Designing the virtual air barrier

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.

RickRick 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

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