To use any of the new prescriptive paths the overall average U-value for
all the opaque doors in the project must be less than or equal to 0.20 Btu/hr-ft2-F°.
This U-value requirement can be met using a door with a default U-value assigned
from the default table in the code, or by using a door with a NFRC tested
U-value. More information on the National Fenestration Rating Council
and their certification protocol for glazing products and doors, including
a searchable database of certified product ratings is available at http://www.nfrc.org .
If a code default U-value is used, the only door type on the default table which can meet a 0.20 U-value is an insulated steel door with a thermal break (i.e. wood edge on the door, and installed in a wood door frame.) Various doors with NRFC test values are available. Many of the tested steel and fiberglass doors do meet a 0.20 U-value requirement; however none of the tested wood doors have U-value of 0.20 or less. Therefore, using the unlimited glazing prescriptive path it is, at first glance, impossible to use wood exterior doors.
However, there are alternatives available to designers who wish to include wood exterior doors.
1. A provision for only one exterior door of any construction having an area of 24 square feet or less is included in the code to allow for a custom front door.
2. Wood frame full-light doors may be used since they are classified in the glazing category rather than the opaque door category. An exterior door that is fully glazed is required to meet the prescriptive vertical glazing U-value requirement, rather than the 0.20 U-value requirement.
3. There is a code provision which allows for opaque doors to be included as glazing in the U-value and glazing percentage calculations. It will always work to your advantage to make use of this provision when using prescriptive path III (the unlimited glazing path). Since the glazing percentage is unlimited, it will not affect your compliance to include the doors in the glazing area, and instead of having to meet the 0.20 requirement for the doors, you may meet the 0.40 requirement for the vertical glazing instead.