The first design step was to determine the most cost-effective and commercially available approaches to minimizing energy losses. The insulation techniques that are commonly available in the United States are shown in the “Cost of Insulating” table below.
Cost data and R-value were collected for each of the construction methods, and a “dollar per R” calculated for each. This calculation reveals the lowest-cost, highest-value insulation techniques for the walls, roof, basement walls, and floors.
The next step in the design process was to optimize the amount of insulation based on the temperature difference between the inside of the home and the temperature on the opposite side of the surfaces being insulated. Then, by using the cost per “R” for commercially available insulation systems, the most cost-effective R-value for each exterior surface can be determined. For example, in northern Illinois, given the temperature difference between the inside of the home and the average midwinter temperatures, a roof or wall can be effectively insulated with an R-45 structural insulated panel (SIP). However, using greater levels of insulation requires building techniques that are less cost- effective or not commercially available in North America. For example, the cost of increasing the R-value of the wall by roughly 20% (R-55) could double the cost of building the walls because SIPs are not commercially available with an R-55 rating and other building techniques are much more expensive per R-value.
Passive solar homes use the sun’s energy to provide significant home heating. It feels almost magical to walk into a 70°F home on a sunny, 0°F day and know that no heating system is operating. Although there is nothing new about harnessing the sun’s heat, the materials and design knowledge for capturing solar heat and storing it for night use have improved dramatically in the last 30 years.
We chose Canadian-made Loewen Access, Douglas fir-framed windows, which look and act like awning windows and can be “flipped” to be cleaned from the inside. The windows on the north, east, and west sides are Loewen’s Heat Smart III—argon-gas-filled, triple-pane units with two layers of low-e coatings and energy spacers. Because standard low-e coatings are meant to block solar radiation, we specified double-pane Loewen windows with a hard low-e coating and a solar heat gain coefficient of 0.65 for the south-facing windows. I have recently started specifying triple-pane windows without low-e coatings for southern exposures. They cost less than the special-order hard-coat low-e windows and the performance is slightly better.
To benefit from passive solar gain, the area of the south-facing windows should be between 10% and 15% of the home’s total floor area. Too little glazing means that you won’t collect as much energy as you could; too much may result in overheating. Pura Vida’s south-facing window area to floor area percentage is 11%.
One of the most important aspects of German Passivhaus design is the level of effort made to eliminate thermal bridging at floor, wall, and roof interfaces. The problem occurs when something without much R-value—like a wall stud—bridges between the inside and outside of the building without intervening insulation. While designing Pura Vida, builder Rick McCanse and I spent many hours discussing the details of how this could be easily—and affordably—accomplished.
Where the floor meets the wall, the 4 inches of extruded polystyrene (R-20) under the 4 inches of poured concrete floor were butted up to the insulated concrete form’s (ICF) frost wall. Since the insulation under the concrete was in direct contact with the ICF insulation that extended below the frost line, there was no path for heat loss from the concrete floor.
At the main floor level, an internal 2-by-4 wall was added on the inside of the ICF to increase the R-value of the walls in contact with the earth to R-35 (R-22 from the ICF plus R-13 fiberglass in the stud wall); provide a space to run electrical wiring; and provide floor support that was inside the ICF and SIP’s thermal envelope.
Besides thermal bridging compromising a home’s efficiency, air leaks can account for 50% of winter energy loss in a typical home. With fewer and easier-to-seal joints, SIPs and ICFs minimize infiltration compared to conventional stud-framed homes. While APHs are built quite tight, infiltration losses can be further reduced by double entries (mud rooms, also known as airlocks). Mud rooms were common in farmhouses, but most modern homes do not include this energy-saving feature. The mud room should be insulated and use a well-insulated, sealed external door and an exterior-quality second door into the interior of the home.