ASHRAE Journal - August 2008 - (Page 65)

with the drapes open, and the room full, and everything on. And, you know that someone in some bureaucracy somewhere wants way too much outside air. Therefore, most of the time your latent load dwarfs your sensible load. So, what do you do? You reduce the amount of outside air by installing timers on exhaust fans, and you ignore the faceless bureaucrat. Then, you only run the through-wall unit to control the temperature (i.e., make it the sensible system) and install a dehumidifier in each unit to control the humidity (the latent system) (Photo 6). It’s a primitive, but effective fix. In new designs we hide the dehumidifier better, or better still, we make it part of the ventilation system. Guess where this technology is making an impact? Condos and apartments. Small houses are next. We are already separating the latent from the sensible on most commercial design. After all, it is the sensible thing to do. North Versus South So, why is it worse in the south than the north? In the north, it still gets cold and the outside cold air is pretty dry—and walls tend to dry to the outside in the north—so the size of the moisture drying sink has not changed. But, in the south, walls also dry to the inside. And, if the inside is humid because of the part-load problem, the walls stay wetter because the moisture drying sink is smaller. Additionally, in the south the moisture drive from the exterior into the wall is greater, while at the same time the moisture drive from the wall into the interior is smaller as compared to the corresponding drives in opposite directions in the north. Let me translate. It is easier to get wet from the outside in the south than it is from the inside in the north. It is also easier to dry to the outside in the north than it is to dry to the inside in the south. With less energy available, it gets worse in the south faster than in the north. Good air-conditioning design leads to dry interiors and dry interiors allow walls to dry to the inside. Part-load humidity problems lead to wetter walls. Higher levels of thermal insulation and lower heat gain lead to part-load humidity problems. Not good. Now What? OK, everybody relax. We want and we need much higher levels of thermal control (airtight, insulated assemblies) everywhere. We just have to be smart about it. We want and need ultraefficient glazing, lights, appliances, and reflective surfaces everywhere. We just have to be smart about it. And, we want and need the right amount of outside air whenever we need it. We just have to be smart about it. This is neither expensive nor difficult. It’s just different. We need to modify our building enclosure and mechanical system paradigms. Get used to it or be steamrolled by the change. Insulate on the outside. Back-ventilate and drain your cladding. Separate your sensible load from your latent load. Don’t overventilate. Existing buildings are going to be a bear. And, have a nice day. August 2008 Photo 6: Hotel Room Fix. The through-wall unit controls the temperature (the sensible system). The dehumidifier controls the humidity (the latent system). Joseph W. Lstiburek, Ph.D., P.Eng., is a principal of Building Science Corporation, in Somerville, Mass. Visit www. buildingscience.com. www.info.hotims.com/16017-5 ASHRAE Journal 65 http://www.buildingscience.com http://www.buildingscience.com http://www.ajmfg.com http://www.ajmfg.com http://www.info.hotims.com/16017-5

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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