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The Thermal Barrier and Ignition Barrier story:

One issue that has confused many is the code requirement for thermal and ignition Barriers and how it relates to spray foam insulation when installed in an attic or crawl space. I’m going to attempt to put the explanation in laymen terms.

First, definitions:

Thermal Barrier – A thermal barrier as far as building code is concerned, is any product that has been ASTM tested and is considered to have a “15-minute thermal barrier” or an “index of 15”. An example of a thermal barrier is 1/2” sheet rock, 1/4” plywood or particleboard, and some fire proof coating.

Ignition Barrier – a product that prevents the ignition of the product which it is applied to from a spark, or from direct heat, but does not protect from direct flame over a period of time. Ignition barriers are usually spray on or brush on coatings.

This analogy might help understand the difference. Lets say a firefighter is wearing his fireproof suit, his suit is our thermal barrier. We are standing next to him wearing a leather jacket. This leather jacket is our ignition barrier. The firefighter can walk though a fire without burning, but we can’t. That leather jacket would burn quickly. But if we were standing outside the fire and a spark come in contact with the leather jacket, it’s doubtful we wouldn’t burn. The jacket would give us a small amount of protection. But nothing close to the amount of protection that the fireproof suit would give us. So therefore, a thermal barrier is a high-level of protection, and an ignition barrier is a low-level of protection.

When does code tell me I need to us a thermal barrier?

Simply put, everywhere foam is applied to the interior of the building, a thermal barrier must separate the foam from the interior of the building. For example, when foam is applied on the exterior walls, your sheetrock on the walls is your thermal barrier. When foam is applied to the roof deck, sheetrock installed at your ceiling is your thermal barrier. Now, if your ceiling is not sheetrock, or does not have a 15-minute fire rating, you must now apply a thermal barrier directly to the foam.

When does code tell me I need to use an ignition barrier?

Code says that anywhere foam is applied in an attic or crawl space, it must also be protected from the attic space from an ignition barrier, in addition to the thermal barrier that is at the ceiling. The way the code sees it is once the foam is separated from the interior of the building with a thermal barrier, the foam does not need the same level of protection from the attic space, but it does require some level. So that is when the ignition barrier, or low-level protection steps in.

What is the truth about different products and their claim to not needing these barriers? How do I know if a foam can be installed without the ignition barrier? Does the same code apply to commercial buildings?

There are no open cell products currently approved to be applied in an attic with out an ignition barrier. Some companies claim they have the ignition barrier “built in”. Ask them to show you the ES report showing you their exemption from needing an ignition barrier. They can’t because there are none. There are a few closed cell products that have passed the test and that can be left exposed. Of course, the cost of open cell with an ignition barrier will be cheaper and a better solution in most cases.


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