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This site provides a short primer on knee wall fires based primarily on a recent fire on Searl Terrace in Co. 11's area, UL Fire Research, and local experience with similar fires. There is simply not enough time to go into great detail on most things, however, what you find here should whet your appetite.
Contributions by:
Assistant Chief C. Bailey, Assistant Chief M. Ryman, Battalion Chief E. Couse, Battalion Chief C. Stroup, Captain D. Hout, Captain J. Masters, MFF D. Hensley
The view from inside looking towards the knee wall on Side Alpha
Background
Montgomery County still has a lot of existing housing stock with knee walls. While this construction feature is most commonly found in the older areas of the County the potential exists in all areas. Fighting attic fires when there is also fire in the knee walls is a riskier than usual venture, primarily because the fire tends to burn, ventilation limited, inside the walls. The smoke and gases are sufficient fuel and they are sufficiently heated to burn, they just lack the necessary oxygen. Aggressive opening of the hidden fire without cooling can lead to severe fire conditions.
Purpose
The purpose of this document is to provide some very general information about fires in knee walls and some basic tactical approaches. This document does not pretend to be comprehensive.
Knee Wall Definition
A knee wall is short wall, typically, but not always, under three feet in height, used to support the roof rafters. Where the roof rafters come over the edge of the structure they are connected to the knee wall forming a void space between the knee wall studs and the exterior cladding of the house.
A short video showing the interior (intact) knee wall framing. Note the continuous void space behind the studs.
The most common and obvious indication of knee wall is the presence of dormer [1] windows.
In many cases the officer conducting the initial on scene report will refer to the top floor as a “half-story.” This structure is commonly referred to as a 1 and ½ story single family.
But beware not all structures are the same. You can find fake dormers and you can find structures without dormers that contain knee walls. See picture below.
The major concern will knee walls is that fire can spread, sometimes unnoticed, most times ventilation limited, through the void spaces.
Knee Walls but No Dormers
As the fire on Searl Terr highlights you can have knee walls without dormer windows. In this house you have a full window on the top (attic level)on Side Delta and more windows along Side Charlie. However, it would be understandable if the unit officer reported this structure to be two stories with a basement instead of what it really is 2 & 1/2 stories.
Fire Dynamics
Fire Dynamics
This section involves a series of statements/observations from a UL Attic Fire Study. The highlighted sections are editorial comments.
Knee Wall Fire Dynamics [2]
Utilizing the upper ½ story of a structure for living space creates unique compartmentation not found on lower levels of the building. The interior living space is surrounded on three sides by void spaces separated only by drywall and possibly insulation.
During a structure fire, it is possible for fire to enter void spaces and surround crews conducting interior operations before they notice a rise in temperatures or see any signs of fire.
Any penetration into the void space from the interior creates a flow path, allowing fire to spread into the interior and exposing the crews. This fire spread may not occur immediately following the opening of the wall or ceiling, as the void space fire is likely ventilation limited. Thus, firefighters may breach a separation and then continue further into the structure. Even though there is a delay between making the breach and the change in conditions, once initiated, the transition to untenable conditions in the area of operation occurs in seconds. In Figure 7. 44 the temperatures at 3 feet in the attic increased 300 degrees in 5 seconds. When things go bad, they go bad fast.
[The most common interior observation is high heat and dense smoke on the uppermost level with no visible fire. When this condition is encountered personnel should use their hose line to cool the upper gas layers and then find an area of relative safety below the fire to regroup and consider options.
This fact of rapid growth highlights the ABSOLUTE importance of having good ladder based secondary egress. There should be a ladder at every window. Further it highlights the need to ensure that interior egress paths are completely open at ALL TIMES. When temperatures increase 300 degrees in five seconds there is no time to negotiate crowded stairs.]
Knee wall construction often provides the potential for ideal fire growth, with air entering low at the eave line and combustion gases exiting the peak through mushroom vents, ridge vents or gable vents. The limited natural ventilation keeps the fire small and it normally becomes ventilation limited. At the same time, the relatively large open space behind the knee wall allows for the heating of large amounts of fuel to near its ignition point. Subsequent ventilation, either by breaching the interior barrier or by venting at the roof, provides the necessary flow path to rapidly grow the fire to flashover.
