R-value

R-value is a term that gets used a lot, but what does it mean? How does it affect your home? And how is it calculated? 

This comprehensive guide to R-value will answer these questions! 

R-value

R-value is a measure of thermal resistance. Essentially, it is a unit of measure based on the amount of heat that is transferred through any given material. The higher the R-value, the less heat that is transferred. 

U-value- the opposite of R-value, it is a measure of thermal transmittance. The higher the U-value, the more heat is allowed to pass through a given material, and the lower R-value. 

Left: High U-value, low R-value;  Right: Low U-value, High R-value

Effects of R-value

The overall R-value of your home indicates the quality and consistency of the insulation. Satisfying recommended R-values leads to:

R-value recommendations

It is best to meet these recommendations to receive the benefits listed above. Exceeding these recommendations will provide little value to the homeowner, and too much insulation can lead to ventilation and moisture issues.

Calculating R-value

The tricky part of R-value- a well-meaning homeowner may pile insulation into their attic, but heat will still be transferred through unsealed and uninsulated parts of the attic. Thermal bridging occurs through materials such as ceiling joists, and an uninsulated attic hatch may cause even more heat loss. 

Example: This is from a recent home I inspected. This home was 1,200 sf and had an attic access hatch of about 10 sf that was uninsulated. The attic insulation had an R-value of about R-38. For this equation, we will assume the attic was perfectly sealed with no thermal bridging. Also, we will assume that the R-value of the hatch was R-1, which is rounding up in this situation.

To find the R-value, we must first find the average U-value. 

For this example, we have 1,190 sf at R 38, and 10sf at R 1

The formula is    ((1/(r-value 1) x (area of r-value 1)) + (1/(r-value 2) x (area of r-value 2))) / (total area)

      ((1/(38) x (1,190)) + (1/(1) x (10))) / (1,200)= .0344

We then use this formula to solve for R-value:  1/(average u-value)

1/0.0344= R-29

That is a 9 point drop in R-value with a small attic access hatch!!

In this article, we defined R- and U- values, described their importance to your home, and provided a basis for calculating these numbers. Questions? Give us a call at (918) 530-6833 or email at jphelan@superiorjoes.com.