ASHRAE Journal - May 2013 - 62

BUILDING SCIENCES

Rocks Don’t Burn*
By Joseph W. Lstiburek, Ph.D., P.Eng., Fellow ASHRAE

f you take rocks and melt them and blow air through them you get
fluffy rocks. And fluffy rocks don’t burn. If you take gypsum and make

it into sheets, you get “sheet rock.” And guess what? Rocks don’t burn.
Also, metals don’t burn. You can look that up. OK, some metals burn,
but you get the idea. If you take wood, which burns, and add rocks
and metals to the wood,† you get wood that does not burn. We could
be on to something here.
There are all kinds of places where we
have to deal with wildﬁres, and all kinds
of places where we build right next to
our neighbors. Where we build next to
our neighbors, we have kind of decided
that it is a good idea, that if they are on
ﬁre, that we should not be on ﬁre. And
vice versa. This makes things very interesting if we want to vent our roofs or
if we want to use continuous insulation
on the exterior of our buildings or if we
want to do both. And things get even
more interesting if we want to vent our
claddings.
If you want to see a ﬁreman’s head
explode, mention roof vents, particularly
the sofﬁt kind, or air spaces behind siding.
It seems that even after 140 odd years
the ﬁre folks can’t seem to get over Mrs.
O’Leary’s cow.‡ Truth be told, good for
them. We have the best ﬁre codes bar
none in the industrialized world. But
every now and then things get irritating

code-wise if we also want to control
other things besides just ﬁre—like water
and energy.
With wildﬁres, sofﬁt vents are a problem because burning embers get drawn
into the sofﬁt vents and you end up losing
the roof. Roof ventilation is prohibited
in many wildﬁre zones for this reason.
And it is a good reason. We have the
technology to build unvented roofs. The
codes allow them. We have covered this
before (“Understanding Attic Ventilation,” ASHRAE Journal, April 2006). So
what is the problem?
Well, in places where it snows a lot
we need attic ventilation to address ice
damming. You can’t just do it with a
highly insulated airtight unvented roof.
The R-value of the snow cover will raise
the snow-roof cladding interface surface
temperature above the melt point of
the snow even with a “super insulated”
unvented roof. We have also covered

this before (“Dam Ice Dam,” ASHRAE
Journal, June 2010).
Now it gets interesting. In a wildﬁre
zone where it snows a lot, if you don’t
vent your roof you will get ice dams.
The only technology available to control
the inevitable ice dams are snow melt
systems. Yech. Is there another option?
But of course. Construct a vented roof
out of non-combustible materials. Really.
Check out Figure 1. Use rock wool
insulation (aka “ﬂuffy rocks”), ﬁre retardant treated plywood, ﬁre retardant
treated wood, metal rooﬁng and ﬁber
cement siding, trim and sofﬁt material
(rocks in sheet form) and follow the applicable physics and you get something
that handles the embers and the ice dams.
The rock wool insulation comes in
rigid boards: 4 ft × 4 ft and 4 ft × 8 ft
and in 5 lb/ft3 to 10 lb/ft3 (80 kg/m3 to
160 kg/m3) densities. This rigid board
insulation is installed over the top of two
layers of standard rigid insulation such as
foil-faced polyisocyanurate or extruded
polystyrene. Steel “hat-channels” are
installed over the top of this rock wool
board insulation. The “hat-channels”
are screwed through the three layers of
insulation into the roof rafters. The “hatchannels” should be at least 2 in. (51 mm)
thick to provide enough airﬂow to handle
ice dams. Fire retardant treated plywood
is screwed into the “hat-channels.” A
fully adhered membrane§ and metal roof
come next.
The fascia, soffit, frieze board and
siding are all ﬁber cement siding. They

*I heard this the first time as a youngster listening to one of the “Masters”—Rene Dupuis—in a class on roofing in Madison, Wis. Rene and Carl Cash were the
guys I learned roofing from. Wow, I was one lucky guy. Rene had me laughing and learning for two days. He was charismatic, charming and drank red wine. I
wanted to be just like him. Rene, if you ever read this footnote, thank you for being such an inspiration. Your son turned out alright as well. Nicely done.
†Add boron, sodium, phosphorous, which are metals and minerals (aka “rocks”) and a bunch of other stuff, and you get fire retardant treated plywood.
‡Catherine O’Leary’s cow is often blamed for starting the Great Chicago Fire. Turns out that this was a made-up story published at the time by the Chicago Tribune.
The reporter made it all up “because he thought it would make colorful copy.” Apparently, journalism has not changed all that much since then.
§Always install a fully adhered membrane under a metal or steel roof. There are only two kinds of metal roofs: those that leak and those that will leak.

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