INSULATING GARAGE CEILING AT ROOF LINE

QUESTION FROM Joe, Madison, WI:

I have an attached garage on my house with exposed Kingpost 2x6 trusses. There is no insulation under the roof. There are no soffit vents or roof vents in the garage either.

Wisconsin gets cold, and despite me having a gas heater in the garage, without roof insulation it is, of course, very difficult to get the garage to a comfortable temperature in the winter.

I'd like to insulate against the roof boards between the 2x6 trusses, but am not clear on A) the need for a vapor barrier or B) if doing so then requires me to drywall the ceiling (which I don't want to do). I have considered various R19 products, including Kraft faced, plastic faced, and foil faced (which I was inclined to use for the reflective qualities as well).

I read your other post about insulating between rafters, but that was regarding living space, not garage. And since I have no soffit or roof vents in the garage, I wondered if the potential vapor trapping was still applicable.

I use the area above for light storage. I do not want to drywall the ceiling as it will limit the storage functionality of my garage (even with a pull-down stair). If a vapor barrier is required, and in turn requires drywall, then I guess I am out of luck. But if there is some way to do this, I'd like to know about it.
 I would appreciate any advice on my particular situation.
 
Thanks,
 Joe

Reply from Joe:

Dear Bruce,
Despite the unfortunate conclusion, I truly appreciate your expertise and fast response.

These are the type of definitive answers I was looking for.  
I'm so glad I found your site before I bought a ton of R19.

Many thanks,
Joe

ANSWER FROM THE INSULATION DOCTOR:

It can be done but there are certain limitations you will encounter.

The first is you will need to install proper vents even though you do not have soffits. The proper vents will need to go from the outside wall all the way up the roof line to the newly installed ridge vent, which you will have to install at the apex of the roof over your garage.

Failure to do this will cause you two problems. The first is the condensation between the insulation and the roof boards will cause your roof to rot. The second is it will cause the plywood roof deck to curl upward right off the roof in the heat of the summer.

As far as installing the insulation, you have a 2x6 rafter (actual size is 1-3/8" x 5-1/2") of which the proper vent will take up 1-1/2", leaving you a balance of 4" to work with meaning you will be able to install 3-1/2" R-13 blanket insulation with a 1/2" stapling flange.

For the covering on the insulation the national fire code requires that you use a non-combustible covering. Standard foil-backed or kraft-backed insulation does not meet that requirement. There is a product on the market which is a fire-proof vinyl backed insulation and is an FSK fire-rated product. This is normally used in commercial applications and is usually only available at commercial supply houses. It is a very expensive product, approximately 4 times the normal cost of R-13.

What you want to do can be done but it is not economically practical. I'm sorry to be so negative, but insulating a roof line is not recommended. Hope this helps. Bruce Jones The Insulation Doctor