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Re: GBlist: What I mean is, wall plastic ain't green



--- Fred wrote:
>Plastic does nothing for a wall because it
cannot be made into a pressure barrier.<

Fred,

Ya gotta watch these absolutes.  The guy said he was using all kinds of
goop.  He might actually have a pressure barrier.  This stuff is so
tricky
to talk about in less than a volume.

Dear Greg,

Of course you are correct. It is theoretically possible to build a
temporary pressure barrier out of polyethylene foil. 

But I am a just practical guy. How long will the goop and even the
plastic last if it really ever worked?

Now tell the truth. If I went around to my clients and said: "we'll put
up some 6 mil, goop the joints, pressure test it, decorate the interior
with T&G pine boards, and everything will be fine", would I be a green
builder or a crook?

Have I mentioned to you that I hate the way you lurk on these lists?
only to swoop down on me when I have to get to bed instead of writing a
"volume" about this stuff. I mean-come on!! We've got Marc saying that
Fglass is fine here. 

Hugs and kisses to you and Al, Fred
--- end of quoted material ---

and Fred wrote:

And speaking of VDBs, they are generally considered useless at best and
destructive at worse.
--- end of quoted material ---

Wow...where to start.  I'm reluctant, 'cuz Fred is so certain about whatever he
says, and after 18 years in this biz I find I get less and less certain about
more and more!

I don't know what Fred actually does for work (something with cellulose
insulation, I would have to presume?) or where his info comes from.  My
experience is based on some education in mechanical engineering, studying the
basics of heat and mass transfer (which includes vapor diffusion), and
experience designing 30+ low energy use homes, and experience as a
troubleshooter of building failures, which mostly concern moisture in cold
climate (heating climate)  homes.

A vapor diffusion retarder needn't be a piece of poly.  Whether poly is "green"
or not is beyond my evaluation capabilities at this time.  Most problems
relating to moisture in building assemblies come down to rate issues: how fast
does the assembly get wet, and how fast can it dry?  The more forgiving an
assembly is, the likelier it will be durable.  The fact that moisture diffuses
from warmer to colder areas tends to favor making the exterior of heating
climate assemblies more vapor permeable than the interiors.  We have learned
one fact that we didn't know originally is that plywood becomes much more
permeable when it gets wet, so many walls that we thought might have problems
may have been saved by this forgiving fact.

So it seems as though retaining the vapor diffusion retarder on the interior of
assemblies in heating climates is a reasonable idea.  To quote Joe Lstiburek,
my personal guru on these topics, who has studied building science at the
doctoral level and also has tremendous field experience as a forensic engineer,
in his latest publication Builder's Guide for Cold Climates (pg 150):
"Cellulose is not a vapor diffusion retarder. That means if you need a vapor
diffusion retarder you should use polyethylene or low perm (vapor retarder)
paint.  Wall assemblies in heating climates need interior vapor diffusion
retarders unless insulating sheathings are installed in sufficient thickness to
elevate temperature of condensing surfaces."  

I agree with Fred that vapor diffusion is a less significant moisture transport
mechanism than air movement.  Joe does too!  But I'm reluctant to abandon
interior VDRs in my climate.

Finally - poly CAN be an air barrier as well as a VDR, if done well.  Does it
and its performance last over time?  Great question.  My experience is that the
homes I've designed and supervised have held up in terms of their energy
performance.  Will they last 50 years?  Great question - let's stay tuned!  I
prefer other insulation materials to fiberglass batts, but I am reluctant to
conclude that there is only one way to do anything.  And in fact some of the
lowest energy use homes in North America (probably most of them) have been
built with fiberglass batts.  As long as you repeat the mantra, fiberglass does
not stop air, you can succeed.

Fire away, Fred, I'm already duckin'!

Marc Rosenbaum


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