Energy Efficient Windows: Firefighter's Friend or Foe?

The truck company captain shouts, "Take the windows!" This is a task we all take for granted. We have all done it and we regard it as just another routine task on the fireground. In our mind's eye, we swing our hook into the window and the glass crashes to the ground, providing relief for interior crews. We finish the job by cleaning out the remaining glass and sashes.

Photo by Jerry Knapp/Rockland County Fire Training Center 1. Interior view of the burn module. The energy-efficient window is on the left and the single-pane window is on the right. The fuel load was a love seat and 10 pounds of paper and cardboard. Thermocouples can be seen running up from the floor to the interior of the windows. These wires did not touch the window glass.

But all windows are not created equal. This series of articles will provide you with a thorough knowledge of the many types of modern residential windows. We will examine: types and construction of windows; types of glass and composites being used for windows; and different types of frames and sashes and their effect on fireground operations. In this first installment, we examine how our firefighting environment has been changed by the widespread use of energy-efficient windows (EEWs). We will also present some surprising test data comparing fire resistance of EEWs and single-pane windows.

New Technology = New Challenges

As our world constantly changes around us, we must continually adapt, upgrade and modernize our strategy and tactics. The energy-efficient window is one of the new technologies that present firefighters with new challenges requiring changes to our tactics. However, before we change our operations, we must first clearly define the problem.

Energy-efficient windows what are they? Energy efficient windows are designed to minimize heat loss from the inside of a structure in winter and to minimize the heat gained by the structure in summer.

An EEW is generally two panes of glass, each 3/16 or 1/8 inch thick, depending on the specific model and use. The panes are spaced about a half inch apart and sealed together. Between the glass panes can be air or it can be filled with an inert gas such as argon. EEWs are sometimes called insulated glass or thermopane windows.

Before we continue, we must agree on several common definitions:

Single-pane window A windows that has only one piece of glass between the interior and exterior environments.

EEW A modern type of window with more than one pane of glass and designed to reduce the transmission of heat through the glazing.

Glazing The glass part of the window.

Sash The wooden, aluminum or vinyl frame that holds the glazing.

Frame The window frame includes the structural members that hold the sashes into the window opening of the wall the window is installed in. The frame may contain the cranks, levers, balancers, springs or counter balance devices necessary to open and close the windows easily.

What's The Problem?

There is widespread belief that EEWs present a danger to firefighters because of the following:

1. EEWs will maintain their integrity under a fire load longer than single-pane windows, increasing the severity and potential for flashover and backdraft.
2. EEWs will hold in more heat in the area of origin, making conditions untenable for interior operations more quickly.
3. EEWs will not fail quickly and therefore hide the real location of the fire, especially in a multiple dwelling.

When firefighters hear statements like those, they tend to believe them as "Golden Truths." These may come from seasoned firefighters and officers for whom we all have great amounts of respect. The first obvious begging question is: "Under the same fire load, how much longer will an EEW withstand the effects of fire as compared to an older single-pane window and what effect does that have on the fire development?"

In gathering data to answer this question, a literature search turned up very little. We could not find any specific testing that was conducted to answer this and similar questions. We did find several articles describing fires in which EEWs played a significant role in hiding fire, keeping high levels of heat inside the area of origin and probably leading to backdraft/flashover conditions. In short, there were lots of war stories and opinions but no hard data. Our effort as reported in this series was to collect data from realistic testing.

We developed a realistic live-burn test method to compare the effect of fire on single-pane versus energy-efficient windows.

Photo by Jerry Knapp/Rockland County Fire Training Center 4. The energy-efficient window is about to fail. The burn-through of the glass along the top and down the sides of the sash is extended to almost the entire perimeter. Except for the one light (pane of glass) that is broken, the single-pane window is intact. Thermocouple readings are 1,145 degrees Fahrenheit in the center upper sash of the single-pane window and 1,050 degrees F in the center of the upper sash of the energy-efficient window.

