I'm insulating the ceiling of the garage. Since trusses are exposed I think I'm going to put polyiso foam board between the truss rafters (top cord). I figure I'l leave a gap of 1.5" between the roof deck and the insulation so that area has air circulation. I can get about 6" of foam board in there giving me ~ R36. I can run foam board the other direction to reduce thermal bridging fron the 2 x 8 rafters. You can buy used polyiso foam board reasonably cheap. Spray foam isn't an option due to coast. Fiberglass insulation isn't an option because of the thickness needed. Heating zone 5. I will be stacking sheets of the foam board to obtain the desired R value.

Any thoughts on doing this? Am I doing it wrong? Can't ustify the cost of spry foam and can't get enough thickness of fiberglass to meet code.

 

You will need to install fire rated drywall over the insulation!

 

You should also have a ridge vent and soffit vents

 

Yes, I have a ridge and soffit vents.

Is foil backed foad board considered fire rated?

 

Thermax is, if you can find it.

 

Since you are looking at meeting code insulation levels it sounds like the space will become conditioned living space. If it will remain a garage with intermittent heating and cooling not sure how much insulation is needed.

You mention trusses, were they designed for your application, 2x8 sounds like they might have been.

Is this an attached garage with expansion room over the garage?

Bud

 

@Bud9051 It won't be kept at a comfortable temp during the winter. I'll probably turn heat on when I'm working out there. Maybe a constant temp of 45*. No air conditioning . Its a detached garage. 44' x 24' x 10' walls. Attic space about half the other half has scissor trusses to accommodate a car lift maybe some day.

ETA: Location is upstate NY heating zone 5

 

Since it isn't going to be conditioned living space my guess is the insulation levels are up to you, unless local building dept wants to specify.

Intermittent heating can be a challenge for tools and metal items inside. Warm it up and the air absorbs as much moisture as it can. Then turn off the heat and that moisture condenses out on cooler surfaces. If you bring in any moisture sources, like snow covered vehicles that water will need to go somewhere. Floor drains are often not allowed in a garage.

The attic ventilation is only for the attic. Pick up a meter and start monitoring the RH and temp inside and if it gets too high you may need a dehumidifier or open a window.

Bud

 

So back to the original question. Should the polyiso be against the roof decking or should I leave a gap so air can move between the ridge and soffit vents?