I would like to glue sheets of polyisocyanurate to one basement wall which is very cold in the winter. Due to plumbing and electrical panels, I will not be able to cover the polyiso with drywall. Are there any dangers to glue this to an open cinder block basement wall and not cover it? Thanks in advance for your guidance.

  • Why not use 1/4" (or whatever size will fit both) fan fold insulation and do drywall in those areas? Or use 1/4" drywall, too.
    – Mazura
    Jul 8, 2015 at 0:51

2 Answers 2


Polyisocyanurate, AKA: PIR, polyiso, or ISO –Wiki

During a fire, polyisocyanurate can release a considerably higher level of toxins than other insulating materials. Other than PVC (which will release dioxin), I can't think of a worse building material to inhale fumes from. Other types of foam might be 'better' but all of them should have a fire barrier. I haven't installed ISO in 20 years, others are much cheaper (like, half the price) and nearly the same R-value (~5 vs. 6).

A 2011 study of fire toxicity of insulating materials at the University of Central Lancashire's Centre for Fire and Hazard Science studied PIR and other commonly used materials under more realistic and wide ranging conditions representative of a wider range of fire hazard, observing that most fire deaths resulted from toxic product inhalation. The study evaluated the degree to which toxic products were released, looking at toxicity, time-release profiles, and lethality of doses released, in a range of flaming, non-flaming, and poorly ventilated fires, and concluded that PIR generally released a considerably higher level of toxic products than the other insulating materials studied (PIR > PUR > EPS > PHF; glass and stone wools also studied).


Is it dangerous to use polyisocyanurate without covering it with drywall

There are dangers:

  1. The toxicity of fumes released during fires, as described by Mazura
  2. Raising the ire of local building inspectors. It may be a code violation to install any foam board (eps, xps, poly iso) against interior walls without a fire barrier. Polyiso is normally foil faced so some jurisdictions permit it without drywall.
  3. The foil facing makes polyiso vapor impermeable. This will slow the walls ability to dry and will likely result in excess moisture in the basement. Wet basements attract unwanted biologicals such as mold, mildew, carpenter ants, and termites. Consider adding a dehumidifier or energy recover ventilator to remove any excess moisture.
  4. Moisture in the wall can be drawn up the wall to the sill plate. This will, if there's no capillary break installed between the concrete and wood (unlikely in older houses), wet the wood. Wood with excessive moisture tends to attract those unwanted biologicals, presenting health and structural dangers.
  5. Unlike EPS and XPS, PolyIso is water sensitive. When exposed to water, it can saturate, severely reducing its insulating ability. Be extra careful about sealing the edges.


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