1、 Humidity Control Design Guide for Commercial and Institutional Buildings Major Sections nor may any part of this book be stored in a retrieval system, or transmitted in any form by any means- electronic, photocopying, recording or other-without permission in writing from ASHRAE or from the original
2、 copyright holder. DISCWMER ASHRAE has compiled this Publication with care, but ASHRAE has not investigated andASHRAE expressly disclaims any duty to investigate, any product, service, process, procedure, design or the like which may be described herein. The appearance of any technical data or edito
3、rial material in this publication does not constitute endorsement, warrantee or guaranty by ASHRAE of any product, service, process, procedure, design or the like. ASHRAE does not warrant that the information in this publication is free of errors.The entire risk of the use of any inforrna- tion in t
4、his publication is assumed by the user. Humidity Control Design Guide For Commercial and Institutional Buildings ISBN 1-883413-98-2 iii Lew Harriman Mason-Grant Consulting P.O. Box 6547 Portsmouth, NH 03802 USA LewHarrirnanMasonG Geoffrey W. Brundrei Cooks Hill, Kingswood Fordsham, WA6 6JT United Ki
5、ngdom Geoff BrundrettW Reinhold Kittler Dectron Internationale 2316 Mariners Mark Way, # 304 Virginia Beach, VA 23451 Larry Berglund Tohoku University, Japan IarryQsa bine.pln.archi.tohoku.ac.jp Donald Gatley D.P Gatley 2001 At the same time, we are humbled by the magnitude of the project we have u
6、ndertaken. Imagine how difficult it would be to produce a book titled “Temperature Control Design Guide.” The possibilities for books on that topic are endless, and the opinions of experienced professionals are often contradictory and strongly-held. We faced a similarly daunting task in writing this
7、 book. With that in mind, we trust that the reader wi consider this book simply a good beginning rather than the last word on humidity con- trol. Much remains to be done. We sincerely hope that the reader will share his or her experiences, observations, questions, correc- tions and considered judgem
8、ents in the form of additional written work, expanding the range and depth of information available to our profession and to the public at large. V I. 2. 3. 4. 5. 6. 7. 8. 9. Major Sections andmaterials must be selected so that interior finishes do not trap moisture in walls, and so that thermal bri
9、dges do not create condensing surfaces. Fig. 2.3 Take Steps To Minimize Air Leaks . Vapor Diffusion Is Seldom A Problem In a typical commercial building, the moisture load from infiltration is 1 O to 200 times larger than the load from permeation. Hence the importance of minimizing air leaks. 80 70
10、I 201 1 31.31 140 160 180 Chapter 2. Humidity Control Basics 11 contacting that surface. The Architect is certainly responsible for de- signing the wall, ceiling and floor so that vapor diffusion does not cause condensation. To avoid that potential problem, many useful calculations and computer prog
11、rams are readily available to assist the Architect and the HVAC deigner. However, the much larger problem is air initration through gaps in the exterior building envelope. Air infiltration produces 10 to 200 times more water vapor leakage than vapor diffusion, as shown graphi- cally in figure 2.3. U
12、nfortunately, engineering tools for predicting and quantng that huge problem are essentially nonexistent, be- cause the problem depends on the number and size of construction gaps, and the relative air pressures on each side of the wall. No con- struction team intends to have any gaps in the waii, a
13、nd the wind pressure varies minute-to-minute. So in the absence of credible val- ues to fill those terms, equations to calculate leakage are educated guesses. But that does not mean that the architectural and HVAC de- signers are without resources to minimize this key problem. Sugges- tions include:
14、 Install a continuous air barrier Regardless of climate, air leaking into the walls will cause prob- lems. In cold climates, humid indoor air can condense its moisture if it gets into cold exterior walls. In hot and humid climates, humid Hot 81 Humid Climate =Air Barrier Locating the air barrier on
15、the outside of the insulation keeps humid outdoor air from entering cool wall cavities, where moisture would condense in summer time Cold Climate =Vapor Retarder Locating a vapor retarder inside of the insulation keeps humid indoor air out of the cold wail during winter time. Fig. 2.4 locating Air B
16、arriers And Vapor Retarders outdoor air leaks in and condenses moisture on cooler surfaces inside walls. Installing an air barrier reduces these problems. The placement of the air barrier is critical. In hot and humid climates, it should be located on the outdoor side of the insulation. In very cold
17、 climates, the air barrier should be located on the in- door side of the insulation. In climates such as Kansas City, Missouri or Seoul, Korea, both very hot summers and very cold winters are typical. In those climates the air barrier should be towards the out- side-unless the building is humidified
18、 during the winter, as in a swimming pool enclosure or a museum. Then the air barrier should be placed towards the indoors with respect to the insulation. The logic is not immediately obvious, but it is consistent: an air barrier is best located where it can keep the most humid air out of most of th