[While we don't often engage in vertical ventilation prior to fire suppression (if at all) this may be one scenario where vertical ventilation of the knee wall space might actually make the fire fight more effective. This is not to say that vertical ventilation is the go to tactic. Vertical ventilation WILL create additional flow paths, it WILL make the fire bigger, but in cases where the occupants are accounted for this tactic may be useful if only to relieve some of the heat on the uppermost floor, allowing for an interior attack. Remember though that vertical ventilation will only provide temporary relief and if there is any delay in getting the stud bays open and cooled things may get out of hand.
Also remember that if your mission is to ventilate over the knee wall you will need some indication of where the knee wall is...this is typically most evident when dormers are present.
Notice in the photo how a cut too high on the roof will miss the knee wall completely. This case was especially hard to judge because there were no dormers to give an indication of where the wall started. ]
When the barrier between the void spaces and the occupied space fails or is breached, crews operating on the interior may find themselves trapped between the new flow path and their means of egress. Conditions will change even more rapidly if windows in the attic were opened or taken out, a common tactic employed to improve visibility and assist the crews in locating the fire and/or victims. Open windows provide even more air to mix with and ignite the rich fuel coming out of the knee walls.
Knee wall construction creates interconnected void spaces where the wooden structural members provide a relatively large surface area of exposed fuel along with air flow conducive to spreading fire (Figure 7. 48). The experiments demonstrated that getting effective stream reach and penetration inside a knee wall and other attic voids is hindered in much the same way as are streams applied into open attics through gable ends or from the floor below (Figure 7. 49). In both cases, structural members effectively broke up the stream before it could successfully penetrate the fire area.
[This means that a hoseline flowed through a single hole or two in the ceiling or knee wall will have a very local impact. You will not be able to put out the whole thing from one spot.]
The experiments also demonstrated that water should initially be applied into the knee walls at multiple locations through small holes, in order to suppress the burning gases, before large sections of the knee wall are opened for complete extinguishment. This tactic will maximize the benefit of energy absorption through steam conversion while minimizing the spread of fire along the flow path created by the openings made for the nozzle. Once the gases are suppressed or cooled, focus on getting water into the rafter bays where air moving from the eave line to the peak will sustain and accelerate the remaining fire.
[I have had personal success with this method of attack. Of course the work area is tight and there MUST be room for quick egress.
So consider having the officer and two fighters working in the space. One firefighter makes small holes in the ceiling and knee wall (just big enough to put the nozzle in when they move on to the next hole the nozzle firefighter applies water into the holes.
This tactic requires close coordination. The person making the holes and the person cooling must by well synced. ]
This is similar to the concept that makes the eave attack successful; the sheathing is where the air and fuel come together most effectively, so this is where suppression efforts should be focused. Don’t wait to see fire in a knee wall before you apply water to the void space or the surfaces. Tests have demonstrated that the most effective way to get a handle on knee wall fires is to control the source fire, cool the gases prior to making large breaches in the barrier, and then aggressively open the knee walls to complete extinguishment, focusing on wetting the underside of the roof decking.
[The Structure Fire Appendix speaks clearly to the fact that we are surface cooling fire department. However, sometimes gas cooling is necessary. What does that mean? That means that we sometimes will put a wide fog into an attic space to cool the gases before we begin to drop all the ceilings. Steam production will have two primary effects, it will inert the atmosphere in the attic and it will absorb a tremendous amount of heat energy.
BUT you have to play this tactic smart. You have to start cooling through small holes at the top of the stairs first, why because the addition of water may cause drywall to fall and create the very high energy flow paths we were trying to avoid. If this happens you will see rapid fire progression break out and it will be useful to have a ready egress from the area.
Stop. Cool (surfaces and gases) and Move. Repeat until fire is under control. ]
The most effective water application takes into consideration the construction of the attic, using the natural channels created by the rafters or trusses to direct the water onto the vast majority of the surfaces. Application of water through open eaves along the entire eave line allows water to impact over 2/3 of the attic space. When solid eaves are encountered or an interior stream placement is chosen, the same tactical concept can be employed. The crew enters the structure and makes its way to an exterior wall that is parallel to the line of the eaves.
