1000sqft 2nd Story Apt Over Garage
 

I'm just fishing for comments/suggestions/confirmation that my plan/system will work as I expect it to.

Zone 5 NWPA existing building is 42'Wx24'D with the long wall facing about 15 degrees off of south with nearly unobstructed southern view of the skies, West, East, and North walls are 90% shade and the building will be at the bottom of a 400' forested hillside to the north of the building ~4:12 slope (4ft rise for 12ft run) Open floorplan, 2-occupants, 1Br,1Bth

I'll be contracting a 2nd story addition to be built on-top of the existing structure with the same footprint, the existing roof will be demo'd. Construction will consist of open web floor trusses with 3/4" OSB T&G subfloor, 2x6 walls with OSB sheathing, energy-heel 4:12 roof trusses with a flat ceiling.

Insulation will consist of contractor installing 4" of XPS in 2" lifts on the walls exterior to the sheathing with staggered&taped seams, I will be installing by myself dense pack insulation in the 2x6 walls and under the floor with loose fill cellulose in the attic I may sub out this work however depending on cost. Attic insulation will be baffled near the eaves to retain free air-flow.

Insulation I calculate without factoring in sheathing or lumber to be R42 in the walls and I'm aiming for R30 under the floor and R60+ in the attic.

The air sealing plane I plan on being the subfloor, wall sheathing, and drywall ceiling. I don't know what to realistically aim for here but an ACH50 of 2.0 isn't unreasonable I feel.

Heating will consist of a single centrally located ASHP mini-split 100% inside the thermal envelope with mechanical ventilation. ERV? HRV? Ventillation returns to be in the bathroom (1) & kitchen (2), Supply in the bedroom closet (1) and living area (2).

Glazing will consist of about 64sqft of tilt and turn triple pane windows of various sizes. only 10% of the windows will be towards the north half of the building. Triple pane is specified because the building is about 250' away from a rural 4-lane highway with an unobstructed view that can be somewhat loud by experience. One 36" glazed man-door. All glazing will be flush with the siding.

Appliances: All electric, No exterior vents required. No Fireplace.

There are a few things I'm not sure about,

• What model/size of HRV or ERV should I use? Supplier?
• What model/size of Mini-Split should I go with? Supplier? I roughly calculate I need 30,000BTU of heat and a resistive baseboard heater backup would be a good idea. Design minimum temperature of -15F 98%, very rare that a night gets below that.

First of all I would advise you not to build on top of a garage if you can avoid it. This is because attached garages are a major source of pollution in homes. In the winter the stack effect inside an above garage apartment will pump pollutants into the living space. These pollutants are put out by vehicles (and gas powered equipment) even when they aren't running. The major source (when not running) is the gas tank but also pollutants come from the crankcase, tires, interiors, etc... The way to combat this is properly sizing continuous negative pressure ventilation of the garage space. There is no consensus on the correct rate. To find out the correct rate you must get a manometer reading while you negatively pressurize the garage and ensure the air pressure is lower at the adjoining plane (wall and/or ceiling) inside the garage compared to inside the living space, while the living space is operating at max likely negative pressure (in your case likely with the kitchen hood running on high and the bath fan on).

As for HRV/ERV the Panasonic intelli-balance is likely the most economical choice, but Zehnder makes an excellent unit with unrivaled efficiency. You may also consider doing away with an HRV/ERV entirely and doing a supply ventilation strategy. This could be a good thing for you as it will help keep the living space at a higher air pressure than the garage.

As for mini-split use whatever your local knowledge base is comfortable with. Both Mitsubishi and Fujitsu tend to have good regional representation, but local knowledge can still be spotty. Both of those companies also have units that work until around your design temperature so baseboard heat may not be needed.