ASHRAE Journal - May 2013 - 62

BUILDING SCIENCES

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

I

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.

62

ASHRAE Journal

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May 2013



ASHRAE Journal - May 2013

Table of Contents for the Digital Edition of ASHRAE Journal - May 2013

ASHRAE Journal - May 2013
Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
VAV Reheat Versus Active Chilled Beams & DOAS
A Stable Whole Building Performance Method for Standard 90.1
Technology Award Case Studies:
PSU Design Build Project
Passive Cooling for School
Standing Columns
Building Sciences
InfoCenter
Refrigeration Applications
IAQ Applications
Engineer's Notebook
Products
Data Centers
Emerging Technologies
Classified Advertising
Advertisers Index
ASHRAE Journal - May 2013 - ASHRAE Journal - May 2013
ASHRAE Journal - May 2013 - Cover2
ASHRAE Journal - May 2013 - 1
ASHRAE Journal - May 2013 - 2
ASHRAE Journal - May 2013 - Contents
ASHRAE Journal - May 2013 - Commentary
ASHRAE Journal - May 2013 - 5
ASHRAE Journal - May 2013 - Industry News
ASHRAE Journal - May 2013 - 7
ASHRAE Journal - May 2013 - 8
ASHRAE Journal - May 2013 - 9
ASHRAE Journal - May 2013 - 10
ASHRAE Journal - May 2013 - 11
ASHRAE Journal - May 2013 - 12
ASHRAE Journal - May 2013 - 13
ASHRAE Journal - May 2013 - Letters
ASHRAE Journal - May 2013 - 15
ASHRAE Journal - May 2013 - Meetings and Shows
ASHRAE Journal - May 2013 - 17
ASHRAE Journal - May 2013 - VAV Reheat Versus Active Chilled Beams & DOAS
ASHRAE Journal - May 2013 - 19
ASHRAE Journal - May 2013 - 20
ASHRAE Journal - May 2013 - 21
ASHRAE Journal - May 2013 - 22
ASHRAE Journal - May 2013 - 23
ASHRAE Journal - May 2013 - 24
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ASHRAE Journal - May 2013 - 28
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ASHRAE Journal - May 2013 - 31
ASHRAE Journal - May 2013 - 32
ASHRAE Journal - May 2013 - A Stable Whole Building Performance Method for Standard 90.1
ASHRAE Journal - May 2013 - 34
ASHRAE Journal - May 2013 - 35
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ASHRAE Journal - May 2013 - PSU Design Build Project
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ASHRAE Journal - May 2013 - Passive Cooling for School
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ASHRAE Journal - May 2013 - InfoCenter
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ASHRAE Journal - May 2013 - 74
ASHRAE Journal - May 2013 - Refrigeration Applications
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ASHRAE Journal - May 2013 - IAQ Applications
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ASHRAE Journal - May 2013 - Engineer's Notebook
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ASHRAE Journal - May 2013 - Products
ASHRAE Journal - May 2013 - 87
ASHRAE Journal - May 2013 - Data Centers
ASHRAE Journal - May 2013 - 89
ASHRAE Journal - May 2013 - 90
ASHRAE Journal - May 2013 - 91
ASHRAE Journal - May 2013 - Emerging Technologies
ASHRAE Journal - May 2013 - 93
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ASHRAE Journal - May 2013 - Classified Advertising
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