Replies

1. GBA Editor
   Martin Holladay | Aug 16, 2013 03:08pm | #1

   Jack,
   I don't have the exact numbers, but here is what I know:

   - The vast majority of new homes do not have a mechanical ventilation system designed to provide fresh air to occupants.

   - Bath exhaust fans are required by most building codes -- at least for bathrooms that lack an operable window.

   - Many, but not all, building codes require that kitchen ranges be equipped with a range hood that has an exhaust fan ducted to the exterior.

   - Among energy-conscious production builders who have worked with the Building Science Corp., the most common type of mechanical ventilation system used, by far, is a central-fan-integrated supply ventilation system.

   - Many custom builders in northern New England install exhaust-only ventilation systems. A smaller percentage install balanced ventilation systems with an HRV or ERV.

   - Many Canadian builders install an HRV or an ERV as standard equipment.

   - Almost all Passivhaus buildings in the U.S. include an HRV or an ERV. But fewer than 100 Passivhaus buildings have been completed in the U.S.

2. 
   jackfbrennan | Aug 16, 2013 03:38pm | #2

   Thanks Martin, based on your comments it sounds like perhaps 90% are a combo of intermittent de facto exhaust system(when the bathroom or kitchen fan is going) and recirculation when the heating or cooling needs to be delivered around the home.

   Any idea about the level of positive pressure (Pa) from a supply ventilation system in the conditioned space vs outside it? Or how much pressure it takes to drive moisture into the walls?

   I'm wondering how precisely balanced the intake & exhaust are in a balanced ventilation system, and how that shows up in any pressure differential between inside & out. It occurs to me that a "balanced system" (i.e. both supply & exhaust) with some small amount of positive pressure would tend to prevent or reduce infiltration of contaminants from an attached garage, and might even provide makeup air which could mitigate backdraft conditions - not including any 1200 cfm "Professional Chef" style range hoods, of course.

3. 
   heidner | Aug 16, 2013 08:38pm | #3

   It really doesn't take pressure to drive moisture into the walls. The moisture moves from wet to dry even through drywall. Moisture movement would still happen in a perfectly air balanced house -- albeit slowly. Air leaks in the wall just REALLY speed up the problem.

   Very (really small) slightly positive houses are a design goal -- but for the garage - plan on adding its own ventilation system -- or if you are building a new house - detach it.

4. GBA Editor
   Martin Holladay | Aug 17, 2013 06:28am | #4

   Jack,
   In a well designed ventilation system, the pressures of an exhaust-only or supply-only system are minor, and have less effect on the house air pressure than wind or the stack effect. For more information on this topic, see Designing a Good Ventilation System.