"Dennis" wrote in message
news:_ZV7i.4$fX4.2@trndny03...
And why aren't manufacturers recommending this? They'd sell more
product.

Manufacturers are manufacturing this product, like all products they sell
to a specific market / end use. This product was designed to comply with a
specific code requirement, as an alternate to two layers of underlayment,
cemented together. It saves time and labor; money in other words.

However, that's the code, and most prescriptive codes are designed as
minimums and are usually exceeded when there is a good reason to do so.
The reason that the manufacturers do not state that it is to be used
everywhere is simple, whatever they put on the package is considered a
requirement, legally binding in court. Any local building official would
then be permitted to make a full installation mandatory.

That's just not true. Some building officials are power-mad bureaucrats.

The answer is probably two fold: it has little or no value in
practice, and second, ice and water shield is a vapor barrier, and
unless your ventilation is good, you could wind up with rotted roof
sheathing.

You're worng on both points. First it has obovious value in offering
additional protection to a roof deck. Only a fool would argue that
overkill has no value.

That's plain silly. "Overkill" needn't always be benign. Look at bolt
tortion, for a simple example. Ever heard of "too much of a good thing?"