Zone 7 – Insulating a garage over living area
We’re squarely in Zone 7, ground snow can be up to 13 ft deep, and our roof will build up to 8 feet of snow regularly. Built in 1966.
After 30 years of digging out cars, stairs and entries, it’s finally time to add a garage. We’re on a steep downhill slope from the street, so the garage will be above the living area.
What do you recommend for the garage roof and garage floor insulation? Here are some details…
The house roof will be insulated to R49, but is there any sense in insulating the attached garage to R49, too? Or do we insulate to R38. The garage doors (2 bays) will be R18.
We plan to use the garage a bit during the winter (crude gym, ski prep). But it’s really for cars and storage. We might heat the garage to 50F for an hour or two to exercise or work sometimes, but not otherwise.
The garage has an entry to the main house below via elevator (ADA, we’re not getting younger). Probably a well-insulated and sealed elevator door. Not sure if and how you air seal an elevator shaft other than at the doors and the equipment area. Do people insulate elevator shafts?
The garage floor is concrete with floor drains for each bay. The floor drains are a significant thermal intrusion, and sit right above the living area.
How much insulation do we put in the floor? Where do we put the air barrier? Do we treat the garage floor like a roof (seems “yes”), but does that mean we want to have an R49 garage floor? The garage will probably hover around 25-35F during the winter (3-4 months). Not quite as brutal as direct snow pack and weather. Maybe the garage is more like Zone 6 (which I think is still R49 for a living area roof)?
With a two big R18 (maybe R20) garage doors, is there anything to be gained by having much more than R21 or R23 walls? Or are the doors so thermally weak that there’s really no point of putting more than R18 in the walls?
The living area beneath the garage will be a family room, which will get a lot of use. We want it to be comfortable. Since the family room will be new construction, we want to do our best (within financial limits) for air barriers, insulation, and tight windows and doors. Maybe even tight enough for a cold-climate heat pump for heating.
In addition to R values, I’m open to any experience people have, including elevators accessing a cold garage. I’ve googled high and low, and living areas over garages are plentiful, but not the other way around.
Thanks in advance!
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Replies
I think you should treat the garage as being outside the building envelope, and insulate the boundaries between living space and the garage like they were exterior walls/roofs. That means air barrier and vapor barrier as well. It's a good idea to have an tight air barrier between a garage and living space anyway. The floor of the garage is going to have to be waterproof, which means it probably will have to be treated like an unvented roof.
If you then want to insulate the garage as well that will make it more comfortable and shouldn't add that much to construction cost. But unless you get really good garage doors don't expect it ever to be like interior space.
For the elevator, you have to decide whether you want it inside or outside the building envelope, and then put the whole thing on one side. This means an exterior-quality door and walls around one end of the elevator.
@DC_Contrarian - thank you, I like your comments on how to structure the insulation (i.e. treat the garage as exterior with vapor and air barriers at the garage floor accordingly). Same with the elevator shaft.
This would mean treating the garage as an unattached garage. I figure since the garage is directly on top of the living area, it would be easiest to match the garage and living area wall insulation, with additional consideration for thermal bridging/breaks.
Any thoughts on roof insulation for an unattached garage (R18 doors, R21 to R23 walls, fairly airtight), that is heated occasionally to 50F while outdoor temps are 15-20F? I suppose I could hammer out a thermal analysis based on heater BTU, but if there's a rule of thumb...
Any heat that flows up through the floor of the garage will flow through the garage and heat it for free. The temperature of the garage will reflect the ratio of insulation above to below. As a simple example, if you have the same amount above and below the garage will be halfway between indoor and outdoor temperature. If it's 20F outside and 70F inside the garage will be at 45F. But you probably won't have the same amount above and below, you have to take into effect that the garage has more surface area, with walls and roof.
The formula is heat flow equals area times temperature difference divided by R value. Once you know the R-values and areas you can solve for the temperature that gives the same heat flows. Insulation is pretty cheap in new construction so I would shoot for an insulation level that keeps the garage relatively comfortable without heat.
In theory you could reduce the insulation in the floor of the garage to compensate for the insulation provided by the garage itself, but I don't trust garages to stay warm. You're probably going to need thick floor joists to support the weight of cars anyway, filling that space with insulation is relatively cheap.
@DC_Contrarian - I don't trust garages to stay warm either.
I think you're right - stuff the floor joists with the right insulation and try to minimize thermal bridging and make the living area airtight. Do what makes sense in the garage (match the wall R-value of the living area since they share all the same walls, code R-value for Zone 7 in the roof). Probably add a ceiling mounted radiant heater when we need some warmth.
If your local government needs to approving the plans and inspect the site the elevator will be a big problem so long as it connects the garage to the living space.
Just how thick is this R49 waterproof concrete garage floor? Is there a center support underneath or does it need to span 25 feet?
You do understand the R values on garage doors meaningless jokes. The “R20” door would be unlikely to pass independent lab test at R4.
Walta
@walta - tell me more about potential inspection / approval issues with elevators connecting garage and living space, and any direct experience you might have. Is it fire? Toxic exhaust fumes? Would a vestibule separating the garage from the elevator help?
