Virtual air barrier and dynamic buffer zone for a heritage building envelope

by sadia_badhon | December 6, 2019 10:08 am

By Rick Quirouette, B.Arch., Roger Mitchell, SAA (Ret.), FRAIC, and Robert J. England, P.Eng.

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Historic buildings in Canada are often 100 years old or more. Some of these structures are designated as heritage buildings and continue to provide occupancy, but the indoor conditions are inadequate for current residency and production facilities. The indoor conditions are hot and humid in summer, and cold and dry in winter. These buildings also exhibit high energy costs due to little or no insulation and uncontrolled air leakage through porous exterior walls and roofs. The restoration of claddings and exterior walls of heritage buildings is a growing activity in many Canadian towns and cities. However, to be acceptable for modern-day occupancies, these restored structures need to support an adequate indoor temperature, relative humidity (RH), air quality, and controlled building pressure. Additionally, a heritage building envelope requires some consideration with respect to future energy conservation features.

Modern indoor comfort requires relatively uniform temperature distribution and air movement in the room. It also needs supplementary indoor humidity in winter and dehumidification in summer. From an energy point of view, additional insulation is also required for exterior walls and roofs. Heritage buildings may need new windows and doors and the addition or upgrade of HVAC systems. When the indoor conditions are renovated, it presents a new set of challenges for the envelope such as:

• air exfiltration due to building pressurization;
• condensation within wall cavities from humidity in the exfiltration air; and
• greater thermal movements of the cladding surfaces due to increased insulation.

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Excess condensation causes various damages including:

• staining of façades and corrosion tracks;
• corrosion of anchors;
• spalling of mortar joints and masonry; and
• efflorescence.

Increased insulation is not easily accommodated as most heritage buildings do not include expansion or contraction joints in their cladding surfaces to accommodate greater surface temperature excursions.

Architectural restoration strategies do not consider the new loads imposed on the building envelope when the heritage project includes a major indoor fit-up. The principal challenge to mitigate potential condensation damage to the building cladding is to reduce and limit moist air exfiltration. If it is considered, in many cases, the architectural efforts have failed to perform adequately. Air sealing against air exfiltration through a building envelope is a complex problem poorly understood by construction professionals. Well-developed solutions to air leakage control are rare and strategies to mitigate air pressure resistance in the construction of exterior walls and roofs are almost non-existent.

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The application of a virtual air barrier and dynamic buffer zone (VAB/DBZ) concept mitigates, if not eliminates, all of the above concerns including pressure-induced air leakage and moisture loading on heritage exterior walls, as well as reduces rain and/or condensation damage to restored claddings. It can also alleviate the thermal and moisture expansion and contraction issues of heritage cladding and exterior walls.

The Viterra building restoration

The Viterra building was originally designed as a department store for the downtown core of Regina, Sask. It was built in 1913 for C. W. Sherwood Co. The lower building of the Viterra complex is a three-storey structure attached to a newer 11-storey office facility. It was designated as a heritage building by the City of Regina.

A major restoration of the exterior walls and cladding as well as a comprehensive indoor fit-up was completed on the low-rise portion of the Viterra building in 2018. The restoration project is also fitted with a state-of-the-art VAB/DBZ system for the protection of the cladding and exterior wall restoration and to better support control of the indoor environmental conditions.

The building is constructed with a cast-in-place concrete structural frame with cast-in-place concrete slabs. The exterior walls are masonry. The cladding is terra cotta stone with sculptured column covers and parapets. The original, wood-framed windows were replaced with modern, curtain wall-style aluminum glazing. Currently, the building is used as an office space with a small portion of the first floor dedicated to a grain laboratory.