19、e wall for most of the year. In addition, for any air barrier to be effective it must be continu- ous. All seams and joints must be sealed with tape, not just over- lapped. This includes the joints between the air barrier Eim and ail doors and window frames, and the whole length of the joints where
20、waiis meet the roof and foundation. A good WAC design and installation is also necessary, but the Architect and Owner should recognize that the foundation of humid- ity control is a tight building. Without that foundation, humidity con- trol will be very difficult and costly to achieve, no matter ho
21、w well- designed the HVAC system might be. Avoid suction inside the exterior wall One reason that commercial buildings leak so much air is that HVAC systems often create suction near the exterior wall by accident. Then air outdoor leaks in continuously. Although the WAC designer and contractor are r
22、esponsible for avoiding this problem, the archi- tectural designer can also help avoid such suction. When the HVAC equipment does not have ducted returns, the plenum above the ceiling is under negative pressure constantly, which leads to very high outdoor air inltration rates2 Figure 2.14 shows 12 C
23、hapter 2. Humidity Control Basics this situation, along with what each member of the team can do to minimize the problems it creates. The WAC designer can reduce the problem by specdymg a re- turn duct system which is hard-connected to the rooftop WAC unit. The architectural designer can also reduce
24、 the potential problem by specdjmg that the interior wallboard must continue all the way to the roof, and must be sealed to the underside of the roof decking. Specify a whole-building leak test After construction is complete, the Owner, Architect, Engineer and maintenance staff assumes it has been b
25、uilt to specifications, and will not leak an excessive amount of air. Where humidity levels are especially high or low, as in museums, swimming pools or ice arenas, quantng the air leakage after construction is useful. This can be done by specng a whole-building leak test. ho leak test procedures ar
26、e often used by indoor air quality consultants and by energy auditors. Smaller buildings can be tested at low cost with a blower door. In that procedure, the door is re- placed with a moveable partition containing an air blower. The build- ing is “inflated, and the blower air flow is adjusted until
27、the pres- sure inside the building is either 4 or 50 Pascals higher than the pressure outside the building either 0.016 or 0.2 in. wc. higher inside the building . The air flowing into the building at that pres- sure difference is defined as the leakage.5 Larger buildings are usually tested by measu
28、ring the dilution of a tracer gas. A harmless tracer gas, usually sulfur hexafluoride, is re- leased inside the static, closed building with the WAC system off. The concentration of the gas is measured periodically and compared to the original concentration. The dilution of the gas over time allows
29、calculation of the air infiltration rate.5 After the leakage volume is quantified, it can be compared to the amount of excess air provided by the ventilation system. In hot and humid climates, the WAC designer and contractor can adjust the ventilation air to maintain a positive internal air pressure
30、, which en- sures that dry air will be leaking out-rather than humid air leaking into the building. In cold weather, ventilation and exhaust air (be- yond the code-required minimums) should be adjusted to maintain a neutral or slightly negative internal air pressure, to keep humid indoor air from le
31、aking outwards into cold wall cavities. Vapor retarders In most commercial buildings, air barriers are more useful than vapor retarders. Humid air inftration into cold walls carries far more damaging moisture than what vapor permeation can push through solid materials. Also, many walls will gain exc
32、ess moisture at some point through air or water leakage. Placing a vapor retarder in the wall makes it more fragile, because the water may become trapped. Avapor retarder may prevent the seasonal drying that avoids a major problem. On the other hand, vapor retarders are quite important in cold clima
33、tes, and for any building with a high indoor moisture level, such as museums and swimming pool enclosures. The logic for plat- ing vapor retarders is the same as for air barriers: keep high humid- ity out of the walls. Here are some guidelines which may be useful: Locate vapor retarders only on the
34、warm side of the wall In cold climates and in humidified buildings, the vapor retarder should be located inside from the insulation. Vapor will be held inside the occupied space, rather than in the cooler wall cavities, where it could condense. Avoid doubled vapor retarders Some of the most widespre
35、ad and costly building failures have been caused when vapor retarders are doubled by accident, creating a vapor tight “pressure cooker” by sealing both sides of an exterior wall. Then any moisture that gets into that wall never dries out, and major damage follows. The classic example is when the architec- tural designer and owner decide on a face-sealed external glass fiber insulation wall for the exterior (EIFS), while at the same time the interior designer places an impermeable vinyl wall covering on the inside of a hotel room in a humid climate. Tens of millions of dollars