Generally, the code will require a fire rated assembly between any living space and any garage.
Even when no one is inspecting and forcing you to have a fire wall I would consider it a good idea.
Cars catching on fire is one of the more common causes of house fires and likely to become more common when more people are charging electric cars in garages.
An elevator shaft is more or less a chimney that allow a fire to move quickly between floors.
I am sure you can buy elevators that are fire rated but my wild guess it to would almost double the price.
Yes I think getting the elevator out of the garage is a better plan.
To my ear this sounds like a recipe for a flooded living room! “We’re on a steep downhill slope from the street, so the garage will be above the living area.” Maybe it is not as bad as it sounds.
Before you put to much money in your old house consider is it a good investment? Will your house be over priced for your neighborhood? If life happens and you had to sell the house the day after this project is complete how bad would it be?
To my ear living space under the garage sounds desperate at best and unlikely to be a show place no matter how well done.
Consider a new build. It sounds to me like you really want a passive house and the current home will never get you there short of a gut rehab and I think you could build something new and better for less money.
Walta
We spec'd fire-rated elevator doors at each level, and the shaft is also fire-rated along with building egress alternatives near the elevator at each level. No need for a lobby. ADA compliant.
We may negative pressurize the garage or positive pressurize the shaft and vent it to keep exhaust fumes out of the living area.
Passivehaus doesn't make sense in this build - it's attached to a 1966 main house with single pane windows, very drafty, R9 walls and an R4 roof. We'll up the roof insulation and seal against the draft in the existing house, and the addition will use mostly stock materials and standard sizes (some LVL in the roof). Maybe someday when we have the resources, we'll upgrade the windows and the wall insulation in the original house.
Some context... (little or no real engineering content, skip if you like):
The structural engineer is designing the roof on the garage for 420 lb/sqf loading. We need R49 in the roof just to meet code (R49 doesn't even count as passivhaus up here - we have just over 9000 heating-degree-days at our elevation of 6600 ft, at the low end of Zone 7).
We are highly constrained by terrain. The garage floor elevation will be at street level, but with a bridge to get to the garage - because the lot is so steep. The family room directly beneath the garage will have 12-ft floor-to-ceiling windows on 3 out of 4 sides - because the lot is steep. The bedroom below the family room will have 8-ft floor-to-ceiling windows on two sides - because...
This is why an elevator is a necessity if we want to age-in-place. Digging out the stairs and the propane tank throughout the winter will either keep me young or make me die young.
The hillside is granite - no risk of mudslides. The dig-out will be a bit gnarly here and there. In spite of the steepness of the terrain, the existing house is as dry as a bone. Not too worried about flooding. We actually have a sewer instead of septic - probably because of the granite and the adjacent creek. Geothermal is not an option.
The existing home is semi-historic, built by the founder of a local ski resort (we didn't know that at the time we bought it), designed as a mid-century hexagon without square corners anywhere (a few closets perhaps). It's so overbuilt in terms of heavy timber framing that it would cost a fortune to either tear down or replicate. The roof is supported by 32-foot long 8x24 beams placed end-to-end, mortised into 8x18 columns. Our contractor refers to the framing as "bomber". The chimney is giant - 12 feet across.
15 years ago we thought about scraping the building and starting over. But you just can't build something like this anymore unless you have deep pockets. Especially with the terrain.
We bought it from an architectural firm that used it for inspiration (so they say), but it was mostly for skiing boondoggles. It's 2300 sqf of living space, and the architects reconfigured it into a 4 bedroom that sleeps 12. They had a hard time finding a buyer (looked like a poorly maintained airbnb if airbnb existed back then, the lot was steep and difficult to build on, and the custom cabinetry was designed for the homeowner who was 4'-10") - but we fell in love with it. Go figure. Must have been them big bones. It's next to a creek that flows year-round with snow melt that feeds into the local river. Never get tired of listening to it. Some years you can ski up to the 4th of July. The neighborhood feels like Mayberry, right out of the Andy Griffith Show (dating myself). Except when the Gazex near the house is fired for avalanche control. At least they warn you and close the roads.
The home was in rough shape from hard use (and a LOT of bear break-ins). Over the past 30 years we've slowly brought it back. I'm just getting too old (did I really just write that?) to see myself digging through 10-13 feet of snow each visit for another 20-30 years (we average 450-500 inches of snow per season, this year looks like it might be 700 with the atmospheric rivers). Can't get a machine on the hillside (and no garage to put it in), so the snow's gotta be dug by hand. Digging out the propane tank 2-3x each season is fun because it ends up being an 8-foot deep pit that takes a ladder to get in and out of.
Our neighborhood of 110 homes has a Cat Wheel Loader for a snow plow. Because we need it.
So you're right, we're absolutely desperate for a garage. :-) And we're doing the elevator because this is our only opportunity to connect the garage to the house while we can still walk. The extra rooms under the garage will be for the overflow of kids and grandkids.
GBA is an awesome resource. I've learned a ton over the years, and still learning with every project. I hope to share photos of the build and performance statistics to help others building in deep snow country.