A Green Builder’s Dream Come True
At long last, plans for my first Passive House—a new home in Derwood, Maryland for Anne and Elliot Grant—are ready to submit for a building permit. It is truly exciting to have clients who are committed to what is possibly the highest standard for sustainable design: the Passive house standard. Completing our Design Build team on this project is Joseph Klockner and Company of Takoma Park, Maryland.
Passive House Planning Package
The design includes preparation of the Passive House Planning Package (PHPP), an extensive energy modeling program that analyzes every important element of the house and its mechanical systems. The program also considers characteristics of the site including micro-climate, tree cover, and exposure to wind. Our PHPP was submitted to Passive House Institute United States (PHIUS) for pre-certification, and after some serious review and refinement, is nearly ready for approval.
Achieving Passive House Pre-Certification Requirements
To be granted pre-certification the completed passive house planning package must demonstrate a house that will use
- a maximum energy for heating no greater than 4.75kBtus per square foot per year
- a maximum total energy for all purposes no greater than 38kBtus per square foot per year.
To achieve these energy efficiency requirements, the house will be insulated to the following levels:
- roof: R-69 (code minimum is R-49)
- walls: R-54 (code minimum is R-20)
- floor: R-37 (code minimum is R-19)
- windows: triple glazed U-0.16 to U-0.21 (code minimum isU-0.35)
Further, the house must achieve an extremely high degree of air tightness. During construction, the house will be inspected several times with particular emphasis on air tightness. Passive House requirements permit no greater than 0.6 air changes per hour (ACH) measured at standard test pressure. Conventional building code allows up to 3.0 ACH. That would be five times higher than the Passive House value.
Using Natural Assets of the Site to Achieve Passive Standard
The lovely 3 acre wooded site is adjacent to Rock Creek Regional Park and Lake Frank. The actual building location is in a small clearing, surrounded by tall tulip poplars and other hardwoods. Although the dense tree cover will help keep the house cool in summer, it created a challenge in making the numbers work for the Passive House heating requirements. Every window and door opening had to be individually analyzed to determine potential for net heat gain (or net loss) over the course of the season. By trial and error, each opening was adjusted in size and glass type for higher or lower solar heat gain coefficients (SHGC) until optimum modeled performance was achieved.
Currently, International Energy Conservation Code prescribes a solar heat gain coefficient maximum of 0.40, to limit overheating in summer. Passive Houses–no surprise here–must be in the 0.50 to 0.60 range. The Grant Residence windows range from 0.53 on the east, north, and west, to 0.62 on the south. To use code compliant windows would have a severely detrimental effect on the modeled performance.
NEXT POST: Does a Passive House have to look weird?
For those who love decimal points, grid lines and spread sheets, please feel free to geek out on the statistical breakdown of our passive house planning package in the chart below.
Passive House energy modeling figures for new green home in Maryland