Where should insulation and vapor retarders be installed in an unventilated attic?

In an unventilated attic, the proper installation of insulation and vapor retarders is crucial for maintaining energy efficiency and preventing moisture problems. Insulation should be placed above or below the roof sheathing and at gable walls because this placement serves several important functions.

When insulation is installed above the roof sheathing, it helps to minimize heat loss from the living space below while also reducing the risk of ice dam formation in colder climates. This setup keeps the roof's temperature more consistent, preventing the snow from melting too quickly and then refreezing at the eaves. Similarly, having insulation at the gable walls helps to protect these areas from moisture intrusion and thermal bridging, ensuring that the entire roof assembly remains well-insulated.

Additionally, vapor retarders play a critical role in this context. They should be placed in locations where they can effectively prevent moisture from migrating from the living space into the attic area, which could lead to condensation issues within the insulated areas of the roof assembly. By positioning them along with the insulation in the area above or below the roof sheathing, you create a barrier that helps maintain a dry environment in the attic.

Placing insulation on the attic floor or interior walls is not advisable in unventilated attics due