Hello Tim:
The situation does look as if you are asking to supersaturate the  
cellulose insulation.
I would suggest you provide a contiguous ceiling, wall and floor  
waterproof barrier under the tile.... or a fiberglas shower unit....  
combined with a pretty good exhaust fan.

I've always been nostalgic about the switched half-window 30x30 half- 
window bathroom exhaust fan my wife and I had in our first apartment  
in an Art Deco building. It really worked to suck out the vapor in a  
short time.

Jonathan Miller, FCSI, AIA
JMMa Specs

On Dec 18, 2008, at 7:27 PM, Tim Yandow wrote:

>> 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.