Page 9 - CodeWatcher Fall 2016 Issue

Option 1: “Prescriptive” NO TRADE-OFF
Ceilings with Attic Spaces

Insulation at full Raised heel trusses (aka, energy truss)
thickness over allow for the full uncompressed R-value to
be over 100% of the ceiling area and
exterior extends over the wall top plates,
therefore quali?es the ceiling to reduce
wall top plate the R-value as follows:

Insulation •?R-30 complies where R-38 is required
•?R-38 complies where R-49 is required
Credit: DOE’s Building Energy Codes University

Insulation
All photos courtesy of TopBuild
NOTE: The U-Factor Alterna(ve approach to
mee(ng the code does apply to this
Spray FoamreRdoouf Acs(soemnb lay lterna(ve.

Figure 2

Look at Figure 2 above. In this scenario, a raised heel truss Vented Roof Slope Assembly
(or energy truss) allows for the full uncompressed R-value to
be over 100 percent of the ceiling area while extending over Dense Blown
the wall top plates, therefore qualifying the ceiling to reduce Roof Slope
the R-value as follows: Assembly

¦¦ R-30 complies where R-38 is required.
¦¦ R-38 complies where R-49 is required.
It is the top plate that is the defining factor, if you insulate
over the top plates you can reduce the R-v alue over the entire
ceiling assembly.

Ceilings Without Attic Spaces

When you are building homes where there is no attic above
the ceiling (slopes, roof assembly insulated, etc.), there are
a few things to consider.

Where the insulation levels are required to be greater than
R-30 (i.e. R-38, R-49, etc.), there won’t be sufficient space
to meet the higher levels. R-30 is allowed for 500 ft2 or 20
percent of the total insulated ceiling area, whichever is less.

If the ceiling in question does not meet these requirements
then either the U-Factor or UA Alternative will need to be
calculated to determine compliance.

If all of the above methods don’t work, the Performance
Method must be used to determine compliance. This is
where more trade-offs are allowed and help get the building
in compliance with the energy code. For those states and
municipalities who have adopted 2015 IECC, there is one
more alternative which is the Energy Rating Index (ERI). CW

Stephen Carr is Regional Building Science Manager for
TopBuild Home Services. Have a question?
Email him at Stephen.carr@topbuild.com

www.codewatcher.us Fall 2016 / CodeWatcher 9