The Effects of Insulation on the Fire Resistance of Wood-stud Walls

Transcription

1 The Effects of Insulation on the Fire Resistance of Wood-stud Walls Takeda H. 1 and Kahsay F. STRT The fire resistance of gypsum-board protected wood-stud walls with either glass-fibre or rock-fibre insulation filling the stud cavities is examined in this paper. Forintek anada orp. has developed a series of computer models to predict the fire resistance of wood-framed walls and floors. The models utilize two-dimensional heat-conduction equations and thermo-physical property data to describe heat transfer through the assemblies. The model discussed in this paper shows the effects of the fire resistance of wood-stud walls which result from filling the cavities between the studs with either glass-fibre or rock-fibre insulation. When not filled with insulation, radiative and convective heat transfer through the stud cavities play significant roles in the fire resistance of walls. When the cavities are filled with insulation, the insulation impedes both radiative and convective heat transfer, thereby shielding the studs from some of the effects of the fire. Rock-fibre insulation is relatively stable at high temperatures: glass-fibre insulation shrinks and melts. The volume of glass-fibre insulation decreases with temperature. t the same time, an empty space of steadily increasing volume is formed between the gypsum board and the insulation. series of small-scale experiments were carried out to measure the amount of shrinkage in glass-fibre insulation at elevated temperatures. ased on those experiments, algorithms were incorporated into Forintek s heat transfer model for wood-stud walls to simulate the melting behaviour of glass-fibre insulation. INTROUTION Forintek has developed a computer model called WLL2 to predict heat transfer through wood-stud walls protected on each face with gypsum board when one side of the wall is exposed to fire (Takeda and Mehaffey 1995). WLL2 is based upon two-dimensional heat conduction equations for heat movement within the studs and gypsum board, and complete sets of thermo-physical property data for those two materials. Very good agreement was observed between the model s predictions and full-scale test data (Takeda and Mehaffey 1998). Since then, WLL2 has been upgraded to analyze the effect of placing batts of either glass-fibre or rock-fibre insulation in the stud cavities. lthough rock-fibre insulation is relatively stable at high temperatures, there is a small amount of shrinkage at temperatures greater than 7. However, glass-fibre insulation shrinks and melts at significantly lower temperatures. Small-scale experiments were carried out to measure reductions in the volume of glass-fibre insulation when heated at various constant temperatures ranging from 3 to 75. It was found that glass-fibre insulation begins to melt at Its volume then decreases almost linearly with temperature. Figure 1 shows the volume of glass-fibre insulation and rock-fibre insulation as a function of temperature. ecause of the shrinking and melting characteristics of insulation, an empty cavity is created between the gypsum board on fire-side of the wall and the insulation between the studs. Heat transfer within walls containing batts of insulation therefore, is much more complex than walls without insulation. WLL2 considers heat transmission through four different layers, 1) the gypsum board on the fire-side, 2) the empty space in the cavity between the gypsum board on the fire-side and the layer of melted insulation, 3) the layer of melted insulation, including algorithms for the melting process, and 4) the gypsum board on the ambient-side. Heat transfer equations for each of the four layers are solved using finite-difference methods. computer program to solve these equations was written in Visual ++ and MF (Microsoft Foundation lass). The predictions of the model developed in this study were compared to full-scale test data, and also to the model s predictions for an un-insulated wall. 1 Forintek anada orp. Suite 41 TT, 1125 olonel y rive, Ottawa, Ontario K1S 5R1, anada

