ASHRAE Journal - August 2008 - (Page 62)

Photo 4 (left): Wedgie. It’s so simple and obvious after the problem is understood. Wedges created gaps that reduced capillary uptake of surface water and promoted drainage and back ventilation. Photo 5 (right): Drainage and Ventilation Mat Behind Cladding. This is easy for new construction and re-cladding. Existing buildings are going to be a problem. Don’t say you weren’t warned. we insulate cavities. It gets worse when the cavities also have no airflow or convection.‡ Sheathing get colder. If the sheathing is hygroscopic, its equilibrium moisture content also goes up. Why take the risk? Insulate on the exterior of the sheathing. This makes the problem go away. The risk is so high in cold climates that folks are no longer insulating the cavities in steel stud walls in places like Ottawa and Montreal. All of the insulation goes on the outside. This is why insulating sheathing is pretty much the name of the game for highly insulated wall assemblies. Or, at a minimum, claddings should be back-ventilated and back-drained. Or better, do both. This is easy for new construction. The problem is what to do with the existing building stock? What are we going to do if oil ever goes above $100/barrel? Dumb comment. Yes, but now we have a real problem. Folks cannot blow/spray/fill cavities with a bunch of insulation without appreciating the consequences. This is not just a cold climate problem. It is a problem everywhere. In fact, it gets worse when we go far south. Reduced Heat Gain I am going to let you in on a little secret that we mechanical engineers don’t often share with the rest of the world: airconditioning systems only dehumidify when they run. When they don’t run, they don’t dehumidify. If there is no load, there is no dehumidification.§ Well, just make them run longer. How? They still make the air cold no matter how you run them. That is good if you need cold air. But, what if you don’t need cold air? What if you only need dry air? Well, you still have to make the air cold to get it ‡Airflow (air change and convection) increases energy exchange, which increases drying potential. My Canadian friends’ heads are going to explode because they have been taught that air leakage leads to wetting from interior sources. Yes, it does in extreme climates with high interior moisture loads, such as humidified buildings in Canada. It doesn’t occur everywhere else. Come visit the United States. The dollar exchange now works for you, and air leakage improves drying potentials. §Sensible load. I know you knew that I knew this, but some of the e-mail I have been receiving from you folks has been quite picky, which is good. But, I want to head off the obvious e-mails, so continue to be picky, but not on this. www.info.hotims.com/16017-10 62 ASHRAE Journal August 2008 http://www.bapihvac.com http://www.bapihvac.com http://www.info.hotims.com/16017-10

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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