Designing the virtual air barrier

July 21, 2013

Photo © BigStockPhoto/Darryl Brooks[1]
Photo © BigStockPhoto/Darryl Brooks

By Rick Quirouette, B. Arch.
Since 1985, the National Building Code of Canada (NBC) requirements for an air barrier system have been adopted for all buildings in this country. While the air barrier concept is widely accepted by industry, its application can be particularly difficult and challenging when it comes to structures predating that code.

With just over 25 years of this technology’s development, there remains a large proportion of existing buildings that have no air leakage control (i.e. no air barrier system). The problem is these facilities are subjected to various types of conversions (e.g. factories converted to condominiums) and mechanical upgrades to include new ventilation and humidification equipment without a complementary building envelope upgrade. These include many older (and some newer) hospitals, museums, schools, offices, apartments, and heritage buildings.

The provision of an air barrier system for existing buildings is technically difficult and costly to incorporate. For this reason, the concept of a virtual air barrier (VAB), or dynamic buffer zone (DBZ), has been applied, tested, and researched over the last decade by this author and the late Michel Perrault. It has been applied fully, partially, or as a feasibility approach to 13 building projects in Canada.

Anatomy of a VAB
The VAB concept involves the creation of a pressure zone (or cavity) within the roof, exterior wall, window (older double pane windows, not sealed units), or below-grade construction of a building. This pressure zone has no material substance (hence it being ‘virtual’), using the attributes of the existing exterior wall or roof to construct part of the VAB system. The pressure zone must be adequately sealed to contain an increased pressure between the zone and the building interior.

To pressurize a VAB zone, a fan, ducts, and an optional in-line heater are required. The pressure zone is supplied with outdoor air, pre-heated if necessary, and pumped into a wall or roof cavity. By adjusting the VAB supply air fan speed, the air pressure within the zone is forced to rise slightly above the indoor air pressure. This situation then prevents moist, indoor air from entering the VAB zone of the exterior walls or roofs.

Figure 1[2] illustrates the concept as it is applied to an exterior wall or roof cavity. There is no return circuit for this pressurization (ventilation) system, as it would be detrimental to the VAB operation and add considerable cost. It must not be viewed as a cavity ventilation problem, as this approach has been found problematic or, in some cases, disastrous.

A VAB system may comprise many different components, but it generally uses the existing roof or exterior wall components to define a pressure zone. This zone may be inboard of the wall or roof insulation or outboard of the insulation. It is this zone (i.e. cavity) that is pressurized to create a positive pressure with respect to the indoor side of the room. Parts of the exterior cladding, or where there are large openings or holes on the indoor side, may need to be air-sealed. Small cracks, openings, or holes do not significantly affect the VAB system’s performance.

For the VAB to correctly perform, the cavity must be pressurized with dry air from outside—not indoor spaces, elevator shaft exhaust, or mechanical rooms. This is because the VAB supply air must have a dewpoint temperature (i.e. air temperature when condensation occurs) below that of the outdoor air temperature. In this way, the pressurization air can never condense in the VAB zone or anywhere in the roof or wall cavities, behind claddings, or below roof membranes.

The VAB zone pressure is supplied by a small pressurization system designed to function continuously over a period of five to seven months. The VAB zone pressure difference between the zone and the indoor side need not be large, but it must be sustained. In this way, the moist indoor air is prevented from entering the roof or exterior wall cavities. The VAB system then reverses stack effect and building ventilation pressure differences at exterior walls, window, and roof systems to prevent air exfiltration at the outside wall and roof.

CC_July13.indd[3]CC_July13.indd[4]The VAB concept has been applied to several buildings over the past 15 years. They include a heritage masonry office building over 75 years old in Toronto, a hospital less than four years old in Edmonton, a conference facility and roof of an eight-storey building in Ottawa, and several apartment buildings also in the capital.

A research and development project
The VAB’s proof of concept, as well as the architectural and engineering design criteria, was determined in a research and development project involving a six-storey heritage building. The building was clad with heavy masonry walls, metal-framed windows, and a convection hot-water-heating system or radiators at the windows.