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The restoration as well as the design of the architectural requirements of the VAB/DBZ portion of the project was undertaken by this article’s co-author Roger Mitchell, SAA (Ret.), FRAIC, of Regina. The design of the VAB/DBZ mechanical system and controls was provided by Robert (Bob) J. England, P.Eng., of R. J. England Consulting (mechanical engineer), also of Regina and a co-author of this article. Consultation for the application and performance evaluation of a VAB/DBZ for the Viterra building was provided by Rick Quirouette, B.Arch., of Quirouette Building Specialists of Ottawa, Ont. The construction of the cladding and wall restoration, the indoor fit-up, and the VAB/DBZ project was undertaken by PCL Construction as construction manager supported by Brxton Masonry and Tierdon Glass.

The VAB/DBZ concept

The VAB/DBZ is an advanced architectural and mechanical system technology. It has evolved over the past 30 years, from an exterior wall or roof cavity ventilation approach to the more efficient and less costly cavity air pressure control concept. It provides effective and efficient protection to the building façades for an indefinite period of years if operated as prescribed. It is well-suited to masonry heritage buildings as well as other types of structures requiring higher than normal indoor humidity conditions. It has been applied to buildings in Montréal, Ottawa, Toronto, Edmonton, and, most recently, in Regina.

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The concept of the VAB/DBZ technology is to lightly pressurize the interior (or cavity) of the exterior wall (or roof cavity) of a building slightly above the indoor pressure at all times during late fall, winter, and early spring seasons. The exterior wall and cladding cavity pressure is provided by a mechanical pressurization (ventilation) system. When operating, the VAB/DBZ system controls an exterior wall (or roof) cavity pressure even as the indoor levels rise or fall with stack effect, wind pressure, mechanical ventilation, and barometric pressure. In this way, the moist indoor air cannot and does not leak (exfiltration) into the cavities of the exterior wall (or roof) to cause a buildup of condensation in winter.

A VAB/DBZ system uses outdoor air for its pressurization (ventilation) air supply through a rooftop unit. The supply air must be taken from the outdoors because the dewpoint temperature (DPT) of the outdoor air is almost always below the temperature of any surface within the exterior wall components. Thus, the outdoor air cannot contribute to condensation anywhere within the exterior wall as the wall cavity surface temperatures are above the outdoor DPT.

To reiterate and emphasize, the VAB/DBZ system’s supply air must never be drawn from the interior of the building, such as a mechanical room, elevator shaft, or stairwell. This is to ensure the new air introduced into a wall (or roof) cavity is dryer than the air in the building and wall cavities of the VAB/DBZ. There are times when the outdoor temperature is cold, but not below freezing (e.g. during rain). When this happens, the VAB/DBZ supply air would exhibit a slightly higher than wall cavity DPT. However, this condition is short-lived and does not contribute significant condensation moisture to a wall cavity.

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The VAB/DBZ supply air is pumped directly into the exterior wall cavities at specified intervals around the floor perimeter from supply ducts on individual floors. There is no return air in this approach, thus eliminating a significant portion of the assembly cost, not to mention a more effective and efficient system operation.

Viterra building envelope assessment

Prior to implementing a VAB/DBZ on the Viterra building, a review of the architectural conditions of the building envelope was undertaken. To estimate the initial VAB/DBZ supply air requirements, an empirical rule is used. It was determined from previous projects. A heritage masonry wall requires between 12 L/s/10 m² (25 cfm/100 sf) and 19 L/s/m² (40 cfm/100 sf) of exterior wall including windows. The project team’s first task was to determine the areas of exterior wall to be serviced with a VAB/DBZ.

A set of architectural documents were found
from the original construction. They provided general information on elevations, building sections, and floor plans for all three floors including an attic-type mechanical room. The building has a 2114-m² (22,750-sf) floor footprint with high ceilings. Since few detail drawings of the exterior walls were available, test openings were undertaken to determine its composition and cavity conditions. The tests revealed the following:

• terra cotta exterior is 102 or 203 mm (4 or 8 in.) deep;
• exterior wall cavity has masonry rubble infill;
• narrow alignment cavity;
• precast spandrel beam at the ceiling and floor level of each storey;
• two layers of terra cotta masonry on the indoor side below the spandrel; and
• plaster and lath indoor finish (Figure 1).

