Understanding Rolling Door Thermal Resistance and Air Leakage

Thermal resistance is a huge factor that affects the energy performance of insulated roll up garage doors when they are in the closed position. Thermal resistance is measured using R-Value and U-Factor.

Air Leakage on the other hand refers to the seal at the overhead door guides, head and hood while the rolling door is in the closed position. Without the proper seal, air will have the ability to pass through the rolling door. Regardless of how high the R-Value of an overhead coiling door may be, that is irrelevant if an improper seal allows air to pass through.

The Shocking Reality of R-Value vs. U-Factor of a Rolling Door!

R-Value may seem like a pretty straightforward measurement to determine the quality and energy performance of insulated roll up garage doors, but that is not always the case. Unfortunately, the R-Value is often misunderstood and given more credit than it deserves. The technical definition of R-Value is “the inverse of the time rate of heat flow through a body from one of its bounding surfaces to the other surface for a unit temperature difference between the two surfaces, under steady state conditions, per unit area.” Doesn’t this sound like an extremely technical in depth scientific calculation that will help you determine the best insulated rolling door for you…… well it’s not that cut and dry. In plain English, the R-Value is the capacity of an insulating material to resist heat flow. The higher the R-Value, the greater the insulation power. With that being said, we would like to share with you a common misconception of R-Value and how accurate it really is.

Insulated Rolling Door Myth:

The most common R-Value myth is that it is the best indicator of energy efficiency for an insulated coiling door.

Insulated Rolling Door Reality:

The reality is that R-Value is a good measure for layers of wall insulation, not products comprised of multiple components.  This means that stated R-Values for insulated roll up garage doors only reflect the insulation in the slat cavity’s resistance to heat transfer.  An insulated rolling door slat on an otherwise breezy roll up door assembly is the equivalent of putting a wool hat on when you’re wearing shorts.  One component might be right for snow, but the whole outfit isn’t.  This is just like a commercial garage door that has excellent insulation but an improper seal around the roll up door guides, head and hood, which then allows for air leakage. An insulated overhead door with air leakage completely voids a high R-Value. And, R-value is calculated, not tested – meaning that the commercial door insulation within the roll up door slat itself should perform at that level, but it isn’t tested as a complete assembly (hence the seal around the door guides, head and hood not being included in the measure of R-Value.)

Energy Efficient Overhead  Door Fact:

U-Factor measures the rate of heat transfer, and is appropriately applied to the thermal efficiency of windows which, similar to insulated roll up garage doors, are assemblies comprised of multiple components. 

Now that you understand why U-Factor is a better energy performance indicator, lets discuss the details of it.  U-Factor is a tested value of heat transfer via conduction and radiation. The underlining and bolding of tested was not a typo. Unlike R-Value, the U-Factor it is an independently tested value – that’s a huge deal!  Also, the testing is performed on the full assembly, not just the insulation in the slats. Both of those items make the U-Factor a much more relevant number to use when considering what rolling door to purchase. That means that air leakage is taken into consideration when testing for U-Factor. And don’t forget when purchasing your next insulated roll up garage doors, the lower the U-Factor the better!

 Cornell is the industry leading rolling door, security grille and closure product experts since 1828.
To inquire how Cornell's long history of innovation and customer service can help you with your next rolling door project, call 1-800-233-8366!