Sounds like a lot of work! Retrofit is always more difficult. It would have been much simpler to attach the insulation and then the ½" plywood to the rafters. But hindsight is 20-20.

One very important consideration could be the shingle manufacturer. Many of them won't give a warranty unless an attic space is properly ventilated. Trapped air (with leaks) becomes a one way moisture pump. You will occasionally see this effect in automobile taillights sloshing a few ounces of water in one (and not the other). You want to avoid that situation.

Another consideration is whether the shingle nails are penetrating through the ½ plywood. If not now, then what happens when the next roof is applied? Also consider that you will have a small gap between the insulation and the rafter unless you install a mounting strip like a 2x2 on both sides of the rafter. At least that way you would have a flat surface for the insulation board to seal to a mounting strip. But the air gap between the insulation and the under side of the plywood would need an opening at the eave and a ridge vent the entire length of the roof to prevent a condensation buildup. At least that way you could install two solid layers (overlapping the joints so that one is not in line with the other). And you will probably use a few cases of caulk, between the edges of the insulation and the rafter, for both layers.

Look for an architect to get a better opinion, but don't neglect the shingle mfg. requirements.

Thanks for the response, Joe.

I don't know if this is retrofit, remodel or "new construction". The roofing is 23 years old and will probably be replaced in the next few years, possibly with metal, but the house has never been insulated or drywalled. (If I knew how to post a picture I would and that might help.) The nails do penetrate the plywood so it would punch into the foam which is not a problem unless I need a vapor barrier under the decking (one contractor suggested plastic between the deck and the foam but it would get a bunch of holes punched in it).

If I put the foam board against the decking using cans of expanding foam I was hoping to eliminate any trapped air and I was planning on using the expanding foam to fill any gaps and adhere the foam to the rafters. I found a reference to a study that said foam insulation sprayed directly on the decking between the rafters does not make the shingles hotter than a conventional vented roof, but I could not find the original research report.

Good tip on the shingle mfgs. Thanks again.

The most important thing is to ventilate the roof. The roof deck should always be as close to the outside air temperature as possible.

The best suggestion I can give is to install a ridge vent the entire length of the roof:

http://www.lowes.com/lowes/lkn?action=howTo&p=Improve/ridgevent.html

With the roof deck isolated from the space below, you will need to install soffit vents between all the rafters- continuous soffit vents would be the best way to do this:

http://www.tamlyn.com/index_files/ezvent.htm

This will allow air to flow naturally under the roof deck, keeping it cool in summer and in winter (which prevents ice dams).

Keeping the roof cool and well-insulated from the house will save you money on your air conditioning also. I would suggest a vapor barrier between the rafters and the sheetrock to keep your warm or cool air in.

Just a question- why did you pick rigid foam and not fiberglass? More R-value for a given thickness, I assume?

I'm trying to keep the ceilings as high as possible, particularly over a staircase and I will have triangle windows in the gable ends, which I would like to be as large as possible. I found the foam sheets for seven dollars each, so I bought 100 of them. Enough to do two layers on a 15 square roof.

I have seen manufactured sheets of foam and plywood laminated together and the top layer has 3/4 inch airspace under it. That seems like very little ventilation. I can do ridge and soffit vents, but what do you think the minimum space under the deck could be for a rafter about 12 feet long.

If it were my roof, I would nail 1x2s (or strips of wood/plywood ripped to size) along the rafters (making them thicker) and then use both layers of foam panel like you originally planned on. That will give you an inch or so of air space, and only cost you 1/2" or 3/4" of ceiling height. A plastic vapor barrier between the insulation panels and the sheetrock would be a good idea also.

This is a good idea to add to the bottom of the rafters. I could even rip 2x4s in half. The way things are built now, I could easily drop the ceiling a few inches without too much trouble. Thanks! A local builder/neighbor suggested putting the plastic on top. Does the location of the vapor barrier vary by geographic location.

Don't use a vapor barrier on ceilings, the gap from eave to peak between sheathing and insulation materials is the means to vacate vapor.

There is a 'closed cell' spray foam, made from soybeans, that expands. CPI also sells spray on roofing. Might be an option.

I was going to suggest the same thing.

It's like the expanding foam insulation you can buy in hardware store for sealing cracks in wall. They have different formula for different purpose. You may need to call someone to spray it.

I'm aware of the spray on foam and it is an excellent product. They spray this directly to the bottom of the sheathing on the roof between the rafters and it works well for cathedral ceilings. They can do this without any ventilation (as I understand it) because it is not air/moisture permeable. I already have 3200 square feet of 2.5 inch thick foam boards I would like to put directly on the bottom of the roof sheathing in a similar way using the cans of foam if possible. I just can't find much info on insulating this way and the long term results.

Look at this from Cor-A-Vent, it is designed for use under siding but I can see no reason the design wouldn't be applicable in your use too.

Of course anything such as furring strips, lattice, half or quarter rounds glued to the underside of sheathing would work too.

I have a similar situation which I'm thinking to use a corrugated material as spacing.