Spray Foam Insulation R-Value

*Warning*- This is a series of blogs regarding R-value of insulation and Air Barriers in reference to different types of insulation.  The first blog will be about R-value of different insulation types.  There are a lot of comparison of spray foam insulation R-values to fiberglass, or blown in fiberglass insulation

R-values.  Unfortunately current R-value test don’t accurately compare spray in foam insulation to fiberglass insulation or the like.  For brevity sake, going forward, we will consider the following as interchangeable; fiberglass batts, blown in fiberglass ad fiberglass blown-in-blankets (BIB’s).

Conventional R-value testing for both fiberglass and spray foam insulation test how long it takes heat to pass through each medium.  The conditions are perfect, there is no air flow in the testing room, simply a heat lamp on one side and a thermometer for temperature testing.  Unfortunately there is no condition in residential construction that emulates this testing facility.  So here is how the testing fails to compare fiberglass insulation to spray foam insulation.  Say the same test mentioned above is performed, but this time, air flow is added to the testing room, instantly the “relative” R-value of fiberglass insulation decreases by approximately 50%.  Because fiberglass is not an air barrier, the flow of air more or less “pushes” the heat through the fiberglass insulation at a faster rate than if there was no air flow.

On the other hand, if the R-value test is performed on spray foam insulation and air flow is added to the test, the “relative” R-value of foam insulation does not decrease at all.  The reason that the “relative” R-value of spray on foam insulation does not decrease is because once three inches of open cell spray foam insulation is applied it creates an air barrier.

In most modern home designs with conventional blown in fiberglass insulation in the attic, ridge vents as well as soffit vents are installed.  These vents are installed to create air flow in the attic in order to decrease the temperature in the attic.  While it’s a good idea to decrease the heat in the attic, the air flow supplied to the attic through the vents instantly decreases the “relative” R-value of the blown attic insulation.  Most homes built in the 1970’s have little to no insulation in the attic, and homes built today typically have about an R-30 in the attic.  If there is air flow in the attic, the R-30 blown in fiberglass insulation decreases to approximately R-15.  When using spray on foam insulation, there are no vents and the entire attic is sealed which brings the attic inside of the thermal envelope of the house.  The result of spray in foam insulation being applied to the roof deck creates an attic environment that is only about 10 degrees warmer than the house.  Anybody that has blown fiberglass insulation will tell you their attic in the heat of summer can reach temperatures upwards of 130 degrees.  Since the attic is so hot and the “relative” R-value of fiberglass insulation decreases significantly due to the air flow from the vents, it makes the home a lot less energy efficient and increases energy cost…. Next blog will talk more about R-values of different insulation products and how heat can impact “relative” R-values.

Wall cavity spray foam insulation
Wall cavity spray foam insulation