I suspect there is more to this than there seems at first glance. PIR
foam without any fire retardant additives burns moderately though not as
fast as PU foam which unmodified burns like a firelighter. Both produce
dense black smoke with copious CO and traces of cyanide in it. I'd be
more worried about the hot soot and CO to be honest.
I happen to have some rigid PU building insulation foam (ICI prototype
stuff) from the late 1960's in my parents loft. I took a piece and tried
to set light outside (taking appropriate safety precautions). It had one
of the earliest fire retardant formulations in it (which actually makes
the smoke even more toxic if it burns). The backing paper burned off and
the surface of the foam charred but that was all. I was quite surprised
since I was worried that the foam might have been original raw batch
without any additives. If they could make PU foam resist fire at that
level back in the 1960-70's I am sure they can make PIR do the same.
Snag is that the blowing agents have changed and some today may well be
using pentane (which is itself a flammable gas).
However there is still a possibility of someone buying a cheap grade
that is only suitable for cavity wall insulation where it is surrounded
by inert brick/breezeblock walls on either side excluding the air and
then strapping it to the outside of a block of high rise flats.
Structural timber doesn't catch all that easily unless you are very very
unlucky (once a fire takes hold all bets are off). Paper and cardboard
and thin wooden furniture are much more vulnerable. Very old foam
settees from the pre fire retardant era are potentially lethal.
Now is good time to check that yours has a fire certificate stitched
into the underside of the cushions (especially if you are a smoker).
It might be appropriate to ban the sale of the cheapest nastiest variant
which is only safe to use *inside* the cavity of an inert brick wall or
behind plasterboard (or at least add warnings to that effect).
I notice Celotex have voluntarily stopped selling their RS5000 grade as
used on Grenfell tower.
Quite - for a fire to take hold, the insulation material would, I'd have
thought, be the least of your worries. In my case:
On solid walls, mechanically fixed behind 12.5mm plasterboard. There's
simply nothing to burn so I can't see how it would add to a fire. Once
hot enough and exposed, there is the issue of poisonous gas, but I think
the house would be pretty much gutted by that point;
Ground floor, between joists, fixed with fire retardant foam. Assuming
the fire started in the cellar (quite a few electrical bits and pieces
down there, so possible) I'd have thought if anything it would inhibit
the spread of fire by protecting the floorboards - until the point that
the fire had reached such a heat that the foil fails, the foam melts,
and the floorboards are exposed. In the meantime, the joists can
smoulder but there's not that much showing to actually burn;
In the roof space, between rafters, fixed with fire retardant foam,
faced to the inside with plasterboard, open to the roof space. The fire
would have to have spread from another property, or timber in the roof
space would have to in some way catch fire, maybe from an internal fire.
Again, I'd have thought the insulation would in the short term inhibit
the spread of fire because the timbers are less exposed.
I suppose they could look to developing something completely inert, even
if that does effectively turn the home into an oven-furnace.
But as things stand, I don't see PIR as a prime source of fire hazard in
low rise (< 4 storey). Timber and furnishings and other 'developments'
like laminates would be a bigger problem, and households should make
sure they have working fire alarms fitted in the first instance.
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