Seeking advice on insulating attic loft garage

[pb4uski]

I’m building a 24’ x 28’ two-car garage on a slab with an attic loft that will be 24’ x ~18’. We will use the attic loft as a hobby room and it will have a ½ bath.

We live in a cold climate region at about 1,600 feet and do not need air conditioning. We plan to use the attic loft in the winter so it will need to be heated. We’ll probably use a direct-vent thru-the-wall propane heater on one of the gable end walls (the 28' wall).

My question has to do with the best way to insulate the garage. We’ll probably use spray foam insulation (like we used on our home). Option A is to foam the walls at the garage level (where the cars will be parked) and the roof and gable end walls in the attic (similar to our home where the whole exterior envelope is spray foam insulated). Option B would be to spray foam the garage ceiling under the attic deck floor and the attic loft roof and gable walls so the level where the cars are parked is uninsulated and the attic loft is insulated.

After accounting for doors and windows in the garage space, I think the area to be foamed under either alternative is about the same so the cost of the insulation should be comparable between the alternatives.

Which alternative do you think is better? Why? Any direct experience with either option at your house?

[samclem]

Some things to consider:
- Will there be any water pipes in the lower area? Anything stored in the garage that needs to be kept above freezing (paint, etc). These factors would argue for insulating the whole structure.
- Vehicles in the garage? If salt is used on the roads where you live, I've heard some folks say it's cars rust less if they are >not< warmed up in a garage between driving. Of course, it's better for the engines to be kept warmer.
- If you heat the whole structure, every time the garage door is opened your attic will get cool. If you do decide to heat the entire structure, you might want to at least seal off the attic to some degree so that the temp swings are less.

- Cars and the things typically stored in a garage (fuel, pesticides, lawn products, paint, etc) aren't great for air quality. Keeping them outside the conditioned "envelope" of your hobby room (and maybe even actively ventilating the lower space) would make for cleaner air where you'll be.

I like the spray foam, but it ain't cheap. Consider the mixed approach of a light coat of spray (approx 3/4") against the exterior sheathing, then fill the stud bays with fiberglass (or, expanded polystyrene with another bit of spray foam to seal against the studs). This gives you the fantastic air-sealing qualities of the foam and uses cheaper insulation for the "bulk" areas where they can do a good job at low expense.

Sounds like a fun project, good luck!

[meierlde]

Quote:

Originally Posted by samclem Some things to consider: - Cars and the things typically stored in a garage (fuel, pesticides, lawn products, paint, etc) aren't great for air quality. Keeping them outside the conditioned "envelope" of your hobby room (and maybe even actively ventilating the lower space) would make for cleaner air where you'll be.

If I recall correctly Holmes on Homes said that at least in some juristictions it is against code to have air from a garage get into the living space, they were looking for opportunities for air infiltration. Because the space is above the garage, and assuming the door is weatherstripped, little heat should escape (foam the stair well walls as well) Putting the door at the bottom of the steps should also minimize heat loss as the cold air would have to rise to get to the living space.

[heyyou]

My opinion is to insulate both the first floor walls and the attic floor, just because you can so easily while you are already doing the rest of the garage. I like the suggestion of a thin coat of airtight foam plus the cheaper, conventional insulation.

Look into using those foam sandwich panels called structural insulated panels (SIP) for the garage ceiling and the roof. They are not cheap but they are very strong and go up fast which may be a savings. If done with care, the resulting triangular structure is very air tight along with the excellent insulation. Ask about the right heater due to the air tight room.

Having taken the SIPs class, I tried to use them with an inexperienced contractor. He hadn't even opened the book that showed how they are joined, so instead I hired the guy who builds with insulated concrete forms. That cost more for no extra benefit over the SIPs.

You could stick frame the garage shell, then just build an interior room in the attic space using SIPs. If you don't use the lower sloped space, near the floor, for drawers in the attic, consider having a 4' knee wall using 4' x 8' panels sideways. That would save on SIP material and save some labor on insulating the room, but you would lose the lower storage space. Some contractors prefer to use materials that save labor.
Did you do your own spray foam insulating on the house?

For anyone building a new place, build your garage first using SIPs then the workers know how to use them on the house. They aren't much different than building a stud wall on the floor then tipping it up into position. You can buy one piece SIP walls, delivered on a big trailer, that require a boom (light crane) to set them into position, but that speeds up the framing.

[pb4uski]

I had explored using SIPs for the house and for the garage but they were pretty expensive compared to stick build and foam and the builders around here that I trust are not familiar with SIPs so I didn't push it.

