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Foil-faced batts under radiant heat floor

Hunter Mantell-Hecathorn | Posted in Energy Efficiency and Durability on

3200 sq. ft. passive solar home in southwest Colorado, climate zone 6a. The home will be well sealed (around 1.3 ACH @50 Pascals) and will have a stand-alone Venmar EKO 1.5 HRV. It will also have a Tamarack whole-house fan for cooling in the summer.

The home will have a 4.2-kwh PV system. Walls are a combo of stucco and El Dorado faux stone, double layer building felt, 1″ EPS foam, Delta Dry rainscreen, ZIP sheathing, blown-in fiberglass, 5/8″ drywall. Ceiling is 18″ raised-heel trusses with vent baffles at eaves, R-60 blown fiberglass.

Conditioned crawlspace has R-20 ICF stemwalls, 2″ XPS extending 4 feet horizontally from stemwall on inside of crawlspace covered with gravel and Stego wrap sealed to stemwalls etc.

The home will have a 95% efficient propane boiler for the radiant heat, tubing will be embedded in 1 1/2″ lightweight concrete on top of subfloor. It has been proposed to install foil-faced batts under the subfloor in between joists with the foil facing the subfloor, with the thought to radiate the heat from the tubing back towards the living space.

I have some concerns about this approach:

1. Is this necessary? With the crawl space being conditioned with transfer grills from the house and a variable speed Panasonic Whisper Green exhaust fan exhausting stale crawlspace air.

2. Will this possibly create a moisture problem with the foil face batts being a vapor barrier and trapping any moisture in between the batts and subfloor?

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  1. TJ Elder | | #1


    A point about foil faced batts: you would want to install them tight to the subfloor with the foil facing down. The foil layer reduces heat escaping from the warm side and should face into an air space on the cool side. Adding this insulation should improve the efficiency of the radiant system.

    In your climate and with the construction you describe, the crawl space is unlikely to be damp.

  2. GBA Editor
    Martin Holladay | | #2

    I have heard of some people conditioning their crawl space by running a continuously operating exhaust fan. The system works, but it isn't particularly energy efficient.

    You basically have two mechanical ventilation systems. Your WhisperGreen fan is an exhaust ventilation system; if your envelope is tight, it will be pulling fresh outdoor air into your house through cracks in your envelope.

    In addition, you have installed a Venmar ERV. It will also be pulling outdoor air into your house. You will probably end up with an over-ventilated house, and all that ventilation will increase your energy bill.

    I would operate the house for a year without turning on the WhisperGreen fan. As long as you don't have moisture problems in your crawl space, you may not need it.

    (Radon is a whole separate issue, of course. You may want to monitor your house for radon and adjust your crawl space strategy accordingly.)

  3. Hunter Mantell-Hecathorn | | #3

    Thomas- Thanks for the insight on the foil faced batts, makes sense.
    Martin- Radon is an issue here in my area and we have installed a radon mitigation system with the ability to convert it from a passive system letting stack effect pull the radon out to an active system and installing an in line fan. The radon is the main concern about not using a crawl space fan. My concern is that although we will have sealed the crawlspace with Stego inherently there will be some holes somewhere and with the HRV pulling air from the crawlspace I don't want to contaminate the living quarters with this. Your right though it could be monitored and adjusted accordingly. Although the Panasonic fans are super efficient they still do use power therefore incurring an energy penalty. Would you also agree with Thomas on the use of foil faced batts being an effective and efficient way to help the radiant heat stay above? Thanks

  4. GBA Editor
    Martin Holladay | | #4

    Since your PEX tubing is all inside your thermal envelope, the insulation installed under the PEX tubing is not critical. The batts will work fine, and you won't have any moisture problems.

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