Passive House: Home Building for the 21st Century
Climate — of the planet and indoors — is the product of heat interacting with air and moisture. Accordingly, indoor comfort depends on controlling (1) the amount of heat lost to the outdoors, (2) outdoor air infiltration, (3) indoor moisture, and (4) the amount of heating energy supplied. How well we deal with the first three factors determines the quantity of energy required.
Unfortunately, 20th century houses were usually built to maximize ease of sale and builder profit, not energy efficiency. Building materials and the building process were optimized to minimize first costs. Selling points, like attractiveness, were often delivered at the expense of quality, e.g. with leaky joints and hurried, sloppily installed insulation. The resulting poor building performance was routinely offset by an oversized (inherently inefficient) heating system. In short, builder profit was maximized, with problems like high heating costs “externalized” for the occupants to deal with. Houses were seemingly “affordable” to buy, but, as fuel prices rose, increasingly expensive to occupy. This was accepted as “normal” because, next to a larger mortgage, the spread-out cost of the then-low priced fuel (12 to 20 cents per gallon of heating oil!) appeared negligible.
Fuels are no longer cheap, especially when the climate-impact of burning them is factored in. Fortunately modern, 21st century houses built to international passive house (PH) standards are optimized, using computer simulation, for maximum energy efficiency and performance. After plans are made, a computer program selects the best combination of building materials and construction methods at electronic speed. The results are evaluated and the design is revised as needed. The sequence is easily and cheaply repeated until the optimal combination is identified. The result is maximum building performance at the lowest possible first and operating cost. As a side benefit, a library of good materials and practices is established over time, which speeds up subsequent design simulations.
Passive Houses meet minimum performance specifications, based on which they are tested before occupancy. The PH standard specifies the maximum amount of heating energy, (15 kWh/meter2/year), total primary energy consumed (120 kWh/meter2/year), and the maximum amount of air leakage allowed (0.6 air changes/hour at 50 Pascals indoor- outdoor pressure difference as measured by a blower door). Last century Maine houses typically burn over 69,000 BTU/ft2/year in fuel, compared to 4,774 BTU/ft2/year for PH.
When building new, PH is a no brainer, even if it costs more than conventional construction. Climate-impact is minimized, comfort is much improved and reductions in energy use make up for the first-cost increase through permanently reduced occupancy costs. For example:
|2,000 foot2 house||Conventional||Passivhaus|
|Cost/ foot2||$125||$144 (+15%)|
|Avg. gallons oil||1,003||100|
|Avg. energy $/month||$294||$29|
|Annual energy cost||$3,528||$348|
What’s not to like?