Insulation Entirely Above Roof Deck

Insulation entirely above the roof deck.

Insulation entirely above roof deck.

Insulation entirely below the roof deck. Ashrae 90 1 defines a roof with insulation entirely above deck as a roof with all insulation 1 installed above outside of the roof structure and 2 continuous i e uninterrupted by framing members table 1 compares prescriptive requirements of the 2004 and 2007 editions of ashrae 90 1. Placing all of the iecc code required insulation above the roof deck with a vapor retarder will provide acceptable hygrothermal performance in all climate zones figure 7. The ratio of insulation above the roof deck or air permeable insulation below the roof deck is a matter of climate and interior conditions.

Also the higher the indoor humidity the more insulation is needed to avoid condensation. Generally more insulation is needed for condensation control in colder climates.