Passive house is a rigorous building standard which reliably reduces energy consumption by 80% from the International Building Code. The German based Passivhaus Institut estimates approximately 900 million square feet have been built, and it is the fastest growing sustainable building standard worldwide because of the enormous energy savings and improved comfort.
When I began the design and building process for what would be the first certified International Passive House in Colorado I quickly learned that its success would depend on integrating the design with the physics model, and it would have to be practical to build while avoiding bespoke materials and systems. Because passive house takes into account virtually every way heat is gained and lost the emphasis is placed on the building envelope. This means significantly more insulation, but equally as important is air tightness, elimination of thermal bridges, well placed very high performance windows, and other important aspects like a simplified form factor and efficient heat recovery ventilation (HRV).
Utilizing the energy model Passive House Planning Package or PHPP, the design is informed by the energy saving realized, giving the designer control on how to best achieve the standard’s threshold of 4.75kbtu per square foot annually in heating and cooling. Once a basic envelope and program is settled upon details such as the heating system, insulation and airtight layers can be detailed. In my project I decided to eliminate foams to reduce the embodied energy and to emphasize healthier materials.
Because Passive House requires an air tightness test of 0.60 ach50, which is about four times tighter than most updated codes the focus is on designing a continuous air barrier that is cost effective and simple to achieve during construction. Air tightness has become a hallmark of high performance buildings as it has been demonstrated that it is as critical to reduce energy loss as insulation. With an air tight enclosure wind washing of insulation is eliminated, moisture laden air leaks cannot wet the wall assembly, and of course you don’t need to heat all the uninvited frigid air on a winter night.
On the exterior I applied a truss system which was covered in 2 3/8 inch rigid mineral wool board and then a fiber cement board and baton. This low cost system provides deep cavities for cellulose insulation, is vapor open to the outside, and is fire resistant. The cellulose was dense packed to 4 lbs. per cubic foot and has shown no signs of settling in an 18 inch cavity wall.
The European tilt and turn triple pane windows are placed near the middle of the wall assembly to improve thermal performance at the frame and taped to eliminate air infiltration.
The heat recovery ventilation trickles constant fresh air into the living space and extracts from the kitchen and bath. The heating system is a simple hydronic loop in the wall and post heater at the HRV. Only during frigid and cloudy days is it called upon and according to the energy model the house needs a scant 6000 BTUs of heat. After two winters at 6800 feet I can attest to this exceptional performance.
Andrew Michler is a Certified Passive House Consultant and author of [ours] The Hyperlocalization of Architecture. He has lived off grid in the Colorado Rockies for 20 years. He may be reached at baosol.com.