October 1, 2002
by Paolo Pasqualini and Kim D. Pressnail
The building envelope is primarily an environmental separator, which allows indoor spaces to be maintained at different conditions from the outside environment. Particularly in cold regions, interior relative humidity is a very important environmental factor that needs to be considered when evaluating the suitability of a building envelope that is subjected to cold weather. Intentional humidification during the heating season is a common practice in cold climates. Buildings are humidified for reasons that may include human comfort, operation of electronic equipment, and the control of dimensional changes of materials due to fluctuation in relative humidities in buildings such as hospitals, schools, libraries, office buildings, and museums. Moisture escaping from a humidified building via air leakage due to flaws in the air barrier system can negatively affect the durability of the building envelope. Hence, an effective air barrier is an essential component of any building envelope exposed to large condensation potentials unless the wall assembly can manage condensation appropriately.
Design deficiencies and construction imperfections have proven that the conventional design and construction of air barrier systems are not, in many instances, effective in controlling air leakage. With building envelope retrofits, the potential occurrence of air leakage is compounded by the large number of joints and protrusions, which can make the installation of a continuous air barrier system difficult and expensive to install. The Dynamic Buffer Zone method of air leakage control can overcome such difficulties in both new and retrofit construction.
The Dynamic Buffer Zone Concept
The Dynamic Buffer Zone (DBZ) performs the function of the air barrier in a building envelope, which protects the exterior faade from exposure to interior air moisture. The DBZ system creates conditions in an existing or purpose built air space (DBZ cavity) located within an exterior wall that effectively separates the interior and the outdoor environments. Conditions within the DBZ cavity that need to be controlled are air pressure, moisture content, and temperature. Moisture content of an air mass is the actual amount of water vapour (gas) that is contained within the air mass. Relative humidity is the ratio of the actual amount of water vapour in the air mass to the maximum amount of water vapour that can be held in the same air mass.
To effectively prevent exfiltration of humid interior air through the building envelope during cold weather, the air pressure of the DBZ cavity is maintained slightly higher than the interior air pressure. Theoretically, the cavity air pressure needs only to be nominally higher than that of the interior space to prevent air leakage from the interior. The actual amount of cavity pressurization that should be used is dependent on the specific project and is related to the relative air tightness of the components that make up the exterior wall. Typically a five to 10 Pascal pressure difference should suffice. As reference, the National Building Code suggests a 500 Pascal air pressure difference be used for the design of interior partitions subject to normal mechanical ventilation, stack effect etc. During winter conditions, outdoor air will have a low moisture content which makes it an ideal air supply for the DBZ cavity. The requirement for pressurization of the DBZ cavity will ensure that interior humid air will not leak outwards into the building envelope. If any leakage of air from the cavity to the outside occurs, the low moisture content of the DBZ air will eliminate the threat of condensation within the building envelope.
For exterior wall retrofits, the necessary DBZ air space can be an existing interstitial space, if appropriate, or it can be constructed on the warm side of the building envelope. For example, a 38mm to 90mm DBZ cavity can be constructed such that the interior side of the wall forms one side of the DBZ cavity and the interior finish provides the other boundary of the cavity. However, one must be cognisant of any hazardous materials in the walls such as asbestos or mould. A fan is required to introduce the air into the cavity from the exterior and a source of heat is necessary to pre-heat the incoming DBZ air. To ensure that the size of both the fan and the heat source are minimized, the interior finish that forms the inside boundary of the cavity should be sealed at all intersections with ceilings, window frames, columns and other intrusions. Furthermore, the existing wall forming the exterior boundary of the cavity also should be reasonably air tight to reduce the DBZ air flow that is required to pressurize the cavity.
There are two possible modes of operating the DBZ system. The first option is known as a “balloon” system where air is pumped into the cavity with no intentional exhaust air. That is, when the pressure inside the cavity drops below a specified lower limit, fans are activated to pressurize the cavity to the specified upper limit. The air inside the cavity will then leak out through the cracks of the exterior wall and through any imperfections in the interior wall finish until the lower prescribed pressure limit is reached, after which the process will repeat itself. The alternate mode is the exhaust system. This system entails supplying a continuous flow of air into the cavity and intentionally exhausting air from it so that the pressure within the cavity is maintained at a predetermined level. How and where the air is exhausted is dependent on a number of factors that are largely determined by the particular building involved.
The exhaust mode of operation allows for a great amount of versatility to the DBZ system. By controlling the flow rate and the initial temperature of the DBZ air, the system can act as both an air barrier and as a dynamic insulation system that can provide greater thermal efficiencies when compared to the same envelope without airflow.
Additional advantages of the exhaust DBZ system are its ability to promote drying of the masonry following rain penetration, to allow airborne contaminants to be contained in a space and controlled, and to increase the temperature of interior finishes of the exterior wall by supplying warm DBZ air. This will eliminate drafts associated with uninsulated walls and will increase thermal comfort.
In addition to walls and roofs, the DBZ system can also incorporate windows. Additional thermal comfort and condensation control can be provided by increasing the interior surface temperature of windows thereby allowing higher indoor humidities. This system can also eliminate the need for convection units below windows that are used to mitigate cold drafts.
