Others factors being equal, a dry building is more sustainable than a damp building because dry buildings last longer. In a damp building, nails, screws, bolts and steel framing can corrode quickly, which puts the building at risk of early structural failure. Image 1 shows an example of what can happen after only 20 years when moisture and humidity accumulate inside the walls of an air-conditioned building.
In less than 20 years, high humidity and rain leaks ruined the structure and grew enough mold to require complete rebuilding—not a very sustainable building design.
Credit: LEW HARRIMAN
A dry building also has better IAQ than a damp building. Damp building materials degrade, generating and releasing a wide range of indoor air pollutants. According to the National Institutes of Health, Bethesda, Md., the health risk of a damp building is real and measurable. Biologists and health professionals believe mold and bacteria growing in damp buildings are responsible for many negative health effects. According to an article by David Mudari and William J. Fisk in the June 2007 issue of Indoor Air, the Washington, D.C.-based U.S. Environmental Protection Agency and Berkeley, Calif.-based Lawrence Berkeley National Laboratory calculate these health issues probably cost the U.S. about $3.5 billion in treatment annually.
The short roof overhang sheltered this Roman temple for 2,000 years—a bargain-basement investment for long-term resistance to water-related problems.
Credit: LEW HARRIMAN
Occupants are more comfortable at a higher temperature in a dry building than in a damp building, so a dry building’s cooling systems require less energy. In one measured test of a big-box retail super center, chronicled by John Spears and James Judge in the October 1997 issue of the Atlanta-based American Society of Heating, Refrigerating and Air-Conditioning Engineers Inc.’s ASHRAE Journal, when the indoor air dew point was held to 55 F (13 C), the temperature could rise to 79 F (26 C) before the staff or customers noticed. That saved about 15 percent of the building’s annual cooling costs.
ARCHITECTURE CONTROLS THE WATER
The first and most important element in achieving a dry building is to keep rain off and out of the exterior walls. Overhanging the roof above the walls allows very little rain to flow across the wall’s joints, particularly over leak-prone seams above and around windows. Overhanging the roof by as little as 2 feet (0.6 m) likely will reduce the exterior wall’s lifetime water load by more than half, greatly decreasing the risk of leaks.
The second step is to provide and integrate drain pans, or sill-pan flashing, into a waterproofed and well-drained wall underneath every window and door. Installing a watertight sill pan with sealed-end dams and back dams will force any rain leakage out of the exterior wall. Sill pans prevent most common problems caused by exterior-wall water leakage around windows.
The largest remaining load is the water vapor carried into the building by ventilation air and by humid outdoor air leaking into the building. While less immediately risky than rain leaks, humidity loads often cause greater long-term damage. High-humidity loads from ventilation air persist for thousands more hours every year than the shorter, intermittent loads from rain water.
HUMIDITY CONTROL IS NOT A GIVEN
With traditional budgets, ventilation air often is allowed to stay damp as it enters the building. Also, most HVAC designs have neither measured nor reliably controlled ventilation and exhaust air flows. Finally, most building codes do not require sealed duct connections, which is the major reason that most buildings pull in so much humid air. The combined effect of these shortcomings means traditional HVAC systems not only over-ventilate, but also allow tons of excess water vapor to flow through the building each year.
Credit: ASHRAE HUMIDITY CONTROL DESIGN GUIDE
The largest humidity loads are imposed by ventilation and infiltration air. That’s why it is important to dry the ventilation air and provide enough of it to keep the building under positive pressure.
It takes extra effort and money to regulate, dry and distribute ventilation air. Given typical HVAC budgets, the designer usually is forced to assume—rather than measure and verify—that on installation, ventilation and exhaust airflows will be set accurately and permanently, keeping the building under a net-positive air pressure for all time. The designer also must assume the building’s cooling systems will remove the ventilation humidity load. Unfortunately, such budget-driven assumptions usually are false hopes.
A building often gets damp from ventilation air because most cooling coils operate intermittently—when the space temperature rises above the thermostat’s set point. With short, intermittent operation, cooling coils do not remove the nearly continuous humidity load from ventilation.
There are energy-efficient cooling systems, but unfortunately, as Don Shirey and Hugh Henderson point out in their April 2004 ASHRAE Journal article, “Dehumidification at Part Load,” cooling efficiency does not provide effective dehumidification. In the most commonly applied systems, high-cooling efficiency means the equipment cools the room so quickly that the air will not be chilled deeply enough nor long enough—at least 40 continuous minutes per cycle—to remove any significant amount of water vapor.
Problems multiply when occupants feel muggy. In their attempts to improve comfort, they turn down the thermostats. The building gets cold, which consumes more energy and makes the indoor environment even less comfortable for occupants while also increasing the risks for the building. Humid infiltration air condenses and grows mold on cold surfaces.
DEDICATED DEHUMIDIFICATION AVOIDS PROBLEMS
During the last two decades, national policy has mandated and achieved higher cooling efficiency to save energy. Unfortunately, no similar laws require effective dehumidification. With no codes or laws requiring dry buildings, cooling equipment seldom provides reliable dehumidification, resulting in buildings that are cold, damp, uncomfortable and at risk for mold.
The solution is to monitor and control the amount of ventilation air and dry it. For lowest energy consumption and effective drying independent of the need for cooling, the ideal solution is to dry the ventilation air separately with a dedicated system so only dry air enters the building. To avoid discomfort and mold problems, buildings designed for the U.S. government have required this approach since 2003.
The cooling system only ran intermittently, so it did not dry the ventilation air. Mold grew because of near condensation. The air’s dew point was high, even when its relative humidity was low.
Credit: HUGH MCMILLAN
When the building is filled with dry air, the owner can raise temperatures during hot weather to save energy. The low dew point provides excellent comfort and reduces the risks of indoor mold and structural corrosion. Dedicated ventilation systems also have gained LEED credit from the U.S. Green Building Council, Washington, D.C., for superior IAQ. This approach no longer requires site-built, custom-made equipment. As seen in Figure 5, low-cost packaged rooftop ventilation dehumidification systems are now available for light commercial buildings.
The question of “how dry is dry enough” for the ventilation air is important. Drying the air too deeply uses excessive energy. Not drying it deeply enough leads to condensation and moisture absorption into cool indoor surfaces. For new U.S. government buildings, the answer is simple. Chapter 5 of the Washington-based U.S. General Services Administration’s ”Federal Facility Standard“ requires all ventilation air be dried to a 50 F (10 C) dew point any time ventilation air is allowed into the building. Consistent with that requirement, the recent ASHRAE Guide for Buildings in Hot and Humid Climates suggests a 55 F (12.8 C) dew point as a prudent maximum humidity level for the building as a whole during the cooling season. When dried to a 50 F (10 C) dew point, ventilation air usually is dry enough to keep the building below a 55 F (12.8 C) dew point by absorbing the small internal humidity loads seen in Figure 3. This approach avoids the all-toocommon problem seen in Image 4. In this case, the building may be green, but perhaps not in the way its owner would wish.
Lew Harriman is director of research and consulting at Mason-Grant in Portsmouth, N.H. He is a member of ASHRAE Technical Committee 1.12 for Moisture Management in Buildings and was the project manger for the recently published ASHRAE Guide for Buildings in Hot and Humid Climates. He can be reached at lewharriman[at]masongrant.com or (603) 431-0635.