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Figure 1 illustrates the wall composition of the south and west elevations of the building. The north and east elevations are similar but with a terra cotta stone cladding instead of a brick veneer. Insulation or well-defined wall cavities were not found in the test openings. It was hypothesized from the crushed masonry infill the exterior wall would transmit cavity pressure around the building envelope. In a separate investigation of the wall cavity porosity, a pressure test of the wall cavity was undertaken.

A commercial vacuum cleaner acting as an air pump was connected to an exterior wall test opening on the north side of the third floor. All other test openings previously drilled on the same floor were sealed with a poly sheet, about 1.5 m (5 ft) square, to the plaster finish. When the air pump was turned on, it was noted the poly sheet covering the test openings on the opposite side of the building inflated slightly to the indoor side of the floor, an indication the cavity was exhibiting pressurization even with all manner of unknown leakage paths through the exterior wall cavities.

Architectural interventions

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In order to prepare the building envelope for VAB/DBZ, several architectural modifications were undertaken to seal the cladding and exterior wall indoor finishes to better contain the cavity pressure including:

• the restoration and repointing of the exterior terra cotta mortar joints and cladding stones;
• two vertical compartment seals through the exterior wall at the junctions of the main building with the adjacent tower;
• vertical air seals between the exterior wall and elevator shaft to minimize the loss of cavity pressure into the shaft;
• air seals between the exterior wall rough openings and the new window frames;
• foundation to exterior wall connection and the exterior wall to roof/attic slab were both sealed for the purposes of pressure containment; and
• indoor foil faced gypsum board with narrow cavity between gypsum board and existing plaster finish was installed to seal the exterior wall on the indoor side (the foil faced gypsum provides a small insulation value, [radiant cavity] to raise the indoor surface temperature in winter for occupancy comfort).

Mechanical pressurization system

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Based on the previous determination of exterior wall areas an initial estimate for the supply air volume was determined to be about 3540 L/s (7500 cfm). The fan head pressurization of the VAB/DBZ ventilation system was limited to 250 Pa (1 in. H2O). Figure 2 (page 38) illustrates the VAB/DBZ duct system from the outdoor fresh air intake to the connections in the exterior walls in the horizontal sectional view. The DBZ mechanical system comprises a fresh air inlet with rain penetration control, fan as air handling unit (AHU), gas heater for the outdoor supply air, and all associated ductwork (coloured grey and pink). The ductwork is fully insulated from the air inlet to the duct heater. The ductwork downstream of the heater to the duct connections did not require insulation. Since the VAB/DBZ supply air is heated well above the indoor winter DPT, the risk of downstream ductwork surface condensation is minimal. This limited the indoor RH to not exceed 50 per cent during winter season.

The final DBZ mechanical system specifications for the Viterra Building and makeup air unit data are:

• maximum supply airflow – 3540 L/s (7500 cfm);
• maximum fan head pressure – 250 Pa;
• heating capacity – 188 kW (649,000 Btuh);
• a bypass to allow adjustment of the unit volumetric flow rate during commissioning of the air-side system;
• pressure independent variable air volume (VAV) boxes in order to control supply air to each floor and wall cavity pressure tied to a near constant pressure-looped duct main;
• a ‘balancing’ duct to equalize pressures in dual risers where appropriate;
• duct insulation of 76 mm (3 in.) on unheated air (i.e. upstream of the makeup air unit);
• manual duct dampers at exterior wall connections for balancing;
• fire dampers at fire separation penetration; and
• system controls, including monitoring sensors and data collection devices.

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Figure 3 illustrates the DBZ duct layouts installed in the suspended ceiling of each floor of the building. The yellow diamond shows the location of the temperature, RH, and all the pressure sensor stations.

VAB/DBZ operation

The VAB/DBZ system is operated from mid-October to April-end for Regina’s climate conditions. It consists of starting the system pressurization fan and indirectly fired natural gas makeup air unit heater and letting the system operate according to a program protocol for the duration of the fall startup, winter, and spring periods to the shutdown event around April 15.

