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

[John Messerschmidt]

Lawrence,
I may have miswrote.  Vapor barrier is probably not what I meant.  One of
the attractions of expanding foam is that it will stop the flow of air or
wind into the building.  I believe Icynene does not claim to be a vapor
barrier but a vapor retarder or semi barrier or vapor resistant.  Some of
this stuff goes over my head, I admit it.  I'll tell you this, after 15
phone calls back and forth, they were able to convince our local building
inspector that it's not necessary to leave a space at the roof of the
cathedral ceiling for air flow.  A grand achievement in this area.  

Tell me if I've got this right; you want a vapor retarder on the outside of
your wall only, so the wall can dry to the inside and outside.  Now if you
use rigid insulation and Tyvek as your vapor retarder, why do you need the
foam?  It seems like recycled cellulose which has the same r value per inch
would work just fine.  I love the idea of expanding foam, but it's a
petroleum product and they still haven't figured out what to do with the
left over foam that they shaved off the studs.  


Cheers,


John



-----Original Message-----
From: Lawrence Lile [mailto:LLile at projsolco.com] 
Sent: Friday, January 12, 2007 1:47 PM
To: John Messerschmidt; Greenbuilder list
Subject: RE: [BULK] Re: [Greenbuilding] Green real estate market -- spray
foam, and geothermal

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

-----Original Message-----