Controlling heat in detached garage - where to begin?

Hey everyone. I figured I'd post this here and then move the topic over to a sub-forum once I figure out what to do. I've relied on this board for a while now to learn how to repair a lot of my house, so thank you.

I've got a detached 2 car garage (smallish, ~19x19). I'm in inland Orange County, CA. In the summer it gets up to 95-100 outside. Last year we got up to 115 one day. Even hotter in the garage obviously. No cars are kept in the garage. It's a music room, electronics workstation, storage for paint/chemicals, etc. I'd really like to avoid the extreme heat out there. It's not good for the stored stuff. But it seems like murphy's law. Every fix I can think of has major roadblocks. Right now I have a window AC unit out there and it works fine, but I don't really want to let that thing run all day every day of summer.

There's no ceiling. Joists are exposed, but they're spaced at 4'. The interior walls are drywalled, but there's no insulation, just drywall on studs. No gable vents or powered vents, just a whirly bird on the roof. Oh, and the garage door faces West. So that makes for a nice radiator all afternoon...

That's where I'm at, I'm stuck. I thought about adding ceiling joists every 2' to finish the ceiling. But that really only seems to make sense if I'm going to tear out all the drywall, insulate the walls, and re-drywall. And then it really only seems to make sense if I want to also build a new fourth wall inside the garage door, otherwise I'll have 3 insulated walls and 1 hot garage door. I've seen people cut thermal insulation foam and glue it to the inside of the garage door but I don't know how helpful that really is.

I thought about putting powered roof vents up (maybe even the solar ones), but my fear is with no insulation they aren't going to reduce the temperature more than a few degrees.

Any thoughts or input you guys have would be much appreciated. I'm curious what you've done or would do in your own garage. Thanks!

I’ve seen powered exhaust fans work but they need to be big fans. Little bathroom exhaust fans aren’t going to move enough air to make a difference. You would also want a fresh air intake so you can pull in cooler air. The temperature of the air entering has a drastic effect on how effective it will be. You will never get it cooler than the outside air temperature using this method, and you should expect it to be a fair bit warmer still.

I think if you want to be comfortable then mechanical cooling is your best bet. A ductless split unit would work well if you board up the ceiling, and you could adjust the temperature set point higher when you are not in there so you save energy but still don’t spoil your items being stored.

Edit, corrected autocorrect

What kind of a budget are you looking at?


There is no reason to condition the air in the truss / rafter space. But you do need to provide intake air if you close it off.


Do you want the space to look finished or just cool and sealed off and insulated?


If you don’t want to rip down the drywall you could attach foam sheets to the inside. You could also attach foam sheets to the ceiling joists filling the cracks with can spray foam.


Not sure what code says about that but as it’s not a living space just storage it might fly.

If this garage is "required covered off-street parking", then it must be maintained as such, unless you want to do a lot of demolition.

Is the garage overhead door the only access, or is there a pedestrian door?

From your description the structure was stick framed and the member that occurs on 4' centers, is a rafter tie, and hold the walls together.

What is the landscaping material on the north, east and south side of the garage?

Wow guys, thank you for the fast responses.

pjones said:

A ductless split unit would work well if you board up the ceiling

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Yea that's not a bad idea. I would think then "finishing" the ceiling doesn't even matter. Any reason I couldn't just put up plywood on top of the rafter ties (w/ 2' spacing) and then just attach insulation to the bottom of the plywood? It would be visible from inside the garage but I can't imagine that changes effectiveness.

bud16415 said:

What kind of a budget are you looking at?

There is no reason to condition the air in the truss / rafter space. But you do need to provide intake air if you close it off.

Do you want the space to look finished or just cool and sealed off and insulated?

Click to expand...

Don't really know what the budget is. I guess: whatever it costs - without spending extra for cosmetics or optional things that won't affect the temperature that much. I skimp on A LOT of projects, but this seems like one where it's not worth spending anything if I'm not going to do it the right way. If the rafter space were closed off, would I need to install gable vents? That seems more complicated. Or could I just put roof intake vents and maybe another whirly bird? My father in law's a roofer, so anything on the roof he can take care of easily. And I don't care about the look at all.

Snoonyb said:

Is the garage overhead door the only access, or is there a pedestrian door?

From your description the structure was stick framed and the member that occurs on 4' centers, is a rafter tie, and hold the walls together.

What is the landscaping material on the north, east and south side of the garage?

Click to expand...

There's a pedestrian door as well. And yes, sorry about the terminology. I looked up rafter ties and you're right. The rafters are spaced at 8' and there's a tie every other rafter on 4' centers. So I think putting the additional rafter ties on the other rafters would be pretty simple.

The East and South sides are dirt, but the South side has about 5' to the property line. The North side has a little patio, an extension of the same slab the garage is on.

I guess I missed if you are trying to make it cooler than the outside air, or just cooler than what it is when it is hot outside... I'm going to assume the latter.
Ventilation is going to be huge, with your FIL a roofer, he might be able to install a ridge vent for you (assuming there is not one already) which will allow the hot air to escape naturally. The next thing would be a fan for an intake, preferably pulling in air from the coolest side of the building - which should be the north side. Considering you have a patio over there, your outside air on that side should be even cooler.

Another question - what color is your garage door? White is preferable to reflect the most heat. Same goes for the roof itself, but light colored roofs are never really fashionable...

Speaking of the roof, shade would be good to keep direct sun off of the roof and the garage door if possible. If not, you could always go full hippie and plant a garden in your roof...

Seriously though, if your garage door has all of that afternoon exposure, you may want to think of ways to keep it from getting all of that direct sun. An awning would be ugly, but could work. Maybe a trellis and vine that provides at least some level of shade to the door...

In short, your biggest natural wins are more airflow and blocking as much direct sun as possible.

You should already have eve vents and it would be uncommon not to have gable end vents, and a couple wind driven whirlybirds would suffice.

Rather than the expense and labor of installing additional rafter ties, you might investigate a "T" bar system and unfaced batts above.

Doors, especially the pivot, are tough to seal from air infiltration and a coat of high gloss paint or lacquer will be reflective.