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 PATHChat
| SUBJECT: |
RE: unvented attic
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| FROM: |
CAMERON DUNCAN
Cameron@nahbrc.org
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| DATE: |
4/12/2000 12:22:00 PM |
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The 1997 ASHRAE Handbook of Fundamentals
addresses this issue. The real issue is
minimizing moisture condensation (e.g.,
surfaces in the attic that drop below
the dew point of the attic air
temperature/moisture mix. The
temperature of the roofing nails and
roof sheathing are often the coldest
surfaces in an attic/cathedral ceiling.
(Winter) Warm indoor air that bypasses
air sealing efforts and escapes into the
attic may condense on these surfaces.
(Summer) Warm, humid outdoor air may
condense on the back of cold ceilings
(during AC use).
Hot Roof (insulate at rafters; no attic
ventilation): If you foam insulation to
(fully covering) the roof sheathing, the
first condensing surface of the assembly
becomes the interior surface of the
insulation. The insulation should
significantly raise the temperature at
this location to the point that it will
remain above the dew point temperature
making condensation unlikely. This is
similar to a SIPs roof. While Icynene
is not a true vapor diffusion retarder,
the approach appears a viable option in
your climate.
Negatives: The challenge will be dealing
with the building code requirements.
Building unvented roofs is accepted,
however, in Tucson, Phoenix, and Aspen.
Asphalt roof shingle temperatures may
be expected to rise a few degrees, and
insulation installed in this fashion may
void your asphalt roof shingle warranty
(not metal, etc. however). Cooling
penalties of 3-5% from not venting the
attic are offset by documented savings
of up to 30% (from airtightness,
humidity control, ducts in
insulated/conditioned space, etc.).
Conventional Attic (insulate attic
floor; ventilate attic): Installing the
foam at the attic floor will provide
effective air sealing along with R-value
and will likely be easier to achieve
code acceptance (since the attic may
then be vented). Negatives: Future
access to wiring runs, etc. will be
fowled with adhered insulation unless
initially protected. Attic space is
less usable than if insulated as a hot
roof. Attic-located ducts can be
inefficient due to duct air losses and
conductive loss.
Both approaches are viable for your
locale, winter or summer. They both
provide an air retarder/pseudo vapor
diffusion retarding, thermally
insulating, building envelope membrane.
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Partnership for Advancing
Technology in Housing (PATH)
451 7th Street, SW, Rm. 8134
Washington, DC 20410-0001
Telephone: 202 708-4370 Fax: 202 708-5873
E-mail: pathnet@pathnet.org |
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