Building Envelope -- Waterproof Design: Mechanical Systems

As we mentioned earlier, gravity, as well as temperature, humidity, and pressure differential, affects water vapor. This is referred to as vapor drive, with the driving forces being the physical laws and chemical properties. We have discussed the desire to control moisture in the built environment by the use of membranes, barriers, flashings, and coatings, coupled with good design principles and practices.

This section addresses the role played by powered mechanical systems, including HVAC equipment. These mechanical systems work with the envelope materials to keep people comfortable, healthy, and happy. We have found that the way to do this is to provide some local control of an effective building HVAC system.

This contributes to the happy part-individuals like to have the ability to adjust settings. This is especially true in government and commercial buildings, where a large number of people spend the majority of their waking hours with a lot of other people around.

Creature comfort is often as much psychological as physical. However, this can be a double-edged sword because you can't give people too much control.

If you do, they will make other people miserable. It just happens. We have seen placebo controls work well in some settings, whereas they increase complaints at other times or in other places. The building HVAC system is only as good as the sum of its parts. This means that all major components must be capable of functioning at an acceptable level of performance at all times. The sensing and controls systems must be communicating properly with the control valves, variable-speed drives, dampers, heaters, and other parts. The fans have to be able to move enough air, ductwork must resist the pressure and convey the air, and the louvers have to allow outside air into the duct. Compressors, pumps, and piping must be able to meet the demand, and coils must have sufficient gas and liquid levels and pressures. The electrical power must be available. All these pieces need to be ready to work when needed.

Temperature and humidity control are the two primary functions of a building HVAC system. In order to minimize condensation in the envelope, you also must control pressure. You may remember Charles' and Boyle's laws from chemistry class, P1 V1/t1 = P2 V2 /t2

This expresses the mathematical relationship of pressure and temperature in a fixed volume such as a building.

Since the volumes are the same, they cancel out. What you are trying to do is maintain positive pressure on the building skin to minimize outdoor air and possible contaminants from coming in at the perimeter. Such air infiltration is bad; it can introduce hot, humid air in the summer or cold, damp air in the winter that can lead to localized condensation and other unwanted effects.

You want your building envelope to be tight and slightly positive (relative to outside air). The tighter the envelope, the less pressurization air is required to maintain positive pressure. This equates to less energy consumption. It also means a better opportunity for a potentially healthy indoor environment for your people.

It is always a good idea to look at your overall building balance before you begin to look closely at any one space or area in the building. This can be as simple as a sum of in’s compared to out. It lists values (in cubic feet per minute) of outside air coming into the building minus the total being exhausted, which leaves the amount for pressurization. This is a quick and important step in assessing any building HVAC design, which every architect should check before the building plans go out to bid. Make sure that you have pressurization air. If the exhaust is greater than the outside air intake, you have a negative pressure building. This can cause problems. Don't let it happen.

Pressurization air can be as little as 0.03 to 0.07 inches of water column static pressure. This can be measured with a manometer and two small plastic hoses.

Your mechanical engineer can calculate it for you and should indicate the value in a table on the plans (or specifications). Too much pressure can result in exterior doors staying open; too little can result in normal wind velocities causing infiltration on the windward side of the building envelope. An easy way to test it's to tear off a small strip of paper from a notebook and hold it in front of a set of double doors. If the strip blows out straight, the pressure is probably too much. If the paper curves less than 45 dgr, it's about right. Your test and balance report should list test values or at least calculate the value for you.

Outside air mixed with return.

The main advantage of pressurization with outside air after it passes through the HVAC system is that it contains less moisture. This is especially true in the summer months. --a section for a four-story representative building with positive pressure. Outside air is collected at the facade and ducted through a filter section and passed through an enthalpy wheel. An enthalpy wheel is made of a good heat-transfer medium such as honey combed plastic. Outside air passes through the wheel, where it's preconditioned. In this example, building exhaust air passes through the opposite side of the out side air intake. In the summer, the exhaust is at around 75ºF, whereas the out side air may be at 95ºF or higher. The cool, dry air being exhausted lowers the temperature of that section of the wheel in the path of the air, and as the wheel turns slowly, it moves in front of the outside intake airstream.