The VAB/DBZ mechanical system controls the outdoor air supply, heater conditions, and air temperature settings, amount of air to the risers for each floor, and VAV boxes. Additionally, the ducts connecting to the wall cavities were supplied with manual dampers for the initial balancing of the supply air and pressure difference measurements.

VAB/DBZ SUPPLY AIR ON THE WARM OR COLD SIDE OF INSULATION

The virtual air barrier/dynamic buffer zone (VAB/DBZ) supply air can be pumped into a wall cavity on the warm side of an insulated exterior wall cavity or on the cold side. If a building envelope is composed of a brick or stone veneer with a drainage cavity, insulation, and vapour barrier, the VAB/DBZ supply air is directed into the wall cavity on the cold side of the insulation. In this case, the supply air need not be pre-heated but the ducts may require insulation if routed on the indoor side of the building to prevent condensation from dripping onto a suspended ceiling or a floor below. To augment the cavity air pressure in a pressure-equalized rainscreen wall, the drains and vents of the brick cladding and any other openings leading into the cavity of the exterior wall must be sealed. The exterior walls can still be drained with tubes if required. The tube drains do not represent a significant uncontrolled leakage area. Alternately, if the VAB/DBZ supply air is pumped into a cavity on the warm side of the insulation, as it might occur with an insulated precast wall cladding, it is best to pre-heat the supply air to a level determined by a temperature gradient analysis. This maintains indoor finish surface temperature at ambient comfort conditions and the entire wall cavity dry (Figure 5). If an exterior wall is uninsulated, such as with most heritage buildings, and there is no clear cavity condition, then the supply air temperature may be set to halfway between the indoor and outdoor average monthly temperature. As the outdoor temperature varies dynamically, an average temperature difference for the coldest month is adequate to identify a fixed temperature setting for the wall cavity under consideration. For a more efficient heating system, a temperature control approach will save energy without affecting DBZ performance. Lastly, if the VAB/DBZ cavity is well sealed then there will be little heated air supplied. If the cavity is more porous than expected, the heating system will provide more heated air to the exterior wall cavity.

The VAB/DBZ system is controlled by the pressure difference between the wall cavity of each floor and the interior pressure. The wall cavity pressure is determined by the amount of air pumped into the wall cavity. In Viterra, the DBZ supply air to each floor wall cavity is limited to a maximum flow of 810 L/s (1716 cfm) for the first floor, 575 L/s (1218 cfm) for the second, and 520 L/s (1101 cfm) for the third.

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The wall cavity pressure is governed by the pressure difference induced by the DBZ supply air and the indoor pressure. To ensure cavity pressure does not rise too high, the wall cavity pressure difference with the indoor side is controlled between zero and 10 Pa (0.04 in. H2O). If the cavity to indoor pressure difference rises near the maximum pressure limit of 10 Pa, the VAV box for that floor limits the DBZ air supply to the cavity. If the wall cavity to indoor pressure difference operates near zero pressure variance, the DBZ supply air is increased to near maximum flow for that floor. In this way, the wall cavity pressure difference for each floor is controlled independently of the exterior wall cavity airtightness conditions. VAV boxes allow the DBZ system to respond independently to rises in barometric pressure, stack effect pressure, wind pressure, and the building ventilation pressures on each floor.

The condensation control for the wall cavity is the same whether operated at high or low pressure difference but the cost of heating the supply air (with a temperature of 10 C [50 F]) increases if the wall cavity is inadequately sealed. The performance of the system is quite forgiving, so much so the concept has been dubbed fail safe. To better explain this important concept feature, if the cavity-to-indoor pressure difference hovers near its maximum, it indicates an airtight cavity and the pressure difference prevents humid indoor air from entering the wall cavity. If the cavity-to-indoor pressure difference hovers near zero, it indicates a less well-sealed cavity. Should a small quantity of indoor air find its way into the cavity, the DBZ air then dilutes and flushes the cavity moisture gain with dry outdoor air into the wall cavity. The only time the system does not perform adequately is if it is turned off for maintenance and not turned back on for the remainder of the operation cycle or season.

