What is Annular Space?

This post provided by Sharron Halpert at Halpert Life Safety Consulting

ANNULAR SPACE- This is a term used only in a discussion of through penetration firestop not in rated joints. It is basically the gap. More specifically it is the distance from the inside edge of the opening to the outside edge of the penetrating item. It is actually a critical and often overlooked part of a firestop assembly.
When measuring the annular space, sometimes it gives a “nominal” measurement. If the detail says nominal ½”, then the tested and listed detail expects the field condition to have a pipe that is centered in the opening. That can happen, and it snows in Las Vegas…sometimes. More often the annular space will offer parameters defined by a minimum and maximum annular space. If the annular space lists 0” to 1” this means that the penetrating item does not need to be centered in the hole. It also means that its okay if the penetrating item makes contact on one side.
This does NOT mean that when an electrician runs a 1” conduit, they can use a 1” hole saw. Some contractors see the 0”-1” and think that the pipe can squeeze into the opening and the firestop contractor can firestop the application. This happens all the time, but that doesn’t make it right. It only makes it common.
When the opening is just barely big enough to allow the pipe through, this creates a condition known as CONTINUAL POINT CONTACT. Another time this can occur is when a 6” sleeve is run for a 4” pipe that will have 1” insulation on it. There is enough room to get everything through the sleeve, but there will not be enough room to install the firestop detail that should have been submitted.
There are very few tested and listed systems that allow CONTINUAL point of contact for a bare metal pipe, let alone for a combustible penetration such as insulation or even plastic. This gap is critical to the proper performance of the firestop assembly. If the tested and listed detail calls for 0”-1” then it assumes there will be some space into which the sealant can be installed. For a typical 1-hour gypsum wall the required sealant thickness will likely be 5/8”. If there isn’t at least ¼” gap, then the sealant depth cannot be achieved. This is critical to the performance of the firestop installation.  We will go into this in depth, but for now we are not finished with the discussion about annular space. How do you measure it?
If there is a square duct in a square hole, measuring the annular space is pretty simple. If it is a round pipe in a round hole, its simple again. What about when you have a round pipe in a square hole? Do you measure to the longest distance, which would be to the corner or do you measure from the edge? According to UL, the measurements should be made to the edge, so basically at a 90-degree angle from the edge of the opening to the side of the pipe.
That covers annular space pretty well for now, but there is more to consider. If you have any questions feel free to reach out to us and we are happy to help if we can. Next up we will talk about the hose stream test. This will help clarify why the annular space is such an important element to verify during a firestop inspection. You will know how a continual point contact installation will likely fail and much, much more. Thank you for taking the time to learn more about firestop.