Opening up a trough along this wall exposes the roof deck in much the same way as opening up the eaves. Alternatively, when the building layout makes the peak more readily accessible, (for instance, a building with a center hallway) a trough can be opened along the centerline of the structure immediately below the line of the peak (Figure 7. 36). Water is then directed toward the peak at as severe an angle as possible, alternately flowing water down both sides from the peak to the eaves, wetting the sheathing and raining down on the other combustible contents in the attic.
[specifically in older structures unit officers and crews need to keep in mind that the typical 6’ New York hook is too large to operate in many of these spaces. Halligan bars are often the shortest and most readily available to open up. *Also note that the wall coverings in this house was a thick coat of plaster over wire mesh. This type of covering best pulled using a boston rake or maybe even a trash hook .
The video below focuses on drywall and it will be hard to translate this technique 100% with the wire mesh. However, the general principle is the same. Short, measured hooking looking for efficiency not speed.
Some Other Thoughts
The Thermal Imager: Use the imager from the outside, to identify where the fire is. Use the thermal imager from the inside to identify where the fire is. Either way use the thermal imager.
Cooling the Atmosphere: Crews encountering high heat conditions, even where there is no visible flame, will benefit from flowing the line. The smaller part of the cooling comes from water droplets interacting with the heated gases in the space. The greater part of the cooling comes from the handline to cooling ceiling and wall surfaces. Cooling the surfaces will not put the fire out in this case BUT it will cool the working area, delaying flashover, and increasing the chances of survival.
Searching: When searching the ½ story you have to be super careful about opening doors. You may come across "doors" as you are searching and there is a high likelihood they will lead into the knee wall space.
If you open one of these doors you could experience rapid degradation of conditions-especially if the compartment you are searching is already hot. Some clues that a door leads to knee wall storage space would be half doors, odd shaped doors or lower than usual door knobs.
Hopefully this search is occurring behind the line, which reduces the threat of fire exiting an opening and threatening crews.
Personnel should also be aware that in many cases the knee wall void space is full of storage. The stuff in the storage area poses two problems: 1. It makes it harder to get water onto the burning surfaces and 2. It provides more fuel for the fire. Recall though that the additional fuel loading in the absence of sufficient oxygen will not add too much to the actual fire until more oxygen is added-usually after the wall/door is opened.
Its Hot! When its hot to you in your gear you are already in trouble. We hope to have demonstrated that the fire can be above and all around you when you arrive. You either start cooling fast or start leaving fast. There really is no good third option.
Command Considerations
Command Officers should consider:
- Limiting the number of personnel assigned to the area with knee walls. These spaces are almost always tight and having too many people will restrict both efficiency and egress safety.
- Using the “eave” attack to knock the fire back.
- Using vertical ventilation over the knee wall space to relieve heat. Careful consideration must be given to occupant status and the location of operating crews.
- Adjusting the work/rest cycle (shorter work, longer rest) to avoid exhausting incident resources.
- Getting pictures or video of all sides of the building to better predict construction features and associated fire behavior.
- Assigning crews with salvage covers to save property on the lower levels while being prepared to relieve personnel on upper floors.
Think about having:
- One crew attacking the fire.
- One crew laddering the structure while withholding ventilation.
- One staged on the floor below.
- One conducting searches then salvage.
- One “on-deck” out front.
Example.
- Engine 1 on top floor searching, hooking and squirting.
- Engine 2 staged with backup line one floor below.
- Truck 1 laddering, then staged in front.
- Truck 2 searches of all floors below then conducts salvage.
- Engine 4 staged in front yard.
- RS staged in front.
This approach allows you to manage multiple rotations with just a box alarm. Of course a task force is still indicated but it will likely be all that you need.
Notes:
[1] Dormer Window: A dormer is a roofed structure, often containing a window, that projects vertically beyond the plane of a pitched roof. It is also known as rooftop window.Dormers are commonly used to increase the usable space in a loft and to create window openings in a roof plane. https://en.wikipedia.org/wiki/Dormer
[2] The discussion of knee wall fire dynamics is taken from: UL Fire Service Summary Report: Study of Residential Attic Fire Mitigation Tactics and Exterior Fire Spread Hazards on Firefighter Safety. https://ulfirefightersafety.org/docs/Attic-Fire-FD-Summary-Report-4_2_15-OC-Corrections.pdf My comments are bracketed and italicized.
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