The test facility consisted of a single, long space facing east with multiple windows (Figure 2[5]). In this project, the VAB was designed and constructed for the building’s exterior walls and steel-framed windows. The indoor rooms were equipped to maintain a constant temperature and to produce high-humidity conditions. Part of the exterior wall and windows of the test area was isolated to provide a ‘control-room’ cavity separate from the VAB exterior wall.

This control wall was isolated from the VAB wall by a compartment seal (i.e. interruption of the wall cavity), which allowed part of the existing exterior wall to be exposed to the same indoor conditions while retaining its original exposure and performance attributes. The control and VAB walls were both equipped with visual and electronic monitoring equipment for observation of condensation control performance over a heating season.

The test building’s masonry wall.[6]
The test building’s masonry wall.

The building’s masonry wall was constructed of 100 and 200-mm (4 and 8-in.) granite stone, with two layers of 100-mm terra cotta bricks mortared together with the stones (Figure 3[7]). The stones were anchored to the terra cotta masonry with metal brackets. A 50-mm (2-in.) cavity separated the stone and brick cladding assembly above from an interior layer of terra cotta brick with a plaster finish.

There were many holes and openings in the plaster finish. Pipes were found within the wall cavity servicing a hot-water heater just below the windows, which comprised two single-pane glazed panels in steel frames with a 100-mm (4-in.) space between.

The test exterior wall and windows were then sealed on both the outdoor and indoor sides—the cladding face and indoor plaster finish, respectively. The exterior wall’s continuous cavity was divided into two parts: the ‘control room wall cavity’ and the ‘VAB zone cavity.’ The cladding face or exterior stone work was repointed. Where required, the interior exposed terra cotta blocks were sealed with gypsum board and taped joints (Figure 4[8]). No attempt was made to supper-seal the VAB zone or the control cavity. In other words, pencil-sized holes and fine cracks were ignored, but pipe openings were sealed with gypsum board tape and joint compound.

The test facility’s room side was furnished with a humidifier capable of delivering up to 4.54 kg (10 lb) of water vapour per hour. The room temperature was set and maintained at 20 C (68 F).

To pressurize the VAB zone, an insulated duct was installed through one of the windows. The outdoor side was protected against snow penetration by a grilled opening. On the indoor side, the insulated duct was equipped with an inline heater to preheat the outdoor supply air to a maximum of 10 C (50 F). The duct was further equipped with an inline fan capable of delivering up to 240 L/second (500 cfm). The system supplied and pressurized the VAB zone through duct branches installed through the indoor plaster finishes (Figure 5[9]). The pressurization air was controlled with a variable speed controller and a temperature thermostat. To ensure against over-pressurization, the supply fan was selected with a maximum head pressure of 125 Pa (0.5. in.H20). Most exterior walls or roofs are designed to support five to eight times as much pressure difference from wind alone. There is no need for additional pressure-relief equipment.

To validate the performance of the VAB concept and its application, observations were visually recorded once a week and continuously with electronic monitoring equipment. Visual observations were recorded by noting conditions on the outside of the building near the exterior wall, and on the indoor side of both rooms. The control cavity and VAB zone were equipped with metal door inspection hatches. The hatches allowed a view of the back surface of the wall control cavity and the VAB cladding masonry. Each masonry wall had a 100-mm diameter hole drilled in the layers of terra cotta brick, exposing the back of the granite cladding stone.

Electronic monitoring and observations consisted of measuring the temperature, relative humidity (RH), and air pressure difference at various locations of a wall. These locations included monitoring sensors on the outdoor side, in the cavities of the VAB zone and control wall cavity, and on the indoor side of the rooms. Data was recorded every five minutes and downloaded to data loggers for retrieval at the end of each week. Monitoring continued uninterrupted for nearly six months. The records were then plotted and analyzed.

For the test, the interior exposed terra cotta blocks were sealed.[10]
For the test, the interior exposed terra cotta blocks were sealed.
















