# Firestopping Compression Calculator

## Joint Firestopping insulation material compression calculations per the XHBN UL Guide Specification for Joint Systems.

Have you ever read a UL listing and it requires a 50% or even a 33% compression ratio? What does it mean and how do you calculate it? You can use the firestopping compression calculator below to figure out how much-uncompressed insulation material you will need?

If you have a joint that is 10-inches wide for an edge of slab stuff and spray and the listing indicates a 50% compression. It seems like a simple math calculation right. You would think that 50% of 10-inches is 5-inches, so you would add 10-inches plus 5-inches to have an uncompressed thickness of 15-inches. Thus, when the 15-inches of material is compressed you would fill the 10-inch edge of slab with the compressed insulation material having a 50% compression.

If we dig a little deeper to make sure that we are doing the math correctly you will want to reference XHBN UL Guide Specification for Joint Systems and follow the formula that they provide to calculate the required uncompressed thickness of insulation material that needs to be used in your system. Not an advertised feature of the door gap gauge but you could use it to measure the joint width required for your fire stopping requirements.

A common issue associated with mineral wool usage in firestop systems, and often observed during field verifications, is that the compression of the wool specified in the listed system’s installation instructions is sometimes not achieved. The compression ratio might be calculated incorrectly by the installer, or sometimes even ignored. For instance, a 1-1/2” joint that requires mineral wool as a backing material installed with 33% compression would require 2-1/4” of mineral wool compressed into the 1-1/2” gap. (100% – 33% = 67%. 1.5” /.67 = 2.24” mineral wool).

This calculation is not an easy one for everyone to do, so we created the firestopping compression calculator. You input a couple of simple numbers and the calculator will spit out how much un-compressed mineral wool you will need.

Before the firestopping compression calculator, it was not uncommon to see installers simply compress the wool somewhat, giving it a slight friction fit into the joint, and assume it is “close enough”. Becuase the insufficiently compressed mineral wool provides less resistance to heat conduction, thus leading to the same undesirable result as having a mineral wool with too low of a density. If you are an inspector, superintendent or foreman you can take a few minutes with the installer to educate them on what is required, show them the calculator and have them save it on their smartphones home screen now and have confidence that the calculations are being done correctly. We hope to be able to create more free tools as our membership base grows. Please support our efforts by becoming a member.