PPD Practice Exam - K value

Hi,

I’d like to get help on this question – in terms of concept, the explanation makes sense. K-value and R-value can be understood to have an inverse relationship. However, the ARE also gives the equation “R = x / k” in their exam references. Under this equation, 8" concrete with K-value 10 does have a higher R-value (R-0.8) than 4" brick with K-value 8.4 (R-0.47). There seems to be a conflict between these concepts. Any help would be appreciated!

Thank you!

HI @samantha.lee ! Good question, @coachlizschneider are you able to provide some clarification?

Great question! You are 100% correct. K and R do have an inverse relationship. And while generally a lower k-value will give you a higher R-value, thickness does play a role in this. Generally increasing the thickness of the material will also increase the R-value of the material. So both k-value (conductivity) and thickness are important in determining the material that has the best thermal resistance (R-value). The formula that you identified best explains this relationship. R=x/k, where x is the thickness of the material. So for this question, 8 inches concrete, k-value = 10, would be correct.

@monicafitzpatrick we should correct this on our practice exam.

1 Like

I’ll get that fixed! Thanks!

6300 N. Northwest Highway P.O. Box 31846, Chicago, IL 60631-9998 / All rights reserved © 2020 Black Spectacles.