garage roof insulation question
 

Have a 3 bay garage in the backyard. The two bay garage was built first. Then the previous owner added the 3rd bay. The roof of the 3rd bay is attached to what was the outside of the 2 bay garage and sits on a masonary/stick built exterior wall. The roof is made of straight 2x4's that are 10 foot long. Roof is waferboard with metal roof. The exterior wall does have a soffitt with some vents but I see no value because there is no exit for the heat. So I was thinking of stapling 3 1/2 " kraft faced insulation into the 16 " on center 2x4's. I plan as my final interior ceiling sheathing to use 1/2 " foam insulation board. Does this sound like a decent way to go ? Winters are not that harsh here in Georgia but summer heat is strong. Advice please.

I don't completely understand. There is a dead space between roofs? Ventilating that would be good. Any insulation is good. It's all in the R value. But, you can help it by keeping uninsulated spaces from building up heat.

I've posted here about what to do about a hot garage in the past. Someone mentioned that if the space isn't conditioned, that insulating it will end up keeping it hot than keeping it cool. I believe the best way to fight the heat (short of installing AC) is to have some vents down low, and have exit vents up high. I'm considering adding an inexpensive solar attic fan and some vents near the bottom of the garage doors.

Here's a thread that i found in a quick search.
http://forums.pelicanparts.com/off-topic-discussions/685488-keeping-garage-cool-cooler.html

Milt picture an existing building with a soffitt on the side. The previous owner screwed metal joist hangers on the side wall of the existing garage so the 2x4's and sheathing would go under the existing soffitt. So on that side there is no place for hot air to escape. On the other side ( low side ) of the roof of the 3rd bay is a masonary/stick built wall that the 2x4's rest on. That also has a soffit with vents but I don't see what good they do.http://forums.pelicanparts.com/uploa...1349393835.jpg

Hopefully this picture helps.

I would use 1 1/2 or 2 in styrofoam SM (blue or pink here) not the white low density beadboard.
It should give you about R10 Run it horizontally as your ceiling and leave the 2x4 space above it for some ventilation under the roof. Code here requires a drywall cover for fire safety but you might not be concerned as your original plan was to use white 1/2 as a ceiling anyway. Less expensive would be just stuff the 2x4 space with batt insul and perhaps a 1/2 plywood ceiling. Easily painted and quite durable in a garage. The regular attic area would be best to use batts.
If concerned more about summer heat then a roof fan for the attic would be a good idea.

Just to be clear the 1/2" foam I plan to use for the ceiling is NOT the white crap. I plan on either the pink/blue foam board or maybe the one that has foil on both sides. That I know I will do. What is still in question is additional fiberglass insulation between ceiling rafters or leave the 3 1/2 " air space ? This bay also has two side windows, a side door and obviously the main garage overhead door so ventilation when working in it shouldn't be too bad. I also plan on pulling off the tin roof and going with shingles, so I do have an opportunity to add a couple of roof vents if we think that is needed.

Not much room under the eaves...
There are roof louvers Air Vent: Roof Louvers that can be added on top. Also the spinner-types.
It's a low-pitch, but you won't be worrying about snow.

For the main 2-bay, there are a/c powered venting fans that turn on at a certain temp. Solar too.
Those existing gable vents seem like it's not enough for hot southern sun.

Add a ridge vent at the peak, or louvers there as well?
Drill holes between the bays?
I don't know.

I am not sold on insulation. The best way is to remove the hot air up top. Vents are your friends. Power or solar vents are better friends. Just make sure the fresh air is coming in from someplace down below to create air flow.

Ridge vent. And soffit.

Yes I have considered a solar gable vent fan for the main 2 car garage. You got me thinking about drilling holes in the 3rd bay. The 2x4's are actually screwed to the end boards of the soffitt from the 2 bay. If I were to drill a 3 " hole in the middle of each 16 " center rafters the hot air should flow from the 3rd bay to the 2 bay main. Add ridge vent on main and/or a solar gable fan should do the trick. I may have to add additional soffitt vents on left side of 3rd bay but that's easy to do. I think I have a plan. Thanks