[Greenbuilding] [BULK] vapor retarder/barrier?

[Lawrence Lile]

It is more important to seal for air leaks than install a vapor
retarder.  This statement will no doubt elicit a "Huh?" from your
building inspector.  

First, I would strongly recommend using a Tyvek housewrap type product
under your siding, unless that is what you meant by "building paper".
Tyvek can serve as a drainage plane under the siding, stops a lot of air
infiltration, and allows moisture to escape.  

Second, I would recommend getting away from fiberglass.  Take a look at
the archives here, you'll find that Cellulose has a number of benefits.
It is Greener, being 100% recycled, it has better R value, and does not
lose R value at low temperatures like fiberglass does. Cellulose resists
airflow better than fiberglass, fiberglass can allow convection currents
inside the wall that can reduce effective insulation value.  Cellulose
is slightly more expensive than fiberglass.  But it doesn't make you
itch, a big advantage if you are doing it yourself.  Some suppliers will
rent you the cellulose blower for free if you buy the insulation, and it
is pretty easy for a DIY installation.  

Third, I'd look at the insulation values.  R-21 is a nice, hefty wall
insulation, R-38 sounds a little light for a super-efficient building.
My roof has R-60, but I live in a much colder climate, R-38 is just the
code minimum here.  

Fourth, Forget the vapor barrier on the inside!  [Gasp!]  Concentrate on
sealing gaps and cracks.  Caulk the inside of your exterior sheathing,
at top and bottom plates, any horizontal joints, any penetrations.
Caulk the ceiling sheetrock where it meets the top plate.  Eliminate any
ceiling lighting fixtures - they are just chimneys for air leakage, or
seal up ceiling penetrations tight.  Spray foam behind any electrical
boxes on outside walls (careful don't get any foam inside the box, it
ain't code) and seal them up tight.  Caulk around the edges of
electrical boxes on exterior sheetrock. Spray foam in electrical
penetrations in the top plates.  The point is to prevent warm, wet air
from entering your wall and condensing.  A plastic sheet won't
accomplish this, if it is full of holes for electrical penetrations.  If
the wall ever does get wet, you want moisture to be able to leave,
drying slowly through the gyp board and the exterior wall.  Buy a case
of spray foam and two cases of caulk, and use them up before you start
insulating, sealing the interior side of your walls.  

Finally, consider a layer of foam insulation under your siding.  Any
amount of foam will help mitigate the low R-value through studs, seal
the house up tighter, and provide another drainage plane for bulk
moisture to run off.  1/2" to 1" is a handy size, thicker foams can be
hard to fasten, I've used up to 1-1/2" successfully.  

This is exactly what I did on a house I am building now.  No building
inspector is questioning the installation of no vapor barriers.  

A plastic vapor barrier is just as likely to soak your insulation as
keep out moisture.  Bulk vapor transport through gaps and leakage is
what soaks walls.  Vapor transmission through surfaces is negligible.
This flies in the face of conventional wisdom from the 1970's.  Many of
those houses now have mold inside the walls because of the plastic
sheets.  

Lawrence Lile, P.E., LEED AP



 

-----Original Message-----
From: greenbuilding-bounces at listserv.repp.org
[mailto:greenbuilding-bounces at listserv.repp.org] On Behalf Of
yabberline at opusnet.com
Sent: Tuesday, October 17, 2006 11:25 PM
To: greenbuilding at listserv.repp.org
Subject: [BULK] [Greenbuilding] vapor retarder/barrier?
Importance: Low

Hello,
We are building our passive solar house as we go, on a tight budget . 
After 2 years, we have finally got to the insulation/drywall stage.

This is our vapor profile:
I hour NW of Portland, Oregon-700' el.
6" insulated concrete mono-slab
Hardiplank siding/building paper/ext. ply/R21 fiberglass batts
Attic: vented birdboxes/ridge vent/R38 insulation
Heat: wood stove w/electric baseboard backup (code) 2-storey House

Our bldg. Inspector suggests we use 6 mil plastic on the inside exterior
walls & no retarder/barrier on the ceiling.  This doesn't sound right to
us, although we have no real knowledge.  We have been to the building
science site but are really none the wiser.  We would really appreciate
some advice.
Many thanks,
Lynn



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