What I did do on the house that some people may find interesting is that we studded the house with 2x4 rather than 2x6 which is more common up here since the foam adds structural integrity to the wall. We then put 3/4" thick strapping along each wall horizontally every 2' from top to bottom and then installed the sheathing on top of the strapping. As a result when we foamed, the foam oozed in back of the studs in the gap between the strapping and the sheathing, effectively providing a 3/4" layer of foam along the entire outside of the wall except where the studs and strapping intersect so the only thermal bridging in the walls is at those intersections.

I wish I had thought to do that for the roof as well as during the winter when the roof is covered with snow you can actually see the thermal bridging of the roof joists through the snow.

We had the spray form done and will probably do the same for the garage even though I know it is more readily available to homeowners today that it was when we built.

We have an air exchange system in the house because it is so tight. For the attic loft I'll probably just have a bathroom fan with a switch wired to run 20 minutes each hour which some people used and an affordable alternative to an air exchange system.

[samclem]

Quote:

Originally Posted by pb4uski We then put 3/4" thick strapping along each wall horizontally every 2' from top to bottom and then installed the sheathing on top of the strapping. As a result when we foamed, the foam oozed in back of the studs in the gap between the strapping and the sheathing, effectively providing a 3/4" layer of foam along the entire outside of the wall except where the studs and strapping intersect so the only thermal bridging in the walls is at those intersections.

That's an interesting approach, pretty neat. When I built my shop I just filled the stud bays with FG and used standard OSB sheathing, then put 3/4" XPS over that, then housewrap and vinyl siding. The foamboard does a great job of preventing that thermal bridging through the studs, reduces air infiltration (offset the seams from the sheathing) and having any of these foams on the outside helps keep the air inside the FG above the dew point most of the time, greatly reducing the chances for mold in there. Your strapping-and-spray foam approach does the same thing and results in better support for the exterior wall covering than what I did, though it costs a bit more. I'd be reluctant to do what I did with any exterior covering heavier than vinyl, since it's just supported in shear by the nails, which are cantilevered 3/4" (through the foam to the OSB).

One thing not mentioned is firestopping and building codes. If the area above the garage is considered living space, you'll need (and want) to provide a means of alternate egress and a fire barrier between the lower level and the upper level. The fastest, cheapest way to do that, of course, is with drywall (of appropriate type and thickness) attached to the underside of the joists.

Quote:

Originally Posted by pb4uski We have an air exchange system in the house because it is so tight. For the attic loft I'll probably just have a bathroom fan with a switch wired to run 20 minutes each hour which some people used and an affordable alternative to an air exchange system.

You'll probably want to provide a dedicated route for make-up air to enter the attic if you do this, otherwise when you depressurize the attic with the fan you'll draw the air from the garage area (vehicle oil and fuel fumes, stored volatiles, etc) into your attic.

Some people install a dedicated exhaust system with a make-up air vent in their garages, the fan turns on in response to humidity. When a wet or slushy car is pulled into a garage, it adds a LOT of water to the air for an extended period of time. Providing a means to get rid of that water vapor is good for the vehicle and the air quality in any attached living areas.

[38Chevy454]

I have a 26 x 48 x 12 detached shop garage, but no attic storage or space. I insulated 2x6 walls with R-18, and R-38 above the ceiling. It is very air tight and stays very moderate temps vs outside temp swings. That is one of the big advatages of insulation, the moderating effect. Plus of course saves heating costs, you mentioned you live in colder area and no need for A/C.

I would insulate all, lower and upper levels. Foam is great, higher cost than fiberglass. Once the guy is there, it is only extra cost forthe additional material to put on thicker. Foam makes very air tight. If budget is concern (always is....) then I would do foam upstairs and fiberglass down. My garage has OSB on the walls, and drywall on ceiling. The OSB and drywall all painted, but the OSB allows me to put a screw anywhere, not just at the studs. Only detached structure can have the OSB walls, if attached to house you need drywall for fire rating.

Look over at www.garagejournal.com and you will find lots of info for garages and learn some good ideas. Much easier to do right the first time than retrofit later.

[pb4uski]

As a followup, I decided to go with foam in the attic loft (5" on the roof and 3" on the gable ends). They just completed the foam this morning. It looks goo and I think I ended up with more foam than what I paid for.

The main level, where the cars are parked, will be insulated with Roxul rockwool batts covered with a "smart" vapor barrier.

I just put up a new thread on interior finishes, but the front-runner at this point is 1/2" OSB with a skimcoat of drywall compound, primed and painted so I can hang things where I want without having to worry about finding a stud.