One of the big challenges in bringing in large amounts of outside air is keeping rainwater intrusion to a minimum. On the building diagram, the intake for outside air is at the high point in a gable end condition. This is where wind pressures can be greatest. If it's raining, and you're introducing outside air to the outside air unit, you need to be able to remove as much rain as you can before it enters the mechanical equipment coil or fan sections. This can be achieved through the introduction of a water-trap, such as the one provided in section view.

Water trap at outside air intake.

Pressurization controlled at unit air handlers.

This kind of water separator is low tech and low maintenance if built right. It uses gravity and geometry to remove moisture from the air stream. The intake area is large, with water separator louver blades. Behind that's a drain sump and weirs that cause the air to speed up as it rises, and then slow down as it falls so that droplets collect and gravity pulls them down into the sump. Dryer air is then pulled up once more where it can be filtered prior to the outside air enthalpy wheel. For flexibility, you may choose to have a variable-velocity exhaust fan, along with a variable-speed outside air makeup fan section. Interconnected with building differential pressure sensors and room occupancy and /or CO2 sensors, you can tune the performance for maximum savings while exceeding ASHRAE and mechanical code standards for ventilation. Many outside air units have heating and cooling coils incorporated into their design for optimal discharge air regardless of outside air conditions. We have seen as much as a 15ºF drop in outside air temperature, along with a 15 to 20% drop in relative humidity, through an enthalpy wheel. The savings in energy calculates out to about a 4- to 6-year payback.

The pressurization air now has been preconditioned at the outside air unit (OSA) and is ready for distribution to the air-conditioning units. Because the volume of outside air is only a small portion of the total volume of air (typically measured in cubic feet of air per minute, also referred to as building cfm) being moved by all the unit air conditioners combined, this OSA ducting is not as large as the unit air-conditioning system supply air duct. A diagram for a representative unit air-conditioning system is provided. You can see the OSA makeup duct, supply air (SA) ducting and return air duct indicated. For cost-effective operation, the program that operates the unit and building components all have night setback and non-occupied modes programmed in, as well as normal occupied modes. Regardless of the mode, the program should minimize infiltration through positive pressurization.

Ventilation

Air conditioning and heating systems rely on three principles for creature comfort- ventilation (moving air), temperature, and humidity control. This section will not go into radiant heating or trombe walls for building envelope considerations.

This book is intended to focus more on mainstream building design and construction. Most building types rely on moving conditioned air to regulate occupants' temperatures. For heat rejection, the more air you move over a person's skin, the better is the potential exchange rate. You can either move more air over the person or lower the temperature of the air to increase heat rejection. There is a balance point in air velocity where the air is moving so slowly that it's not noticeable. Some of the air cools the surroundings, which is why you often direct air-conditioning supply grilles at exterior windows and walls.

As the conditioned air moves through the interior space, it moves from supply to return, creating patterns of flow and eddies. This air mixes with the air already there, diluting odors and providing fresh air. This mixing air also carries away moisture from walls, floors, ceilings, people, plants, and animals. This air is returned to the air handler, where it's remixed with outside air, dehumidified, cooled, and resupplied. This cycle continues.

Ducted exhausts from sources of moisture, plan view.

Exhausting

Moisture is moving from the wall to the space as the wall dries, and at the same time, you may be introducing moisture to locations within the building.

This localized moisture can cause problems if it's allowed to move freely. This is why we recommend collecting high concentrations of moist air at the source and exhausting it . Several common sources for concentrated moisture are showers, laundry, and cooking, but there are others too. Hot tubs, clothes dryers and washers, laundry sinks, and indoor clothes lines in the laundry room are just a few. What is important is local collection and exhaust. In buildings with a lot of business machines, it's prudent to locate exhaust grille inlets above them to remove concentration of harmful or undesirable fumes.

