Jack Woolfe wants to build a small, airtight house with an attached garage. The house will have an exhaust-only ventilation system, meaning the system will expel stale air from the house without providing a specific source for replacement air.
That’s one of several options for whole-house ventilation, but Woolfe is weighing the possible risks.
“I’m concerned that the negative pressure in the house may draw unwanted fumes in from the garage,” he writes in a post at GreenBuildingAdvisor’s Q&A forum. “Are there any recommendations for supplying makeup air to the house so that there isn’t negative pressure in the house, or perhaps a well-sealed door between the house and the garage? Any other ideas, besides a HRV system or a detached garage?”
Woolfe’s question is the subject of this week’s Q&A Spotlight.
Consider occupant-sensing ventilation
Peter Smith suggests Woolfe consider something called occupant-sensing ventilation, available through Conservation Technology. The company describes it as a “continuous, multiport, exhaust-only” system. An efficient exhaust fan is connected to bathrooms, laundry and kitchen with a trunk duct and motion-sensing exhaust grilles. When the rooms are unoccupied, airflow rates are modest. When a room gets too humid or is occupied, airflow rates jump sixfold.
John Klingel has two additional ideas: install a high-capacity fan that starts up whenever the garage door is opened and runs for 20 minutes. The system would have an inlet for fresh air specifically for the garage. And second, Klingel suggests, install the door between the house and the garage so it opens into the garage. That way, any negative pressure in the house will draw the door more tightly closed.
Or, make the garage a detached structure
“If it’s not too late,” says GBA senior editor Martin Holladay, “consider a detached garage. Even a garage connected by a short breezeway is safer than an attached garage.”
No doubt this is true. Given the variety and toxicity of stuff that often ends up in the garage, keeping the structure safely away from an airtight house sounds like good advice.
Except that Woolfe is counting on it.
“Martin, you must feel strongly in favor of a detached garage,” he says. “I really like the idea of the garage and the house sheltering and warming each other, and the sharing of the common wall means less foundation, framing, roofing and cladding. Sure would like to find a way to have my cake and eat it too.”
John Linck agrees. “I built my home with a long tandem attached garage along nearly all the cold northern exposure, with attic above,” he says. “The savings in materials and energy help to green the whole, if having a garage at all can really be considered green. “I also turn off my Prius when it’s in the garage so any fumes are very temporary and certainly don’t approach dangerous levels. The garage is totally sealed from the living space, 5/8-inch fire-rated drywall, fireproof steel entry door, etc.”
The Prius factor? Linck isn’t the only one to have a hybrid car. “I’ve found that with our hybrid car we can back out and close the overhead door before the gas engine ever kicks on,” writes Kevin Dickson.
But not everyone owns an electric vehicle, and there also are the many solvents, fuels, adhesives, finishes and other potential sources of foul air in a garage. And, Linck adds, if you’re a fan of starting your car with a remote device, you’re probably better off with a carport.
“If your memory is like mine, you might start it and forget it,” he says. “My CRS syndrome is getting worse every year.”
Is this any way to heat a garage?
Woolfe may be a fan of green building, but what he’s really after is staying comfortable in his garage, not necessarily greening “the whole thing,” as Linck had suggested. “Green?” he asks. “Who’s talking green? I’m mostly interested in avoiding white — as in white fingers when working on my car in the winter!”
James Morgan can sympathize, but adds this: “It doesn’t sound very smart or effective to rely on leakage from the house through an inadequately insulated common wall to heat your garage. It’s only leaky homes that have made attached garages an acceptable arrangement. Tight homes demand a different standard.”
“James, I don’t plan to rely on heat leaking from the house to heat the garage,” Woolfe says. “The garage will be cold unless I specifically heat it, just not as cold as it would be with four walls exposed to the weather, instead of three walls. Likewise, one wall of the house will be warmer because it will be buffered by the garage.”
Heat is definitely a factor. Holladay argued in a previous exchange that basements are far more useful than garages. “Garages freeze; basements don’t,” he wrote. “You can’t store a bucket of drywall mud in a garage.”
Our expert’s opinion
Here’s how Peter Yost, GBA’s technical director, views it:
While the air-sealing details to isolate an attached garage are not easy, they work. For more, see How to Build an Energy Star Home: Building Plans for the Thermal Bypass Checklist. Diagnostic testing can confirm how effective this is.
An excellent workshop by Keefe, Cox, and Williams on the principles, techniques, and procedures for isolating attached garages was presented at an Affordable Comfort conference: The Garage Connection: Solving Indoor Air Quality and Comfort Problems.
I have to admit, I have been pretty dead-set against the idea of attached garages from an indoor air quality perspective, but the sheltering and material savings associated with an attached garage are compelling. If the garage is isolated outside the conditioned boundary, the door to the house is airtight and swings outward, and a dedicated exhaust fan on a trigger and timer is installed. Sure seems as though our GBA crew, collectively, has nailed this one.