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In Regina, the system should not be operated in late spring, summer, or early fall, and especially in a region of high outdoor summer humidity. If it is operated during this period, the VAB/DBZ system will cause the wall cavity materials to store moisture and raise the wall materials’ moisture content to reduce the efficiency of the VAB/DBZ system performance.

It is not very difficult to understand outdoor air at 50 to 80 per cent RH at outdoor temperatures of 15 to 30 C (59 to 86 F) during summer would cause the masonry materials of the wall cavity to absorb and store moisture. This storage will then be released in the fall and early winter within the wall cavity. So, the VAB/DBZ system is always turned off during the summer season.

VAB/DBZ performance monitoring

During the renovation of the indoor spaces of the Viterra building, a system of sensors was installed in the exterior walls cavity to measure the performance characteristics of the VAB/DBZ system. Additionally, sensors for the VAV boxes, supply air heating system, supply air delivery to the building, VAB/DBZ risers, and ceiling duct circuits were also installed.

To monitor the performance of VAB/DBZ, the sensors installed on and in the exterior walls measured the temperature, RH, and the air pressure difference across two planes of the exterior walls. Specifically, measurements of temperature and RH were obtained near the cladding surface, in the exterior wall cavity, and the indoor side of the indoor finish. The air pressure differences were measured from indoor to wall cavity, wall cavity to outdoor, and from indoor to the outdoor side.

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The sensors included the thermocouples for temperature measurements and RH probes and pressure tap (tubes) with associated pressure transducers to determine air pressure differences. The sensors for these measurements were grouped into monitoring stations. There are six stations for the building. Five of them are located in the exterior walls of the building and one is on the roof to obtain local weather conditions. The sensors were wired to a direct digital control (DDC) building management system in the maintenance office of the building.

The data was collected on a continuing basis and stored in a data logger. The data was then tabulated as hourly results. The data collection started in late fall 2017 and continues to this day. For the purpose of this report, the data analysis was limited to December 2017 to end of January 2018.

VAB/DBZ performance analysis

Figure 4 (page 43) illustrates the air pressure differences between the indoor and the wall cavity (red) and the pressure difference between the indoor and the outdoor (blue). The indoor was chosen as the reference pressure (atmospheric) for these difference measurements. Note the cavity pressure (red) is slightly positive or above the indoor pressure (reference) and nearly constant. Cavity air then leaks out of the wall cavity. It is this pressure difference that prevents indoor air from exfiltrating into or through the exterior wall cavities. The indoor to outdoor pressure difference is indicative of the pressure difference on the exterior wall from stack effect, wind, and mechanical ventilation.

The analysis above illustrates the effectiveness and efficiency of the VAB/DBZ of the Viterra Building in Regina. It is performing as expected. During an initial system operation review in December 2017, it was determined the supply air used for pressurization could be reduced without losing condensation control performance. The system was adjusted accordingly with a 17 per cent reduction in air supply. Analysis of the wall cavity moisture and pressure data revealed condensation control performance remained near perfect over the heating season of 2017 to 2018.

AIR SEALING THE FAÇADE AND INDOOR FINISH

Architecturally, the virtual air barrier/dynamic buffer zone (VAB/DBZ) system performance depends on the airtightness of the exterior wall (or roof surfaces) containing the cavity air. It is important to review the characteristics of the exterior walls or roof of the building to be restored to determine the type and extent of architectural intervention required to provide adequate air sealing of the exterior wall. There is a similar requirement for roofs albeit the waterproofing provides an adequate air seal on the outdoor side, but most often there exists a major discontinuity at the roof wall junction or parapets. The challenge for a roof VAB/DBZ is the construction of a continuous indoor side plane of airtightness encapsulating the roof cavity and a major intervention at the roof wall or parapet junction to compartment the roof cavity effectively.

Recommendations

The VAB/DBZ technology is simple in concept but moderately difficult to design and construct. There is little or no literature to guide the professionals in this technology. Further, it does not fall cleanly into one professional camp or the other. Building teams wanting to pursue the design of a VAB/DBZ for a project may be guided by two main objectives.

