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
Open-cell sprayed Icynene has almost the same embodied energy
liability as the fiberglass, but it would result in a less
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
--- On Fri, 12/12/08, Tim Yandow
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I have found that using a vapor barrier with wet spray is
an invitation to disaster though. The walls need to breathe.