This article was originally published in the September/October 1995 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.



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Home Energy Magazine Online September/October 1995


Air Sealing in
Low-Rise Buildings

by Victoria Hayes

Reinforcing the pressure barrier in low-rise multifamily buildings
can increase occupants' comfort and savings.

Multifamily buildings vary widely. They range from houses divided into three apartments to 500-unit high rises. So a weatherization project must be tailored to fit the personality of the building. The first order of business is to assess whether a particular building should be treated as a system or divided into floors or units.

This assessment should take into account how air moves through the building. If there is a lot of air movement between floors, the building starts to act like a tall house. If each floor is sealed off from the others, the building resembles a lot of houses stacked on top of one another. To determine which type a given building is, gauge the stack effect by looking at the temperature differentials throughout the building. Are the top floors overheated while the bottom ones are too cold? Other helpful measurements include pressure differentials and physical tests such as opening a window on the top floor and checking for a strong steady gust of air moving out of it.

The second scenario--in which the floors are compartmentalized--is preferable, because in the first, the apartments at the bottom tend to be cold, while those at the top may have poor indoor air quality. However, it is the first scenario that predominates among low-rise multifamily housing stock. In these buildings, defining a pressure barrier for the entire building, and then making it effective, stops direct convective heat loss and can reduce conductive loss as well. It can be confusing, however, to define the pressure barrier for low-rise multifamily buildings, and very difficult to correct problems with a pressure barrier.

Defining the Pressure Barrier

It is important to establish a pressure barrier to prevent conditioned air from escaping to the exterior. Conductive loss from interiors warms air in the building cavities, causing it to rise toward the attic. If the cavity is open to the attic, the warmed air will escape to the exterior; and if it is open at the top and bottom, there will be a continuous flow of air upward through the cavity as the air is warmed, rises, and is replaced by exterior air at the bottom.

The pressure barrier, then, is the actual air barrier between inside and outside air. It usually consists of the attic floor and the exterior walls. It should not be confused with the thermal barrier, which is the area where insulation has been (or should be) installed to reduce conductive heat loss between conditioned and unconditioned spaces. These two barriers should be aligned, but often they are not (see User-Friendly Pressure Diagnostics, HE Sept/Oct '94, p. 19).

Regardless of whether the thermal barrier has been aligned with it, the pressure barrier can be very difficult to define in multifamily housing. Units share walls, ceilings, and floors with other units. There are generally common areas, such as hallways, mechanical rooms, and group activity rooms, which may or may not be heated. And there may even be areas inside the building that need to be isolated from heated areas, such as stairways, elevator shafts, and trash chutes. Once the pressure barrier has been defined, it can also be difficult to make it effective, since areas that are critical to treat may be inaccessible, and the extensive work required to make them accessible may be too expensive.

In order to make the pressure barrier effective and cost-effective in a low-rise multifamily building, it is important to treat the building as a system. While units may be individually heated, it is usually not cost-effective to try to zone units or floors off from one another. Moreover, the units themselves are often well separated convectively from one another and from building cavities. In houses treated through Syracuse Energy's Demand Savings for Multifamily Buildings project, we often found units so tight that they had blower door readings of 400 and 500 cubic feet per minute at 50 Pascals of pressure (cfm50). In Vermont, an entire four-unit building of 2,370 ft2 was tested at 1625 cfm50. (According to GRASP's energy efficiency rehabilitation specifications, the minimum reading allowed for such a building is 7,110 cfm50, or 3 cfm50/ft2.)

The location of the pressure barrier will depend on how the apartments are laid out. There are two typical configurations: apartments with entrances located in a common hallway that connects to the exterior with a stairway and doors, or apartments with entrances directly to the exterior. In both cases, the exterior walls of the apartments and hallways, the roof cavity floor, and floor of the bottom story probably define the pressure boundary. It is particularly important to maintain a continuous pressure barrier in such tricky areas as second story overhangs, cantilevered porches, and exterior stairways.

Ducted distribution systems present special problems. The crew should attempt to enclose them as much as possible within both the thermal barrier and the pressure barrier. These systems may carry central exhaust air; heating system air; heated make-up air; or individual bathroom, kitchen, or dryer exhaust air. Exhaust systems carrying potentially moist air must be vented to the exterior, not into the attic, as is common practice in multifamily buildings.

Figure 1
Figure 1. Air movement through a party wall versus through a soffited wall.

Finding Pressure
Barrier Problems

After determining where the pressure barrier should be, the crew must inspect it for problem areas. With single-family homes, a blower door can identify pressure problem sites. Such testing is more complicated with multifamily buildings, because there are more opportunities for indirect leakage paths, which may skew results. Depressurizing an apartment with a blower door may induce air to flow through adjacent units or spaces into the tested unit. This will cause the blower door to measure more leakage than is actually entering the tested unit from the outdoors. Conversely, if the units are very airtight as a result of consistent maintenance, which is not uncommon, the test results may make the building seem tighter than it actually is.

