An Insulating Challenge: Scissor Trusses and Sloped Ceilings

TDSA LVING-GARAGE SECTION 11-10-2020Architect Terry Smith of TDSA Architecture in Washington state recently contacted NAIMA with a question about how to properly insulate various types of roof trusses – both conventional and scissor type. Terry rightly pointed out that insulation is intended to be most effective when uniformly applied or layered to cover the full extent of the space being insulated. But there are instances, such as narrow areas near the top plate of scissor trusses and sloped ceilings, where that is not possible.

Terry could not find any guidance in his state’s building energy code to address how to properly insulate the narrow areas near the top plate particularly of scissor trusses where the area is very narrow. Similarly, fully insulating sloped ceilings to meet code requirements can be tricky. There is no specific guidance in the code to specifically address how to do either properly.


General Guidance on Insulating Narrow Areas

Generally, if R-38 is the code requirement, you can install the full height R-38 for the areas where there are no height constraints, and then fill the areas that have constrained heights (but making sure to install eave baffles). Alternatively, you could have a raised heel truss and install a minimum of R-30 across the entire attic floor per R402.2.1 of the 2018 IECC (similar provisions exist in other versions of the IECC).


Guidance for Sloped Ceilings

Insulation Institute spoke with Johns Manville Research Manager and Engineer Francis (JR) Babineau regarding whether state codes provide specific guidance on insulating sloped ceilings. After conducting his own search for answers from state building energy code guidelines, J.R. noted the only written information he found relative to insulating sloped ceilings was from the state of California, which says:

“If the ceiling is sloped (for instance, with scissor trusses), loose fill insulation can be used as long as the slope of the ceiling is no more than 4:12. If the ceiling slope is greater than 4:12, loose fill should be used only if the insulation manufacturer will certify the installation for the slope of the ceiling.” See the reference link here:!Documents/35insulationproducts.htm

Bottom line: the insulation manufacturer must certify the installation for the slope of the ceiling. But is that happening or do code officials simply sign off on the presence of insulation – regardless of whether it is or can be installed at the required thickness given the slope?

When the Codes Do Not Specify

While state building energy codes can provide prescriptive guidance, properly insulating sloped ceilings and scissor trusses – appear to be areas where additional guidance would help clarify that the intended code requirement is met. The absence of prescriptive guidelines could leave much to individual interpretation and put the onus on manufacturers to address a problem that could be avoided with proper guidance from state building energy codes.

We would like to hear from our readers what their own experience has been regarding these issues. Please comment on this post.




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