This is why many condominium design teams stipulate interfacing lights in laundry and bath areas with exhaust fans. These used to be specified only in toilet rooms with no exterior walls, but their use has expanded to include most sources of moisture. We recommend against the use of ductless exhaust fans over stoves in kitchens. These essentially remove (by filtration) some cooking odors and blows the moisture from boiling water or cooking foods right back into the space. You want to get that moisture out of the envelope.

Dehumidification

The obvious advantage of HVAC systems is cool or warm air. It is not so obvious but equally important to consider the role of moisture reduction-dehumidification. Have you ever had an air-conditioning system in the attic of a house or apartment in which you lived? When did you realize that the attic was where it was located? Not when it was working perfectly. Probably when the condensate drain pan overflowed. That big ugly stain in the ceiling won't go away, not even after you unclog the drain line. Well, that drain pan probably leaked at the peak of the cooling season, didn't it? This is when the air is hottest, so it holds the moist moisture. Did you know that air at 95ºF can hold more moisture than air at 72ºF? Well, it can and often does. The condensate drain pan, even when partly clogged, could keep up with the normal rate of condensation in the early summer. But those cooling coils work overtime in the middle of the summer, and the air holds a lot more moisture. So it generates more water as it drips from the cooling coils. The drips overwhelm the 1-inch lip on the drain pan, and look, a brown spot that won't go away! You want your indoor air to be relatively dry in the summer. A good rule of thumb is less than 50% RH. If you can stand it, afford it, and your system is able, you may wish to keep it below 40%. This goes for the unoccupied mode too. By keeping the air that low, you maintain a relatively low moisture level in the walls and continue to draw moisture out of the walls, where it can be removed by the building HVAC system. Dehumidifiers can be installed in buildings as well. We are seeing them in attics, basements, laundry rooms, wine cellars, and mechanical closets (even in residential units). By locating one in the mechanical closet, you can do two things. You can dry the returning air (in an unducted return plenum condition), and you can minimize potential condensation on the unit and walls of the closet. If the air-conditioning unit is cycling with the fan on and not the compressor, the supply air (SA) will be dryer because the return air is dryer. Of course, the air-conditioner and dehumidifier condensate pans need piping to drains. Check with your local codes for the possibility of using condensate water for irrigation or other non-potable uses. Consider at least one air handler and dehumidifier be powered by emergency generator or stand by power.

Sensing and controls

For a modest home or even a small hotel room, the building mechanical system can consist of a single window unit or through-wall air conditioner. It has a compressor, condenser, fan, and cooling coils, along with the condensate drain pan, in one compact package. All you need is a power source, and you can get cooling. Even this basic system typically has a couple of different fan speed settings for cooling and heating modes, along with a temperature sensor to regulate output. This device is called a thermostat. A thermostat is a "smart" switch. Historically, these were made by combining a bimetallic coil with a mercury switch. As it gets cooler, the coil changes length because one of the two metals contracts faster than the other. This changes the position of the mercury switch. Since mercury conducts electricity, the two electrodes are connected when the mercury is level in the glass tube, actuating the relay that powers up the air conditioner. This is the basis of antiquated sensing. It is not very accurate, but it's reliable. It requires no maintenance, repair, or replacement unless the glass tube breaks or one wire comes off. Owing to the poisonous potential of mercury in our environment (in landfills and such), this technology is being replaced by digital sensing devices.

Digital sensors are simple electronic devices that measure temperature by comparing it to a known value; they sense and "think." Their output is typically 24 volts direct current (DC) ranging between 4 and 20 milliamperes. The device receiving the signal converts the signal to a response in the air-conditioning system output. Taking this technology a step farther, digital sensors can be connected to programmable controllers. These controllers have logic built in that can be programmed to make decisions based on preset information. A series of "if-then" scenarios is placed in memory. When the sensor gets information, it decides what to do with it. These devices can be programmed to conserve energy, turn off lights, calculate the number of people in a building and adjust outside air input, and so much more.