The crew should test the entire building, if possible, to get a true picture of the exterior leakage through the building's interior skin--that is, through drywall, plaster, and so forth. A single blower door can accomplish this if the building is tight and all the units open onto a central corridor, but more often it requires two or more blower doors. Two blower doors will more accurately gauge the exterior air moving into the building interior. Unfortunately, this test may overstate the integrity of the pressure barrier, because air movement in the building cavities may not be apparent if the building has a tight interior skin.

Air movement occurring within building cavities can be as important as exterior air penetrating the interior of the building. In fact, interior air movement is usually even more significant in multifamily buildings, because of their construction. Air movement within building cavities lowers the interior temperature, increasing conduction from interior spaces. In effect, interior wall cavities may behave like exterior walls if enough unconditioned air is allowed to move through them.

Why is this leakage so significant in multifamily buildings? In new construction, the walls between units, and sometimes those carrying pipes, ductwork and wiring, are constructed using a double-stud wall system. Both or either of these walls may have insulation, and they may also run the full height of the building. The key, then, is in the attic. Each wall system generally has a top plate, but the gap between the wall systems may be up to 1 ft wide. When there are many units in one building, there will be several of these interior chimneys. The square footage of wall space compromised by this air movement can be much larger than the area of the exterior walls!

Any interior wall that is open to the attic will have such leakage. Building cavities that allow this type of heat loss include soffits for kitchen cabinets, especially those carrying piping; open-core block party walls and the cavities adjacent to them; chases for ductwork including exhaust ducts, dryer ducts, and distribution system ducts; plumbing walls; and walls adjacent to interior stairwells.

Another building cavity that often creates problems is the joist space in any upper story that overhangs a lower story. While the portion of the cavity overhanging the exterior is sometimes insulated, there is normally an inadequate air barrier separating heated from unheated space. Plumbing, mechanical, and electrical systems often pass through these cavities, and then make a turn into upstairs wall cavities. This creates opportunities for air movement from the exterior through the overhang, into interior walls, and out at the top (see Figure 1).

The best diagnostic technique for these problems is a good visual inspection. In the attic, pull back existing insulation and look for party walls, chases, and dropped soffits. Be especially careful to look around fire walls for a full building height cavity, especially at block walls. Check the block walls for open cores, which are usually rather obvious. On a cold day the warm air blowing out of these cavities will be perceptible to an ungloved hand. One excellent indicator of air movement is dirty fiberglass insulation; as the air moves through the fiberglass, it is filtered and the dirt is left behind. When the insulation is lifted or moved, there will be very clear black streaks over problem areas. While in the attic, check the insulation over the exterior walls as well. Fiberglass insulation in the walls will not stop air movement. The air movement will in fact compromise the R-value of the insulation and may render it ineffective if it was not installed correctly. In overhangs, remove a piece of the soffit, if possible. If the cavity is insulated, remove the insulation and check for dirt streaks. The same inspection should be conducted in other cavities, such as basements or crawlspaces (although these are normally included within the thermal boundary, unless they have problems with radon leakage or moisture).

Another common attic problem in rehabilitated buildings can occur when a contractor tries to redefine the thermal and pressure boundaries by lowering ceiling heights. It is common to find the pressure barrier located at the new ceiling height (the drywall), while the insulation may be up in the old ceiling (1 to 12 ft above the pressure boundary) or sometimes even on the roof deck. In this situation, the crew must decide where to locate an effective thermal boundary. The choice will depend on cost, on which building systems run in the dropped ceiling cavity, and on what can be installed in this cavity.

Doing a combination of blower door testing and infrared scanning can sometimes help to diagnose these problems (see Selecting an Infrared Imaging System, HE July/Aug '93, p. 37). An infrared scan is performed on the building's interior to determine how the building appears when no blower door is inducing pressure. A blower door is then used to draw air through building cavities from the attic or the exterior. Air movement will quickly become visible with the infrared camera. The unit can also be pressurized and a scan done in the attic to determine problematic areas of the attic floor. Finally, one new and quite effective diagnostic technique is the use of differential pressure measurements developed by Michael Blasnik and Jim Fitzgerald (see User-Friendly Pressure Diagnostics, HE Sept/Oct '94, p. 19). This technique is still being refined for use in multifamily buildings.

Treating the
Pressure Barrier

Once the crew has diagnosed the problems with the pressure barrier they should prioritize them and develop treatments. Unfortunately not much research has been done to determine which cavity leakage sites have the greatest impact on energy loss. The available computerized building modeling programs are too limited. Most of them allow an input for certain types of convective loss, and the several inputs needed to define building R-values, but they do not have any input options for air movement in interior cavities. In programs that allow some flexibility in data entry, these surface areas can be carefully modeled as exterior surface areas with varying R-values, depending on the severity of the air movement and on the building materials used. What is known is that treating these areas lowers fuel use, and that occupants usually notice an immediate improvement in comfort. Several materials already familiar to the air sealer experienced in single-family housing stock are available to treat these problems.

