[Greenbuilding] [BULK] Re: Green real estate market -- spray foam, and geothermal

[Lawrence Lile]

Cellulose is not a vapor barrier at all. 

The details recommended for cellulose are a little different than
fiberglass.  They recommend no vapor barrier on the interior - no vapor
barrier paint either.  Seal all cracks and gaps in the interior
sheetrock tightly.  This prevents bulk moisture transport due to air
leaks, which could condense inside the insulation (true for any kind of
insulation other than foam)

The exterior can have a layer of rigid expanded foam, which has some
vapor transmission, and a layer of tyvek, which also has some vapor
transmission.  Don't use foil faced foam boards, which have near zero
vapor transmission.  

The idea is, you want to allow the wall to dry out when it can, you want
to block air from moving through the wall with the tight sheetrock and
the tyvek layer. Moisture moves through walls slowly by diffusion, and
this is not enough to worry about.  The old idea of wrapping houses in
polyethylene sheets worked against moisture, trapping it inside the wall
where it would build up and condense, causing mold and rot.  The plastic
was a great air barrier except at electrical outlets, which would allow
bulk moist air to get in and soak the whole wall.  

Read this before you insulate anything:

http://www.buildingscience.com/faq/default.htm
 
 
Lawrence Lile, P.E., LEED AP

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