[Greenbuilding] Subject: mounting rigid insulation above sloped framed un-vented roofs.

[Alan Abrams]

a related issue is the management of water vapor in the framing cavity
beneath an impermeable roof system.

Ken Levenson, a PH consultant and owner of 4.75 High Performance Building
Products, recommends (and sells) vapor barriers with variable permeability
under the wall board.   Permeability drops in winter, helping to keep
moisture out of the cavity (which could otherwise condense on colder,
exterior surfaces).  Permeability increases in summer, allowing the cavity
to dry to the interior.

He also maintains that cellulose is superior to other permeable
insulations, because of its ability to absorb and transfer moisture when it
does enter the cavity.

all this raises the question of how effective such a vapor barrier can be
if painted wall board has constant permeability.  on the other hand, Ken's
approach presupposes that there is a service cavity behind the wall board,
which could be as simple as 2x2's fastened to the framing, with the vapor
barrier in between.

see:
http://www.foursevenfive.com/index.php?main_page=page&id=16

AA                        *

Alan Abrams**
Abrams Design Build LLC*
*une approche durable à l'espace beauté*
*un enfoque sostenible del hermoso espacio
ein nachhaltiges Konzept für schönen Raum
un approccio sostenibile allo spazio bello
a sustainable approach to beautiful space*

6411 Orchard Avenue Suite 102
Takoma Park, MD 20912
office  301-270-NET- ZERO (301-270-6380)
fax      301-270-1466
cell     202-437-8583
alan at abramsdesignbuild.com
www.abramsdesignbuild.com
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