We converted an attic in an 1890's farm house in Southern Illinois to living space 25 years ago . The master bedroom has 2 south facing roof windows and a large east facing 1/2 round window. Since losing several large shade trees our attic bedroom reaches unbearable temperatures during our hot humid summers. We are looking into replacing the dark 15 yr. old asphalt shingles with Energy Star Rated metal shingles. The problem is that we need to insulate above the roof decking with a rigid insulation and we are not sure if we need to create a vented space above the insulation & apply radiant barrier . Could we get good results without the vented space or radiant barrier? Essentially what we have now is a cathedral ceiling with 3.5 inches of fiberglass batting. The goal is to reduce the heat gain in the summer. We have looked into ACfoam CrossVent a vented insulated nailbase that has a radiant barrier option but it runs from 80 - 100$ per 4x8 sheet ! We would need 63 sheets. Also contractors in this area are not familiar with this type of product. Any good suggestions ?

I can't answer the radiant barrier question, but I believe you should vent the roof. The vented nailbase is one option, but as the manufacturer's (Atlas) web site says, you need a sufficient amount of insulation under the nail base panel, which could be one or more additional layers of polyiso under the nailbase. The option is to put down the layers of polyiso foam and install 1x3 or 2x4 strapping over that to hold down the foam sheets and fasten your metal to that. This is probably cheaper material wise, although somewhat more labor than nailbase. I'd be looking for a total of R-50, ideally. Another thing to keep in mind is that you can get condensation on the underside of the metal which you should keep out of the poyiso, so installing a membrane like GAF Deck Amour on the top of the foam sheets can be valuable. Be sure to seal your existing rafter insulation cavity to eliminate any infiltration into that area.

We are hoping that the existing plywood decking is in good shape after we tear off the shingles . Could we put a layer of OSB then Deck Armour on top of the strapping and then the metal shingles ? Would condensation be an issue then ? There is a product called Enkamat by Colbond that is a nylon mesh ventilating underlayment that can be used under metal roofing that could possibly be used to provide some ventilation to prevent condensation. We do not know of anyone who has used it. Also, we will not be able to obtain R-50. We will only be able to add two inches of foam board especially if we allow room for an airspace.

msteger,
I have used the Enkamat over SIPS roofing. It seems to work well. I also have used nailbase over an existing roof. The nailbase can be any thickness you want for desired R value. Follow up with the Enkamat and your metal roof. The biggest problem will be in how to trim out the overlay, and not destroy the aesthetics of the home.
Where are you located in S. IL? We have worked in S. IL as far north as Carbondale, and would be glad to look at your project, if you wish.

Wes, We are in Carbondale. If you will be in this area in May or June you could look at the project. I will email you. Have you used the Enkamat in a residential project ?

msteger,
I am always traveling through s. IL. going to the St. Louis area. It would be no problem to stop and visit sometime.
And yes, I have used Enkamat on a residential project, under a metal roof.

Posted By msteger on 24 Apr 2011 04:17 PM
We are hoping that the existing plywood decking is in good shape after we tear off the shingles . Could we put a layer of OSB then Deck Armour on top of the strapping and then the metal shingles ? Would condensation be an issue then ? There is a product called Enkamat by Colbond that is a nylon mesh ventilating underlayment that can be used under metal roofing that could possibly be used to provide some ventilation to prevent condensation. We do not know of anyone who has used it. Also, we will not be able to obtain R-50. We will only be able to add two inches of foam board especially if we allow room for an airspace.

The amount of air space you'd need depends a bit on the roof pitch- at high angles it doesn't take much.  ANY amount of air space (even through mesh) is a huge improvement over laying metal-on-felt or metal-on-OSB.  It doesn't prevent condensation, but it does allow the roof deck to dry toward the exterior rather than accumulate moisture over time.  Using mesh solutions helps keep the gap from becoming a home for critters too- it's not a bad way to go.

