Seeking HRV Options
I’m in the midst of building a small, 24’x32′ garage-apartment in Climate Zone 6. The building has a two car garage below and about 750 sq. ft. of living area above with about half of the apartment with an open floor plan for the living room and kitchen while the other half are composed of the bathroom and two bedrooms. My insulation plan includes a vented, unconditioned attic. After meeting with my HVAC contractor today, he suggested running the vents from the soffits to the ridge vent and spray foam to code (or higher) to make the attic a conditioned space so he can run ductwork for both a heat pump and a Fantech Hero 150 HRV. I’m not in love with the idea of making the attic a conditioned space because it adds cost and complexity to my insulation strategy and will likely exceed my insulation budget to spray foam. I’d prefer keeping it simple.
I can run the heat pump without ducts — perhaps not idea, but I had planned only to have a single head for the entire living area. The ductwork for the HRV becomes the problem. I’ve read every GBA post and article I could find and am wondering whether anyone out there has found simple ways to bring the HRV ductwork inside the conditioned space of the building or ductless HRV units. I also have space inside a closet that’s along an adjoining wall between the bathroom and kitchen/living room area which could facilitate short duct connections to the HRV (but it could be a pain to service the HRV inside what will be a narrow closet). There will about a 9’x 21′ conditioned space below the apartment (a mudroom/laundry room) that might also be a possible location for the HRV (waiting for HVAC contractor’s reply to that idea).
Any thoughts and suggestions from GBA readers is appreciated.
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Replies
You don't need ventilation channels if installing spray foam (ideally, closed cell in your climate) on the underside of the roof sheathing. The flash and batt method would be more affordable than an all-foam approach. (Seehttps://www.greenbuildingadvisor.com/article/the-flash-and-batt-method for more info.)
But if cost is an issue, a ventilated attic with a deep layer of air permeable insulation is generally the best option.
On the HVAC, a single head minisplit can work, but the occupants would need to leave the doors open (especially at night). If you think the doors are likely to stay closed, a ducted minisplit is probably a better solution. Often, a ducted air handler can be located centrally (say, in a closet) and connected to short runs of ducts. Or you can install bulkheads to cover the duct runs.
A through-wall HRV or ERV could probably work in type of home. Zehnder makes this type of unit and has a great reputation.
Since the area below is a garage, I assume it is not conditioned? If so, that means you are insulating the underside floor system? And installing an air barrier in the ceiling of your garage?
If you really want a ducted (as opposed to through wall) ERV/HRV solution, run your ducts down the wall and in between the joist bays, tight to the underside of the floor deck. Then the insulation goes below that. If the 4" duct offsets your insulation too much, drop a 4" header for more insulation only where the ducts are.
Units like the Panasonic Whisper Quiet have just one supply and one return duct. Ideally you mount the unit in the bedroom as that is the room that most needs fresh air (people don't come and go opening exterior doors and letting fresh air in while they are sleeping like they do in the living areas). Mount it near an outside corner, and drop the ducts down between the joists so any soffit you build is near a side or back wall of the garage.
A 2x4 web truss floor system would make running ductwork easy. I have added rooms over garages and this is what I have used. A 24' span will require a minimum 16" deep truss allowing plenty of room for ducts and insulation.