Passive House FAQ

It's common building wisdom that a house needs to "breathe." Can the passive house's air-tight construction lead to moisture or air quality problems?

Many new homes today are being built tightly in an improper fashion, trapping moisture inside and leading to significant mold and indoor air quality problems. Unlike most other new construction, the passive house employs an appropriate vapor barrier and then a mechanical, balanced ventilation system with heat recovery, which assures superior air-quality and comfort by continually exchanging the indoor air. Moisture is safely removed, as well as other potentially unhealthy pollutants (for example off-gassing from carpets or furniture). Also, people with allergies can easily control the indoor air-quality by specifying filters according to their particular medical needs. 

The passive house standard is based on the climates of Europe. Can it be met in the more challenging climatic zones of the US?

Passive housing principles have a long history in the United States. The Germans actually used many principles pioneered in the States at places like the Small Homes Research Council at the University of Illinois. It is correct that Germany has a more temperate climate than, for example, Minnesota or Florida. The principle, though, can be applied in all climates. Minnesota requires additional insulation levels to meet the standard. In hot climates like Florida, the cooling load becomes the major factor in energy usage. In humid climates, de-humidification is an additional challenge that needs to be considered. But neither cooling nor de-humidification prevent application of the passive house concept, and can be managed by a corresponding design of the minimized mechanical system (balanced ventilation with heat/energy recovery, heating, cooling, de-humidification and domestic hot water). PHIUS is in the process of developing and testing these systems in the variety of US climate zones. 

How does the cost of passive house construction compare to that of standard building?

Based on the construction of the Fairview House in Urbana, some baseline numbers have been confirmed (regarding passive energy efficiency measures, NOT regarding other green features such as active solar or deep green materials).To achieve passive house standard requires an additional upfront investment of approximately 10% of the construction budget, as compared to regular energy code-compliant 2x4 construction. Of course this number can vary depending on the building. The real cost advantage occurs at the passive house heat requirement of 15 kWh/sq.m and year (4750 BTU/sq.ft. and year), when the standard HVAC system can be eliminated. It is then replaced by a smaller ventilation system, a so-called fresh-air furnace. This system's significantly reduced ductwork is used to deliver the remaining 10% of heating and cooling needs. The required duct work for the ventilation system is much smaller and fits into 2x4 walls saving on material and labor.  

Why "passive" house?

A "passive" house achieves overall energy savings of 60-70% and 90% of space heating without applying expensive "active" technologies like photovoltaics or solar thermal hot water systems. Energy losses are minimized, and gains are maximized. Superinsulation and air-tight construction minimize losses. The heat/energy recovery ventilator helps keep energy that has already been generated in the house instead of venting it out. Knowing about thermal storage capacity of certain materials and their "passive" effects on the indoor temperature of a home, the architect/designer can plan for enough thermal storage mass in a house by specifying tile floors, finished concrete slabs, concrete or granite countertops, stone fireplace surrounds, adobe walls or earthen plaster (in a passive house thermal storage mass no longer has to be painted black or directly exposed to the sun! The PHI recommends 5-6 thermal storage surfaces per room for optimal effect). In climates with low humidity and high day/night temperature differentials, it can be very effective to bring in outside air through a pre-warming/cooling "Earth Tube" to passively harvest the energy of the earth (without use of active ground source heat pumps).

It is beneficial to install solar hot water systems in addition to the passive house construction techniques. Next to space heating/cooling, domestic hot water is the biggest energy requirement in a home. Solar hot water can also effectively be used to provide the remaining space heat wherever heating is necessary (hydronic heat coil integrated in supply air of the ventilation system). 

Can you open the windows of a passive house?

Yes. You can open windows and live in a passive house just like in any other house. It is a normal house. You have the tools to do just what you would do in any other house: if it is cold outside, instead of sending the warm air directly outside through opening your windows, one can use the ventilator to do the ventilation, keeping the heat inside the house (some "old world" people insist on sleeping with open windows no matter how cold it is outside. Studies have shown that cracking a window at night during winter has no significant effect on the performance of a passive house. It still works!). 

During the in-between seasons one can bring in fresh air through windows like in any other house, and even turn off the ventilator if so desired. If it is hot outside, it's best to keep the hot air outside and the cold air generated through active cooling or night cooling in. The ventilator then recovers the cold air for you and you still are able to properly ventilate your home. 

Note that studies have shown that most people do not ventilate their home as they should. The desired humidity inside a home should be between 30-60%. If it is lower than 30% most people perceive it as uncomfortable to breathe. If it is above 60% mold growth will be supported and is very likely. Regular home owners should open their windows every two hours for two minutes to ventilate properly (including night hours). The passive house provides proper ventilation continuously, at low speeds and free of dust; creating a superior, healthy indoor air quality. 

Is radon a problem in passive houses?

Radon is a radioactive gas that can come into a house through the ground, primarily through cracks in basement walls or concrete slabs. Because passive houses are built to such a high air-tight standard, the entry of radon is prevented as best as is possible. Beyond that, any possible infiltration is met by a balanced ventilation system that continuously refreshes the indoor air. Passive houses are, by design, well-protected from the dangers of radon.    

Isn't the standard an unreasonable constraint on the freedom, expression and aesthetics that an architect has at his/her disposal?

Once a designer becomes familiar with the principles and learns to incorporate (at the start of a project!) the Passive House Planning Package software as a design tool, one will be able to do (almost) anything as regards aesthetics. The core principle for a building designer is to develop varying solutions based on the specific boundary conditions of the object to be designed. The passive house standard is redefining the boundaries of the building sector. We now desire homes that allow us to be energy independent and that are ideally carbon-neutral. The passive house standard is an opportunity rather than a limitation. It opens a multitude of doors to reshape aesthetics and expression for the needs of this 21st century.