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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.

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    methane -> propane -> butane -> octane -> cetane (diesel) -> styrene -> polystyrene – Harper - Reinstate Monica Sep 11 '20 at 18:19
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    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 '20 at 20:38
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    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 '20 at 20:50
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    polystyrene doesn't melt until about 250C, and ignition temp is even higher. – dandavis Sep 11 '20 at 20:52
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    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 '20 at 20:53
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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.

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  • deleted previous comment because you answered my question – capet Sep 11 '20 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 '20 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 '20 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 '20 at 19:01
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    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. – Fresh Codemonger Sep 11 '20 at 19:31

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