Time Ratings vs Class Ratings

When dealing with fire retardants, there are typically two standards - time ratings and class ratings. 

Class ratings are usually Class A, B or C, with 'A' being the best. This is usually measured with the test for ASTM E84, which measure Flame Spread and Smoke Development. It most often applies to NON-structural building materials, like plywood, fascia, wall coverings, and things like that. 

Time ratings are usually 1, 2 or 3 hours, and apply to STRUCTURAL things like beams, joists, floor/ceiling assemblies, framing. This is usually measured according to the test standard for ASTM E119. They can also apply certain structures like decks or even sheds, if they are within 5 feet of a property line, or another building.

Spray foam insulation projects have 2 standards, 5 minute ignition barrier and 15 minute thermal barrier. Most projects will require 15 minutes. The 5 minute ignition barrier is for inaccessible spaces, and the 15 minute thermal barrier is for everything else.

It's best to find out from your fire code official, architect or engineer specifically which standard they want you to achieve, because it can make a huge difference in costs. For instance, it takes twice as much paint (and twice as much labor) to go from 1 hour to 2 hours. 

We are happy to help you figure out what you need, but since these are code questions, the fire marshal overrules everything!

Mar 23rd 2021 Doug Stafford

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