Passivhaus: Why No One Should Build Anything Less
Paul Kando
I first learned about Passivhaus in 1988 while working in Sweden – two years before the first such building was built in Germany– and have been following the development of this remarkable building system ever since. Passivhaus, or passive house (PH), refers to a strict voluntary standard for energy efficiency in a house or other building. It results in ultra-low energy buildings that require little energy for space heating or cooling and have reduced ecological footprints. The standard applies to any building; homes, office buildings, schools, kindergartens and supermarkets have all been designed and constructed to meet the standard. PH design is not a supplement tacked on to architectural design, but an integrated design-build process. Mostly applied to new buildings, the PH process is also used for upgrading existing buildings, and benefits from an international technical support network anchored by the German Passivhaus Institut and the International Passive House Association.
photo credit: Paul Kando
PH is not a collection of features or technologies; it is a vertically integrated design-modeling- construction process, which results in buildings that have an annual heating and cooling demand of no more than 15 kWh/m2 (4746 btu/ft2) per year, or a peak heat load of 10W/m2 (0.93 W/ft2); and a total primary (source-) energy consumption not over 120 kWh/m2 (3.79 × 104 btu/ft2) per year. A PH building may not leak more air than 0.6 times the house volume per hour at 50 Pascals indoor/outdoor pressure difference as tested by a blower door. Quality control – how well things are done is as important as what is done in constructing a PH.
PH buildings do not need conventional heating systems. However, most do include some means to provide supplemental space heat, which is normally distributed through a low-volume heat recovery ventilation system required to maintain air quality, rather than by a conventional hydronic or forced-air heating system. Some PH technologies were specifically developed to help meet the standard. Others, e.g. superinsulation, already existed, and passive solar heating has been around from antiquity. The standard also builds on experience with earlier low-energy building standards, such as the German Niedrigenergiehaus (low-energy house) standard, and the demanding energy codes of Sweden and Denmark.
In PH buildings, cost savings from dispensing with a conventional heating system help pay for beefing up the building envelope and the heat recovery ventilation system. With careful design and increasing competition in the supply of special PH building products, in Germany it is now possible to construct PH buildings that cost no more than a comparable building constructed to meet the normal German building standards. However, worldwide, a PH building may cost up to 14% more up-front than a conventionally built equivalent (especially where conventional building standards are low or nonexistent).
So, what might a PH home cost compared to a similar conventionally built 2,900 square foot (SF) cape? Obviously costs vary house to house and location to location. According to research published last January by the National Association of Home Builders (perhaps on the high side for Maine), a code-built house of this size, @$153.25/SF will cost $444,425. Of this 61.7%, or $274,137 is the construction cost. Applying the above 14% premium will increase the construction cost of the PH over its conventional equivalent by $38,379 to $312,516. (A 20% down payment will reduce this $250,013.)
According to energy audit data, similarly sized Maine houses burn an average of 1,207 gallons of oil each winter, at current annual cost of $4,848 or $404 per month. In contrast, a PH equivalent built 3 years ago here in Maine has been using the energy-equivalent of only 78 gallons of oil per year, costing $310 annually or $25.83 per month – a 93.6% saving compared to the conventionally constructed house. The monthly energy bill is $378 less for the PH home. As a monthly payment on a 7% 30-year mortgage loan, $378 will finance a bit larger than $56,000 cash expenditure, far more than the $38,379 difference in construction costs.
Extrapolating the historic annual fuel oil price inflation of 6.8% over the next 30 years from the current $4.00 per gallon, the monthly energy bill for the conventional house will rise from the current $404 per month to $523 in 5 years, $727 in 10 years, $1,011 in 15 years, and so on. Meanwhile the $25.83 monthly bill of the PH will be only $35.89 in 5 years, $49.87 in 10 years, $69.29 in 15 years and so on – increasing comparative savings to $487, $677 and $942 per month in 5, 10, and 15 years, respectively. So, even should $56,000 extra be spent up-front for the PH, huge savings will result in financing and operating the PH home over the mortgage term, without increasing mortgage payments above those payable for the conventionally constructed house.
Do your numbers before you build. And remember, a PH is guaranteed to perform as designed – it is tested before occupancy. The savings in total monthly occupancy costs – mortgage payment, heating, cooling, water heating, cooking, electricity, taxes – a PH offers over its conventionally built counterpart far outweigh any first-cost difference. You can lose only by not building to passive house standards. More on PH process applied to our old leaky houses soon.