house. Due to a number of factors, an upstairs shower stall, 4' X 4' ended
up in an outside corner of the house (north west corner). I know this is
not a great spot for it, but there it is. The tiler would like me to place
a vapor barrier over the framing before the hardy backer and tile go on.
walls) behind the shower. This I presume is to keep moisture from the
shower away from the insulation. I am wondering if this is a good idea
given the porous nature of tile and hardy backer. Any input? Thanks.
--- On Tue, 12/16/08, William C Badger AIA
Stain may be fine for certain applications, but white stain does not cut
it on a 19th Century Greek Revival building. There are many applications
where paint is the only suitable coating and our wall sandwich has to
accept that.
Acrylic water-based primers and paints are relatively vapor permeable, as
long as they don't have lead or zinc oxide pigments.
I have been on the scaffolding and seen soggy paper mache that was once
cellulose insulation pulled out of walls.
Without the forensic analysis that would determine the source of the
moisture, that anecdote indicates nothing.
Try telling the owners of a late 18th Century library that all the books
and shelves need to be removed and the wood paneled walls painted with
vapor barrier paint. For that matter, any old house with an historic or
just well finished interior is not a candidate for an interior vapor
barrier.
Many of the early cellulose retrofits had insufficient density to prevent
the air movement that is the primary vector of moisture in walls. If
properly dense-packed, and interior humidity levels are appropriately
controlled (and there are no sources of bulk moisture, such as wet
basements or crawl spaces or ice dam leakage), cellulose retrofits do not
require a vapor retarder. In fact, one cellulose manufacturer - Applegate
- will void their warrantee if a vapor retarder IS used.
More attention is being paid to the "flow-through" concept of moisture
control, allowing drying in both directions.
In 1979 a field study in Portland, Oregon (4,792 degree days) concluded
there is no risk of moisture damage in mild climates without a vapor
barrier
A second major field study was done in Spokane, Washington (6,835 degree
days) by George Tsongas, Ph.D. P.E. Professor of Mechanical Engineering at
Portland State University. The exterior walls of 103 homes were opened, 79
with retrofitted insulation and 24 uninsulated as a control group. “This
study strongly concludes that the addition of wall insulation without a
vapor barrier does not cause moisture problems in existing homes in
climates similar to that of Spokane.” Bonneville Power Administration
provided funding for this study.
A 2004 study released by building scientist Erkki Kokko of Finland,
”Hygroscopic Cellulose Fiber Insulated Structures” found the use of
permeable building materials resulted in improved indoor air quality. The
absence of a vapor barrier, such as polyethylene film, allowed the wall to
absorb and desorb relative humidity. This enables the interior relative
humidity to remain more constant and comfortable to the occupants.
They also found a 30% reduction in the carbon dioxide levels.
The EEBA’s Builder’s Guide for Cold Climates states in Appendix III,
“Polyethylene on the inside of building assemblies in cold, mixed-humid,
mixed-dry, hot-humid, and hot-dry climates is not generally a good
idea.” “A classic flow-through wall assembly should have a permeable
interior surface and finish and permeable exterior sheathing and permeable
building paper drainage plane.” This permits drying to both the interior
and exterior.
In a December 2001 presentation in Proceedings of Thermal Performance of
Building Envelopes VIII, Asst. Prof. John Straube stated “In many
practical situations, a low permeance vapour barrier will not improve
hygrothermal performance, and may in fact increase the likelihood of
damaging condensation or trap moisture in the system. In some cases, a
low-permeance vapour barrier may be called for, but in many practical
high performance enclosures, none is needed, and eliminating them will
actually improve performance by encouraging drying and avoiding
solar-driven diffusion wetting.
I would pose a typical problem building for group comments. A late 19th
Century structure with some timber frame and some stick built walls. The
bulk of the walls are uninsulated with clapboards nailed directly to the
studs (no sheathing). The interior walls are lath and plaster with 1/2"
Celetex over it and 1/4" plywood paneling over that. It has a stone
foundation and slate roof. The attic is vented and the attic floor is
insulated with a nominal 12" of fiberglass, but electricians over the
years have done their best to rearrange it. The paint tends to hold fairly
well, but is a mix of relatively new coatings and what ancient bits have
still hung on. Will dense pack cellulose exert enough pressure to pop
clapboards off (small cut nails are what was used)? Will moisture transfer
peal the paint?
As long as moisture sources are mitigated (stone basement?) and interior
humidity levels are controlled with appropriate ventilation, this might be
a good candidate for dense-pack cellulose. It's unlikely that the
installation would pop the cladding nails, as aged wood has amazing
holding power, unless previous rusting has deteriorated the bond.
Ironically the potential problem in this case might be the presence of an
interior vapor barrier - the Celotex, with its double foil facings. Since
the exterior cladding has no weather-resistant barrier (not even
sheathing), there is a potential for wind-driven moisture penetration,
particularly if there is a high exposure level (no trees or other adjacent
buildings as protection).
This wall structure would have to do all its drying to the outside; but
since there should be little moisture drive from the inside, the outward
drying force may not be sufficient to lift the paint.
But, if I were interested in preserving this building, I would consider
removing (and either saving or replacing) the exterior cladding and
installing a weather-resistant barrier (probably 15# felt or grade D
building paper). Without such a secondary drainage plane, it's likely that
you would be up on that scaffolding again removing soggy cellulose and
rotted wood.
Removing the cladding would also allow the cellulose to be blown in behind
InsulWeb netting for a more complete installation (around knee braces,
etc), then covered with WRB and siding.