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We've covered Mechanical Ventilation with Heat Recovery (MVHR) before here, and it's been the ventilation method of choice for Passivhaus buildings for many years.

But I heard something interesting recently: that some Nordic countries, having been concentrating on MVHR in super-insulated dwellings for some time, were now moving to Positive Input Ventilation (PIV), whereby a fan in the roof space brings fresh air into the building, and the increased pressure pushes out stale air elsewhere.

Is this true, and if so, what's behind the shift?

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  • Citation required. This sounds like a bad idea; it would lead to energy loss due to a lack of heat exchanger. PIV is normally (not always) just done in retrofit situations where it's not always expedient to implement MVHR. – Dan Gravell Aug 9 '17 at 10:16
  • Unlikely, in Sweden there is an energy requirement on new houses between 50-95kWh/m2 depends on the latitude. Without HRV as complement to a heat pump of some sort, meeting this requiremet will be unlikely. Exhaust Air Heat Pumps are very popular in newly built houses. – Aus Dec 18 '17 at 19:05
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I would answer this as a comment, but comments don't accept multiple paragraphs.

On the face of it this is a bad idea. Better to pull air OUT of the house, and let it sneak in where it can.

Scenario 1. The building vapour barrier is perfect. Air put into the house can only escape through faulty weatherstripping at doors and windows. Net result. Too low an air exchange.

Scenario 2. The building vapour barrier is badly detailed. This allows air to escape around electrical outlets, window edges, the joint between wall and floor or wall and ceiling etc. Interior air carries more water. That water is moved into the chilled wall space and condenses. Now you have a problem with wet walls, increasing chance of mildew, and lower insulation values.

Usually for a house in a cold climate there is a natural stack effect. The warmer is lower density and so tends to exit the building through cracks in the top half of the building, and enter through openings in the bottom half of the building. In my house I heat with wood, so there is always some air flow up the wood stove chimneys even when the stoves are not in use. Our makeup air comes through an opening in the basement, keeping it cool enough for storage.

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