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Tight Houses, Blower Doors, and Ventilation Explained in Under 3 Minutes

Built tight and verify. And then ventilate.
June 22, 2016

A blower door is a simple device: basically a fan in a shroud that fits tightly into a door. The fan is calibrated so that to measure exactly how much air is moving through it. By running the fan continuously and adjusting the opening from which it blows air, a pressure differential is created between inside and outside of the house.

The standard pressure is 50 Pascals, which is roughly equivalent to a 20 mph wind pushing on all sides of the house.

Once the 50pa pressure differential is achieved, the blower door measures the resistance of the building envelope to air flow.

From the 'No Surprise' Desk: This passive house has very little air leakage, under 125 cubic feet per minute (CFM) at 50 pa. This is about the same amount of air that a typical bath fan moves.

By dividing CFM by the volume of the house, which can be measured, you can calculate the air changes per hour (ACH). Passive House certification requires .6 ACH, which means the house is very tight.

For the 'Yeah, but ' ... crowd:

All houses need ventilation to keep the indoor air fresh—even leaky ones. Try walking into a teenage boy's room when the window and door have been closed and he has been staring at a screen for a couple of hours. Better yet, do not try that, take our word for it that the first thing you will do is open the window.

Opening the window is a great solution on days with comfortable outdoor temperatures, but it was 120 degrees in Arizona last week. And it was below zero all over the north for a long time last winter. Opening a window is a last resort on days like that.

The other typical ventilation strategy for leaky houses is—wait for it ...—leaks: cracks in the floors, walls, ceiling, and attic. The driving force in these 'naturally ventilated' homes is wind pressure and stack effect. But range hoods, clothes dryers, and forced-air heating and cooling systems can also drive air movement into and out of a house via the musty basement or attic with dead squirells.

If only there was a way to bring in fresh air, exhaust stale air, and not waste energy in the process.

Oh wait, there is: mechanical ventilation.

As with all tight houses, the house in this video has a mechanical ventilation system. Mechanical ventilation often takes the form of a heat recovery ventilator (HRV) or an energy recovery ventilator (ERV), but there are other ways to do it, too.

The tight shell of a Passive House not only controls energy loss (and gain) and improves comfort, but it allows you to control the air flow into and out of the house.

No more radon injection system from the crawlspace or basement.

Instead, controlled mechanical ventilation exhausts old stale air from the house while at the same time bringing in fresh air. HRVs and ERVs transfer the energy between incoming and outgoing air to improve comfort and save energy.

"It is a very efficient and comfortable way to have very high quality fresh indoor air."

—Skylar Swinford

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