My own education in architecture (20 years ago) taught my peers and me little about this important aspect of architectural design and building construction. As it turns out, researching this article was to be my real education on the subject of permeability. It appears old Al was spot-on right about relativity and formal education.
Old man trouble
In recent years, the "breathe-ability" of wall and roof assemblies-that is their ability to allow moisture to pass-through and "dry-out"-has been a hot topic of discussion among design/construction professionals and litigation (e.g. EIFS barrier systems). The purpose of a vapor retarder is to keep water vapor-which is an invisible gas-out of wall/roof cavities where it can condense on cold surfaces thereby accumulating moisture within the cavity. The result of such moisture present within the wall/roof cavity can be mold growth and/or rot-not good. Polyethylene vapor retarders have been used for many years for this purpose, to keep moisture out but they have serious shortcomings, as we shall see.Water vapor naturally seeks to move from areas of high vapor pressure to areas of low vapor pressure. Thus, depending on conditions and circumstances, it can/will migrate from the outside in or from the inside out. A generally accepted rule of thumb concerning vapor barriers is to use only one vapor barrier installed on the room or "warm-in-winter" side of the cavity insulation. Use of more than one vapor barrier hinders vapor flow and the dry-out potential for a wall/roof assembly. Vapor barriers are generally used in heating/mixed climates, not in high outdoor humidity (cooling) climates or in buildings with high-constant indoor humidity (pools, saunas, etc.).
The main benefit of traditional 6-mil thick polyethylene is that it is a very effective vapor retarder during the heating season (winter) due to its impermeability. This very same characteristic (impermeability) tends to trap moisture in a wall/roof assembly during the season's transition and does not allow proper dry-out of the cavity during the cooling (summer) season. This paradox of benefit/detriment is particularly pervasive if the building envelope consists of low permeance materials, such as glass and steel. Another good rule of thumb concerning permeability is, "The amount of moisture that gets into an assembly should always be less than that which can get out of it."
Since water vapor always seeks the path of least resistance (from higher to lower vapor pressure), polyethylene vapor barriers can contribute significantly to "reverse vapor drive." Exterior building surfaces warmed by the hot summer sun can be warmer and more humid than the building interior. Thus, the water vapor is driven into the interior but the polyethylene's impermeability prevents it from passing through where it remains trapped in the cavity. The ability of water vapor to pass through a material and dry-out is known as "vapor diffusion."
Vapor diffusion is a slow process and is contingent upon the material's physical properties, overall thickness and relative ambient air conditions on both sides of the material. Measured in perms, cellular and fibrous materials, such as foam/fiberglass/mineral wool insulation, are most permeable whereas wood is semi-permeable. Vapor barriers, such as polyethylene fall, into the generic category of water resistant barriers. A major advantage of "sheet barriers," such as polyethylene as opposed to surface applied coatings is the fact that the installation process requires an overlap of the polyethylene sheets. This overlap provides a ventilation path whereby some water vapor can pass through-conversely, it can also allow moisture in. Vapor barriers also serve to block out exterior pollutants into the interior environment very effectively.
Smart, very smart
Certainteed has created the first "breathable" vapor barrier: MemBrain Smart Vapor Barrier. By altering pore size (thus its permeability) relative to ambient humidity levels, MBSVB is "smart" in the sense that it changes from impermeable to semi-permeable based on environmental conditions-standard polyethylene cannot do this. During the summer (cooling) season, dry-out can occur since moisture can pass through the opened pores of MBSVB. When relative humidity is plus-60 percent, the microscopic pores in MBSVB expand thus increasing permeability significantly in the transparent polymide-based sheeting MBSVB is made from. Though only 2-mil thick, it has a high tensile strength and is as strong as standard 6-mil thick polyethylene. This high strength-to-thickness ratio is due to the fact that MBSVB is made from a nylon-based material. During the winter (heating) season, when humidity levels are low, MBSVB acts much like standard polyethylene vapor barriers by blocking (impermeable) the passage of water vapor so it cannot condense on cold surfaces within a wall/roof cavity.It is under high relative humidity conditions that MBSVB shows its smarts. Vapor diffusion during seasonal changes and/or moisture intrusion is dependent on the permeability of cavity fill insulation and interior finish materials. Use of vapor retarding paints and/or vinyl wallpaper is therefore not recommended. Fiberglass or mineral wool batt insulation works best as a permeable cavity fill insulation. "Short-peak" high humidity rooms, such as kitchens and bathrooms, are acceptable for use of MBSVB since the interior finishes typically provide a "buffering action."
MBSVB has not been tested with wet-spray insulation systems to date and is therefore not recommended for use with these types of insulation systems (e.g. cellulose, BioBase 501, etc.). It should not be used for exposed applications, ground cover and/or between concrete subfloors/flooring materials.
By changing its permeability, the drying potential for a closed building envelope is dramatically increased. In turn, this reduces the dangers of (and legal liabilities) due to mold/rot growth that can/will occur if significant moisture is present in wall/roof cavities. Though it has a higher initial cost than polyethylene, this is quickly amortized by the reduced risk/liability/callbacks associated with moisture intrusion. MBSVB looks like and is installed very much like standard polyethylene sheeting so it is a material rather than a labor additional cost burden. MBSVB comes in rolls of 100 linear feet in standard widths of 8, 9 10 and 12 feet. Also, it is not adversely affected by gasses and/or organic pollutants.
For seven years in Europe, prior to its North American introduction, Certainteed proved MBSVB's effectiveness in real-world applications and has patented the product. In terms of temperature and humidity, MBSVB is ideal for use in severe climate conditions or mixed/heating climates-not in high humidity (cooling) climates or buildings with plus 50 percent interior humidity levels.
MBSVB was named a "Top 10" green building product by the prestigious GreenSpec Directory for 2003. By blocking pollutants from entering interior spaces from the outside and allowing wall/roof assemblies to "breath" (dry-out)-thus preventing mold and rot to occur, indoor air quality is dramatically improved. IAQ is a significant criteria/measure of a building's green characteristics and is used in the LEED certification program as a major parameter.