Frost accumulated within the terra cotta’s drilled hole.[12]Test protocol
Monitoring began about mid-November and continued through to the end of April of the next year. During this period, the indoor humidity was increased from about 30 to 70 per cent RH in increments of 10 per cent every two weeks. Additionally, the VAB zone pressure was periodically increased and decreased to determine the minimal operating level that would prevent or limit VAB zone cavity condensation.

In addition to the exterior wall experiment, 13 windows were retrofitted to determine the most effective strategies to control surface and interstitial condensation.

Visual observations
During the monitoring period, the conditions of the stone cladding, windows, VAB zone, and control wall cavity were visually observed and photographed each week. In mid-January, the cavities of the control wall and VAB zone were inspected through hatches and conditions photographed. Figure 6 [13]shows the accumulated frost (condensation) within the hole drilled in the terra cotta of the control wall cavity; one can also note the absence of frost in the hole drilled within the VAB zone cavity. The indoor conditions were 20 C (68 F) with a relative humidity of about 50 per cent, and an average outdoor temperature of –10 C (14 F) at the time of observation. The control wall cavity pressure was lower than the indoor side by about –5 Pa (0.02 in.H2O) while the pressure of the VAB zone was higher than the indoor side by about +10 Pa (0.04 in.H2O).

On the exterior side of the wall, the VAB zone cladding was free of any moisture stress. The same was found of the exterior side of the control wall, but stress would not normally appear after a few years of exposure.

Electronic observations
Every week, the data-loggers were downloaded to files and plotted for later analysis. Plots of both the control cavity and VAB zones’ pressure difference history, relative to the indoor side, were examined and analysed. If the pressure difference of the VAB zone was found positive with respect to the indoor side, then the pressure system was functioning as required. It also confirmed the change in direction of any air leakage and the prevention of the indoor air from leaking into the VAB zone of the wall.

These pressure differences illustrate the variation between the control and VAB exterior walls in terms of indoor side.[14]
These pressure differences illustrate the variation between the control and VAB exterior walls in terms of indoor side.

Figure 7[15] is a bar chart of the recorded pressure differences to show the differences between the control and VAB exterior walls with respect to the indoor side. The control cavity pressure is 5 Pa (0.02 in.H2O) below (negative) the reference pressure (the indoor or room side). Further, the indoor pressure is 60 Pa (0.241 in.H2O) higher than the outdoor pressure. This is partially due to stack-effect pressure within the building and also to building ventilation pressure.

The push of the small but negative pressure difference thereby induces air infiltration into the control wall cavity and over the back side of the cladding assembly. The cavity air then leaks to the outside through imperfections in the cladding assembly (i.e. cavity surfaces and at the drilled hole) after depositing condensation and within the control wall (as shown in Figure 6[16]).

Similarly, the VAB zone pressure was found to be 10 Pa above the reference pressure of the indoor side. This is a positive pressure difference with respect to the indoor side. This difference was sufficient to prevent air leakage into the wall cavity of the VAB zone. Therefore, no humid air entered the VAB cavity, and no condensation was found (as shown in Figure 6[16]).

The analysis also involves examining the temperature and humidity records the VAB zone data. An actual data plot of the VAB dewpoint temperature was prepared. The temperature and RH of the air within the VAB was then converted to a dewpoint temperature and compared with the outdoor temperature.

In Figure 8[17], the VAB system was allowed to function for several weeks uninterrupted. It was then turned ‘off’—meaning the supply air fan to the VAB zone was off for 24 hours. The rise of the dewpoint temperature can be seen in the graph as humid indoor air pushed into the VAB zone as the virtual air barrier space’s pressure collapsed. The dewpoint temperature within the VAB cavity rose well above the outdoor temperature, indicating potential condensation conditions.

The dewpoint temperature rose as humid air pushed into the VAB zone.[18]
The dewpoint temperature rose as humid air pushed into the VAB zone.

When the VAB supply fan was restarted, the dewpoint temperature rapidly dropped once again to a level below the outdoor temperature within a few days. It is this effect that prevents condensation from occurring within exterior wall and roof cavities when a virtual air barrier system is installed.

