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Is It OK to Ventilate a Home with Attic Air?

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I’ve seen some crazy things done to homes. It’s usually someone’s attempt to make things better. Sometimes that someone is a homeowner or other occupant. Sometimes it’s a contractor. But it always seems to be a solution to a problem that creates other problems. It’s just like they say: Our biggest problem is solutions. So what’s the problem with this method of ventilating a home?


One of the home energy raters we work with in the Atlanta area, Bruce Kitchell, sent me these annotated photos. He found an air handler for a heat pump in an unconditioned attic. The return plenum had a filter sitting on top of it, as you can see above. The filter was there to filter the “fresh” air they were using to ventilate the home.

Beneath the filter was a hole cut through the fiberglass ductboard plenum. When the system runs, it pulls return air from the conditioned space in the home and it also pulls a significant amount of air from the vented attic.

Hey, fresh air is good, right? And this ventilation system is about the cheapest you can install. All you need is a box cutter and a one dollar filter.


Let’s take a look at the problems with this solution.

Indoor air quality. Attic air isn’t the cleanest air around. It’s got lots of dust and dirt, probably a fair amount critter poop, and maybe even some dead critters. That little filter isn’t going to stop everything. That little filter probably also has some significant bypass around the edges so not all of the “fresh” air goes through the filter.

Comfort. That hole in the return plenum is going to suck in a lot of attic air. The air conditioner certainly wasn’t designed to handle a lot of 120° F air being mixed in with the return air. I wouldn’t be surprised if they have trouble getting the house as comfortable as it should be.

Operating cost. That hole pulls in a whole lot of air, I’m sure. It’s hot and humid in the summer. It’s cold in the winter. Their heating and conditioning systems will run overtime in an attempt to heat, cool, and dehumidify that air. What they saved on putting in a proper ventilation system will quickly be eaten up by high electricity bills. According to Kitchell, the most recent occupants renting this home moved out because of huge electric bills. Not too surprising.

Critters. Any HVAC tech who’s been on the job for a while can tell you about the dead animals they’ve found in HVAC systems. Check out HVAC Hacks and Other Screwups for some photos, if you have the stomach for it. Snakes, squirrels, raccoons, birds. If they can get in your attic, how hard do you think it would be for them to get past that filter?

Get the picture?

An interesting bit of irony here is that they covered the plenum with mastic to seal it tight before cutting that hole. Nice!

Ventilation is a good thing. We want airtight homes and a good system to bring in outdoor air. And yes, it’s certainly possible to use the return plenum to bring in your ventilation air. But it needs to come in from outdoors, not the attic. And it needs controls and dampers and critter screens. The AirCycler system is one that can do that properly.

Ventilating a home with attic air as in the photos shown above isn’t such a great idea after all. It’s a solution that creates a whole host of new problems.

And speaking of solutions, did you know that, in the chemical sense, whisky is a solution? So I guess that’s an exception to the rule about solutions being a big cause of problems! Oh, wait. I think I remember a time or two that it caused me problems.

Allison Bailes of Decatur, Georgia, is a speaker, writer, building science consultant, and the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.

Allison Bailes, III, PhD
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Posted In: Building Performance, Residential Buildings

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