Test Goals

The goals of our live-burn test were:

1. To compare the outward observable performance of a single-pane glass window and an energy-efficient window when simultaneously exposed to the same test fire.
2. To determine at what temperature the windows failed, possibly resulting in increasing the air supply to the fire and probably increasing the intensity of the fire or stimulating flashover.
3. To determine at what approximate temperatures glass cracked and whether this data could be used as an approximation of temperatures inside as had been reported.
4. To collect data that compares the time and temperatures the windows partially and totally failed and fire broke through them.

Using the Swede Survival Flashover Simulator as a temporary laboratory, we installed the windows as shown in the photos on the preceding pages, the single-pane window on the left, the EEW on the right.

Photo by Jerry Knapp/Rockland County Fire Training Center 6. Flames have extended out the entire area of the energy-efficient window. The chipboard cover over the thermocouple digital readout was ignited on the energy-efficient window side due to radiant heat. Flames showing at the single-pane window are the result of flammable gases escaping in gaps between the sashes and the sash and frame, mixing with outside air into a flammable mixture and burning. The wood sash and mullions are burning as well on the exterior of the single-pane sashes, giving the appearance that fire has vented through the window.

The fire room was the burn module of our flashover simulator. The interior is eight feet long, seven feet high and eight feet wide. We framed out the walls and ceiling and sheetrocked it similar to a home using two-by-four wall studs and two-by-sixes for ceiling joists and outside wall (that contained windows) studs. Paneling was added to the interior walls. The fire load was a love seat; 10 pounds of cardboard and paper were added to the center of the module.

We chose this fuel load because it represented a realistic scenario, especially for the small room we were using. This fuel load presented an obvious problem of getting a consistent fire development for each subsequent test fire. We felt it was reasonable to accept this variable.

The first two tests compared a single-pane window to an EEW under the same fire load in the same fire test. Additionally, we are analyzing data collected from these two burns and three additional live burns. We will compare our time temperature curves to data collected at other live-burn studies to verify that our test fires were realistic.

The single-pane window used in the first test had a wooden sash. The window measured 34 inches in width by 53 inches in height. Upper and lower sashes were 34 inches wide and 27 inches high. This window is known as a "6 over 6" with each light (pane of glass) 10 inches wide and 12 inches high. Wood mullions divided the sash and held the glass in place. The window was constructed in 1930 as part of the original construction materials for a local home.

The EEW used in the first test had a vinyl sash. Overall dimensions of the window were 27 1/2 inches in width by 56 inches in height. Upper and lower sashes were 27 1/2 inches wide and 28 1/2 inches high. Glazing dimensions were 26 1/4 inches wide by 27 1/4 inches high. Glazing utilized standard window glass.

Thermocouples were installed to hang one inch off the center of the glass in each sash on the inside of the burn module. Wires were run from the thermocouples down to the bottom of the module into the digital readouts. Four thermocouples were used.

Fire Test 1 Results

At 600-650 degrees Fahrenheit, the single panes began to make a tinkling-type noise. Although only one pane of the single-pane window cracked, the glass continued to make noise. The noise was similar to tapping a spoon on a fine thin wine glass. It was resounding at a rate of about 2 per second.

The first failure of the EEW was at about 700 degrees F at five minutes 58 seconds into the burn. The interior pane of cracked and dropped to the floor of the burn module. The exterior pane remained intact.

At six minutes 47 seconds, one light of the single-pane window cracked and broke out about 20% of its surface area. This was the only failure of the single-pane window.

The outer pane of the EEW held for seven minutes 30 seconds. The photo at the bottom of page 74 shows the window just before failure. Note the frame has melted around the glazing along the top and part way down the sides. This pane fell out as a single piece. Immediately following was a dramatic increase in fire intensity.

It is important to note that flash-over (full room involvement in fire) occurred at about 1,100-1,150 degrees F as recorded by the upper thermocouple. At about this same temperature the EEW began to show signs of losing structural integrity by the glass itself melting or the frame burning/ melting from the inside. Then, apparently when the sash and glass were weakened enough, the entire glass was pushed out from the fire area. This, of course, allowed good ventilation into the fire area and temperatures peaked at 1,800-plus degrees F.

