Below grade is a term that describes a space that is below ground level – usually referred to as a basement.
The default U values for the Below grade walls are referred from:
ASHRAE Standard 90.1 (Energy standard for buildings except low-rise residential buildings) - table 5.5 when respective ASHRAE code version (2007, 2010, 2013, 2016 and 2019) is selected.
New York City Energy Conservation Code- table C402 when the respective code version (2016,2020) is selected.
Table 3-8 when respective California Title 24 code (Building energy efficiency standards for residential and nonresidential buildings) version (2016, 2019) is selected.
Division B-Part 3 when respective National Energy Code of Canada for Buildings code version (2011, 2015) is selected.
Table C402.1.4 from the Washington State Energy Code when one selects the Washington State Amendments (2015) or Seattle Amendments (2015)
Part J1 Table J1.5 from the National Construction Code, Building Code of Australia 2019 when the respective code version is selected
Table 3 Limiting fabric parameters from United Kingdom National Calculation Methodology when the respective code is selected
Since there are different types of material assemblies for Wall Cove.tool makes that assumption for users based on Pacific Northwest National Laboratory (PNNL) building prototypes. Cove.tool auto populates the U value based on the building type and location.
For example if your project (Apartment) is in Atlanta, GA (ASHRAE Climate Zone 3A) and the energy code selected is ASHRAE 90.1 2013, the auto populated value can be seen highlighted in the image attached below
Considering simplification for compliance by ASHRAE User's Manual, the c-factor can be used within the u-value tab for below-grade walls. C-factor is like a U-factor except that it does not include the interior air film, the exterior air film, or the effect of the earth.
Below Grade U value - It is a measure of the overall heat transfer for the walls below- grade of your construction. The lower the U value the slower the rate of heat transfer between the wall and the ground.
Below Grade Area - It is the total area of the Walls that lies below the ground of your building.
Below Grade Depth - It is the distance from the ground floor (Below grade) to the first floor (Above grade) of your building.