2 asically, insulation blocks the flow of heat at the interface between the insulation and the gypsum board on the fire-side and shields the wood studs and gypsum board on ambient-side from the heat. Therefore, placement of insulation in the stud cavities improves the walls fire resistance. On the other hand, the heat that is blocked can not dissipate at the interface between the insulation and gypsum board is accumulated in the gypsum board and increases the temperature of the gypsum board on the fire-side of the wall. This may accelerate shrinkage of the gypsum board and hastens the opening of joints between adjacent sheets of gypsum board. This paper discusses Forintek s heat-transfer model, and the effects on the fire resistance of the wood-stud walls which result from placing batts of glass-fibre or rock-fibre insulation in the stud cavities of walls. 12 ROK-FIRE GLSS-FIRE TEMPERTURE TEMPERTURE Figure 1. Volumes of glass-fibre insulation (circle) and rock-fibre insulation (triangle) as functions of temperature Figure 2. Thermal conductivities of glass-fibre (circle) and rock-fibre insulation (triangle), as functions of temperature. MOEL ESRIPTION Heat transfer through the insulation was considered to be a combination of gas-phase conduction, solid-phase conduction and radiation (Kumaran et.al. 1988). The thermal conductivity of glass-fibre insulation k g, was assumed to be functions of temperature and density: k g = a + bt ct 3. (1) a = a 1 + a 2 ρ + a 3 /ρ (2) b = b 1 + b 2 ρ + b 3 /ρ (3) c = c 1 + c 2 ρ + c 3 /ρ (4) where T is temperature and ρ is the density of the glass-fibre insulation. oefficient a corresponds to conductive heat transfer, c corresponds to radiative heat transfer and b to the interaction between conduction and radiation. oefficients a 1, a 2, a 3 are constants, and have been defined as follows: a 1 = 1.492E-2 b 1 = 2.777E-7 c 1 = 3.557E-1 a 2 = 3.274E-5 b 2 = 6.9E-1 c 2 = 7.84E-3 a 3 =.122E- b 3 = 2.235E-6 c 3 = 2.864E-9 The thermal conductivity of rock-fibre insulation k r is almost independent of density, and therefore, was defined as a function only of temperature.

3 k r = a + bt ct 3. (5) a =.35 (6) b = 1.79E-5 (7) c = 6.394E-11 (8) Figure 2 shows the thermal conductivities of glass-fibre and rock-fibre insulation as functions of temperature. Heat transfer through the solid components satisfies the two-dimensional heat conduction equation Eq. (9). p ρ( T/ t)=k( 2 T/ x T/ y 2 ) (9) Where p is specific heat (joule/kg K), ρ is density (kg/m 3 ), and k is thermal conductivity (W/mK). ll of those thermophysical property data are defined as functions of temperature T (Takeda and Mehaffey 1998). oundary conditions at the exposed surface of the gypsum board on the fire-side are given by balancing heat conduction at the surface with the radiative and convective heat input: k( T/ x) = h f (T sf - T f ) + εσ(t sf 4 - T f 4 ) (1) where T sf is the surface temperature of the gypsum board facing the furnace, T f is the furnace temperature, h f is the convective heat transfer coefficient (W / m 2 K) between the gypsum board surface and the hot furnace gas, ε is the surface emissivity of gypsum board and σ is the Stephan-oltzmann constant. The boundary condition at the gypsum board surface on the ambient side of the wall can be described as: k( T/ x) = h a (T sa - T a ) + εσ(t sa 4 - T a 4 ) (11) where T sa is the surface temperature of the gypsum board on the ambient-side, T a is the ambient gas temperature and h a is the convective heat transfer coefficient (W / m 2 K) between the gypsum board surface and the ambient environment. t the interface between the gypsum board and the insulation in the stud-cavity, convective heat transfer is assumed as a boundary condition when the interface temperature is less than the melting (or shrinking) temperature of the glass-fibre or 1 8 F I R E 6 4 GLSS-FIRE WOO STU GYPSUM OR Figure 3. Temperature-time curves at various key locations in the wall, where is the furnace temperature (STM E-119 standard curve) and points to are those key locations in the wall (see right hand side figure).

