R-Value of Foam Insulation and Fiberglass Insulation

To follow up on the previous topic, this blog will be about the relative R-value of spray on foam insulation compared to fiberglass insulation when they are tested in high temperatures.  This coincides nicely with the previous blog.

I want to make one side note before I continue with this blog.  I’m an insulation contractor, so it might seem odd that I’m talking about faults and or short comings of different types of fiberglass insulation.   But no matter what industry I have worked in, I’ve always believed that educating people was the number one priority.  I’m writing these blogs in an effort to educate people on the benefits of foam insulation.  Being an Austin insulation contractor we receive a lot of questions like “why is spray in foam insulation better than fiberglass”, “what is the difference between blown in fiberglass and foam insulation”, “why is the spray foam insulation R-value lower than fiberglass batt insulation”, as well as a variety of other questions regarding insulation.  So, I write these blogs to create a resource to send my customers to in an effort to educate them.  Also, in the end, I think all insulation products can provide benefits in certain situations, but my goal is to identify the differences, advantages, benefits and disadvantages of different types of insulation.

Alright, lets get back to the topic at hand.  If you haven’t read the previous blog, read about the R-value test because I will reference it in this blog.  When the R-value test is performed the testing room temperatures are moderate, somewhere between 70-80 degrees.  However, most of the time wall cavities as well as attics (as discussed in the last blog), reach temperatures much higher than 70-80 degrees.   Once temperatures reach over 110 degrees the relative R-value of blown in fiberglass insulation starts decreasing.  As the temperature rises above 110, the relative R-value continues to decrease.  The relative R-value can decrease up to 50% if there is a large enough temperature difference.   Depending on the wall construction it is very easy for the temperatures in wall cavities to reach over 110 degrees which is when the relative R-value starts decreasing with fiberglass batt insulation.


When spray in foam insulation is used the attics don’t reach the extreme temperatures compared to when blown fiberglass is used, however, foam insulation is under heat stress.  Depending on the roofing system installed when foam insulation is used, roof decks can get extremely hot.  So, spray on foam insulation is still in contact with extreme temperatures, however the relative R-value of polyurethane foam does not decrease as temperatures increase.   The same case can be said for spray foam insulation in walls cavities.  The relative R-value of foam insulation does not decrease in the wall cavities, so spray foam insulation is still providing the same R-value at higher temperatures.  This is another reason why spray foam insulation out performs fiberglass insulation.


Whatever your insulation needs are, please feel free to call Best Insulation we are an Austin Insulation company, and have a great reputation to stand by.