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Reflecting foil

Gta Member Posts: 138

Is reflective foil worth installing under my floor joists?

I’ve 2000 sq ft of radiant pex attached directly to wood/ covered with a thin lite weight concrete , and ceramic tile on top .

I’m trying to save as much heat as possible, the boiler hardly shuts off .

Any input I’d appreciate. Thx


  • Jolly Bodger
    Jolly Bodger Member Posts: 209
    is the floor over a basement or over a vented crawl? Are the joist bays insulated? Are you talking about staple up plates or tubing over the wood then covered with 1-1/2" of lightcrete?
  • Gta
    Gta Member Posts: 138
    Bays are not insulated .... Tube over wood and yes buried in aprox 1.15 inches of lite weight Crete ... here is a pic
  • Gta
    Gta Member Posts: 138
    Crawl space has no vent and is currently plus 16 c down there and the house is set to 20 c

    The crawl space has no heat source and is always warm ....
  • Jolly Bodger
    Jolly Bodger Member Posts: 209
    forget the foil, insulate those bays with as much fiberglass as you can. Looks like they are 9" TJI? That would be R30.
  • Gta
    Gta Member Posts: 138
    Can pink or roxsul insulation be placed right to the ends ? I ve seen pics of folks that ended up with moisture and mold by doing that
  • Zman
    Zman Member Posts: 7,542
    Reflective foil would serve no purpose in your application.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • kcopp
    kcopp Member Posts: 4,393
    get as much insulation under the floor as you can. Rockwool would be a great choice. The foil is not helpful especially after dust gets on top of it in a few months. Radiant heat will travel downward and that's where your issue is.
    Sealing up the crawlspace from outside air would also be a good plan. Vented crawlspaces are a REALLY bad idea. That where most of the moisture comes from.
  • Gta
    Gta Member Posts: 138
    I have about 1/2 of it done with the “ foil” can I just place the roxsul over it or should I remove the foil first ? Thx
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,567
    I need to draw a picture in my mind's eye. At the top is the floor covering you walk on... Ceramic tile. Then tubing embedded in light-cast that was poured on a subfloor. The plywood subfloor is next. Then there is an air space of about 7" to 10" (the hight of the TGI floor joist). Then you attached the foil to the bottom of the floor joist. And now you want to attach the insulation to the bottom of the floor joist and leave the 7" to 10" air space between the subfloor and the insulation?

    If that is easier than insulating between the joist, that might be OK. But you need to understand the concept of Radiant Heat. Radiant heat is heat from Infrared radiation. That radiation travels down like the heat from the sun! So your crawlspace is hot when the floor is hot. The heat is going down!

    You want to stop the heat from going DOWN. Insulation will stop the transfer of heat by blocking it from going DOWN. If the heat can't go DOWN then there will be more heat going UP.

    Heating the air space between the floor joist and all the wood in the floor joist is a waste of heat. Put the insulation between the floor joist, the more the better! This will stop the radiant (INFRARED Radiation) Heat from going DOWN

    A qualified insulation contractor should know if there is a moisture problem that needs to be addressed or if you need to take special steps around the rim joist, to prevent moisture and mold.

    Yours truly,

    Did I mention that you don't want any of the heat to go DOWN?
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics