Insulation question

[Chris E Smith]

On two sides of my shop I have sections of cathedral ceiling that comprise a shed roof with no attic space. It's framed with 2x6 rafters with 2x4 purlins on top and R-panel metal roofing attached to the purlins. There is a continuous soffit vent and a continuous ridge vent at the top made of Cor-a-vent material. I want to insulate this roof as well as possible but would prefer not to use spray foam. I am in Texas and the sections of roof I am talking about are on the east and west sides of the building. My idea is to attach 3/4" foam board to the bottom of the purlins to form a ventilation layer between the foam and R-panels. That gives me the back side of the R-panel ridges for air to flow through from soffit to ridge vent. I would seal the gaps in the foam with canned foam. Then I would put 6" faced fiberglass insulation under the foam and cover the ceiling with sheetrock or possibly add another layer of foam then sheetrock. The former scenario gives me R-19 + R-5 and the latter gives me R-19 + 2x R-5. I'm thinking I should close the soffit off from the space occupied by the fiberglass insulation so that air only flows above the foam board rather than infiltrating through the fiberglass. Does this sound like a good plan ? I would prefer not to use spray foam because it's less serviceable and I would have to block off the soffits and ridge since the back side of the roof would then become interior space. I'm wondering if foil faced foam should be used for the top layer of foam to act as a radiant barrier. I'm not clear about whether a radiant barrier is useful under a metal roof or whether I would put the foil on the top or bottom. Thanks for any feedback. Chris

[Jim Andrew]

Sounds like a pretty good plan. Ventilating the roof is very important. If you are to install the foil covered foam, I would put the foil to the top side of the panel. Blocking off a space for ventilation is ideal, and then adding insulation is the right way to go. I always block off the overhang, and leave a gap at the top, so wind doesn't pick up your insulation and move it around. Years ago I built some homes, and no one was concerned about the overhang, and I had a call from a customer that he had frost spots on his ceiling next to the outside wall. We had had a wind storm and it blew the insulation away from the outside wall and heaped it up in the middle of the attic. I took batts and pushed them out to the edge of the wall and then pushed the loose insulation back, didn't get that call again, but went to cutting scrap and blocking the overhang except for the top about 3/4". I'm thinking that air should be able to move after you nail up your foam panels, am I correct?

[Chris E Smith]

Yes, the air would move up the corrugations on the back side of the R panels and also have the space between the 2x4 purlins to collect. But to get past the purlin it can only travel through the corrugations. There seems to be at least two large corrugations and a few small ones between each pair of rafters so I'm hoping this will be enough. I guess I could calculate whether this is the same free vent area provided by the same length of CoraVent. I was going to block the space occupied by the fiberglass insulation so that air can only travel in the vent space rather than infiltrate through the fiberglass also. I will either put wood blocking there or a piece of foam board. Thanks - C