Trying to decide on a roofing material for a new roof.  I live in inland southern california in a mostly hot, dry climate.  How would the energy efficiency of an architectural asphalt shingle roof (I would probably use a cool shingle such as the ones from owens corning) with say, 2 inches of polyiso board used as decking underneath compare to that of a standing seam metal roof?

I also found this decking product by Rmax that employs a vented and insulated structure:

http://www.rmaxinc.com/roofing-nailable-base3.asp

I'm referring to the lower photo in the link.

Thanks

If it's a comparison between:

A:  Cool-roof shingles over 2" of iso (maybe vented, maybe not)

B: Standing seam metal (not cool-roof presumably mounted on purlins), no iso, solar absorption/IR-emissivity unspecified

A wins the energy race every time- even if you went with cool-roof metal like Galvalume, etc.  That much iso adds ~R13 to the insulation layers, and cool roof materials will run cooler to boot.  If the pitch of the roof is above 2:12 the relative importance of cool-roof material falls off dramatically due to convection cooling of the roof, but the additional R13 makes a difference. 

How MUCH of a difference this iso make depends on what other insulation you have between the roof deck and conditioned space or ducts.  If you have AC ducts & air handlers in the attic above the ceiling insulation the effect of the R13 will be very significant.  If you have R50 of cellulose in the attic and all AC ducting & equipment is within conditioned space, it might be measurable on the electric bill, but only if carefully correlated with cooling degree-day data for the billing period (real, but not huge.)

Unless the roof has some real pitch to it the cooling effect of ventilated vs. unventilated nailer deck will be very slight, and at pitches below 2:12 probably indectectable- a 3rd or 4th order effect.  (If below 2:12 CA Title 24 2005 required cool roof materials.  Depending on your CA climate region there are limitations on what you can use on steep roofs as well, although it's nowhere as strict as in low-angle or flat roofs.)

Dana, thank you for the excellent reply.  I don't know the exact pitch of my roof, but here's a picture so you can get an idea:

http://img199.imageshack.us/img199/...urrent.jpg

The house is about 2600 square feet, and about half of it has attic space.  The air ducts are in the attic space.  The other half is open beam cathedral ceilings, which is why I was thinking insulating the roof would be a good idea.  I'm very interested to hear that the polyiso and cool asphalt shingle combo would likely give me better energy efficiency.  I originally wanted a standing seam roof, but I'm not sure of how it will look on the house.  I think asphalt shingles would fit the style of the house better, but I know they're not as good as metal (when used without polyiso) in terms of efficiency.  I did look into pricing on a standing seam metal roof a bit, and it sounds like it will be pretty expensive - I think I'm looking somewhere in the $30Ks.  Any idea what I can expect to pay for 2 inches of polyiso and cool shingles?

Now that you can see the pitch of my roof, do you think the venting won't make a significant difference?

Thanks again for the informative reply.

That looks to be about a 4:12 pitch, in which case using a ventilated nailer deck will improve energy efficiency slightly, as well as the longevity of the shingles due to a slightly lower peak temps of the shingles themselves.

Installed pricing for materials is very local-market dependent- I wouldn't want to speculate.  As for the iso with nailer deck already installed, there are several vendors to choose from (Atlas, Hunter, etc.)- it's a competitive marketplace.  Shop it around.  As for the raw material, fiber-faced iso in my neighborhood (no nailer deck) runs ~$0.55-0.60/board-foot ($55-60 per "square", in roofer's parlance) f.o.b. the distributors warehouse (so a 4x8 sheet of 2" is 64 board-feet, or  $35-38/sheet + delivery + installation.)  I haven't priced the stuff with nailer decks applied recently, but IIRC its on the order of ~$10-12/sheet additional  (independent of iso thickness), call it roughly $65-70/square.  But I could be all wet on this- like I say, I haven't priced it recently.

With ducts in the attic space you should see a measurable reduction in cooling costs with an insulated roof deck if you can correlate power use against cooling degree-day data.  Peak attic temps will likely be several tens of degrees cooler.  If you currently have the typical R19 batts between attic floor joists your primary insulation, it's early afternoon performance with a hot roof deck is actually around R10 due to the thermal bridging of the joists and infra-red penetration of the insulation radiated from the hot roof deck (which is why  radiant barriers are sometimes useful.)  The ducts themselves are surrounded by air that's over 110F, probably over 130F  some days.  With 2" of iso above the roof deck there is no thermal-bridging of the rafters, and you get the full R13 out of it, so you'll have more than doubled the effective R-value of the roof, and the space where the ducts live will now rarely exceed 90F.  On a single-story house and latitude like yours the roof accounts for about a third to half of the total solar gain, and this should cut your kwh/CDD use by at least 15%, and could be as much as 25%.  (There are models out there that could predict the load reduction with some degree of accuracy, if you want to be obsessive about it.)

You have sufficient roof pitch that cool-roof materials by themselves won't be a huge factor, but it certainly won't hurt.  CA Title 24 2008  has a spec for your local climate zone for just how dark you can go, but using low-absorption high-emittance roofing doesn't seem like it would pose a big style/aethetic restriction for that house.  I'm sure the roofing contractors are all over what is/isn't allowed in your circumstances under the new rules. IIRC if you improve the insulation by some fraction (and I'm sure you'll be achieving that with 2" of iso) the cool roof rules get relaxed, but there's no reason not to do both.

what about hardie panel and magnum board?