Ventilation Factors We Cannot Control - Part 1

There has been recent talk, debates and training on vertical and horizontal ventilation on the fire ground. The focus has been identifying what we do on the fireground that directly impacts fire behavior, therefore impacting our tactical considerations. 

This focus on ventilation is results of testing, both laboratory and acquired structure throughout the U.S. and unless you have been under a rock you are aware testing has occurred and the scientific data gathered is having a positive impact on the fire service.

The blogs focus is to look at the vertical ventilation study and discuss a piece that has fallen through the cracks for some. We all understand any h*** made in the structure is ventilation. When a door is left open, opened for entry, when a door or the roof burns through, when a window fails or is broken all affect the fire. Regardless of what caused the opening they all affect the amount of air entrained into the environment. All of these examples create a change in the pressure. Remember, flow path is the air movement from the high pressure to the low-pressure area. The fire, smoke (the unburned products of combustion that are just waiting for the right mixture to light off) and heat will move towards the low-pressure area. We should also have a clear understanding that when any opening is made we will get some lift of the products of combustion from the lowest points in a building. However, we will increase temperatures because of the flow path created and the introduction of more air will rapidly raise temperatures. 

Much of the focus has been on what we the fire service does to create these opening. But, few have looked in detail at the Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction study. This study conducted by Under Writers Laboratories, Firefighter Safety Research Institute (UL-FSRI) published their findings in December 2010. When you dive into this paper on page 19 – 62 you will find a section that focuses on door and window failure. We all know doors and windows come in many shapes and sizes. We also understand they are made with many different materials and the focus is usually energy efficiency. 

But, let’s take a look how the building features that we have no control of impact the fire ground.

There should be no questions on the difference of hollow core, solid or metal doors and if the door is a rated fire door or not. Each door will assist in confining the fire but each type will do so for a different period of time. Do you know the operational time frames that a door will hold the fire back? When sizing up your incident and looking to identify current and soon to be flow paths you must know the different types of doors and how those differences will impact your operational times.

In part 1 of this blog we will focus on the window failure testing. In part 2 we will look at door failures and in part 3 and 4 we will look at fireground application.

During the UL-FSRI testing they found the following results on the window burns

There were a number of different window failure mechanisms and degrees of failure observed during the experiments. In order to have an impact on the fire growth there has to be a passage for air to enter the structure, therefore the failure of interest was the breaking out of the glass as opposed to the cracking of the glass. Failure is defined as a passage through the window of 25% or more of the total glass area. In most cases this was the failure of the top or bottom pane(s) of the window but in some cases the top window sash moved downward, opening the window 25% or more. The two legacy windows with single glazing failed later than the four modern windows with double-glazing. These experiments demonstrated a significant difference in legacy and modern windows exposed to fire conditions.

In this series of experiments the legacy single glazed windows outperformed the modern double glazed windows in terms of longer failure times. It is proposed that this occurred for two reasons. First the legacy windows had thicker glazing than the modern windows. The legacy windows had glass thicknesses of 0.093 in and 0.11 in., while the modern window thicknesses were 0.087 in. Second, the method the glass was fixed into the frame differed greatly between the two eras. The legacy window glass was held in place with putty like substance and there was room in the frame for expansion of the glass. The modern glass was fixed very tightly into the frame with an airtight gasket and metal band, to provide better thermal insulation. This configuration did not allow for much expansion and therefore stressed the glass as it heated and expanded.

The results of this testing goes against what my assumptions would have been. I assumed newer windows would have a greater time to failure. However these tests show there will be failures earlier in the incident with a modern type of window. Now consider your response times and crew efficiency. Where will your crews be in their tactical assignments when windows fail decreasing the time to flashover?

Until next time remember, the clock is ticking and we must be able to predict what flow paths we will create and what flowpaths window failure will create that will impact our tactical assignments.

Stop Believing and Start Knowing!