[Greenbuilding] [BULK] Cellulose: vented v. un-vented

[Lawrence Lile]

I agonized over this detail in a cathedral ceiling in my own house.  

I finally chose a vented construction.  I understand that unvented
cellulose is used a lot, and with proper detailing it can work - you
need to make sure there is a path for moisture to leave the insulation
cavity, and that any possible air leaks are sealed.  

Cellulose by istelf isn't a decent air barrier, despite what your
insulation contractor says.  Gyp board will stop infiltration many times
more effectively than cellulose, which is still better than fiberglass
in limiting air movement.  I would worry about any electrical
penetrations in the gypsum board, which might make the sort of air leak
that you are trying to avoid.  

--Lawrence Lile



-----Original Message-----
From: greenbuilding-bounces at listserv.repp.org
[mailto:greenbuilding-bounces at listserv.repp.org] On Behalf Of David
Bamford
Sent: Monday, July 24, 2006 10:24 AM
To: 'Greenbuilder list'
Subject: [BULK] [Greenbuilding] Cellulose: vented v. un-vented
Importance: Low

Greetings. We have a number of projects under construction or on the
boards in the greater Washington, DC area where we are considering using
cellulose insulation in cathedral-style roofs and in the walls. The idea
has been to use an un-vented assembly with a well detailed interior air
barrier that will keep most of the moisture-laden air out of the cavity.
Any moisture that does get into the assembly can "breath" back (vapor
diffusion) through the interior assembly (drywall and latex paint). The
sense among my colleagues has been that this is a better assembly than a
vented one where vents can get clogged, critters can enter and copious
air is moving all over the place (and reducing the thermal performance
of the insulation). 

But I just had a conversation with a large, local insulation installer
about the use of cellulose in these applications. He likes cellulose in
a vented condition (and installs plenty of it) but has concerns about an
un-vented application, particularly in the roof. His primary concern is
that the cellulose will not pack densely enough to act as an effective
air barrier to prevent water vapor from migrating through the assembly
and to the cold roof surface in the winter where it will condense. He is
proposing we use a sprayed in foam (icynene, Biobase, other?)

I am hoping to get some clarity about what is the best practice for
using cellulose in my region.

Thanks for participating in this discussion.

Dave


David Bamford
ELEMENT Design + Build
1422 Varnum Street, NW
Washington, DC 20011
202.436-0956 (cell)
202.318.9167 (fax)
dbamford at usa.net



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