Re: PEX in the attic
Agreed. Since the pipe is between the valve and the showerhead, water should only be in the pipe when the shower is in use. The OP mentioned "diverter valve" but the way I read the post, that's probably incorrect terminology. I'm guessing there isn't a tub spout. As long as the pipe is sloped to minimize the amount of water that can remain in the pipe in unheated space, there should be little chance of damage. Sloped properly, most of the water will drain out when the shower is shut off. Placing insulation above the pipe should be sufficient.
Originally Posted by MSSP
PEX is freeze-resistant. The pipe will not burst or be damaged even if frozen solid. However, the fitting where the shower arm meets the PEX will have to be inside the attic for appearance sake, but if it's placed close to the sheetrock with plenty of insulation above it shouldn't ever freeze. (Fittings could be damaged by freezing, even if PEX pipe wouldn't be.)
The "Senior Member" designation under my name doesn't mean I know a lot, it just means I talk a lot.I've been a DIYer since I was 12 (thanks, Dad!). I have read several books on various home improvement topics. I do not have any current code books I can refer to. I was an apprentice plumber for two years.