In researching articles for The View Up North, we discovered the Passive House movement. This environmentally oriented approach is quite well established in Europe— particularly Germany, where it is called PassivHaus. The goal of passive house design is to reduce the space heating load by 90% and overall energy consumption by 82%, while still maintaining a comfortable living environment. Some of our readers may find the measures extreme, but we found that there are plenty of ideas that can significantly reduce your fuel consumption in a log home—without breaking the budget or sacrificing comfort.
To get a sense of how the Passive House format works, we didn’t need to look far in Maine. We talked with Alan Gibson of G•O Logic in Belfast, Maine. Maine has one of the most challenging climates for a Passive House design, ranking seventh among states for heating degree days*.
*What is a Heating Degree Day? According to the US Dept of Energy, weather has a major impact on heating and cooling a home. A heating degree day is a climatic statistic that engineers and economists use that reflects the severity and length of the heating season. Heating degree days represent the number of hours over the course of a year that the outside air temperature is below 65°F (then divided by 24 to present in terms of the number of days). The average over the 25 years ending in 2006 in Maine is 7,046. Maryland has an average of about half of that: 3,334 heating degree days.
Three Key Elements
To achieve his Passive House certification, Gibson’s company designed a home that relies on three primary elements to conserve interior heat during the heating season:
An airtight, continuously sealed exterior envelope with high insulation values;
- A ventilation system that recovers 95% of heat from exhaust air amn transfers it to incoming fresh air, then distributes it thoroughout the home:
- Well-designed passive solar glazing to use the sun’s rays to heat the interior.
- We’ll examine each of these elements over the next three months and show how they might be incorporated into your Katahdin log home design to maximize both comfort and energy savings.
Passive House Envelope
This month, we review the first element of a passive house design: the envelope. The passive house formula works on the theory that the more airtight and well-insulated a home is, the less energy is needed to keep the interior at a constant temperature.
Passive House Planning Package is the specialized software that enables architects to specify a home that meets the international standards. Like any type of rating system, (including HERS, and IECC 2009) elements and features are assigned numeric value. When these are compiled using the software, a skilled designer can develop a plan that also meets the homeowner’s unique requirements.
In the G•O Logic passive house design we toured in Belfast, Maine, the envelope was constructed on a highly insulated slab foundation with a heavily insulated exterior shell construction in walls and roof systems.
For the foundation, G•O Logic first laid down 8” of rigid expanded polystyrene insulation sheets over a gravel base. The concrete slab foundation was poured over this insulation barrier to form the floor of the home and the first part of the envelope. The walls and roof systems are “hung” on a ybrid timber and stick-built frame. The walls used structural insulated panels (SIPs) that were constructed offsite to computer-specified dimensions. The SIPs were hung onto the framework skeleton and sealed to create an air tight exterior envelope. The interior stick built frame walls were further insulated and finished with traditional sheetrock. Total combined R-Value for the walls is R-50.
The roof system was constructed similarly with scissor trusses filled with 24" of blown cellulose. The exterior used a Galvalume metal roof and was also outfitted with both solar water collectors and photovoltaic panels to harvest the alternative energy benefits from the southern exposure. Total combined R-Value for the roof system is R-80.
Windows contribute to the savings
As with any home, windows play an important role in overall insulation. The G•O Logic home oriented the long side of the home to the south and used oversized fixed windows designed for passive solar applications to maximize the solar gain for heat. These windows were imported from Germany and featured triple glazed glass and highly insulated wood and aluminum frames. Other, smaller casement windows set on the north, east and west walls afforded lots of light and well insulated and tightly sealed frames. The casement windows opened to the interior like a door and had the option for a top tilt-in of about 3 inches for summertime ventilation. The total U-value (a low U-Value number is equivalent to a high R-Value) of the windows and doors is U-0.12.
Designing to Passive
Like any construction project, Gibson says that it’s important to incorporate the passive features into the design at the beginning. “There needs to be a continuous barrier in the envelope such that on a section drawing I can connect the outer shell elements— foundtaion, wall and roof—without lifting my pencil,” he explained. By designing for maximum energy savings from the exterior shell to the home’s appliances and amenities, he is able to build a modest passive home for no more than a 10% increase in square footage costs over conventional built homes. “It’s all a series of tradeoffs to achieve the passive house certification,” he added. A passive house substitutes the cost of conventional heating and cooling system for an investment in the combination of airtight insulation, ventilation with heat recovery, passive solar and other alternative energy sources. For the 1,500 square foot G • O Logic passive home we visited, the annual cost of all energy (electricity) was about $300. That’s pretty impressive.
Passive Log Homes?
As far as the airtight and insulation aspect of passive houses, Katahdin Cedar Log Homes is well on the path to extraordinary savings. Insulation values for the exterior walls are now at a level of R-23 through the Energy Envelope System™, and increasing insulation on the inside of exterior walls is certainly feasible. Katahdin’s attention to providing dry Northern White Cedar logs another essential element in air tightness. Achieving the passive house airtight envelope and reducing thermal bridging has been done to a dramatically low HERS rating in Katahdin Log Homes, but to date not to the ultra standards of Passive House certification. Roof insulation systems in the passive house configuration typically achieve an R-100 level of insulation. The cathedral-style truss and purlin design that many Katahdin Cedar Log Homes have is easily adaptable to accommodate the additional insulation level and to complete an airtight envelope.
Next month: Windows in a Passive Solar Home
Photos courtesy G•O Logic.