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--- 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.