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Old 07-24-2021, 09:19 PM   #16
John Mercier
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They aren't comparable because they are not used for the same thing.

One is an ice & water shield, the other is a synthetic acting something like a housewrap (Tyvek).

Ice & Water does not allow for transmission in either direction, while the synthetic allows for water vapor to move from one side to the other.

The synthetic underlay is like a high tech felt.

Synthetic is usually used on steeper sloped roughs (5:12 over above) with an ice & water shield applied to the last three feet near the eave. The I&W protects the hot/cold transfer spot were ice dams are most likely to occur.

It can be used on low slope roofs (heated or cold) to stop wind-driven penetrations. But rolled roofing is preferred to prevent shingles from getting wind damage when applied at such a shallow angle. This would be like the GAF Liberty.

But a lot of determining the best product is the slope, what the waste factor will be, and what the final product will look like.
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