The first single-family Passivhaus in the U.S. was completed by Katrin Klingenberg in 2004. Klingenberg’s superinsulated home in Urbana, Illinois includes two unusual features: a ventilation system that pulls fresh outdoor air through a buried earth tube, and walls that include an interior layer of OSB. These details were not invented by Klingenberg; she adopted practices that were commonly used by European Passivhaus builders.
Although many North American designers have copied Klingenberg’s details, earth tubes are beginning to fall out of favor. Yet even in 2012, earth tubes are still being promoted by some Passivhaus builders.
By December 2007, when I first interviewed Dr. Wolfgang Feist, the director of the Passivhaus Institut, Feist was no longer recommending earth tubes. When I asked Feist whether earth tubes in Europe had experienced problems with condensation and mold, he answered, “There were problems in northern Europe, especially in Scandinavia. In Central Europe we haven’t had any hygienic problems so far. Actually, I’m not sure why we don’t have these problems in Central Europe. But I don’t advertise these systems any more, mainly because they are too expensive.”
The classic European wall
European Passivhaus builders (and Klingenberg) favor the use of a superinsulated wall system with three distinguishing characteristics:
- Vapor-permeable exterior sheathing made of fiberboard.
- Thick insulation — usually cellulose or blown-in fiberglass rather than spray foam or rigid foam.
- A layer of OSB on the interior side of the wall to act as an air barrier and vapor retarder.
Many American Passivhaus proponents (including Albert Rooks of Small Planet Workshop) endorse this type of wall assembly. They often claim that such a wall assembly is a better choice than one with thick exterior foam, arguing that it’s best to use vapor-permeable exterior sheathing so that a wall can dry to the exterior.
A case study in Belgium
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.
A recent case study of a house in Belgium highlights the importance of getting these details right. The home’s problems were discussed in an academic paper by Dr. Hugo Hens, a professor in the Building Physics unit of the Department of Civil Engineering at the University of Leuven in Belgium. His paper, “Passive Houses: What May Happen When Energy Efficiency Becomes the Only Paradigm,” was presented at the 2012 ASHRAE Winter Conference in Chicago. (I’d like to credit Richard Kadulski, the editor of Solplan Review, for alerting me to Hens’s paper.)
The Belgian house was designed to meet the Passivhaus standard. Here are some facts about the house:
- Completion date: December 2005.
- Area: 2,034 square feet (1,561 sq. ft. of treated floor area using the PHPP definition).
- Number of stories: 2 stories plus an unoccupied conditioned attic.
- Foundation: Slab on grade.
- Foundation insulation: 7 inches of foam (R-36).
- Wall construction: Wood framing.
- Wall details, inside to outside: drywall, OSB vapor retarder/air barrier, 12 inches of cellulose insulation (R-43), vapor-permeable wood fiberboard exterior sheathing, 1 inch air space, brick veneer. (The type of fiberboard sheathing used in Europe is impregnated with wax; like Zip System sheathing, it is considered a WRB if the seams are taped, so no housewrap or asphalt felt is required.)
- Roof assembly: Insulated sloped roof with 12-inches of cellulose (R-43).
- Windows: Triple-glazed low-e windows with argon gas fill.
- Space heating system: Air-to-water heat pump with electric resistance backup.
- Space heat distribution system: Hydronic coil in the ventilation supply duct.
- Mechanical ventilation: HRV with fresh air intake drawn through a buried earth tube.
- Air leakage rate: 0.68 ach50.
Something is terribly wrong
The homeowners (a family of five) moved into their new home in December 2005. Hens wrote, “Shortly after moving into the just-finished dwelling, the first complaints surfaced about degrading health with [these] main symptoms: coughing, shortness of breath, headache, dry throat, pain and weakness in the legs, painful muscles, fever, diarrhea, paleness, nausea, tiredness and a loss of taste. Also, visitors staying in the dwelling for a couple of days developed analogous symptoms. The complaints were confirmed by a medical diagnosis of [the] parents and children at a university hospital.”
The family complained that the house was uncomfortable, with “very high relative humidity indoors. … They mentioned too low room temperatures in winter and unpleasant high living room temperatures in summer. … The inhabitants confirmed they were forced to open the two windows in the attic space to keep the indoor environment ‘habitable.’ ”
Several problems were discovered, including the following:
- The air space between the brick veneer and the wall sheathing was not ventilated at the top and bottom of the wall.
- The insulation contractor left insulation voids at the top of the exterior walls.
- There was standing water in the earth tube.
- The OSB installed on the interior walls was damp.
- The HRV air flow was less than the design specifications.
