I would like to use foil-faced expanded polystyrene boards to insulate my window at night.* That window is located a foot or two above my electric baseboard heater.** To me it seems like this could create a fire hazard, or at least risk deforming the polystyrene.***

Is there anything simple that I could do to mitigate or eliminate this fire/deformation hazard (if any)? I was thinking that maybe I could make a heat-resistant barrier between the heater and the window, or maybe put a flame-retardant material or coating on the polystyrene. But any solution would be welcome.

Also, is there a good way for me to test the extent of the hazard under "normal conditions"? For example, could I put in a shelf as described above, stick a memory thermometer underneath, and monitor the air temperature below it over an extended period to see how hot it gets?

* That may be stupid, feel free to let me know. I will probably set up a separate question about that.

** Which I always run on "low"

*** In the past, this has not been an issue with the plastic (polyolefin?) film that I put around the window.

  • 1
    methane -> propane -> butane -> octane -> cetane (diesel) -> styrene -> polystyrene Sep 11, 2020 at 18:19
  • 1
    if your space heater can ignite things feet above it, you've got bigger issues to worry about than your window insulation.
    – dandavis
    Sep 11, 2020 at 20:38
  • 1
    Yes, there should be multiple redundant safety mechanisms to ensure that never happens; thermostat, bimetallic thermal cut-off (re-settable), and a one-time thermal protection switch that's basically a relay held shut with wax calibrated to melt at a certain temp that should never occur in normal use.
    – dandavis
    Sep 11, 2020 at 20:50
  • 1
    polystyrene doesn't melt until about 250C, and ignition temp is even higher.
    – dandavis
    Sep 11, 2020 at 20:52
  • 1
    perhaps, but the other two mechanism should be in place. you can test it by putting a moist towel over the front while running, but that's a potentially expensive way to find out it's only got a one-time thermal cutout instead of both fall-backs. that said, a heater w/o a thermostat is really not supposed to be used unsupervised or long-term.
    – dandavis
    Sep 11, 2020 at 20:53

1 Answer 1


Why not use Roxul ComfortBoard instead of the polystyrene? It is fire proof.

Alternatively you could make a sandwich with the comfortboard on the roomside and the foil faced EPS on the other.

Are you trying to achieve the highest r-value? what r-value per inch is the foil faced EPS you are considering? How many inches do you want this removable insulation panel to be?

I'd just go with a pure comfortboard solution and not have any EPS.

  • deleted previous comment because you answered my question
    – capet
    Sep 11, 2020 at 18:51
  • I am trying to achieve the highest r-value "within reason" at low temperatures. In the "EPS scenario" I am envisioning 4 inches, which I think would theoretically give me an r-value of 16 but I'm sure I won't actually achieve that.
    – capet
    Sep 11, 2020 at 18:53
  • There are two reasons why mineral wool has not won out definitively for me yet: 1. I would prefer to get thicker board than I can find. 2. I haven't looked into the potential health unknowns/risks. Although being burned alive is also a health risk....
    – capet
    Sep 11, 2020 at 18:54
  • While we're on the subject of mineral wool and what's available: I often see mineral wool on the internet marketed for high-temperature insulation, soundproofing, or fireproofing. Would products marked for these purposes be similar to/have similar thermal properties to mineral wool that is marketed for insulation from cold?
    – capet
    Sep 11, 2020 at 19:01
  • 1
    yes they have similar properties. roxul carries a sound dampening version of their product - the difference is that the sound version is thinner allowing an air gap which is more effective for sound dampening. less r-value but more sound resistance. mineral wool is spun basalt (volcanic rock) - it should be relatively inert and safe. the batt version can be dusty. I'd expect the board version once cut to be fairly stable but I am not sure. high-temperature/low-temperature insulation is just insulation.. it resists the movement of heat. Sep 11, 2020 at 19:31

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.