What is R-value for Spray Foam Insulation vs. Fiberglass Insulation?

Posted on: November 21st, 2012 by

Being in the Chicago spray foam insulation market, this question comes up a lot when comparing spray foam insulation to fiberglass. To understand R-value, you need to understand the ASTM 518 testing performed in a lab AND how the materials actually perform in the real world outside of the lab. The effects outside the lab that are not incorporated into the testing include: thermal bridging, air movement, air infiltration, thermal mass, moisture content, solar radiation, and performance at various temperatures. What is tested is conduction of energy from one plate through the test material (insulation) to another plate. Conduction is one of three ways heat moves, convection and radiation are the other two which are not tested in R-value.

Studies done by independent research facilities funded by the US Department of Energy have shown that fiberglass loses 8% of its R-value before it’s installed. When perfectly installed with access on both sides of a stud wall to ensure all six sides of the insulation sit square and are in contact with all surfaces, fiberglass loses 11% of its stated value. When commonly installed it lost 28% of its stated R-value in a completed wall assembly. This study did include thermal bridging but did not take into account air infiltration, moisture, and greater temperature differences which would reduce this number even more.

Spray foam insulation has shown to eliminate air movement and air infiltration which address convective currents in the wall cavities along with moisture. When tested at greater extremes than the ASTM 518 calls for, spray foam maintains its R-value and as temperatures drop, spray foam insulation actually preforms better than its stated R-value. Spray foam also reduces the effects of solar radiation and when thought has been put into wall framing, thermal bridging can also be addressed making SPF a logical choice in super insulating homes.

R-19 Fiberglass actual performance with lower climate temperatures.

Loose Fill Fiberglass

Metering Chamber (F)

Climate Temperature (F)




















5 Responses to What is R-value for Spray Foam Insulation vs. Fiberglass Insulation?

  1. Sarah Wold had this to say about that:

    Adding insulation later on will be more expensive and difficult. Spray foam insulation is costlier but has a higher aged R-value and you can recoup you investment with savings.

  2. Alfonso had this to say about that:

    Its a meaningful post on spray foam insulation, great post and thanks for sharing.

  3. Dunkin had this to say about that:

    Great website, lots of great pictures and articles. Thanks!

  4. Teodoro Parch had this to say about that:

    Great information about how bad fiberglass actually insulates by just being installed.

  5. Que Jalo had this to say about that:

    This web site is really a walk-through for all of the info you wanted to know about spray foam and fiberglass insulation and didn’t know to ask.