Naturally ventilated airtight problems

Sean M. O’Brien, PE, LEED AP
Natural ventilation has typically been the means of providing fresh air for occupants. Operable windows and doors allow occupants to control the amount of air that enters and leaves a building, or individual spaces. Although the ventilation a space receives also depends on wind speed, operable vents have typically been sufficient for residential spaces, given the fairly low ventilation rates required.

When all doors and windows are closed, these buildings rely on ‘incidental air leakage’ through the enclosure. Before continuous air barriers, many buildings were moderately to extremely ‘leaky,’ allowing enough air to flow into and out of the space through gaps in the enclosure to make up for the closed windows and doors. For this reason, most residential buildings, especially single-family homes, were rarely provided with mechanical ventilation.

As buildings became more airtight, incidental air leakage was typically reduced to the point where it was no longer enough to provide fresh air. Operable windows were still sufficient to ventilate the spaces, but very few people open their windows in January in New York or August in Miami. The result is high interior relative humidity (RH) levels, as moisture from occupants (respiration) and activities (cooking, bathing) can quickly build up due to the lack of air exchange with the exterior. This is the same as getting in a car in the winter—moisture from respiration builds up due to the cabin’s airtight construction, condensing on the cold glass.

Condensation buildup on a thermally broken aluminum-framed window system in a residential building. Relative humidity (RH) levels in the adjacent living space exceeded 60 percent. Photo courtesy SGH
Condensation buildup on a thermally broken aluminum-framed window system in a residential building. Relative humidity (RH) levels in the adjacent living space exceeded 60 percent.
Photo courtesy SGH

This author first came across the problem in 2001, shortly after continuous air barriers were incorporated into the Massachusetts State Energy Code. A project involved the complete recladding of a residential building, new windows and doors, and new mechanical systems. The mechanical engineer was directed to select heat pump units with an option for outside air supply to account for the improved airtightness.

At the time, options for ventilation in heat pump units were limited to direct vents to the exterior with a small operable damper—similar to what you would find on a window-mounted air-conditioner. Many occupants would close the fresh air supply (despite a large sticker warning them to keep it open) in an attempt to lower heating bills, which, even with ventilation, were already much lower following the building enclosure upgrade. During the first winter of operation, there were multiple claims of condensation on the new windows, which were thermally broken and vastly improved from the previous systems. RH levels beyond 50 percent were measured in units with closed dampers, while those with open dampers were 20 to 25 percent (with no condensation).

This project was limited by the available mechanical systems at the time. However, this author recently investigated a 2010 building with the same problem. Its units used water source heat pumps (WSHPs) with no direct air vents to the building exterior. The building design, in full compliance with the New York City Building Code at the time, relied on operable windows for ventilation of residential units. RH levels of 60 to 70 percent were measured, with condensation on windows and doors, even during relatively mild weather, observed. Opening windows quickly dropped the RH to more reasonable (i.e. ~30 percent levels), but, again, few open their windows in the middle of winter to provide ventilation.

The main problem was occupants had no options for fresh air other than opening windows. The airtight building, designed and constructed in accordance with both building and energy codes, experienced significant, difficult-to-resolve moisture problems. A more functional design would have included either local or centralized mechanical ventilation to provide fresh air to the units, with heat recovery to improve efficiency. Unfortunately, heat recovery and centralized ventilation were both expensive and not code-required at the time.

Ventilation requirements are slowly catching up to accommodate airtight building design, but designers must review systems to ensure appropriateness to the enclosure and interior spaces. Coordination between building enclosure and mechanical system designers is critical for avoiding conflicts in the overall design.

The opinions expressed in Failures are based on the author’s experiences and do not necessarily reflect those of The Construction Specifier or CSI.

Sean M. O’Brien, PE, LEED AP, is an associate principal at national engineering firm Simpson Gumpertz & Heger Inc., specializing in building science and building enclosure analysis. He is involved in both investigation/forensic and new design projects. O’Brien is a member of American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) and the New York City Building Enclosure Council (BEC-NY) and a frequent speaker on topics ranging from condensation resistance to energy efficiency. He can be reached via e-mail at

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