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Community and Q&A

Insulation Behind Cast Iron Radiators?

pghbob | Posted in Energy Efficiency and Durability on

My Zone 5 house is about 100 years old and is heated with hot water circulated through original cast iron radiators, which all have wooden covers. Almost all of them are under a window on a exterior wall. The walls are brick, ~1″ air gap, ~1″ Celotex or similar fiberboard attached to stud framing, then lath and plaster inside.  Last heating season, I tried sliding Radflec reflective sheets behind the radiators This produced a small, but noticeable improvement in how warm rooms felt. I’m guessing this simply because more of the infra-red energy was reflected inward.

There is about 3″ between the back of the radiator and the wall. I’m wondering if it would be worthwhile to cut a rectangle of Tuff-R polyiso (or similar), and then drop it in with the foil side next to the radiator.

Thanks in advance.

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  1. Jon_R | | #1

    Almost all foam needs to be covered with a thermal barrier. Consider a radiant reflective layer (no foam) and then a piece of rigid mineral wool. Or Thermax.

  2. pghbob | | #2

    I'm not planning to rip out the walls - just wondering if a sheet of polyiso dropped behind the radiator would/could work better than the reflecting sheet.

    1. Jon_R | | #3

      Understood and it will help reduce heat loss. Proceed as specified in #1.

      1. wastl | | #4

        Its a Code Thing.. Don't risk your insurance!
        What you can do is to put some aluminium foil on Rockwool, then you have decent insulation and the reflective surface

  3. Expert Member
    BILL WICHERS | | #5

    I would use a thin layer of rigid mineral wool here instead of rigid foam. A radiant barrier on top of that may help with the "extra warmth" you felt when you tried the radiant barrier alone. Make sure you don't put in so much insulation that the air space gets blocked off -- you need air to be able to freely circulate around the radiator for the radiator to function properly.

    Note that there are some specialty rigid insulating materials with a shiny foil facer that are used to insulate things like generator enclosures. I have found them available from Aircraft Spruce before. They are not inexpensive materials.

    BTW, ANY rigid foam is going to degrade due to constant exposure to heat. That's in addition to the code issues with thermal and ignition barriers.


  4. tommay | | #6

    ....or you could consider removing the wooden boxes that block the heat from radiating during the winter.

  5. pghbob | | #7

    Wasn't aware of the code requirement for foam board - thx for the heads up. So, the assembly would be something like: Radflec sheet glued to 1.25" Comfortboard 80 or similar, and the board attached with Velcro or minimal adhesive to the existing wall behind the radiator. This would leave about 1.5" open space between the assembly and the radiator, all of which would be enclosed within the wood cover, as aesthetics matter.

  6. Expert Member
    Michael Maines | | #8

    I'm in the process of renovating a bathroom with a cast iron radiator. It had a sheet of foil-faced bubble wrap stuffed between it and the wall, which is old but insulated with R-11 fiberglass. There was a lot of mold between the bubble wrap and the plaster. If you decide to add a layer of insulation, keep an eye on it to make sure you aren't creating the kinds of conditions mold likes--warm but not hot, with a source of moisture.

    1. wastl | | #9

      Yes - that Plate is vaportight - you would have to seal it (run the foil around the board the plate) against the wall or give enough room for ventilation . IF the wall can dry to the outside...

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