Tiny Tuesday: What Makes a Passive House?

The goal of a passive house is to perform its mechanical functions passively – with as little energy input as possible. This approach to homebuilding minimizes a house’s ongoing impact on the environment, which is one way to live a tinier life. Here are the five principles that govern passive house design.

1. Insulation. A building envelope with high R-values all the way around keeps the indoor temperature constant, so it doesn’t fluctuate with the temperature outside.

2. Windows. Usually the weak points of a building envelope, windows in a passive house are typically triple-glazed. They’re positioned to maximize sunlight in cold months and minimize it in hot months.

3. Ventilation and heat recovery. A ventilation system provides continuous fresh air year-round. A heat recovery system transfers thermal energy from the outgoing air to the incoming air, preserving that all-important indoor temperature.

4. Air tightness. The building envelope has no punctures where air could leak. Even accidental nail holes are caulked closed during construction.

5. Full thermal break. Weak points where heat can transfer through the building envelope, such as continuous window sills, are known as thermal bridges. These are eliminated to maximize the effectiveness of insulation. A full thermal break between inside and outside also prevents moisture accumulation and protects against mold.

PassiveHouse2

A new passive house built by Shelterwood in Waitsfield, Vermont.

Some Passive House principles seem backwards at first glance. Doesn’t a ventilation system require energy to operate, while a drafty wall lets air pass through for free? My answer: ventilation doesn’t exist in a vacuum. (That’s an amusing tautology!) A drafty wall lets in so many things other than air – heat or cold, moisture, unwanted breeze – that a sealed house is considerably more efficient to operate. Up to 90% more efficient, if the Passive House Institute is to be believed.

Also, the thick insulation and HVAC systems only have a job to do when the air outside is radically different from room temperature. If you live in a passive house and you want to enjoy a mild spring day, it’s OK to open the windows!

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