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VGBNTALK  December 2008

VGBNTALK December 2008

Subject:

isolated vapor barrier

From:

Tim Yandow <[log in to unmask]>

Reply-To:

VGBN Discussion <[log in to unmask]>, [log in to unmask]

Date:

Thu, 18 Dec 2008 19:27:27 -0500

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text/plain

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>I am interested to know if anyone has an opinion about or experience with
the following situation:
I am currently constructing a 2000 sq ft double wall, dense pack cellulose
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.
There is 12 inches of cellulose in the walls with a thermal break (2- 2x4
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.
Tim Yandow



I am not sure what makes the most sense here
>
> --- On Tue, 12/16/08, William C Badger AIA
> <[log in to unmask]> wrote:
>
>
> 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. 

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