Clamping Down on Attics

The several penetrations normally found in the attic are treated depending on their size and on the material to which the treatment will be applied. Smaller gaps, such as those typically found around wiring and piping, can usually be caulked.

Larger gaps, such as the area between the top plates and the adjacent drywall, as well as larger utility penetrations, should be foamed--with or without a backing, depending on the size of the gap. At plumbing stacks, especially those using polyvinyl chloride (PVC), however, foam treatments usually fail, because PVC tends to expand and contract with changes in temperature. A better air sealant for these areas would be one that fits tightly around the pipe but is flexible and allows movement, such as the material used in roof jacks or ethylene-propylene terapolymer (EPDM) membranes. The crew can seal this to the attic floor, and then fit it to the pipe, which will allow it to move while providing an air barrier.

At the tops of party wall cavities, the crew can use either rigid board insulation or drywall, depending on their preference and on the local fire code (some codes prohibit the use of foams or rigid board on a fire-rated barrier).

For open wall cavities, such as those found around soffits, the crew should create a continuous barrier. This can be done either by air sealing the entire top of the dropped soffit or by capping the wall cavity at the lower soffit level and providing continuous insulation that follows the pressure barrier (see Figure 2).

For any chases around chimneys or areas that require clearance around combustibles, the crew must use a noncombustible material such as metal flashing with a high-temperature rated sealant.

For open core block walls, they can open a series of cores and seal the cores with either foam or dense-packed cellulose. Dense packing is an insulation technique in which cellulose is installed in a cavity at a density of 3.5-4 lbs./ft3. At this density, the cellulose provides an air barrier that also functions as insulation.

In addition to air sealing treatments in the attic, the crew may want to consider capping loose-fill fiberglass insulation with a layer of cellulose insulation. The cellulose tends to inhibit air movement through the fiberglass.

If the attic has a flat roof, making it inaccessible for treatment, and the crew finds air sealing problems, the only options may be to dense pack the entire cavity, or to treat what can be accessed and dense pack what is inaccessible. If this seems too expensive and there is no existing venting on the roof, the crew may consider dense packing the perimeter of the attic cavity only. Since the air movement out of an attic cavity with no roof venting would be at the eaves, this can stem the heat loss. It will be necessary, however, to make sure that there are no large air leakage areas elsewhere on the roof deck. If there are, moist air may be drawn up into the attic cavity and trapped there, where it can cool and condense, creating moisture problems.

Figure 2
Figure 2. Two options for sealing a soffited wall.

Plugging Wall Problems

The most effective way to treat walls and overhangs is, again, to use the dense-packing technique. For overhangs, the crew can access the joist bay from either the interior or the exterior and install the insulation in the top plate area of the lower-story wall. Strategic dense-packing techniques can also be used in the band and rim joist areas of walls in which they have detected air movement with the infrared camera.

If the infrared camera shows either voids in the insulation or insulation severely compromised by air movement through it, the entire wall cavity may need to be dense-packed. This can be done even if there is existing insulation in the walls. If the existing insulation consists of faced batts, the cellulose should be installed to the interior side of the batt facing.

Treating the Ducts

Effective duct treatments use a combination of mastic and fiberglass mesh tape for sheet metal ducting (see Diagnosing Ducts, HE Sept/Oct '93, p. 26). The crew should replace worn and ripped flex duct and should insulate any ductwork carrying conditioned air, except when it passes through conditioned spaces. If ductwork is in soffits, they should air seal and treat the soffits first, if necessary; they should then seal and insulate the ductwork. The crew may also need to insulate any exhaust ducting carrying moist air, to prevent condensation problems after the attic treatment is complete. After the attic is treated, the temperature above the insulation may drop substantially during the colder months.

The crew should also check exhaust ducts for effective dampers. Multifamily buildings, because of their height and the number of exhaust systems in the building, can exhibit substantial heat loss through undampered or poorly dampered exhaust systems.

This article does not address motor-driven or mechanical air leakage due to faulty or overfunctioning exhaust systems. For more information on these, see Evaluating Ventilation in Multifamily Buildings, HE July/Aug '94, p. 23.

Continuing Challenges

The challenge posed by multifamily housing stock is to understand conductive heat losses caused by movement through building cavities. This type of heat loss is most severe in multiunit buildings because of the large amount of interior surface area involved. It affects not only energy use, but tenant comfort and building durability as well. Traditional blower door diagnostics may not find the major problems associated with multifamily buildings. Computer programs don't easily allow for modeling the energy use associated with them. Visual inspections do reveal the same problems as those found in single family housing, but the extent of the problems take on a new priority in these larger, taller buildings. The treatments are the same, but more of them are required. Still, considering the high energy use of many of these buildings, the impact of these treatments on the fuel bills and comfort of the buildings' occupants, and the tremendous economies of scale due to low auditing and production costs, air sealing multifamily buildings can be an attractively cost-effective project.

Victoria Hayes is senior building technology specialist with the National Center for Appropriate Technology, where she is working on a project addressing energy efficiency in HUD multifamily buildings. She was formerly housing director at GRASP.


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