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The first objective is an architectural intervention providing the best cavity seal possible with the cladding system on the outdoor side, sealing the indoor side with the indoor finishes linked continuously from foundation to roof, and creating an airtight seal around the perimeter of the building assembly cavity. A continuous single cavity is not always required. If the building is a difficult form, the exterior walls may be separated into individual cavities. Perimeter conditions of a wall cavity include sealing at roof wall parapets, connections to adjoining buildings, header, jams, and sill of windows and doors, floors and partitions, and major penetrations from the indoor or outdoor side. It is unnecessary to seal the wall cavity perfectly. If the total leakage area from a wall cavity can be quantified after the architectural intervention, the design criteria would require a leakage area of no more than 3000 mm² (4.65 si) to 4839 mm² (7.5 si) per 0.1 m² (1 sf) of exterior wall. There are numerous ways to determine the location and severity of exterior wall leakage characteristics and these include pressure testing with a smoke generator to locate leaks and test openings for a positive visual confirmation.

From the mechanical engineering point of view, the supply air fan capacity to create the wall cavity pressure required may be targeted between 12 L/s (25 cfm) and 19 L/s (40 cfm) per 9 m² (100 sf) of exterior wall in masonry heritage buildings. Other types of exterior walls need to be assessed on an individual basis. If the pressure difference between the wall cavity and the indoor is close to zero, additional architectural intervention may be required to better seal the exterior wall cavity enclosure elements. If the pressure difference of the cavity to indoor side is higher than 10 Pa, the fan supply air to the exterior wall cavities must be reduced. Additionally, it is recommended the maximum fan head pressure not exceed 125 Pa although ducted distance and flow resistance may require a slight increase in head pressure as in the Viterra project. These rules of thumb have met the requirements of several VAB/DBZ projects undertaken in Canada including the Viterra building.

OBSTRUCTIONS IN THE WALL CAVITY

The virtual air barrier/dynamic buffer zone (VAB/DBZ) concept is designed to pressurize an exterior wall cavity. A clean, well-defined exterior wall cavity is not essential. Cavity pressure distributes quickly even in a rubble-filled masonry wall. Since pressure travels at the speed of sound, (approximately 330 m [1000 ft]/s), a pressure pulse is immediately distributed even in the most obstructed wall cavity of an exterior wall. This was clearly demonstrated at the Viterra building, Regina, Sask. It has an obstructed wall cavity with infill masonry. A test opening undertaken in the north wall indicated the immediate appearance of a pressure pulse on the other side of the building. The flow of air through the wall cavity was irrelevant.

The VAB/DBZ technology is a collaboration of both architecture and engineering as are many elements of building design. However, it takes a knowledgeable multidisciplinary design team to recognize the need for, and potential effectiveness of such a system. This technology, the VAB/DBZ concept, and application is so forgiving for exterior walls and roof condensation control, that even with incomplete architectural intervention, an undersized pressurization and/or heating system, and imperfect construction sealing, it will function well, but may consume more heating energy.

[15]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 (ABEC) and a past-president of the National Building Envelope Council (NBEC). Operating as Quirouette Building Specialists Ltd., he can be reached at rick.quirouette@gmail.com[16].

[17]Roger Mitchell, SAA (Ret.), FRAIC, is the principal architect in his eponymous firm Mitchell Architect. During his 43-year career, Mitchell has provided specialized consulting in building accessibility issues and sustainable design. He has extensive experience in effective management of planning, design, and production team. He can be reached at rmitchellarch@gmail.com[18].

[19]Robert J. (Bob) England, P.Eng., offers specialized building mechanical consulting services through his boutique-styled firm R. J. England Consulting. England has worked in the mechanical engineering field for most of his career in geographically varied building projects. He enjoys heritage building work, as each commission presents unique engineering challenges requiring innovative solutions. He can be reached at bob@rjengland.com[20].

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