GARAGE DOOR

I have decided to have a new insulated garage door installed.
My next concern is to chose the R-rating I need for my locale.
I live in Arlington, Texas. A city between Dallas and Fort Worth.
The winters here are not too awfully cold, but the summers can have many days of 100 degrees or hotter.
I've seen doors with R-rating from 5 to 18. I guess the 18 would be best, but in my area do I really need that high of a number? Will the 5 or 9 rating be enough to keep my garage comfortable?
Thanks,
Hound Dog
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I used to live in Dallas, and a regular wood overhead door worked fine, but the door was facing north, so it never had direct sun on it. If the garage is unheated and unairconditioned, then there is no way to get it comfortable, but I would go with the lower R values, as they are cheaper and work fine, unless you get full sun on it - then you might go for the higher value.
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