Why 75 degrees is NOT effective for measuring R-value performance

By: John Cowan, Regional Sales Manager

Cowan 75 Degrees

Are you curious why 75 degrees is used as the temperature for measuring R-Value insulation levels? Were you aware of this standard? You may be thinking no one cares. If you are a building owner paying  the bill, you should care. Let’s look at roofing insulation.

The FTC mandates that each products R-Value is tested at  75 degrees and will be the standard design criteria for residential construction. This standard is part of the building code and energy code which also encompasses commercial standards.

Let’s take a quick look at Polyiso rigid insulation. Polyiso accounts for approximately 2/3 of all the insulation used in commercial roof construction. For starters, Polyiso has had declining published R-Values for years. This is due in part to changing the blowing agents used in the manufacturing process over the last 20 years. The design R-Values, LTTR aged R-Values, and recommended “in service R-Values” are all different.

These R-Values will vary from R-6 per inch to R-4.5 per inch for a 50 year LTTR . The NRCA (National Roofing Contractors Association) has recommended using an R-Value of R-5 per inch of thickness in all climate zones when specifying Polyiso (Tech Bulletin 3001). There are numerous  commercial structures where the design R-Value used for Polyiso was R-6 per inch of thickness and higher.  If the standard used is the  R-Value at 75 degrees for all products, what difference does it make what the actual climate may be like?

It does matter. Polyiso and Expanded Polystyrene behave differently at temperature extremes. For example, Polyiso actually loses R-Value as the temperature drops. At 75 degrees, the R-Value will be approximately 5.6 per inch. As the temperature drops to 40 degrees, so does the R-Value. It is now R-5.0 per inch. As it continues to get colder the R-Value continues to drop. At 25 degrees it dips to R-4.5 per inch. The highest R-Value for Polyiso is at 75 degrees, when you least need to condition the space (Tech Bulletin 3016).

Not only is Polyiso losing R-Value due to aging and out gassing, it also loses R-Value when you most need it, as the temperature drops.  Expanded Polystyrene on the other hand is predictable. Not only is expanded polystyrene stable, it does not out gas or lose R-Value over a period of time. The R-Value actually increases as the temperature drops. At 75 degrees Foam-Control® PLUS+® 150 has an R-Value of 4.2 per inch. At 40 degrees, the R-Value increases to 4.6 per inch. At 25 degrees it is 4.8 per inch (Tech Bulletin 3016). Foam-Control® PLUS+® 150  out  performs Polyiso. It is more prudent to use R-Values measured at temperature extremes and not at 75 degrees.

When designing and specifying insulation for commercial roofing,  Expanded Polystyrene should clearly be the preferred choice.

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