Radiant barrier in air gaps less than an inch or two don't add much.  But using a high solar-reflectance high-infra-red emittance finish on the exteior cladding does a LOT during the cooling season.  For comparing apples to apples/pears, use the database of product testing data available at http://www.coolroofs.org/. 

At 2" you get ~R12 out of iso during the cooling season, maybe R10.5-11 during the depths of January, but that would be more than sufficient to allow you to do a high-density full-cavity fill on your 2x4 rafters rather than just the batts without any risk of rotting the roof deck from below with wintertime condensation from air leaks & vapor diffusion from the interior.

Low and mid-density batts leak air and convect like crazy lowering their effective R during the winter, and are translucent enough to infrared that the hottest part of the batt ends up being about an inch-in from the outer surface, making them effectively R7-R8 under a 120F roof deck in July.  Filling the same cavity with cellulose or high-density new-school blown fiberglass (1.8lbs density Optima or Spider) eliminates the derating due to convection loss, giving you a stable & honest R12-R14 year round.  With R10+ on the exterior you don't need (and shouldn't use) interior vapor retarders, but if they're there (kraft facers on batts etc) it still probably isn't worth ripping down the ceiling to remove them. It's often possible to dense-pack over batts without removing them, but if they're easy to pull, it's easier to install the blown goods without them.  With R10+ on the roof deck the temperature difference across the batt is lower though, so it's R value derating is also lower- it might be comfortable enough with a just a cool-roof w/ R10-ish exterior foam solution.

Also note, R10-R12 over the roof deck improves the whole-assembly R more than you might think, since t's placing a thermal break over the rafters which would otherwise be conducting both directions.  R12 foam + R13 batt/blown may only add up to R25 center-cavity, but it's the energy-equivalent of ~R30 between rafters or joists due to the much-reduced thermal bridging at the rafters.

I appreciate all the input,

So I am wondering what scenario would keep the attic living space cooler ( even though I may still have to add a ductless A/C unit) in the hot humid summers here in Southern Illinois ? The roof pitch is ~ 9:12 and roof is currently unvented with black asphalt shingles. I am not able to add to or remove insulation between the rafters. The house is 110 years old. Attic was convetred to living space 25 years ago.

1)Remove existing shingles
Add ventilated nailbase(1-2" polyiso,1-1.5" airspace, 7/16 OSB ( Atlas Crossvent) to existing decking
Add proper underlayment
Add Energy Star Metal Shingles
or

2) Add insulated naibase XPS or polyiso
No airspace
Add underlayment
Add the metal shingles
or
Same as 1 or 2 but add a product like Enkamat mesh on top of the underlayment as airspace and/or for moisture removal

or ??

Thanks

What's the pitch/angle of the roof? If it's lower than 3:12 go with #1 (or do a DIY ventilation & nailer deck scheme with furring.) Over that mesh underlayments would be fine. (Some manufacturers say you can go as low as 2:12 with mesh, but I wouldn't use it on a pitch that low on my house. YMMV.)

BTW: It's pretty cheap & easy to drill in from the exterior to blow cellulose into the rafter cavities as you strip the roof, prior to adding the rigid goods on the exterior. The additional air-retardency of even low density cellulose would mitigate what is likely a very real thermal bypass of wind-driven air movement under the roof deck. Blown fiber would also fill in the myriad gaps that exist in every batt installation.

Pitch is 9:12. Do I still need an airspace with that pitch ? Would the mesh product provide enough ventilation so that I could go with #2 scenario ?

9:12 is PLENTY of pitch to go with an mesh underlayment approach rather than a separate fully vented nailer deck. It's the lower angle roofs that are more susceptible- high pitch roofs have a significant stack-effect working to drive air currents through the mesh to keep the deck dry.

Be sure to tape the foam as an air barrier. Consider that the roof absorbing heat helps you in the Winter and better AC might be the best offset for continuing to use dark shingles. If you plan on using faced iso, use reflective foil covered if it doesn't cost much more.