Passive Houses are lauded for their low energy use, but does that come at the expense of occupant health?
According to Bo Gleditsch of the Norwegian asthma and allergy association, they can.
“We get many questions about passive houses," he noted. "They are almost hermetically sealed houses where the air does not change. This causes dust to accumulate and high temperatures."
Regulations in Europe will require all new buildings to be nearly carbon neutral by 2020. Passive Houses are one way to reach that target. However, Gleditsch said, “in our eagerness to solve an environmental problem, we are causing a health problem.”
The Passive House standard was developed in Northern Europe and is optimized for that climate, with specifications for total energy use, heating and cooling energy use, and air-tightness of the structure. The US branch of Passive House broke away from the German branch and has developed its own standards for North America that vary the requirements by climate. Nonetheless, both versions of the Passive House standard require a very tight structure combined with extremely low energy use.
The standard does not prescribe methods or materials, but typical approaches have evolved to meet the three mandatory standards. Those include using thick insulation, and usually call for an active ventilation system, such as a heat-recovery ventilator.
Another technique that aims to reduce energy use is earth tubes. Tubes buried under the house or elsewhere on the site are used for the fresh air supply and are connected to a heat-recovery ventilator. Their location in the soil helps to cool the incoming air in hot weather, and warm the incoming air in cold weather. In theory, this simple approach lessens the amount of supplemental heating and cooling needed for a comfortable indoor temperature.
Problems can arise when they don’t perform as expected, or are not installed as designed. In those cases, earth tubes can harbor mould and condensation, leading to degraded and unhealthy indoor air. The air-tight construction and thick insulation required in a Passive House may then exacerbate air quality problems resulting in health issues for the occupants.
To cite one example, a home in Belgium was eventually declared uninhabitable after the family living there experienced myriad health problems immediately after moving in. Investigators determined that the earth tubes had standing water in them, leading to mould growth. The tubes also contained construction dust.
In addition, the air flow created by the heat recovery ventilator underperformed design specifications, and mould built up on the HRV filter.
Another problem with this home is the wall design, which used an unventilated air space between the exterior brick veneer and the wall sheathing. Investigators determined that after rain, some of the moisture absorbed by the exterior brick cladding migrated inward where it dampened the wall sheathing. This process is known as “solar driven vapor flow,” which increases with temperature. The investigator concluded that high relative humidity, along with high temperatures, facilitated the release of formaldehyde from the sheathing into the interior of the house.
These issues caused health problems for the family, including respiratory problems, diarrhea, muscle aches, and headaches. The earth tube contractor attempted to correct the problem twice, but without success, and the home was declared uninhabitable about two years after it was built.
In this case, the Passive House approach coincided with other problems, but was not the cause of the problems. Any nearly air-tight home would be susceptible to air quality issues caused by mouldy, humid incoming air, along with formaldehyde off-gassing. Those air quality issues were the direct result of design and construction flaws in the earth tubes and the walls.
The investigator, Dr. Hugo Hens, a professor in the Building Physics unit of the Department of Civil Engineering at the University of Leuven in Belgium, drew some conclusions in his paper, Passive Houses: What May Happen When Energy Efficiency Becomes the Only Paradigm.
“Though solely promoted as energy savers, their impact on energy consumed is too minimal [for] the possible risk they create in terms of degrading supply air quality,” he remarked.
Martin Holladay, a noted writer on green building, wrote in Green Building Advisor that “Homes with earth tubes and vapor-permeable exterior sheathing can perform well — but only if the designer uses a whole-building approach that considers moisture movement from all angles. Moreover, if a builder makes installation errors, all bets are off.”