Efficient roofs
rodmartin77
| Posted in Energy Efficiency and Durability on
Aloha,
My name is Rod and I live in Waialua on Oahu. It gets pretty hot here. We have vaulted ceilings and are about to replace the roof so we can put a new solar hot water system and 18 pv molules up there. What roof would you use to repel heat from coming in the house?
Thank You,
Rod Martin
808 854 9636
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Replies
Rod,
To keep heat out of your house, you need to install a roof assembly that includes insulation.
If for some reason you don't want to include insulation, at least choose a "cool roof" (that is, roofing with high solar reflectance and high thermal emittance).
In some cases, including a vent space between the roofing and the sheathing can help -- but venting is never as effective a measure as including insulation.
The portion of the roof with the PV now has a nice shading factor on the roof-proper, as long as there's sufficient ventilation space between the PV and roofing.
A CA Title 24 compliant cool roof material or coating relevant for your roof pitch would work. The highest solar reflective index (SRI) goods tend to come in one color- BLINDING WHITE, which is restricted by code in some locations. But there are an array of pretty-good products that reflect primarily in the infra-red spectrum. There are searchable third-party tested products listed on:
http://www.coolroofs.org/
http://www.coolroofs.org/products/search.php
The higher the aged SRI numbers, the better the heat rejection. For anything other than white you won't find anything with a 3 year aged SRI >80, but there are some off-white finishes with SRI >70.
In a HI climate adding mass to the roof can be almost as important as adding insulation- ceramic and concrete roofs do quite well, but if the house wasn't designed for it a retrofit would be a bit daunting on cost. If metal roofing is an option for you (not cheap, but not heavy like concrete roofing), there are some very high SRI finishes >80 for metal roofing out there.
Thank You
Mr. Martin:
This article might also be of some help to you:
http://www.rdmag.com/news/2012/09/ornl-roof-and-attic-design-proves-efficient-summer-and-winter
Aloha!