GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Does Loosefill Attic Insulation Require an Air Gap Between Insulation and Roof Decking?

AtticAdventures | Posted in Energy Efficiency and Durability on

Does loosefill attic insulation (specifically fiberglass or cellulose) require an air gap between the insulation and roof decking to be effective? Once I’ve installed baffles to protect the soffit vents I would like to blow insulation all the way to the roof decking at the eaves on houses with shallow pitch roofs. My goal being of course to maximize R-value. Any reason not to do this? Thank You for sharing your knowledge.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. nrj | | #1

    I asked a similar question recently. I plan on blowing in cellulose in the rafter cavities with no ventilation. From what I understand you can do this but you then need to add insulated sheathing on the other side of the roof decking to make sure the decking doesn't get cold enough to cause condensation issues.

    From the California building code 806.4.5.2: Air-permeable insulation only. In addition to the air-permeable insulation installed directly below the structural sheathing, rigid board or sheet insulation with an R-value of R-4 shall be installed directly above the structural roof sheathing for condensation control.

    You may need to increase the R-value depending on your location.

    The smart guys on this site will hopefully back me up on this one.

  2. GBA Editor
    Martin Holladay | | #2

    One way to categorize roof assembly types is according to whether they are vented or unvented. If the assembly is vented, you need to have an air gap between the top of the insulation and the roof sheathing. (This would be the case, for instance, in most attics.)

    Once you fill that gap with insulation, you are building an unvented roof assembly. (This is typically done in some cathedral ceilings.) If you go the unvented route, you are restricted to either spray polyurethane foam under the sheathing or to a a hybird approach that includes rigid foam insulation above the sheathing. More details here: How to Build an Insulated Cathedral Ceiling.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |