How to determine if solar reflective asphalt shingles are worth it?
I’m building a small house (800 sq ft) in Phoenix, AZ. That matters a very great deal because it gets egregiously hot here on a very regular basis (109 F today) and the sun is omnipresent. I have decided to roof the house with asphalt shingles, largely because the metal and tile options are many times more expensive, for likely little benefit to me. The question I’m left with is what shingles to get… and specifically, does it make sense to spend more on solar reflective shingles? HOW do I make that determination?
The house already has a radiant barrier on the roof — the sheathing is OSB with a radiant film on the underside of it. The house will also be very thoroughly insulated — R50 or R60 (largely depending on what I can fit) over the ceiling. I’m shooting for rather extreme air sealing. Cooling (and heating, but that almost doesn’t even matter) will be handled via a mini-split, so no ducts will be in the attic.
Okay, there are roughly three “levels” of SR shingles — the base level typically has an SRI of 17-21. The next level is Energy Star certified and so needs to start out with SRI 25 or higher. Then there are a very few candidates that are in the SRI 40+ camp, which is getting solidly into even metal roof territory! I haven’t yet gotten any real quotes on any of them but general articles on the topic suggest that the prices (compared to non-SR shingles) will be more expensive; much more expensive; and a lot more expensive, resp. I honestly wouldn’t be surprised if the SRI 40+ lines are in the metal roof pricing territory, too.
Does it make sense for me to get any of them? My reading is coming up inconclusive. In general, Energy Star says that an SRI 25+ shingle roof will save between 7%-15% in energy costs for a year. Those are nearly useless numbers to me since that doesn’t say how many cooling days there are; how much sun; and if other tactics like radiant barriers and much-higher-than-code ceiling insulation are in play. Honestly, I’m not finding any evidence either for or against if a SR shingles will have literally ANY appreciable effect on my house.
I have found some articles that make the blanket assertion that no, it wouldn’t make sense for me. Alas, they give zero evidence to support that assertion.
So… if there is obvious “yes” or obvious “no” to if it makes sense in my case, then I’d like to hear it and (more importantly) know why it is obvious. If it’s not obvious, then what steps need taking to determine if it is worth it?
Thanks
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If you can find the absorptivity and emissivity numbers you can enter them along with cost per square foot and computer model your house with different insulation values. You will likely spend 20 + hours to watch the training videos and build the model but it is the only way to get a real answer and not a guess/estimate.
https://beopt.nrel.gov/
Walta
Oh! I forgot about BEopt. I did spend the time early on in the process to make an accurate (at the time) model of the house, just to see what kind of effect certain wall constructs would make as well as what would happen if literally 40% of the North wall was glass. I didn't look too carefully at the roofing material, though, and I kind of forgot that it even existed.
Time to dust off my model!
One thing to keep in mind is that you are in a climate that probably represents less than about 1% of the US population. What I'm saying is that if there is any advantage to those types of shingles then your climate would be where it applies. I wouldn't overlook that being in an atypical climate is often disregarded in articles that give general advice. I think you should go for it since you are doing everything else to resist heat infiltration and ignore general advice on those types of shingles by people who don't live in your climate.
Make sure the attic insulation is blown in cellulose. It is known that fiberglass insulation doesn't work well in hot attics. It's also greener. Also, consider tile floors for their coolness.
Thanks. Plus, check and check for the blown cellulose in the attic and tile floors.
Modeling it correctly is dependent on many details. For example, roof slope and wind effect how hot the shingle gets. SRI/heat also effects shingle life - probably a bigger $ effect than the AC cost.