Page 61 - Green Builder November Issue Codes Update

Option 1: “Prescriptive” NO TRADE-OFF www.greenbuildermedia.com/building-science-central
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:

•?R-30 complies where R-38 is required
•?R-38 complies where R-49 is required
CREDIT: DOE’S BUILDING ENERGY CODES UNIVERSITYInsulation

Insulation
ALL PHOTOS COURTESY OF TOPBUILD
NOTE: The U-Factor Alterna(ve approach to
Spray FoammRoeoef (Asnsegm btlhy e code does apply to this

reduc(on alterna(ve.

Figure 2 Vented Roof Slope Assembly

of the ceiling area while extending over the wall top plates, therefore Dense Blown
qualifying the ceiling to reduce the R-value as follows: Roof Slope
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-value over the entire ceiling
assembly.

WCEILINGS WITHOUT ATTIC SPACES
HEN 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 (e.g., 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). GB

Stephen Carr is regional building science manager for
TopBuild Home Services. Have a question?
Email him at Stephen.carr@topbuild.com

www.greenbuildermedia.com November/December 2016 GREEN BUILDER 59