# R-Value and U-Value in HVAC

R-value and U-value are two important concepts in HVAC. R-value is a measure of a material’s resistance to heat flow. The higher the R-value, the better the insulation. U-value is a measure of a material’s ability to transfer heat. The lower the U-value, the better the insulation.

Equations

R-value:

Thermal resistance is a measure of a material’s ability to resist the flow of heat. The higher the thermal resistance, the better the material is at insulating. Thermal resistance is measured in R-value, which is expressed in Imperial units as Hr. Sq. Ft. °F/Btu and in SI units as m².K/W.

$$R = \frac{1}{C} = \frac{1}{K} \times \text{Thickness}$$

U-value:

U-value is a measure of the rate of heat transfer through a material. The lower the U-value, the better the material is at insulating. U-value is measured in Imperial units as Btu/Hr. Sq. Ft. °F and in SI units as W/m².K.

R-value and U-value are inversely proportional to each other. This means that the higher the R-value, the lower the U-value, and vice versa.

$$U = \frac{1}{\Sigma R}$$

where:

• R is the R-value (hr-ft²·°F/Btu)
• U is the U-value (Btu/hr-ft²·°F)
• C is the conductance (Btu/hr-ft²·°F)
• K is the conductivity (Btu·in/hr-ft²·°F)
• ΣR is the sum of the individual R-values

Units

R-value and U-value can be expressed in both Imperial and SI units.

Imperial units:

• R-value: hr-ft²·°F/Btu
• U-value: Btu/hr-ft²·°F

SI units:

• R-value: m²·K/W
• U-value: W/m²·K

The following conversion factors can be used to convert between Imperial and SI units of thermal resistance and U-value:

Example

A wall is made up of three layers: a layer of siding with an R-value of 0.5, a layer of insulation with an R-value of 10, and a layer of drywall with an R-value of 0.5. The total R-value of the wall is:

Rtotal = Rsiding + Rinsulation + Rdrywall = 0.5 + 10 + 0.5 = 11 hr-ft²·°F/Btu


The U-value of the wall is:

$$U = \frac{1}{\Sigma R} = \frac{1}{11} = 0.091 Btu/hr-ft²·°F$$

Conclusion

R-value and U-value are important concepts in HVAC. By understanding these concepts, you can better design and operate HVAC systems to improve energy efficiency and comfort.