U-Factor/R-Value




U-Factor stands for the overall heat transfer coefficient and it is representative of a material’s ability to conduct heat. Similarly to thermal conductance, a higher U-factor value has a higher ability to conduct and transfer heat. U-factor is related to thermal conductance by the following formula.

Heat Transfer U-Factor for the Mechanical PE Exam

This equation assumes that U does not vary based on temperature. For purposes of the exam, this is a safe assumption.

R-Value stands for thermal resistance and it is representative of a material’s ability to resist heat. This is opposite of the U-Factor and thermal conductance which are measures of a materials ability to conduct heat. The relationship between the R-Value, U-Factor and thermal conductance is shown in the following formula.

Heat Transfer R-Value for the Mechanical PE Exam

This equation assumes that R does not vary based on temperature. For purposes of the exam, this is a safe assumption.

R-values are typically used in the HVAC and Refrigeration field to describe building insulation and materials. For example, insulation manufacturers provide product data for their various products and the key value shown on the product data is the R-Value based on different thicknesses.

Heat Transfer R-Value for the Mechanical PE Exam

Notice that the unit R-Value is 5 for 1” of insulation. The corresponding R-values for various inches of thicknesses are found by simply multiplying the thickness in inches by the R-value for 1” of insulation, refer to the below equation.

Heat Transfer R-Value for the Mechanical PE Exam

A must have skill for the aspiring professional engineer is to be able to calculate the overall heat transfer coefficient, U-factor for a wall, roof, duct or pipe. The method in which the overall heat transfer coefficient will be described through this wall example.

Heat Transfer R-Value for the Mechanical PE Exam

It is important to be able to follow the flow of heat from the beginning to the end of this diagram. (1) The first method of heat transfer is convection, warm outdoor air moves across the outer surface of the concrete wall causing the outer surface of the wall to heat up. There would also be radiation loads acting upon the surface of the wall, but for simplicity it is assumed that there are no radiation loads. (2) Next the heat travels from the outer surface of the concrete wall to the inside surface, (3) then to the outer surface of the insulation and through the insulation,(4) then to the outer surface of the gypsum board and through the board. (5) Finally the outer surface of the gypsum board transmits heat both convectively and through radiation to the indoor air.

In order to find the overall heat transfer coefficient, all of the resistances must be summed. It is the opinion of the author, that each method of heat transfer should be converted to its equivalent R-Value in order to keep it simple.

Heat Transfer R-Value for the Mechanical PE Exam

Heat Transfer R-Value for the Mechanical PE Exam