Passive House
The term ‘passive house’ may easily give the wrong impression of being 100% passive. A passive house improves and emphasizes the static components and design of a house (walls, windows, insulation, orientation) so that the active components (heating, cooling) can be minimized. This results in significant energy savings as well as improvements in comfort.
My brother Bjørn Kierulf at Createrra has built over 100 passive houses in Slovakia and elsewhere in Europe, so when we started this project, it was natural that we would build this way. I’m not an expert, but have been learning a lot, so I want to give a layman’s overview on how to design and build a passive house.
Creating a passive house
As I see it, the main steps in achieving passive house standard are as follows:
Improve insulation: Step one in creating a passive house is to make sure it’s really well insulated: thick walls, triple-pane windows, good roof insulation, and insulation under the foundation. And avoid thermal bridges where the heat can leak out.
Make it airtight: Another step in keeping the heat from getting out (or in) is to make the envelope of the house as airtight as possible. Of course, there will always be leaks, but these will be minimized by making this a priority during construction, and by using a blower door test to ensure leaks are fixed, and that the house performs as designed.
Together, this helps keep the heat in during the winter and out during the summer. But an airtight house doesn’t make for comfortable living.
Add heat-recovery ventilation: Ventilation with heat recovery ensures that the house always has fresh, clean air, without losing much energy. The goal is to exchange about a third of the total air volume of the house every hour.
During the design phase, there are a number of decisions that can be made to reduce heating and cooling needs:
Optimize sun exposure: Large windows toward the south help heat up the house during the winter; overhangs keep the sun out during the summer. Overhangs are not as effective towards the east and west, due to the lower angle of the sun, so you’ll want to place those windows strategically.
Keep shape compact: For a desired floor area, a square will have less surface area than a long-stretched rectangle, and will thus lose less heat. However, a boxy shape may not work for the house you want to build, so just something to keep in mind.
After all these steps, calculate how much heating and cooling is needed for your climate. Most likely, a small, efficient heat pump will be sufficient.
Official standard
There are actually two official passive house standards: PHI and PHIUS. The goals of both are very much in alignment, and the differences are not significant for our purposes; we plan to get certified by both.
As my brother is based in Europe, we’ve been using the PHPP spreadsheet of PHI to calculate and optimize our house design. In the spreadsheet, you specify all the details, including size and angle of every wall and window, and it calculates how the house will perform in the given climate (using specific climate data for Kanab). After a number of iterations on the house design, our results are as follows:
- Heating demand: 10 kWh/(m2a) – max 15
- Cooling demand: 7 kWh/(m2a) – max 15
- Energy demand: 41 kWh/(m2a) – max 60
So assuming the house passes the blower door test, we should have no problem achieving passive house standard.
Comfort
Have you ever felt chilly inside a house even though the temperature says you should be comfortable? If so, it’s likely due to a combination of drafts and cold surfaces around you (see here and here for a detailed explanation). A passive house keeps you comfortable year-round: no drafts, no cold walls or windows.
The thick walls and triple-glazed windows also keep noise out. (And let you turn up the volume without bothering the neighbors.) Not crucial for our house, but an important consideration in other places.
In later posts, I’ll look at the walls we’ll be using to achieve superb insulation, and how floor plan and windows were influenced by passive house thinking.