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mattiasc 01-29-2008 02:09 PM

Garage project - ventilation? insulation?
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Hi everyone,

I have a garage that I will use as a work room several hours a day with the garage door closed. I live in a very mild temperature which rarely goes above 80 and gets down to 60 each night on average.

It is a 20' x 20' garage with a 4' pitched roof above that. So far, I have insulated the three walls and put up sheetrock. I stopped at the ceiling point (8') because I am stumped as to how to best handle this space above. (see picture below). I want to keep the attic area open and not drop an 8' ceiling, but I am worried about 2 things: ventilation and air conditioning.

How should I tackle this open space? Should I do something between the rafters such as insulation and more drywall? What about vents? There are no ventilation soffits, gables or otherwise to the open space above (this is an old house). Are they necessary if the ceiling goes all the way to the roof? I have considered a portable air conditioning unit with intake and exhaust to the outside for fresh air.

Thanks for any advice.

Ron6519 01-29-2008 03:13 PM

Are you going to leave it cathedralized or are you going to frame in a floor for storage?
If you're framing the floor, just insulate between the floor joists. Install roof vents to get rid of the hot air.
If you're going to leave it cathedralized you still need to do framing for the drywall as those roof joists are pretty far apart. Many other places do not have a nailing surface for sheet goods. You can't put batt insulation up there, the rafters are too far apart. You would also need an air space on the underside of the sheathing.
One possibility would be to frame it for drywall and sprayfoam the underside of the roof decking. There's a foam that will act as a vapor barrier and can be sprayed directly on the sheathing without leaving an air space.
I wouldn't want to be a roofer who has to deal with bad sheathing in the future though. It would be a pain to deal with all the foam attached to the underside of the sheathing.

mattiasc 01-29-2008 04:07 PM

Thanks Ron. I am leaving it cathedralized. I guess I'm wondering about ventilation if I go ahead and drywall up there. Do you ventilate a space like that? If I drywall up there it essentially becomes a big bedroom with no attic space. Should I worry about ventilation between the drywall and roof? (4 inches maybe.. insulation would take up a good portion of that if not all)

When you say frame for drywall, what do you mean? I am adding another joist across the center of the garage parallel to the roof line.

bigMikeB 01-29-2008 04:28 PM

Insulating the rafter bays will rot out the roof deck. Be careful how you handle that. You need moving air in those spaces to avoid that situation. That spray foam is very very expensive.

Ron6519 01-29-2008 04:31 PM

Currently you have no place to attach the drywall at the gable ends. The vertical studs project out past the roof rafter. You would need to build out on both sides of the roofline, on both gables probably, so you could attach the drywall. Drywall is also made to span 16" on center. Those roof rafters almost look like 24"+. If you don't fill in between, the drywall will sag between the joist spacing you currently have. That issue aside, what width insulation were you planning to install ? And how were you planning on keeping it in place?
There needs to be at least a 1" space for airflow between the wood sheathing and the insulation. Otherwise the sheathing gets baked by the hot air trapped there and it shortens the life of the roof. I would at least frame in a common area near the ridge beam so the vents you install will be able to get rid of the hot air up there.

BleachCola 01-29-2008 07:07 PM

i am doing the same in my garage,

I decided to do a drop cieling out of drywall

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