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I believe that a plastic vapor barrier is an invitation to
problems in almost any thermal envelope. It was promulgated by
code when most houses were insulated with fiberglass and there was
little attention to (even recognition of) the need for
air-tightness.
In addition to preventing a thermal assembly from breathing
(diffusion drying to the inside), it also prevents a wonderfully
hygroscopic material like cellulose from performing as a moisture
buffer to stabilize indoor RH, and likely creates a static charge
which draws negative ions out of the living environment.
Now that it's been proven that, with reasonable indoor RH
levels, diffusion contributes as little as 1% of the total
moisture load in a thermal envelope during the heating season, and
that stopping air movement is the key to preventing
indoor-generated moisture problems in the structure, the air-tight
drywall approach solves all the problems without creating
more.
Unfortunately, particularly for the sustainable building
community, sprayed foam is being touted as a solution to moisture
problems, when it (much like plastic VBs) creates its own set of
negative consequences, since closed-cell foam has no moisture
storage (buffering) ablity and open-cell foam can trap moisture
and cause wood rot and mold. This, of course, in addition to the
non-renewable resource depletion, embodied energy and carbon
contribution issues.
I have calculated that, compared to a 2000 SF reasonably
well-sealed code-standard fiberglass-insulated house (0.5 ACH), a
urethane sprayed house (with insulated gable walls and roof
assembly rather than ceiling), even accounting for an increase in
air-tightness (min. 0.35 ACH), will have an additional embodied
energy cost that would require 23 years of operation to pay
back.
The same fiberglass-insulated house with 1" of exterior XPS
and 0.35 ACH would have an additional embodied energy payback
of less than 1 year, in large part because it results in much
greater energy savings than the typical sprayed urethane, mostly
by eliminating thermal bridging.
Open-cell sprayed Icynene has almost the same embodied energy
liability as the fiberglass, but it would result in a less
energy-efficient house.
A similar house with 2x8 framing 24 oc (instead of 2x6 16 oc
for the others), which uses no more total wood, and made very
tight with the air-tight-drywall system (0.25 ACH, which is
adequate in a non-toxic breathable house), would have 41% of the
insulation embodied energy of the fiberglass (less than 5% of the
urethane) and use 39% less heating energy. --- On
Fri, 12/12/08, Tim Yandow
<[log in to unmask]> wrote: I have found that using a vapor barrier with wet spray is
an invitation to disaster though. The walls need to breathe.
Tim Yandow
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