A networked system of digital sensors and programmable controllers can be set up to operate building security, heat up the hot tub, answer the phone, and much more. Here, however, we want to focus on building envelope sensing and actions. Beyond the capability of a programmable controller, there are now building automation systems that are computer-based. These have exponentially more decision-making capabilities and interface opportunities. They have sensors that can detect motion, fingerprints, thermoclines, smells, gases, and more important, pressure differential and humidity. Differential pressure is referred to in the controls jargon as DP. You use DP sensors in water and air systems, as well as compressed gases in large chillers. You can use DP sensors in the exterior walls to regulate building pressurization and minimize infiltration.

Taking this a step further, you can use moisture sensors, or humidistats, to sense moisture in the exterior wall and roof cavities. Rather than rely on room air sensors to regulate moisture in a wall, you can measure moisture in the wall. Rather than keep room air at 45% RH in an attempt to minimize condensation in the wall, you can use dew point sensors in key locations in the wall. For buildings where you are conditioning ceiling cavities, attics, or basements, you can measure conditions accurately and limit the amount of conditioned (precious) air being introduced for pressurization. This will result in efficient use of two of our most valued resources-energy and money.

Sample unit supply and return ducting.

There are two common means presently being employed for providing ventilation air for occupied buildings. There is the old method of providing around 15 cfm per person that has been the standard basis for gross ventilation calculations for decades. This number varies depending on the person and his or her activity level but was accepted for years as an acceptable basis for calculating outside air in sizing building air-conditioning systems. Some less conservative designers conclude that the 15 cfm per person does not have to be all outside air.

In their designs, a portion of the room air being supplied into the space is air that has not come from the air handler directly. Instead, air that has been in the space is not exhausted or returned to the air handling unit and it's mixed with a lower percentage of air from the unit. This recycles indoor air and results in lower cost per square foot of energy. It also results in a potentially higher CO2 level and a higher concentration of other indoor smells. It means more off-gassing from building materials and machines in the occupied portions of the building will be staying in the air the occupants breathe.

The second scenario being used in some buildings today relies on CO2 sensors to control the ventilation air. This can result in the lowest volume of outside air being introduced into the building air-conditioning system. In theory, this results in lower operational cost for the client or building owner without allowing the air to dip below acceptable levels for CO2 in the airstream. This system should be combined with a system for capturing less desirable air, such as previously mentioned capturing of exhaust air from large copiers or other equipment, at the source for removal from the building. The choice as to which methodology to use as the basis of design should be made in the programming stage so that the associated costs are figured in from the beginning stages and so that when the project is finished it meets or exceeds the client's expectations. It is important for clients to weigh the pros and cons of the two ventilation schemes. More outside air generally means healthier inside air, unless, of course, the air outside isn't very healthy.

Fortunately, with variable-velocity fans and adjustable dampers on ducts, pro grams can be changed to increase or decrease outside air and tune performance. Unfortunately, building automation systems also make it easy for operators to totally change the way a system runs. They can close off outside air with the push of a button. They can disable heat components and ignore sensors if they choose. As responsible designers, we don't want those decisions made by the local mechanic who may not be trained in or care about occupant health and safety. He or she may be trying to save operating cost or not know how to replace and calibrate a 4-milliampere output from a humidistat. This is why we must consider the operational and maintenance aspects of the building in the future before we design a system for a project. It may be that the low-tech thermostat is the only thing that the client's maintenance crew is able to keep working.

This is why we recommend that you specify building materials that can handle the small amount of occasional water that can result from condensation if all your precautions are not enough during extreme conditions. Furthermore, this is why we have discussed the importance of providing a drainage pathway for moisture that's formed at the vapor or moisture barrier.

There are many ways to design a building, and each results in a different level of performance, as well as a different initial and operating costs. By starting out with clear intentions and keeping focused on what is important, you can develop a good foundation for a good envelope. By careful consideration of operating and ambient conditions, you can look at ways to prevent condensation from forming in walls, roof systems, or interior surfaces. If you provide great illustrations and communicate your details well, you can improve the weather tightness of the skin. You know that there are excellent products for controlling air and moisture intrusion, and you can use them in concert regardless of the weather conditions. Roof, wall, floor, and site design decisions that are made with regard to gravity, geometry, and technology should result in a composite whole that prevents water intrusion and mold and mildew growth in buildings.