4 rock-fibre) insulation. The heat-transfer coefficient at the interface is a function of the thermal conductivity of the insulation. Once the temperature at the interface reaches the melting (or shrinking) temperature of the insulation, the surface of the insulation begins to away from the gypsum board. This result creates an empty space between the gypsum board and the insulation. onsequently, both radiative and convective heat transfer must be considered. Thus, the overall fire resistance of the wall assembly is strongly affected by melting of the insulation, and the radiative heat flow from the gypsum board on fire-side attacking the surfaces of the woods. RESULTS N ISUSSION Glass-fibre insulation The model s predictions were compared to the full-scale test data. The full-scale tests were carried out on a wall constructed with nominal 2 x 4 (SPF) wood-studs, 4-mm o.c., and lined with 12.7-mm Type X gypsum board on both the fire and ambient sides. In one wall, the stud cavities were filled with glass-fibre insulation (see Fig. 3 right). The density of the insulation was 12kg/m 3. Figure 3 shows the temperature-time curves at key locations in the wall (locations are shown in the right figure). Solid lines show the theoretical results and dashed lines show the test data. Good agreement was found between the theoretical prediction and test data. In this prediction, the effects of joint opening were considered. When gypsum board shrinks and bends at high temperatures (Takeda 1999), the joints between the adjacent sheets of gypsum board on the fire-side open and hot gases enter the stud cavities from the furnace. This increases 1 8 Joint not open T Joint open Figure 4. Thickness of glass-fibre insulation as functions of time (solid line: joint open, dashed line: joints not open) Figure 5. Temperature-time curves at locations to when joints are not open temperatures within the cavities, accelerates the charring of the wood studs and accelerates calcination of the gypsum board (Takeda 1999). The model predicts the joints between the adjacent sheets of gypsum board on fire-side to open at 15minutes 17seconds. Prior to this event, the filler compound covering the joints begins to fall from the wall at 13minutes 9 seconds. Finally, the fire-side gypsum board was predicted to lose its strength and pull away from the nails attaching it to the studs at 37minutes 2seconds. Glass-fibre insulation in the stud cavities was predicted to begin to melt at 2minutes 22seconds. The thickness of the insulation then decreases with time. Figure 4 shows the thickness of the glass-fibre insulation as a function of time (solid line). It was predicted that the thickness of the insulation would be less than 2cm by 36minutes. onsequently, the temperature at point in Fig. 3 would reach about 4. If the joints between adjacent sheet of gypsum boards are not open, the model s predictions are quite different. The dashed line in Fig.4 depicts the thickness of the glass-fibre insulation when the joints are not open. In this case, the model predicts that the insulation will retain more than half of its

5 initial thickness (8.89cm). The temperature at point (Fig. 5), thereby, is much lower, in comparison with those situations in which the joints are open (Fig. 3). The temperature at point in Fig. 5 is also lower when the joints are not open. This is because the interface between the gypsum board on the fire-side and the surface of the wood-stud (point ) is protected by the gypsum board when the joints are not open. When the joints are open, the surface of the stud at the interface is exposed and directly attacked by the radiative and convective heat from the furnace. Thus, char formation within the wood studs is seriously affected by the opening of these joints. Figure 6 illustrates char formation in the studs when the joints are open (left) and not open (right). The results show that the wood studs are shielded by the insulation for a much longer period of time when the joints are not open. Joints open Joints not open har Melting front of glass-fibre har Figure 6. har formation in wood studs 36 minutes after exposure. Left: joint open, right: joint not open. Rock-fibre insulation Rock-fibre insulation shrinks between 7 and 8 (Fig. 1). However, in comparison to glass-fibre insulation, the amount of shrinkage is relatively small. Figure 7 shows the models predictions of the temperature-time curves at several key locations in a wall insulated with rock-fibre insulation. In the figure to the left, the joints are open, and to the right, the joints are not open. The density of the insulation was 48kg/m Figure 7. Temperature-time curves at locations to in walls insulated with rock-fibre insulation. Left: joints open, right: joints not open.

6 The greatest difference that can be observed between model s two predictions, as shown in Fig. 7, was the temperature at point. When the joints are open (left), the temperature at point is much higher than when the joints are not open (right). This is because the surface of the studs at the interface between the gypsum board on the fire-side of the wall and the wood-stud surface is directly exposed to the fire when the joints open. On the other hand, there is no significant difference in the amount of char formation in studs, irrespective of whether the joints are open or not. Figure 8 shows the amount of char formation in the studs after 5 minutes of fire exposure (left: joints open, right: joints not open). Joints open Joints not open har Rock-fibre insulation shrinking front har Figure 8. har formation in wood stud after 5 minutes exposure. Left: joint open, right: joint not open. This demonstrates that even when the joints between adjacent sheets of gypsum board open, rock-fibre insulation continues to shield the sides of the studs from the fire and thereby improves the overall fire resistance of the walls. No insulation Figure 9 provides a comparison of the model s predictions for insulated walls with those for walls with no insulation in the stud cavities. oth figures show the temperature-time curves at locations to in the walls. The left-hand figure illustrates the effects of joint opening, while the right illustrates those effects when there is no joint opening. The temperatures at point are lower than that at point and close to for both figures, left and right. This is much different from what is shown in Figs. 3, 5 and 7. lso the temperatures at point, compared to those shown in Fig. 7, are much higher. This depicts the fact that the radiative and convective heat transfer through the stud cavity are serious factors, when the stud cavities are empty. TEMPERTURE TEMPERTURE Figure 9. Temperature-time curves at locations,, and when there is no insulation in the stud cavity. Left: joint open, right: no joint open

