A number of people recommended John Straube’s article on buildingscience.com to me. I appreciate that PH is being discussed on buildingscience.com and John’s in-depth assessment of PH in cold climate settings.
I thought I’d offer some perspective from my corner of the world (Minneapolis, MN = cold climate). Being a certified Passive House Consultant, I obviously believe in the model and favor it. Having said that, I do agree with John regarding the feasibility of PH in cold climates. At the same time, there are only very few examples of Passive House in cold-climates to date making a real-world analysis and comparison difficult. I feel that we need to do a lot more work to assess the feasibility and potential of PH in cold climates. At the same time, I am fascinated with leap-frog approaches to building and severely reducing energy consumption of buildings. Therefore, PH appeals to me on a number of different levels.
I am currently working on a Passive House for our cold climate. This one is a custom-design, and therefore falls into the custom home category as outlined in John’s article. I have been in talks with people about a production-builder, or partial prefab PH model for cold climates. With Passive House requirements being rigid, these buildings end up looking different in every climate zone—making Passive House a much more local model.
I quickly put together my personal take on John’s article. Take for what it’s worth. I am not a Ph.D. though I do hold a german engineer’s title in architecture. I respect John’s thorough article greatly, and encourage others to look at PH in detail. The comparison with local climates and standards is vital, and exciting stuff to get into.
First off, I’d like to offer my comments on a couple of specifics, that I understand differently from John, according to my training and studies of the PH requirements.
Re: Treated floor area
The “treated floor area” (basis for the energy calculations) is measured as the conditioned space between finished interior surfaces—wall-to-wall, so to speak—not total conditioned area inside the cladding, which would suggest that the footprint of walls counts as treated floor area.
Re: Basement and treated floor area
The basement of a Passive House does not automatically count at 60% of its actual area for the floor area calculations. The floor area calculations are actually based on the “Wohnflächenverordnung”—a German set of rules on how to measure useable square footages inside buildings. Living space with a ceiling height of 2 meters (6′-6 3/4″) or more counts at 100%, living spaces or areas within living spaces with a ceiling height between 1 and 2 meters (3′-3 3/8″ – 6′-6 3/4”) count at 50%. Utility spaces and ancillary spaces count at 60%. There are more boring details to this document that I won’t get into 😉
Re: Thermal bridges
Thermal bridge are to be avoided per the PH requirements. This starts in the design phase and continues during construction. Thermal bridges (for example in existing structures or retrofits) can be accounted for in the energy calculations.
It is true that a simpler building envelope is likely to achieve higher air-tightness (fewer details, less room for error). At the same time, if properly designed and executed, in theory any building shape can be executed virtually air-tight.
Re: General assessment of PH in cold climates
I agree with the author that cost for PH in cold climates versus PH in modest climates is inflated. To what extent is what we are trying to find out right now. At the same time, I feel that PH in a cold climate provides great insurance for durable, long-lasting assemblies, that significantly reduce risk of moisture problems in assemblies. The benefit of the increased insulation values, and resulting thicknesses of insulation packages, is insurance that dew-points are being kept in outer layers of insulation, even at very high temperature differences between inside and outside. In addition, interior surfaces of exterior assemblies tend to stay very close to room temperature, virtually eliminating radiant heat-loss and potential for condensation and mold growth, even in areas that have poor exposure to moving air (i.e. inside corners and areas behind furniture).
The PH requirement for ventilation is partially founded on hygienic principals. According to John’s article, ASHRAE standards require less ventilation per inhabitant. It appears that different cultures have a different take on what is more suitable for human beings. The same thing is true for heat-delivery over ventilation systems. The PH standard provides a definitive cut-off. It is my understanding that hygiene also plays a role in the limits of heat delivery over ventilation air. Having said all of that, I feel that a higher ventilation rate can contribute to better IEQ through mitigation of pollutants. At the same time, there is a greater potential for dehumidification that comes with higher ventilation rates as well. I experienced the IEQ in German Passive Houses and found it to be terrific. The result is virtually no odors—even in buildings that were just finished where one would typically anticipate a “new home smell” of paint and furnishing’s VOCs. I cannot say that I have experienced a Building America building, or one ventilated to ASHRAE standards, so that comparison still needs to be made.
In my opinion, any builder or building designer can learn a lot from the PH paradigm and requirements. It encourages more detail-oriented design, greater consideration for building assemblies, as well as more careful systems-integration (envelope and mechanical response) and sizing. The PHPP energy modeling tool is very helpful during the design phase. It helps create a predictable structure. It shows potential and weaknesses of existing buildings, as well as new designs. It makes unknowns known, and sharpens our vision for higher performing buildings.
I feel that PH standard produces incredible buildings, both from a performance and a value standpoint. That’s what appeals to me and my clients. We look forward to building Passive Houses in a cold climate, and to learn from them for a more sustainable building future.