When the winter design temperature is below 60ºF, the International Residential Code requires a dwelling to
have heating facilities capable of maintaining a minimum room temperature of 68ºF in habitable rooms.
Portable space heaters cannot be used to meet this requirement. A permanent heating system must be
installed.
In buildings we refer to heat flow in a number of different ways. The most common reference is the R-value, or
the resistance to heat flow. The higher the R-value of a material, the more it will restrict heat loss or gain. Ufactor (sometimes referred to as U-value) is a measure of the flow of heat (thermal transmittance) through a
material, given a difference in temperature on either side. In the inch-pound (I-P) system, the U-factor is the
number of Btus (British Thermal Units) of energy passing through a square foot of the material in an hour for
every degree Fahrenheit difference in temperature across the material (Btu/ft2hr°F or BtuH). In metric, the Ufactor is usually given in watts per square meter per degree Celsius (w/m2°C).
Calculations of heat loss are made to determine whether a proposed heating (or cooling system) is adequate to
supply and maintain the desired temperature within a structure as specified by code. These calculations are
also used to estimate the annual heating or cooling costs of a system.
● Use the 99.6% percent values from Appendix D of the International Plumbing Code or from the NOAA
Engineering Design Data publication (https://www.ncdc.noaa.gov/data-access/land-based-station-data/datapublications). Alternatively, you may use a design temperature that reflects local climate or local weather
experience as determined by the building official.
● The International Residential Code requirements/rule of thumb for a new house with good insulation is 1.25
watts per cu. ft. or 4.25 Btu per cu. ft.
● Important conversion information:
● Watts x 3.21 = Btu/hr
● Btu/hr x .2931 = Watts
Procedure
Heat Loss from the Utility Shed
You will calculate the total heat transmission load measured in Btu/H for the Utility Shed shown in Drawing A1.
Assume the following:
● For this exam the floor will be ignored
● One double door, 72in. x 7ft
● Two single-glazed windows, 2ft x 4ft
● Desired inside temperature of 70°F
● Outside winter design temperature taken as the 99% value (refer to Table 4.6 Page 110, Weather Data Sheet
provided, International Plumbing Code, or local data reports)
● Location: Syracuse, NY.
R-Value
Calculate the total R-value for each surface including the four walls, the door, the windows, and the ceiling/roof
structure. Use the R-value chart provided to find the R-value for the individual layers within the wall. Drawing
A1 shows the different layers that comprise the wall and ceiling/roof. Include an inside air film layer.