Inward solar vapor drive
As builders in the U.S. have known for many years, the combination of a so-called “reservoir” cladding like brick veneer and a vapor-permeable sheathing like fiberboard is extremely risky. Hens noted, “During warmer weather after a rainy period, part of the moisture [absorbed by the brick veneer] diffuses across the inside leaf to the inside where it humidifies the OSB, a phenomenon called solar-driven vapor flow. … In fact, temperature at the cavity side of a wet west … [or] south looking brick veneer wall may pass 35°C (95°F) during warm summer days, giving water vapor saturation pressures up to 5260 Pa (109.5 lbf/ft²), high enough to create an additional daily vapor flow to the inside, which further increases humidification of the OSB. Solar-driven vapor flow may thus have activated global aldehyde release by the OSB during the summer months. … Solar-driven vapor flow is not considered by most of the designers and builders.”
Hens concluded that “high relative humidity, in combination with quite high summer temperatures indoors, must have boosted formaldehyde emission by the oriented strand board sheathing.” The concentration of formaldehyde in the indoor air was measured at 43.4 to 70 µg/m3 (35 ppb to 57 ppb). According to Hens, “the formaldehyde concentration is much higher than allowed in most countries.”
This same phenomenon — inward solar vapor drive from brick veneer through vapor-permeable sheathing — caused a disastrous cluster of building failures in Cincinnati, Ohio. The Ohio problems began surfacing in July 1999. Hundreds of homes suffered wall rot, and the enormous repair costs drove the developer, Zaring Homes, into bankruptcy.
If only Zaring Homes had specified rigid foam sheathing instead of fiberboard sheathing, these walls would have been fine.
You call that fresh air?
As it turned out, the earth tubes installed at the Belgian house were a big mistake. Hens wrote, “The search for causes [of the IAQ problems] started with a camera inspection of the overall ventilation system. … [The camera] revealed the presence of stagnant water and construction dust in the supply ducts.” According to Hens, the source of the water was rainwater intrusion.
Hens continued, “In 2006, the contractor renewed [repaired] the ground pipe, however without success. Again, it collected water. A second renewal in 2007 brought some upgrade; however, still, the ground pipe did not remain dry. In fact, in summer, relative humidity in the pipe passed 80% during prolonged periods of time, creating favorable conditions for mold to germinate on the pipe walls.”
IAQ testing “revealed the presence of spores of penicilium, aspergillus fumigatus, aspergillus versicolor and aspergillus niger.” Hens noted, “High relative humidity in the supply air leaving the ground pipe also caused humidification of the filter at the supply side of the heat recovery unit, turning it into a preferred medium for mold to germinate and sporulate. That most probably explained the presence of aspergillus spores in the indoor air.”
Hens concluded, “Ground pipes are potentially a risky technology. 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.”
The ventilation rate was low
According to Hens, the air flow delivered by home’s mechanical ventilation system was less than the design specification. (Considering the fact that there was standing water in the fresh air supply duct, it’s hard to know whether this was a problem or a blessing in disguise.)
The designer called for 182 cfm of ventilation. By U.S. standards, this was a very high ventilation rate. (According to the ASHRAE 62.2 formula used in the U.S., the home should have been ventilated at a rate of 53 cfm.) The supply air flow was measured with the HRV operating at high speed; it was only 62 cfm. Although this air flow rate exceeds the ventilation rate stipulated by ASHRAE 62.2, it was less than the rate called for in the construction documents — indicating (according to Hens) that the ventilation system was never commissioned properly.
Local authorities conclude: it’s uninhabitable
Less than two years after the family moved into their new home, the house was declared uninhabitable. “Indoor environmental quality … was so health-threatening that the passive dwelling was declared ‘uninhabitable’ by the communal authority in 2008. After the family left the house, the health complaints mentioned above disappeared.”
Since the home was vacated, contractors have been working to correct the building’s flaws. I haven’t been able to learn any details about the ongoing corrective work. Lawyers are now involved; according to Hens, it is now “a court case which demands correct privacy.”
What are the lessons?
Hugo Hens’s case study is certainly valuable, but it’s hard to draw firm conclusions about what went wrong without knowing all of the facts. It would be interesting to know, for example, whether the buried earth tube was sloped to a sump. Here’s my guess: no, it wasn’t.
Even with imperfect knowledge, I’m willing to stick my neck out and offer my own conclusions:
- If earth tube ventilation systems are well designed, properly installed, and regularly inspected, they can perform well. However, earth tubes are expensive to install, provide limited energy savings, and incur a significant risk of developing moisture and mold problems.
- Most walls with vapor-permeable exterior sheathing and interior OSB perform well. However, it’s probably not a good idea to specify OSB with formaldehyde on the interior side of your walls.
- Vapor-permeable exterior sheathing does nothing to slow inward solar vapor drive. If you plan to install a “reservoir” cladding like brick veneer, it’s safer to use rigid foam sheathing than a vapor-permeable sheathing like fiberboard.
- Homes with high-R walls and low air-leakage rates are vulnerable to a variety of problems if details aren’t well thought out or if contractors are sloppy.
Last week’s blog: “Window-Mounted Air Conditioners Save Energy.”