Average Rating: ( 0 votes)
The passivhaus movement started in Germany in the late 1980s with the goal of building hyper energy-efficient buildings. The thermal image above (from Wikicommons) shows the results. The house on the left, leaking heat like a sinking Titanic heat island, is a standard built structure while the house on the right follows the tenants of passivhaus. A passivhaus tends to have (1) good insulation (U-value lower than 0.15 watts per square meter per Kelvin degree [R-value higher than 44.5 feet squared degree Fahrenheit per btu]; see note at end of post on conversions and use), (2) southern orientation and shade considerations, (3) energy efficient windows (U-factor lower than 0.80 [R-value higher than 7]), (4) tight building envelope (air changes per hour at 50 pascals less than 0.6) but forced ventilation (of at least 40 percent of the haus volume per hour) for air quality, (5) passive pre-heating of fresh air, (6) a heat recovery ventilation system, (7) hot water from solar or heat pumps, and (8) energy efficient appliances. Overall, the haus must not use more than 15 kilowatt hours per square meter per year (1.4 kilowatt hours per square foot per year) for heating and have total energy consumption less than 120 kilowatt hours per square meter per year (11.1 kilowatt hours per square foot per year).
Probably the most atypical aspect of a passivhaus is its aggressive insulation. The walls and roofs are thick with insulation and even the slab or basement is heavily insulated. Consideration of passive solar is taken seriously. The windows are highly energy efficient and often triple paned (although double-paned will work). And the haus is nearly airtight.
A home typically has 10 air changes per hour with a 50 pascals pressure difference between the indoors and outdoors. Under normal pressure differences, this amount to half the air being changed every hour. A passivhaus has an air change per hour with a 50 pascal pressure difference of 0.6, 94 percent less than a typical home. Under ambient pressure conditions, a passivhaus, without mechanical ventilation, will only change out 3 percent of its internal air per hour. If the air change per hour falls below 0.4, air quality in the home suffers (the air becomes stale). Therefore, a passivhaus will vent out internal air and bring in external air to achieve an air change per hour of 0.4. You may be thinking to yourself: Say what?!?!!? I just made my house super airtight and now I’m going to leak it!!! However, unlike a normal home where that fresh air is coming from wherever the home’s perimeter has been compromised, a passivhaus knows where the air is coming from and can filter and pre-condition it.
When a passivhaus exhales, it can harvest the heat from that air and feed it back into the home. A passivhaus in Germany is typically just heated by body heat and heat from lights and appliances. That’s crazy cool (or hot [or something]).
Most of the 25,000 houses that meet passivhaus standards are built in Germany and Scandinavia with only a couple dozen in the United States. Note the northern climatic bias. So how do you build a passivhaus in a hot and humid climate like Austin where cooling is more of an issue? Instead of a heat recovery ventilation system, warmer climates can use an energy recovery ventilation system. This system removes heat and humidity from the incoming air in the warm season and then acts as a heat recovery ventilation system during the cool season. The energy recovery ventilation system doesn’t cool the incoming air—it preconditions it so the house can get by with a smaller air conditioning system. A passivhaus has been built in Louisiana, and a passivhaus remodel/rebuild is being finished in Austin (see links below). The Passivhaus Institute US has a spreadsheet (for a less than nominal fee) that a designer can use to calculate the overall efficiency of a potential haus.
Proponents suggest that building to passivhaus standards only adds 10 to 15 percent to standard building costs but with a 90 percent reduction in energy use. Hmmm. That doesn’t sound too bad. Interestingly, many examples of passivhau have lots of windows, so such a house doesn’t have to be a cave. And the results are quite impressive:
(chart from the Passivhaus Institute)
Some building suppliers:
Note: R-value is the inverse of U-value; however, both values, although generally reported without their units, are unit dependent with U-values in metric units (watts per square meter per Kelvin degree) and R-values in English units (feet squared degree Fahrenheit per btu). To convert U-values to R-values, take the inverse of the U-value and multiply by 5.68. Your metric U has now been English R’ed!