 AM ET
Edited Jun 19, 2014 11:02 AM ET


6 Answers

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The fiberglass is behaving as insulation, just as it should. It has very little thermal mass and isn't "...absorbing the heat...".

Reflective bubble pack reflects heat, but only the radiated fraction of the heat transfer. Since the tubing is covered with an extremely low emissivity metal (bare aluminum), the radiated fraction of heat transfer is less than 10% of the total. Bubble-pack performs at about R2 in this type of stackup if snugged up against the aluminum heat spreaders. If you install it with an inch or two of air between the bubble pack and aluminum heat spreaders you gain the insulating advantage of the air films, and it would perform at about R6, R8 tops. But three inches of fiberglass runs about R10-R12 (R13+ if it is batts designed for 2x4 framing, and actually 3.5", not 3".) Replacing it with bubble pack would be a step down in performance.

Three inches of fiberglass is fine for separating zones, but if the space below is unheated basement or crawlspace it can be worth adding another 3". If it is a VENTILATED crawlspace it's worth putting in MUCH more, but it may be cheaper/easier/better to insulate and air-seal the exterior walls of the crawlspace, converting it into conditioned space.

The only time reflective materials have any advantage is when the hot surface is highly emissive (not sheet metal), and the temperature differences quite high (say, a 150F roof deck above an uninsulated 60F air conditioning duct.) But even in those situations there are usually more cost effective solutions than reflective bubble-pack.

Answered by Dana Dorsett
Posted Jun 19, 2014 10:52 AM ET


If I were you, I would remove the polyethylene on the underside of your joists. If you want to install an air barrier under your joists, the best product to install would be foil-faced polyiso (rigid foam) with taped seams.

Answered by Martin Holladay
Posted Jun 19, 2014 11:03 AM ET


Thank you. That makes since and saves me a lot of work and money. Maybe I will put in more. It is above a basement but the pellet boiler is down there and keeps it warmer there.

Answered by Rodney Coryer
Posted Jun 19, 2014 11:06 AM ET


I have a garage under part of the house and it is finished with sheet rock. The are above it seems to have a more even heat where the areas not over it are not. Should I just finish the basement?

Answered by Rodney Coryer
Posted Jun 19, 2014 11:21 AM ET


If the standby losses of the pellet stove are heating the basement, you're better off insulating the basement walls. The best bang/buck is usually a combination of rigid foam trapped to the foundation with a fiber-insulated studwall. An inch or so of foam under the bottom plate of the studwall as a thermal & capillary break is recommended (it's not structural, it's just holding up the insulation & wallboard.) The foam/fiber R ratio needs to be considered, and is relative to climate.

For isolating the radiant over in unconditioned garage you'll need to add more fiber insulation- fill up that bay, and add rigid foam as a thermal break on the joists. Since the foam/fiber ratio would have to be significant for dew point control there, it's probably better to use 1.5-2" of EPS, which is vapor permeable rather than foil faced polyiso. (Foil faced polyiso would be fine in the basement though, since the space is passively heated, and would not create a condensation/mold issue.)

Answered by Dana Dorsett
Posted Jun 19, 2014 11:44 AM ET


So the best path to go I should put up EPS, more insulation, and try to insulate the walls. I thank everyone for the advice.

Answered by Rodney Coryer
Posted Jul 7, 2014 8:22 AM ET

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