[Greenbuilding] Cellulose: vented v. un-vented

[David Bamford]

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