insulation and roof vent question

[Stepper]

Newly built 2x4 3 car garage. (24 x 32 Detached)
Already have a NG heater installed.
I plan on running electrical and intalling insulation soon.
I'd like to keep the rafters open, as I like the open feel of the garage.
So, what I'm trying to think through is:
Obviously I need to insulate the garage before I can truly utilize the heater.
I'm considering installing batting insulation on the underside of the roof iteself, rather than installing drywall on the ceiling and insulating on top of the ceiling.
I know quite a bit of my heat will rise anyway, but I prefer the open feeling.

My question is, what should I do with the insulation as it approaches the roof vent at the very peak top of the garage?
- Should I stop a certain distance short of the vent to allow airflow?
- Should I run insulation across the roof vent and "close" it off with the insulation?

 

[Scott H in Wheaton]

What type of roof? Asphalt shingles or metal?

I would keep the insulation 3-4 inches away from the ridge vent. Don't close off the vent, its there to allow moisture to escape in the winter and trapped heat to escape in the summer.

Your workspace should be like a styrofoam cooler, and a cooler only works if it has a top to go with the sides and bottom. No insulation is the same as no top. Ideally you would install insulation leaving a 2 inch gap between the insulation and the roof deck, staying away from soffit vents and the ridge vent, then cover the entire ceiling with solid panels, your 'lid' to the cooler. Air will flow in thru the soffit vents, travel up the roof deck cooling the shingles, and exit out the ridge.

 

[rsa]

Green Building Advisor, Building Science Corporation, Journal of Light Constructions are good sources for research.

Here's a good first read from GBA, How to Build an Insulated Cathedral Ceiling. The comment section is valuable also. Two more, Rules for Venting Roofs and A Crash Course in Roof Venting.

The time to think about insulation is before you build. Rigid insulation above the roof deck could have been the right answer.

 

[Falcon67]

The time to think about insulation is before you build. Rigid insulation above the roof deck could have been the right answer.

Or radiant barrier decking and rafter size 2x8 to allow for R19 insulation. I also thought about doing that since I like open rafters too. But went on with the conventional setup. The cathedral ceiling write up is what you need. If the rafters are 2x6, I'd just do R13 with drywall over it and a ridge vent. That should leave a 1" air gap above the fiberglass. Ceiling insulation works - but it works 10x better if it's covered with a draft free cover like taped drywall, caulked OSB, taped vapor barrier, etc.

 

[Stepper]

Scott h...thanks. That made perfect sense. Thanks for all of the other replies as well. This gets me on the right path.