Insulation help for my garage with pics
Wouldnt that create condensation between the foam and the roof sheeting though?? Is there not supposed to be an air gap??
That type of insulation pretty much creates a sealed volume. I've seen some of these "exposed beam" or log homes where the "sheeting" is decorative paneling with the beams below. The insulation is a sandwich made of some sort of board, as in plywood, one on top, one below, and foam in between.
The secret is vapor barrier. The reason you need soffit and attic vents on a building with a ceiling, is that most insulation leaks enough heat that it can allow snow/ ice to melt where the warm corner of the wall/ rafters meets, melts in that area, runs down to the edge of the roof and re-freezes because it's now colder. Makes ice dam, backs up water, gets under the roof and causes water damage.
In your case you have the opposite problem. You need to seal the rafter cavities to keep building moisture out of the insulation
I used to do HVAC service. We had an RV park with a large indoor pool room. An exhaust fan in part of the building was building negative pressure in the building, causing some gas fired heaters to "not vent", IE was sucking the vent gases backwards down the vent and out into the building. I checked around for other units, and found the pool utility room full of vent products from the three pool boilers. Still looking for problems/ solutions, I took a look in the huge attic space above the pool.
1 An incorrect vapor barrier of plastic had been laid on top of the ceiling
2 I could find NO evidence of effective attic ventilation
3 The rafters had been insulated with paper backed fiberglass batting. The 100% relative humidity in the attic had eventually worked it's way through the paper backing, and SATURATED the fiberglass. You could see glass bats had ripped completely off and fallen, others were hanging down and DRIPPING WATER
I told the manager that if he didn't do something with this, he wouldn't have a building to worry about. Soon after that, I left this job to work for Motorola, so I never found out what they did with the problem.