The VAB was also examined to determine the range of air pressure differences governing its ability to control condensation. It was found the VAB zone’s pressures difference could be so low it was undetectable, yet it was still effective in controlling condensation. Further consideration noted when the wall cavity or VAB zone is airtight, very little supply air is required to obtain adequate pressure to prevent the entry of humid room air into the wall cavity.

Alternately, when the VAB pressure difference was found to be small and the fan supply was near its maximum, the VAB air was diluted with outdoor air to a dry condition, which also produced a dewpoint temperature below the outdoor temperature—a win-win situation when both pressure and flow produce the desired results. The only time the VAB system did not perform as expected was when the VAB system supply was shut down.

Background discussion
The object of the virtual air barrier concept is to prevent, or at least limit, wall or roof cavity condensation when other measures would fail. Condensation occurs when the indoor humid air finds its way into the building envelope cavities and encounters a surface at a temperature below its dewpoint temperature.

The indoor air’s dewpoint temperature is determined by the air’s actual temperature and relative humidity. When these two conditions are plotted on a chart, the dewpoint (condensation) temperature of the air can be determined. The dewpoint temperature is always lower than the dry bulb temperature of the air being considered.

If the dewpoint temperature of the indoor air is above that of the outdoor air, condensation can accumulate within the wall and roof cavities by air leakage, vapour diffusion, and thermal and atmospheric pumping (not to mention rain penetration). However, when a VAB system is installed, it always lowers the wall or roof cavity dewpoint, preventing or limiting condensation from occurring and even drying out a wetted wall or roof cavity.

To achieve this performance in the VAB wall or roof cavity during cold periods, only outdoor air must be supplied to the VAB Zone. This is because outdoor air almost always has its dewpoint temperature below the outdoor temperature (except during short periods of snow or rain) when they are almost equal. When cold outdoor air is supplied to a humid cavity, it tends to dilute the cavity air with dry air while pressurization arrests further through-wall air leakage from the indoor side.

During the fall, winter, and spring, the virtual air barrier system allows much higher indoor humidity conditions for occupancy needs while minimizing or completely eliminating the risk of condensation in roof and exterior wall cavities. This is a particularly useful technology for heritage buildings.

The test facility was designed and constructed using conventional work forces and the remedial repairs to the outside wall were of average quality. The performance of the VAB system is forgiving of minor design discrepancies and construction deficiencies. The design and construction of a VAB for an existing building need not be perfect. The only time it fails is when the building operator forgets to turn it on in the fall or if it is turned off during a cold period.

It was also determined from the performance results the VAB system can easily cope with stack effect and building ventilation pressures, but not wind. Wind will occasionally overcome the cavity pressure and cause a minor amount of indoor air to enter the VAB cavity for a short period. However, wind effects were not found to be detrimental or significant to condensation control.

It was determined this wall (or any similar construction) would be adequately served with 9.5 to 19 L/second (20 to 40 cfm) of supply air per 10 m2 (108 sf) of exterior wall. The choice of fan output pressure for a supply system should always be limited to 125 Pa (0.5 in.H2O). It should also be equipped with a speed controller for periodic adjustment.

The exterior wall of the test facility was well-sealed as most cladding systems are maintained to protect against rain penetration. This is a bonus for a VAB application. However, partitions can be problematic if their cavities connect directly to exterior wall cavities. Partition wall cavities must be interrupted for an effective VAB system.

The VAB also has other benefits. It allows raising of the indoor relative humidity to new levels, even as high as 60 per cent RH in extreme cold climates (providing all indoor surface temperatures such as window glass, window frames, doors, door frames, wall/floor, and wall/wall junctions are maintained above the dewpoint temperature of the indoor air). Additionally, it prevents mould buildup in construction cavities, as it tends to push out and dry any rain penetration in the non-freezing periods.

Rick[19]Rick Quirouette, B.Arch., is a senior building science specialist with almost four decades of experience in building science and technology. He is a life member of the Alberta Building Envelope Council and a past-president of the National Building Envelope Council. Operating as Quirouette Building Specialists Ltd., he can be reached at[20].

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