ASHRAE Journal - August 2021 - 36

COLUMN BUILDING SCIENCES
fire retardant or fire resistant cavity
insulation and a non-combustible
interior lining. The fiber cement
siding is installed over a 1/2 inch
(12.7 mm) drained and vented cavity.
This cavity should not be vented
into soffit assemblies. It should be
capped at the top. The air leakage
typical for lap siding provides the
necessary air change in the cavity to
provide back cladding ventilation.
Figure 3 (page 33) and Figure 4 (page
34) show a building section connecting
an unvented roof assembly constructed
with fire resistant and fire
retardant materials to a wall assembly
with similar materials.
Now for the " confounding issue "
mentioned earlier. In places where
it snows a lot attic ventilation is
needed 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).
In a wildfire zone where it snows
a lot, if you don't vent your roof you
will get ice dams. The common technology
used 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 (and
yes we were also here before, but I
updated the information to cover
what I think is a better way of doing
things: " Rocks Don't Burn, " ASHRAE
Journal, May 2013).
In Figure 5 we use rigid mineral
wool or fiberglass insulation (aka
FIGURE 7 Vapor Diffusion Port for Shingles. Any
off-the-shelf ridge vent works. The only change
is vent opening is covered with a vapor-open
but airtight layer such as a " housewrap " with a
perm rating higher than 20 perms.
PHOTO 1 Vapor Diffusion Port. The vapor diffusion
port covering must be non-combustible such as
gypsum board.
" fluffy rocks " ), fire retardant treated
plywood, fire retardant treated
wood, metal roofing and fiber
cement siding, trim and soffit material
(rocks in sheet form). Steel " hatchannels "
are installed over the top
of this rigid board insulation. The
" hat-channels " are screwed through
the layers of insulation into the roof
rafters. The " hat-channels " should
be at least 2 inches (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.
What if I do not want a cathedralized
attic? Take a classic " vented "
attic and convert it to an unvented
attic with a vapor diffusion port
(Figure 6). In the classic vented attic
air change removes water vapor
from the attic. In an unvented attic
with a vapor diffusion port water
vapor is removed via vapor diffusion
rather than air change. And yes we
were here before, twice( " Venting
Vapor, " ASHRAE Journal, August 2015
and " No Sweat, " ASHRAE Journal,
April 2016). The key difference is
that the vapor diffusion port covering
must be non-combustible such
PHOTO 2 Vapor Diffusion Port. Non-combustible
membrane covering for the vapor diffusion port.
The vapor diffusion port materials also need to
be vapor open (20 perms or greater, wet cup
method), and the vapor diffusion port area needs
to be 1:150 of the attic area.
as gypsum board (Photo 1) or a noncombustible
membrane (Photo 2).
Oh, by the way, the vapor diffusion
port materials also need to be vapor
open (20 perms or greater, wet cup
method), and the vapor diffusion
port area needs to be 1:150 of the
attic area.
Are we having fun yet? We need
to pay the cost to not have the loss.
Apologies to James Brown.†
† " I've Got Money, " Song written and recorded by James Brown, 1962: " Havin' fun, got money to burn, 'cause I paid the cost to
be the boss. "
36
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