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Radiant under wood floors

baylis Member Posts: 6
I'm restoring an old house and thinking of putting radiant heat under the wood floors because all the joists are accessible. Does anyone have experience with how the PEX shoud be attached and if insulation should be installed under the PEX. If so what type of insulation?
Thank You


  • Big Ed_4
    Big Ed_4 Member Posts: 2,780
    edited August 2017
    Aluminum plates on an retrofit , and yes you have to cut off all the nails . The plates attach to the underside of the sub flooring and the Pex pipe clips into the track on the plate .. Yes you want to insulate below the system to direct the energy upward , I first staple up reflective bubble wrap , then and non barrier roll of insulation ..

    First you want to make sure tar paper or horse hair is not used between the floor and sub floor or any material to give off an smell when heated ..

    Your also heating Wood , and extra care is needed to protect it ..
    You don't want to constantly heat and cool the wood .. Outdoor resets and floor sensors will help maintain an near constant water temperature supplying the floor .

    Humidity control is also an must , since the wood is an sponge you need to maintain moisture in the wood . Me I like an good steam humidifier and an humidistat with and outdoor sensor .. You will need to drop the percentage of humidity when the outside temperature drops below freezing as to not condense in the walls..

    Me, I am not an fan of wood radiant , I love radiant in Kitchens and baths . Kitchens with its lack of wall space.. Baths for it lack of wall and construction materials ..
    I have enough experience to know , that I dont know it all
  • RxRoy
    RxRoy Member Posts: 22
    I did this with my house and it works well. I kept the forced air heating for back-up. (Haven't needed yet.)

    Plates. You want to attach the pex to the underside of the floor using the heaviest plates you can find. I used Thermofin plates from Radiant Engineering. While not inexpensive, they make installation a breeze, and ensure no noise while operating. Do not use the thin, tinny aluminum sheets, you will have poor heat transfer and obnoxious noises from the pex moving around. Also, conduction is key. The underside of the floor must be flat. You need to have a good area of contact with the plates.

    Insulation. I put R19 fiberglass batt insulation, tight to the underside of the tubing/plates. No air gap. Make sure you joist sills are tightly sealed and insulated. You want the heat to conduct up, not down or out the sides. I installed a Tyvek type of barrier under the insulation to keep the insulation from dropping (and because it is a friable material, which I don't want to be breathing in/contaminating my basement).

    Thickness. My system is a combination of wood floors and limited carpet. Most of it is 3/4" plywood + 3/4" white oak. The thicker the floor, the harder it is to drive the btu's through the floor.

    Make sure you perform an accurate heat loss analysis before committing yourself to this endeavor. Most systems won't do well to go above 30 btu per square foot. There are other radiant options as well--ceiling and wall.

    A word of caution. This is not a project to jump into without good planning and sound design. And, you may become addicted to hydronics. I'm a pharmacist by trade, but read this website every day. (Hardly ever comment though, everyone else on here is far more knowledgeable.)

  • modconwannabe
    modconwannabe Member Posts: 49
    Second Roy's comments. Thick plates (I used Uponor) cost plenty but make the difference. Make sure your PEX is the correct type (aluminum). I'm not a pro and so researched like crazy in advance and my understanding was like Roy said--you have to make it harder for the heat to penetrate/radiate in the wrong direction and so insulating a higher R value below and at the sides than above is essential. I believe I"d read on site like Greenhomebuilding t the bubble/foil wrap wasn't necessarily as effective as high R-value but this is one of the many points people like to argue.
    Making sure you have a system that monitors and controls the temperature to the floor and that it doesn't go too high is essential. Our plumber didn't install floor sensors and we have areas where the floors start to pop.
  • Harvey Ramer
    Harvey Ramer Member Posts: 2,239
    What type of subfloor do you have? Is it plywood, rough cut boards or smooth boards? Aluminum transfer plates depend on maximum contact with the subfloor to conduct heat efficiently. If the underside of the floor is rough, you would be better served going on top with a panel system or an overpour.

    The insulation should go up tight to the bottom of the subfloor and the rim joists should be insulated and sealed with spray foam. Some people cut blocks of rigid foam to insert in the rim joists cavity and then use spray foam around the edges to seal it.

    I would stay away from a suspended tube system.
    Reflective foil backed insulation is a waste of money.
    Design the system to run on the lowest possible water temp and control the water temp via outdoor or indoor reset control.
    Design the system to run about a 10 degree DT.

    Go for it!!! Done right, you'll love it!
  • hot_rod
    hot_rod Member Posts: 22,107
    Ideally you would run a heat load calculation to see what the space requires and if the floor is capable of covering the load. You will find cases where radiant floor heat alone is not sufficient to heat the space.

    Rooms with a lot of furniture, throw rugs, or high loads need to be calculated carefully.

    It is also quite possible that a system like UltraFin is adequate in low load rooms or areas. Without a load calc it i all just guesstimates.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • baylis
    baylis Member Posts: 6
    Thank you all very much for the info!!!