ASK THE EXPERTS: Permeability

Straw bale walls provide "breathability" relative to vapor permeability, but still require a means of exchanging fresh air.

I’m a bit confused by statements in the “Ask the Experts” piece in HP171 regarding wall permeability and breathability.

One statement says “...moisture in vapor form is able to transpire from one side of the wall assembly to the other. This is a highly desirable trait, as it helps prevent excessive moisture buildup in the home and in the walls, and allows for dynamic drying of the wall to take place year-round.”

I’m a retired general contractor, and all my research contradicts this. In addition to research, both building codes and requirements for the “Super Good Cents” homes I built back in the day require vapor barriers on the inside of walls and ceilings. (Ceilings are not required to have vapor barriers if the attic is adequately ventilated.)

The reason given is because, in cold weather without these vapor barriers, warm, humid air inside the house (in winter) will often reach dew point somewhere within the wall cavity, leading to condensation, wetness, mildew, dry rot, and reduced R-value in insulation. My building/designing career was in southwest Oregon. Are there other areas that can safely do without vapor barriers?

Malcolm Drake • Grants Pass, Oregon

This question comes up a lot when I am discussing vapor-open wall systems, especially with code officials and builders. The inclusion of a vapor “barrier” (an unfortunate term; I prefer “vapor control layer”) is one strategy that can prevent moisture damage in a wall system, and it is intended to prevent moisture from condensing at a dew point within the wall when the wall materials (especially the insulation and the exterior sheathing) have no storage capacity and/or limited permeability. If water is “pouring” into a wall assembly faster than it can escape, this will lead to moisture damage, mold, and rot issues. Mineral-based insulation materials like fiberglass and rock wool have no ability to absorb humidity, and sheathing materials like plywood and OSB have a very limited ability to let humidity through, and hence the solution of trying to keep that moisture out of the assembly by using a plastic liner on the inside face of the wall to prevent it from getting in there.

The natural wall insulation materials we use in vapor-open wall systems have a vast storage capacity. These natural fibers absorb high levels of humidity in damp conditions and release that moisture in dry conditions. They can cycle through endless repetitions of this moisture loading, as long as they return to a reasonably dry state. A straw bale wall, for example, can safely store more than 1 pound of water vapor per square foot of wall area. That is vastly more than will make its way into the wall from diffusion and even from direct air leakage over a typical (Ontario, Canada) heating season. As long as the plaster on both sides of the wall has a reasonable perm rating (I recommend nothing lower than 4 U.S. perms), a natural-fiber wall insulation has no problem handling the moisture load it will experience as that moisture makes its way through the wall to the outside (or inside) atmosphere.

This strategy—combining permeable sheathing materials with natural fiber insulation and no vapor barriers—will work in virtually all climate zones. But it can’t be done partway; the wall assembly must either be fully vapor-open, or it should follow conventions if conventional materials are being used. The worst scenario would be some mix of the two; that’s a sure-fire moisture problem. But building with a completely vapor-open strategy using the right materials has been successful for many builders in many climates—some of us even hit Passive House levels of air tightness without using plastic barriers! And an increasing number of building scientists are starting to realize that this strategy has merit, and can actually result in a simpler and more resilient wall system.

Chris Magwood • Endeavour Centre

I state something similar in my original answer. “There is no need for a vapor barrier because the clay in the wall has a very high capacity to take on vapor when conditions are humid, to store it without any damage to the wall system and then to re-release it when the indoor or outdoor climate has changed. It is this ability that has enabled the historic buildings made of clay and fiber to endure for hundreds of years all over Europe and Asia.”

I would add that the vapor barrier requirement is a fairly new one in the history of building, and now appears to be on its way out, refuted by most building scientists for all but the very coldest climates. If air cooling is used in the hot summer months, which is often the case even in a cold climate, then the vapor barrier can cause problems because the moisture drive—now from outside—can condense on the backside of that barrier. In-wall condensation issues have developed repeatedly in light-frame construction, which is most often insulated with materials that are negatively impacted in the presence of moisture.

Air-driven and diffusion-driven moisture is not an issue with mass walls with high clay content. The vapor is never driven far enough into the wall to reach dew point because the clay absorbs the moisture and because solid mass walls do not have the same kind of air-driven moisture issues as hollow-wall systems. How do we know? There are many hundreds of thousands of clay-based mass wall buildings all throughout Europe and Asia that have stood in cold and wet climates for hundreds of years without deterioration.

Paula Baker-Laporte • Econest Architecture

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