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.
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
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
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
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.