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NC State Extension

Attic Decking – Defined

Attic decking is often used to provide additional storage space or a platform for an HVAC unit or hot water tank installed in the attic. The decking is often installed directly on top of the ceiling joists; this limits the amount of space available for insulation and lowers
the attic’s average R-value.
To ensure proper insulation depth, the attic decking must be raised above the ceiling joists. To accomplish this “edge-nail” 2×4’s or 2×6’s to the tops of the ceiling joists where the decking is to be located or, as an option for trusses, lag 2×4 members to the truss webbing at the desired height. Install the decking securely to the top of the raised lumber after the insulation has been installed.

For many products, an insulation depth of 10 to 14 inches is needed to achieve an R-30 to R-38 insulation value. Thus, a 2×4 or 2×6 extension needs to be added to a 2×6 joist to provide sufficient depth before installing decking.
Source: Technology Fact Sheet