Here's why:

* U-value measures heat transfer: U-value represents the rate at which heat flows through a material. It's a measure of thermal conductivity.

* Vapor barriers are primarily for moisture control: Their main function is to prevent water vapor from migrating into building assemblies. They're not designed to resist heat flow.

How Vapor Barriers Impact U-Values:

While a vapor barrier doesn't have a U-value, it indirectly influences the overall U-value of a wall or roof assembly:

* Preventing moisture buildup: By stopping vapor intrusion, vapor barriers help keep insulation dry. Dry insulation performs better thermally, resulting in a lower U-value (better insulation).

* Improving thermal performance: In some cases, a vapor barrier might be incorporated into a reflective barrier system, which can contribute to a lower U-value by reflecting radiant heat.

Key Takeaways:

* Vapor barriers are not rated for thermal performance (U-value).

* Their primary function is moisture control, which indirectly impacts overall thermal performance.

* U-value is determined by the entire building assembly, including insulation, cladding, and air gaps.

To determine the U-value of a wall or roof assembly with a vapor barrier, you need to consider the specific materials used and their respective thermal resistances. A building professional can help you calculate this accurately.