Outdoor air is an ideal source for the DBZ air because of its low moisture content during winter conditions. Since winter outdoor air temperatures are typically below -10C in northern climates, some heat must be added to the DBZ air before it is introduced into the cavity. Although the temperature of the cavity air supplied is not important when dealing with the prevention of air exfiltration, it is important from the perspective of thermal comfort, surface condensation, and operating costs.
When utilizing the DBZ cavity as an air barrier or as a dynamic insulation system, it is desirable to minimize the amount of heat that is initially added to the incoming outdoor air. Dynamic insulation can reduce the cost associated with initial heating by allowing the DBZ air to capture some of the heat that would otherwise have been lost to the exterior environment. When changes are made to a building envelope, an important aspect of the change is its effect on wall surface temperatures. Exposing people to a relatively cold wall may cause them to feel uncomfortable because of their net-radiant heat loss to the colder interior surfaces. The temperature of the wall surface that is in direct contact with the DBZ air must always be maintained above the dew point temperature of the interior space to prevent surface condensation from occurring. Furthermore, the temperature of t
he DBZ air as it travels from the point of entry into the building and the point of entry into the DBZ cavity should also be above the dew point temperature of the interior air. This will prevent condensation from occurring on the exterior surfaces of the distribution ductwork used to transport the DBZ air within the building.
No testing was found regarding the effect that the DBZ has on wall surface temperatures. Recognizing the lack of information regarding the affect of an exhaust DBZ system on interior surface temperatures of an exterior wall, Yolles Partnership Inc. commissioned a study.
The research was conducted at the University of Toronto’s Building Science laboratory. The study involved the construction of an exhaust DBZ wall assembly that was exposed to controlled temperature conditions. After carefully considering the factors affecting the performance of the system, a testing program was designed and executed to investigate the effects of various parameters on wall surface and DBZ cavity temperatures. These parameters included: initial DBZ air temperatures at 5, 15, and 20C; DBZ air flow rates at 0.01, 0.04, 0.1 m3/min (0.35, 1.41, 3.5 cfm); DBZ cavity widths of 45 mm and 90 mm; addition of interior insulation on the warm side of the DBZ cavity with an insulation value of RSI 0.79 (R-4.5); and exterior ambient temperatures of -20 and -10C. The initial DBZ air temperatures noted above were considered representative of temperatures used in field applications. The DBZ flow rates investigated were based on the maximum flow rate specified for a previously installed DBZ system in a high rise building. The DBZ cavity widths of 90 mm and 45 mm were chosen because they are conventional framing sizes and would be representative of typical cavity widths used in new construction. In retrofit construction, the addition of interior insulation is often contemplated. Thus, the effect of adding insulation on the warm side of the DBZ cavity was also investigated.
As the DBZ air travels through the cavity, it will loose or gain heat energy to achieve thermal equilibrium with its surroundings. The heat transfer mechanisms involved in the flow of heat through the DBZ wall are conduction, convection and radiation. Among other factors, the contribution of each mechanism is dependent on the DBZ air velocity and cavity construction.
The testing demonstrated that the DBZ air temperature equilibrates quickly with the boundary conditions because of the small heat capacity of air. Generally, the DBZ air reached the average static (i.e. no flow condition) equilibrium temperature of the cavity for the various wall constructions tested. In general, the relatively large variation in average cavity temperatures observed did not significantly affect warm side surface temperatures of the wall. With DBZ inlet temperatures greater than the average static cavity temperature, wall surface temperatures were slightly warmer (<1.5C) than static conditions. A significant change in the surface temperatures of the wall was not measured. It must be stressed that the experimental results obtained are specific to the fluid and thermodynamic conditions of the DBZ cavity tested. For the flow rates and wall construction investigated, an initial DBZ air temperature of approximately 5.0C below the average static cavity temperature of the wall appears to be a viable temperature. However, the DBZ air temperature should never be colder than the dew point temperature of the interior air space. This restriction will prevent condensation from occurring on the surface of the distribution system used to transport the DBZ air from the exterior to the cavity and on the wall's surface in the vicinity of the DBZ air inlet.
The DBZ is a facade technology that is ideal for high humidity structures located in cold climates. In the past five years, the DBZ concept has been incorporated in a number of retrofit projects, including the Canada Life Assurance Building (331 University Avenue, Toronto), Roger/Cantel Campus (333 Bloor Street East, Toronto) and both the East and West Memorial Buildings in Ottawa.
Paolo Pasqualini, P. Eng. is an engineer with Yolles Partnership Inc. and Kim D. Pressnail is an Associate Professor in the Department of Civil Engineering at the University of Toronto.
A Dynamic Buffer Zone also was incorporated in the exterior walls of the recently renovated West Memorial Building in Ottawa. The historically designated building was partially converted to an archive centre where relatively high humidity was required.
A Dynamic Buffer Zone was incorporated in the exterior walls of the recently renovated Canada Life Assurance building in Toronto so that greater humidity levels could be maintained in the interior without impacting the integrity of the limestone walls. Above: