Why Do We Apply Spray Foam Insulation in Roof Decks at only 5.5” deep?

Lapolla Open-Cell Spray Foam Insullation Graph

If the code states that attic insulation should be R-30, why are we spraying the roof decks of attics to R-20, (5.5”)?

Are we short-changing or deceiving our clients?

New Construction Trade-Offs

Well, the short answer is this: In New Construction, the trade-offs are easy to hit. In our milder southeastern climate, the high performance windows we install in new homes exceed the code
requirements, and the “excess” points in the compliance calculations are used to offset the points deficit from insulating the roof deck with R-20 open cell spray foam, (when the code clearly states attic insulation must be R-30).

Increasing the foam to an R-30 from R20 will improve the efficiency by less than two percent.

Here at Bird Family Insulation, we only serve homeowners in existing homes, so I have no experience with builders and only a handful of my projects have included code inspections. However, without exception, 100% of my inspections have approved 5.5” (R-20) spray foam in the attics!

But I do have over 18 years experience of upgrading many 1000’s of uncomfortable Atlanta homes to the satisfaction of these wonderful homeowners. Today, about half of our clients get spray foam in their attic, or basement, or crawlspaces. It has truly revolutionized home comfort! To quote my friend Tom Callahan, “ The code does not take into account the efficiency of foam, only the results of a laboratory test chamber.

“These documents help make the case. You see the foam is 96% efficient at stopping heat transfer at 5.5”. Increasing the foam to an R-30 from R20 will improve the efficiency by less than two percent.

“The application of fiberglass needs to be perfect to perform as an R-30. The attached third party document shows how the application will reduce the effectiveness of the R Value.

“Also, the use of SPF (spray polyurethane foam) at R-20 is not arbitrary. The heat reduction table provides the efficiency, and the State of Alabama has adopted R-20 in spray foam acceptable where the code required R-30 in air permeable products, such as, fiberglass (see attachment page 2-4-10.)”

Why do we spray roof decks with open cell foam insulation to a coverage of R-20 (5.5”)?
Because it works great in our climate zone!

● Spray foam insulation is sprayed on as a liquid, so it has very, very few seams, gaps, voids to compromise it’s thermal performance.
● Because the properties of foam insulation are like epoxy glue, it tenaciously adheres to the surface of your roof deck. So there are very few if any air gaps, voids, channels where energy can escape.
● One huge difference between spray foam insulation and other types of insulation, is that it is an effective air barrier. It BLOCKS airflow! So your attic is sealed and insulated to protect your home against energy loss. (The secret to high performance insulation is stopping air infiltration.) That’s why the Alabama code stipulates that it must be “impermeable insulation” to qualify for the R-20 provision. Impermeable = Air Barrier.

So Much Confusion!

As Tom says, states are slow to recognize the obvious. Alabama understands the high-performance nature of spray foam insulation. Soon, hopefully very soon, Georgia will agree with Alabama, and amend our code to recognize the same obvious fact… as 1000’s of Atlanta homeowners have already experienced!

Maybe then, fewer homeowners (and contractors) will be confused! Have questions or comments? You can reach Bob Bird at Bird Insulation!

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