7 s for the effects of joint opening, it was observed that the temperatures at points, and were all higher when the joints were open (see left figure comparing to right figure). The overall effect however, is not as great as that observed in walls filled with either glass-fibre or rock-fibre insulation. The time when the joints were predicted to open was 16 minutes 23 seconds for un-insulated walls, 15minutes 17seconds for walls insulated with glass-fibre insulation, and 15 minutes 16 seconds for walls insulated with rock-fibre insulation. Thus, the time of joint opening, when walls are filled with insulation, is more than one minute earlier. ONLUING REMRKS Placing insulation in the stud cavities of walls protects the wood studs and the gypsum board on the ambient-side of the wall by blocking heat transfer at the interface between the gypsum board on the fire-side and the insulation. Therefore, insulation generally improves the fire resistance of wood-stud walls. However, insulation shrinks and/or melts at high temperatures. This has a negative effect on the overall fire resistance of walls. Gypsum board also shrinks at high temperatures. onsequently, the joints between adjacent sheets of gypsum board open. The heat blocking effect of the insulation in the cavity accelerates this opening of joints between sheets of gypsum board, and is another negative effect resulting from placing insulation in the stud cavities. Rock-fibre shrinking front Glass-fibre melting front No insulation Figure 1. har formation in studs after 5 minutes exposure when the joints are not open. Left: with rock-fibre insulation, center: with glass-fibre insulation, and right: without insulation Rock-fibre shrinking front Glass-fibre melted away No insulation Figure 11. har formation in studs after 5 minutes exposure when the joints open. Left: with rock-fibre insulation, center: with glass-fibre insulation, and right: without insulation

8 Figure 1 illustrates char formation in studs after 5 minutes of fire exposure when the joints are not open and there is rock-fibre insulation in the stud cavities (left), glass-fibre insulation (centre) in the stud cavities, or no insulation (right) in the wall. Obviously rock-fibre insulation offers the best protection: glass-fibre insulation offers the next best protection. oth insulation products protect the wood studs and improve the fire resistance of the walls. On the other hand, when there is no insulation in the cavities (right figure), the wood studs are attacked by the radiation from the hot surface of the gypsum board on the fire-side of the wall, and a deep char layer is formed along the sides of the studs. Figure 11 shows the same results when the joints are open. gain, rock-fibre insulation provides good protection, but glass-fibre insulation does not. Glass-fibre insulation melted away after 47 minutes. Then, the exposed side-faces of the wood studs were attacked by radiation from the gypsum board on fire side of the wall. s a result, deep char layers were formed in the studs. Therefore, it can be concluded that glass-fibre insulation does not seem to improve the fire resistance of wood-stud walls when the joints open. REFERENES Kumaran, M.K and Stephenson.G Heat Transport though Fibrous Insulation Materials, Journal of Thermal Insulation, 11, Takeda, H Model to Predict Fire Resistance of Wood-Stud Walls - The Effect of Shrinkage of Gypsum oard -, Proceedings of Pacific Timber Engineering onference PTE99, 3, Takeda, H and Mehaffey, J.R Predicting Heat Transfer through Wood Stud Walls Exposed to Fire, Proceedings International onference on Fire Research and Engineering,, Takeda, H and Mehaffey, J.R WLL2: Model for Predicting Heat Transfer through Wood-Stud Walls Exposed to Fire, Fire and Materials, 22, KNOWLEGEMENT Forintek anada orp. would like to thank its industry members, Natural Resources anada (anadian Forest Service), and the Provinces of ritish olumbia, lberta, Ontario, Quebec, Nova Scotia and New runswick, for their guidance and financial support for this research.