If I were to have a standing seam metal roof with radiant barrier would that adversely effect PV performance if panels were mountedusing S-5! standoff mounting? Any thoughts or experience?

There won't be much of a difference in performance no matter how reflective/absorptive the roofing material is. Mounted on standoffs or standing seams the PV panel is back ventilated, and convection cools. The portion of roofing under the PV panel is by definition in the shade, so it won't have much radiated heat flux to reflect OR absorb.

Thanks Dana. Have been lurking for a while as my project goes through desgin development and I have come to appreciate your knowledgeble contributions to these forums.

Do you have any opinons on the cool roof coatings such as the Hyperseal line?  http://hypersealinc.com

Cool roof matierials & coatings are all about the ratio of their solar reflection & infra-red emmissivity. I browses the Hyperseal site quickly in search of a spec sheet, to no avail. Properly formulated these products work reasonably well in cooling dominated climates. There's an Energy Star online calculating tool to guesstimate the performance in your application if you have the specifications for the product:

http://www.roofcalc.com/RoofCalcBuildingInput.aspx

The relative importance of cool-roofs varies by many factors other than climate- the pitch of the roof is HUGE. The lower the slope, the less convective cooling you get, so the greater the importance of radiant rejection. Cool roofs are mandatatory for re-roofing & new construction now under California Title 24 (or will be, in a coupla weeks), but specifications vary with roof pitch, shading factors, etc. See:

http://www.energy.ca.gov/title24/toolkit/documents/presentations/2008_Standards_Cool_Roofs.pdf

If the price is about the same, go with a white roof in any situation that uses air conditioning.

It depends- in heating dominated climates with snow-country pitched roofs the net savings of cool-roof materials will be negative due to lower solar gain during the heating season raising the average heating load. (This is well modeled, and somewhat built into the DOE calculators.)

I don't think so - I'd like to see figures.

Knock yourself out:

http://www.ornl.gov/sci/roofs+walls/SteepSlopeCalc/index.htm

Whether it's a net savings or a net loss depends on the average R-value of the roof, the cost of electricity & efficiency of your AC, the cost of heating fuel & efficiency of the heating system, but in say, Grand Rapids MI, assuming 10cent/kwh & 2.0COP, $1/therm & 80%AFUE and a typical R30 roof,  high reflective (SRI>50)/high-E (E>70) roof is cost-negative.  The lower the whole-roof R value is the bigger the net loss.

Bump the electric rates to 20 cents/kwh, it's cash postive.  Lower the R value, it's cash negative.  It's all small-money per foot, but even when there's saving to be had in heating dominated climates with high electric costs, it's not worth the time to even look up the the reflectance & emissivity numbers.

But move the same numbers to Tucson or Phoenix it's reliably a savings, even on high pitched roofs.

> in heating dominated climates with snow-country pitched roofs the net savings of cool-roof materials will be negative

The later post is getting closer to accurate - it may or may not be negative depending on the specifics (and which model you use).

> when there's saving to be had in heating dominated climates with high electric costs, it's not worth the time

Try http://www.roofcalc.com/RoofCalcBuildingInput.aspx (for a commercial building) and find thousands of positive $ in lifetime savings for most areas (even in Grand Rapids).

Gee Jon, from the http://www.roofcalc.com/RoofCalcBuildingInput.aspx calculator what does

"Conditioned flat roof area (sq. ft.)" "(Min: 3,000 Max: 1,000,000) "

as compared to my comment:

"It depends- in heating dominated climates with snow-country pitched roofs the net savings of cool-roof materials will be negative due to lower solar gain during the heating season raising the average heating load. (This is well modeled, and somewhat built into the DOE calculators.)"


???

(The latter to which you responded " I don't think so - I'd like to see figures." )

There's little question about what happens with flat roofs, particularly LARGE flat roofs where convection cooling is all but absent.

To me flat means flat, and "snow-country pitched" roofs means something considerably more than 2:12 ( CA Title 24's definition of "low pitch", which may be common in parts of the south, but not the most common in private MI residential structures.) Did I need to define the terms better?

You'll note I was responding to your comment of,

"If the price is about the same, go with a white roof in any situation that uses air conditioning."

To me "any" means"ANY, all", not "flat roofed commercial buildings anywhere". If you were restricting the statement to large flat roof anywhere, I might have responded differently.

Even in cooling dominated climates the savings of cool roof materials are small-to-inconsequential for roof pitches over 2:12, and savings are often negative in heating dominated climates when some of the solar gain of darker material lowers the heating load.

DO take a look at the rational methods & restrictions built into CA code: http://www.energy.ca.gov/title24/toolkit/documents/presentations/2008_Standards_Cool_Roofs.pdf (There are some issues to be argued in there too, especially on retrofits.)

BTW: "Steep Slope" in the DOE and CA Title 24 parlance means anything over 2:12 pitch, not restricted to 12:12 and above or anything, and the average recommended solar reflectance for pitches above based on modeling is 0.25initial (aged, 0.15), vs. 0.65 (aged 0.50) for low pitch roofs.

An aged solar reflectance of 0.15 is a 85% solar absorber (which would be better than a glazed solar collector's performance.)