I am interested to know if anyone has an opinion about or experience withthe following situation:I am currently constructing a 2000 sq ft double wall, dense pack cellulosehouse. Due to a number of factors, an upstairs shower stall, 4' X 4' endedup in an outside corner of the house (north west corner). I know this isnot a great spot for it, but there it is. The tiler would like me to placea 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- 2x4walls) behind the shower. This I presume is to keep moisture from theshower away from the insulation. I am wondering if this is a good ideagiven the porous nature of tile and hardy backer. Any input? Thanks.Tim YandowI 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 cutit on a 19th Century Greek Revival building. There are many applicationswhere paint is the only suitable coating and our wall sandwich has toaccept that.
Acrylic water-based primers and paints are relatively vapor permeable, aslong as they don't have lead or zinc oxide pigments.I have been on the scaffolding and seen soggy paper mache that was oncecellulose insulation pulled out of walls.
Without the forensic analysis that would determine the source of themoisture, that anecdote indicates nothing.
Try telling the owners of a late 18th Century library that all the booksand shelves need to be removed and the wood paneled walls painted withvapor barrier paint. For that matter, any old house with an historic orjust well finished interior is not a candidate for an interior vaporbarrier.
Many of the early cellulose retrofits had insufficient density to preventthe air movement that is the primary vector of moisture in walls. Ifproperly dense-packed, and interior humidity levels are appropriatelycontrolled (and there are no sources of bulk moisture, such as wetbasements or crawl spaces or ice dam leakage), cellulose retrofits do notrequire 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 moisturecontrol, allowing drying in both directions.
In 1979 a field study in Portland, Oregon (4,792 degree days) concludedthere is no risk of moisture damage in mild climates without a vaporbarrier
A second major field study was done in Spokane, Washington (6,835 degreedays) by George Tsongas, Ph.D. P.E. Professor of Mechanical Engineering atPortland State University. The exterior walls of 103 homes were opened, 79with retrofitted insulation and 24 uninsulated as a control group. “Thisstudy strongly concludes that the addition of wall insulation without avapor barrier does not cause moisture problems in existing homes inclimates similar to that of Spokane.” Bonneville Power Administrationprovided funding for this study.
A 2004 study released by building scientist Erkki Kokko of Finland,”Hygroscopic Cellulose Fiber Insulated Structures” found the use ofpermeable building materials resulted in improved indoor air quality. Theabsence of a vapor barrier, such as polyethylene film, allowed the wall toabsorb and desorb relative humidity. This enables the interior relativehumidity 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 goodidea.” “A classic flow-through wall assembly should have a permeableinterior surface and finish and permeable exterior sheathing and permeablebuilding paper drainage plane.” This permits drying to both the interiorand exterior.
In a December 2001 presentation in Proceedings of Thermal Performance ofBuilding Envelopes VIII, Asst. Prof. John Straube stated “In manypractical situations, a low permeance vapour barrier will not improvehygrothermal performance, and may in fact increase the likelihood ofdamaging condensation or trap moisture in the system. In some cases, alow-permeance vapour barrier may be called for, but in many practicalhigh performance enclosures, none is needed, and eliminating them willactually improve performance by encouraging drying and avoidingsolar-driven diffusion wetting.
I would pose a typical problem building for group comments. A late 19thCentury structure with some timber frame and some stick built walls. Thebulk of the walls are uninsulated with clapboards nailed directly to thestuds (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 stonefoundation and slate roof. The attic is vented and the attic floor isinsulated with a nominal 12" of fiberglass, but electricians over theyears have done their best to rearrange it. The paint tends to hold fairlywell, but is a mix of relatively new coatings and what ancient bits havestill hung on. Will dense pack cellulose exert enough pressure to popclapboards off (small cut nails are what was used)? Will moisture transferpeal the paint?As long as moisture sources are mitigated (stone basement?) and interiorhumidity levels are controlled with appropriate ventilation, this might bea good candidate for dense-pack cellulose. It's unlikely that theinstallation would pop the cladding nails, as aged wood has amazingholding power, unless previous rusting has deteriorated the bond.
Ironically the potential problem in this case might be the presence of aninterior vapor barrier - the Celotex, with its double foil facings. Sincethe exterior cladding has no weather-resistant barrier (not evensheathing), there is a potential for wind-driven moisture penetration,particularly if there is a high exposure level (no trees or other adjacentbuildings as protection).
This wall structure would have to do all its drying to the outside; butsince there should be little moisture drive from the inside, the outwarddrying force may not be sufficient to lift the paint.
But, if I were interested in preserving this building, I would considerremoving (and either saving or replacing) the exterior cladding andinstalling a weather-resistant barrier (probably 15# felt or grade Dbuilding paper). Without such a secondary drainage plane, it's likely thatyou would be up on that scaffolding again removing soggy cellulose androtted wood.
Removing the cladding would also allow the cellulose to be blown in behindInsulWeb netting for a more complete installation (around knee braces,etc), then covered with WRB and siding.