The burn was terminated by a hose stream at 10 minutes 45 seconds. Results to this point showed the single-pane window remaining intact except that one light cracked and a piece fell out. All glass from the EEW fell out. To our surprise, failure of the EEW included the glass, the sashes and even the frame burned down to about six inches from the sill.

Lessons Learned

• Our original goal was to collect data on to show that EEWs would hold fire longer than single-pane windows and to document approximate times and temperatures of failure of each type of glass. We were very surprised with the data and results just described. A repeat test, exactly as the first, was performed about two hours after the first test. Results were almost identical.

The unexpected results of these two test burns posed several other questions. Did the single-pane window stand up to the fire load because of the smaller panes of glass? Did the smaller panes and wood mullion allow the glass to expand with the heat of the fire? Was our testing method flawed?

Whatever the reason, the generalization that "all insulated windows will contain fire longer than single-pane windows" appears to be untrue for the types of window we used in our tests. Will a single-pane window with similar-size glazing fail before an EEW? We have scheduled our next burns to help answer that.

• Single-pane windows began to crack and "sing" at about 600-650 degrees F. Much has been written about what glass can "tell you" about the fire condition. It is reasonable to conclude that if the glass is singing, temperatures are in the 600 degrees F range.
• Several questions remain about the failure of the EEW. Was this method of failure the entire center portion of the glass falling out as a unit a result of the type of sash the glass was contained in? Apparently the vinyl sashes melt and cause the window to fall out in a catastrophic manner. This and the subsequent test showed a dramatic increase in fire intensity caused by this failure.

Will wooden framed (sashes) windows (insulated glass) react differently? What about EEWs with aluminum frames/sashes? We have acquired many windows from local suppliers to use in our live-burn testing to help answer these questions.

• The wooden frame of the single-pane window, although actively burning, did maintain structural integrity for the entire length of both burns.
• Although not related to windows, flashover occurred at or near 1,100 degrees F in both burns. This reinforced the concept that flashover is driven by the ignition of carbon monoxide whose ignition temperature is 1,100 degrees F. Full fire compartment involvement was not achieved until this temperature was reached on the highest thermocouple.

The results of our research and live-burn testing contradicted our original hypothesis. Much has been discussed detailing how energy-efficient windows keep in the heat of a fire and make the fire environment more dangerous, based on real fireground experience. The apparent lesson learned here is that it may not be correct to take some fireground experience and make blanket conclusions. Other factors such as frame and sash material, type of glazing and rate of fire development may play significant roles in window integrity under a fire load. It is clear that, based on our limited observations, sudden failure of EEWs can quickly create deadly conditions.

Issues raised in our initial testing will be examined in future tests. Windows have been acquired and further live burns have been scheduled to obtain more realistic test data regarding EEWs under a fire load.

Thanks to: Rockland County Fire Training Center, John Cole, Sunnycor; Diana Robinson, New York State Fire Academy; Nora Jason, NIST; Charles Lanzer, Dan Leghorn Eng. Co.; Chief Ken Hetrick, West Point Fire Department; Paul Byrne; William Saraceni; Chris Reynolds, Rockland County Fire Training Center; and Douglas Danielson, Woodbury, NY, Fire Department.

Jerry Knapp and Christian Delisio are New York State certified instructors working at the Rockland County Fire Training Center and teach and write on a variety of fire service topics. Knapp is a 23-year veteran firefighter/EMT with the West Haverstraw, NY, Fire Department and is a nationally certified paramedic. He holds a bachelor of science degree in environmental science from Albany University and an associate's degree in fire protection from Rockland Community College. Delisio has been an FDNY firefighter for the past seven years and a firefighter/driver in the West Haverstraw Fire Department for five years. He is a staff sergeant in the 105th Air National Guard Fire Department at Stewart International Airport in Newburgh, NY. The authors are available via the Internet at: [email protected] or at the Rockland County Fire Training Center, Pomona, NY, 914-364-8800.