Breathability of Radiant Barrier & Reflective Insulation Products

Condensation forms when two different temperatures meet on a single surface. Since warmer air holds more humidity than colder air, when the warm air meets the cold surface, the humidity condenses into small droplets of water. In a metal building, condensation may just result in water annoyingly dripping down from the ceiling, but in a home, the last thing you want is water condensing and getting trapped in the ceiling or walls, which could potentially lead to mold or mildew if the moisture doesn’t escape.

Perforated MetLam

Since radiant barriers and reflective insulations are typically made from a metal foil or metalized film, they also act as excellent moisture vapor barriers. ASTM E96 is used to test moisture vapor breathability, and foils and metalized films typically score below 0.05 perms. According to ASTM C1313 (Standard Specs for Radiant Barriers) and ASTM C1224 (Standard Specs for Reflective Insulation) products intended to act as a vapor barrier or vapor retarder should have a perm rating of 1 perm or less. 

Now, there are appropriate uses for a vapor barrier or vapor retarder, like on the interior ceiling in a very cold climate to prevent moisture from every getting into the insulation to begin with. There are also applications where it will make no difference whether the material is a vapor barrier, like under roof rafters in an attic where ventilated air is allowed to move freely around the material. It’s in other applications, like on the exterior of insulated walls or directly on top of the insulation in an attic, that can cause serious problems.

Radiant Barrier

The solution to the problem is to make the radiant barrier or reflective insulation product breathable in applications where a vapor barrier or vapor retarder would be harmful. While some manufacturers have experimented with metalizing a naturally breathable surface, like a breathable film or nonwoven fabric, most manufacturers make their products breathable by perforating it with holes by running the sheet across sharp pins. While the pin holes are a simpler solution, certain applications, like house wrap, may also require water-tightness, and holes may not be acceptable, which is why a few companies offer breathable, non-perforated, non-woven alternatives.

In 1991, with funding and oversight from the US Department of Energy, Oak Ridge National Laboratories evaluated the moisture accumulation of an unperforated foil, a perforated foil, and a metalized high-breathability non-woven fabric. Their conclusion was that a breathable radiant barrier (either perforated or non-woven) “...did not reveal any significant moisture condensation problems, [and] it is expected that moisture condensation will not be a problem in warmer climates”.

Unfortunately, what the Oak Ridge test didn’t consider is that the perm rating of materials, in reality, are much lower than their assumptions. Oak Ridge modeled their results on a material assumed to be 20 to 30 perms, and a material assumed to be 100 perms. While the exact perforation pattern and frequency of the perforations can vary between manufacturers, they seldom get that high. Our research found a product with perm ratings as low as 1 perm to as high as 7 perms. We are given specific criteria in ASTM C1313 and ASTM C1224 which state that any product intended to be breathable should have a perm rating greater than 5 perms. It is important to note that some radiant barrier and reflective insulation products on the market do not meet this +5 perm standard.

Radiant Barrier

The reason perforated materials differ in perms has to do with a wide variety of factors, including the size of the holes, the pattern and frequency of the holes, and the thickness and composition of the material. For example, one perforated reflective insulation product with aluminum facing on each side of foam has much larger holes than many thin, sheet products, but its perm rating is 4.9 perms, just under the +5 perms required for breathability. There may be many reasons that, despite the larger perforations, the breathability is less than the sheet products, one of which achieved nearly 7 perms. For example, the thicker perforated materials may have fewer overall perforations spaced further apart from each other, or maybe the interior insulation was so thick that the pins had a difficult time poking all the way through, or maybe the interior insulation began to close up around the holes. It can be difficult to judge the perm rating of any product just by looking, and the purchaser should look for the testing results rather than make assumptions.

Perforated Foam
Perforated Materials

While radiant barrier and reflective insulation manufacturers are not required to list the perm rating of their products by the FTC, any radiant barrier or reflective insulation product compliant with ASTM standards should have the perm rating stated on their spec sheet and their perm testing available upon request. Importers, wholesalers, and distributors who wish to test the products, whether for the first time or to confirm what their manufacturer has told them, can send the material to an independent test lab themselves. ASTM offers a list of labs that are willing to perform the ASTM E96 perm test here. The test is relatively inexpensive, usually costing between $250 and $500 per sample. Of course, this also means there’s little excuse for manufacturers putting barely breathable radiant barrier and reflective insulation products out on the market.