I'll be soon starting a conversion of a screen porch to interior space and my main issues concern proper insulation of the ceiling. It is a catherdral ceiling that connects directly to the roof rafters. House is NE Oklahoma, so we get long 100deg+ summers and I'm concerned about radiant heat. Spray foam contractor wants to do just 6in of open cell and he says that's plenty. Would a radiant barrier be helpful? I'm thinking something like Rmax board which would give me another R5 and the reflective side would help with heat radiation from the roof. Any advice is very appreciated.

To get anything out of radiant barrier you'd need to leave at least a 3/4" air gap between the RB and the adjacent layer, and to get the MOST out of radiant barrier would take gaps on both sides, and it would barely add up to the same cooling-season performance as the 1.5" of open cell foam that it displaced, and would perform LESS well than the foam in winter.

To get the RB benefit out of the foil faced Rmax you'd either have to leave the gap(s) or leave it unfinished, if on the interior side. Putting it below the rafters would require you to leave a 1.5" ventilation gap between the open cell foam an the roof deck (not a easily buildable or worthwhile assembly) since the foil facer is also a vapor barrier, and there would be no drying path for the roof deck otherwise.

If you put some rigid foam above the roof deck it would do some good by insulating over the thermally bridging rafters. To hit R38 (code min under IRC 2012 for your climate zone) would take the 5.5" of open cell foam (assuming it's 2x6 rafters) and 3" of R-max (polyisocyanurate) above the roof deck, which would also mean you'd have to apply a non-structural nailer deck above the foam- half-inch OSB works. But to keep the OSB nailer deck from running into moisture issues it's usually worth holding the foam down with 2x furring (though-screwed to the rafters 24" o.c. with timber screws), and using Cor-A-Vent or similiar to vent underneath the OSB. In total that adds about 5" to the roof stackup- and my require some adjustment to the flashings where the roof meets the main house.

If you take the above-the-roof-deck approach, use a membrane such as Grace Ice & Water on the structural roof deck, and an appropriate slip-surface material designed for roofing foam between the membrane & foam. (If it's short enough you can skip the slip material.)

There are vendors who trade in reclaimed roofing foam, but usually with a different facer made of fiberous materials (paper, fiberglass, felt, etc), and blown at a higher density than the usual foil-faced goods. It's usually 1/4-1/3 the cost of virgin stock, and it's more rugged than the lower density stuff. The key is finding a local vendor. If you need a LOT of it places such as insulationdepot.com will ship it by the truckload, but for just a porch-roof, probably not. (search the local craiglist for "rigid insulation", you might get lucky. There are several people in my area trading in used roofing foam advertizing there.)

If the roof is on the sunny side of the house and gets a lot of mid-day summer sun, using a CRRC rated "cool roof" shingle with an aged SRI>40 (assuming it's a pitch greater than 2:12) would do as much or more for lowering the cooling load than any single layer radiant barrier solution. see: http://www.coolroofs.org/

If it's a lower angle roof a finished membrane or metal roof with an SRI>80 would be better.

Thanks for the detailed reply. Seeing as the roof was done with the house I'm not interested in tearing it out for the above the roof deck approach you recommended. How would it even be possible to hit R38 only insulating from the inside. 5.5 in of open cell only gets me to R19?

Posted By dkinion on 16 Feb 2013 10:57 AM
Thanks for the detailed reply. Seeing as the roof was done with the house I'm not interested in tearing it out for the above the roof deck approach you recommended. How would it even be possible to hit R38 only insulating from the inside. 5.5 in of open cell only gets me to R19?

It's basically NOT possible to hit R38 in 5.5".  If you have space to add thickness below the rafters you can get there, but it has to be semi-permeable to water vapor to avoid creating a moisture trap.

OK (like most states) has yet to adopt IRC 2012, and your statewide building codes are currently based on the IRC 2009.

Under IRC 2009 the legal code-minimum is currently only R30 for all but the panhandle region.  With 2.5" of unfaced rigid EPS insulation under the rafters & spray foam behind the ceiling gypsum you'd be there, and it would still have sufficient vapor permeability to avoid trapping moisture, yet sufficiently vapor retardent to not have wintertime accumulation issues.   If you can, use two layers (1" + 1.5") with the seams staggered, which mitigates against heat leaks opening up should the EPS shrink a tiny amount over the next couple of decades (which it usually does.)

Alternatively you could use 2" of XPS (pink, blue, green, doesn't matter, as long as it has no foil or plastic facers) it would get you there too, but it would be a the absolute limit from a moisture release point of view. If you have the clearance for the extra half inch, unfaced EPS would be better, with about 4x the drying capacity.  XPS is often available with ship-lap edges, which would gets around the double layering/shrinkage issue.  While there are also tongue & groove variants, ship-lap edges are better for this purpose.

With an R10 rigid foam between the ceiling gypsum  and the R19 open cell cavity-fill in 2x6 rafters you would have nearly the same performance at ~R30 as you would get from low density R38s in a 2x12 rafters, which isn't bad. Assuming a 15% framing fraction, with the 2x6 + R10 foam it's ~R25.5 for  "whole assembly-R" with all thermal bridging factored in, compared to ~R28  for the 2x12 /R38 situation.   By contrast, the code-min R30 batt in 2x10 rafters runs about ~R23 after thermal bridging, so the foam-under is ~10% higher performance than a typical R30 installation.

Do NOT use Rmax under the rafters, since it has aluminum facers, which would create a severe moisture trap.