ASHRAE Journal - August 2008 - (Page 61)

Photo 1 (left): The Typical House. It was built in 1965 in Cleveland with no cavity insulation in the exterior wood-frame walls. No problems occurred for 20 years. Insulation was added to exterior walls in 1985 and within one year the paint began to fail. Photo 2 (right): Retroﬁt Cellulose Cavity Insulation. Dry blown cellulose packed into exterior wood-frame walls. It was injected from the exterior at top of cavity. This author, very young at the time, is amazed at how well the insulation worked. Cavities were dry when opened. and convection. That results in less energy exchange, meaning less drying potential; and, materials on the exterior side of the enclosure are colder in the winter. Building materials being colder on the outside lead to something most folks don’t consider. Many building materials are hygroscopic (Figure 1). This means they absorb moisture based on relative humidity. Even more strangely, they don’t care about vapor pressure except if it affects relative humidity. This is a big deal. In fact, it is a huge deal. Quick, pop quiz on psychrometric chart stuff: as the temperature drops, and vapor pressure is kept constant, what happens to the relative humidity? Time’s up. Yes, folks, you are correct, the relative humidity goes up. The implications are staggering. Just making the outside materials of your building cold, makes them wetter. Period. But, the moisture content of the outside air in the wintertime is low. Yes, it is, but its relative humidity is high. The amount of moisture in the absolute sense is low, but in the relative sense it is high. And, hygroscopic materials don’t care about the absolute sense; they only care about the relative sense. Why? Second Law stuff. Isn’t it always? Check out Photo 1. Nice, normal, everyday type of house in Cleveland: 20-years-old, no insulation in the wall cavities, built in 1965. The house was perfectly happy at this point for 20 years. Then, the homeowner decides to do something crazy. He insulates the exterior walls to save energy and has the cavities blown with dry cellulose. What a dumb idea in 1985, eh? But, nevertheless, it was a beautiful job, no voids—yes, it can be done, and it was done. No voids, no convection (Photo 2 [notice how young this engineer once was]). Next thing you know, the paint falls off, and the siding gets smelly, moldy, and begins to decay (Photo 3). Huh? The insulation did this? Yup. The drying potential is reduced due to the addition of the insulation so that the moisture entering the cladding assembly from the exterior due to rain and capillarity does not evaporate August 2008 Photo 3: Wet Cladding: Problems with mold and decay were worse on north and east elevations. Problems with peeling paint were worse on west and south elevations. Solar radiation cycled the cladding moisture content more on the west and south elevations stressing the paint. UV cross-linked the paint making it less ﬂexible. Less ﬂexibility meant more movement on the south and the west led to the peeling paint. Less energy on the north and the east led to the mold and the decay. and this is further exacerbated by the colder siding having a higher local relative humidity and a higher equilibrium moisture content due to the sorption properties of wood. Bummer. How to ﬁx this? It’s easy. Wedges were used to create a gap at the overlaps of the siding reducing capillary uptake at the laps and increasing evaporation despite the cladding being colder (Photo 4). This early lesson in sorption and drying potentials taught this engineer to drain and back-ventilate claddings, especially those on highly insulated assemblies (Photo 5). Let’s extend this discussion to cavity insulation in general. The same thing happens to the cavity side of sheathing when ASHRAE Journal 61

Table of Contents Feed for the Digital Edition of ASHRAE Journal - August 2008

ASHRAE Journal - August 2008 Contents Commentary Industry News Letters Meetings and Shows Maintain to Sustain—Delivering ASHRAE’s Sustainability Promise Ultraviolet Germicidal Irradiation: Current Best Practices Improving Humidity Control With Energy Recovery Ventilation Single- or Two-Stage Compression Data Center Cooling: Using Wet-Bulb Economizers Building Sciences InfoCenter Practical Pointers Products Emerging Technologies Washington Report People Special Products Classified Advertising Advertising Index

ASHRAE Journal - August 2008

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