PVC-U windows in fires
PVC-U exhibits excellent fire behaviour and it does not burn once the source of heat or flame has been removed.
UK Building Regulations do not stipulate any fire performance standards for the materials used in window frames. Whilst no degree of fire resistance (as defined by BS 476 : Part 8) can be achieved by PVC-U window units, the large scale fire tests carried out show no difference between PVC-U and wood under the conditions of test.
PVC-U can, when correctly formulated, achieve high ratings under the Building Regulations (1985) Approved Document B when performance is assessed by BS 476 : Parts 6 and 7.
PVC-U is very difficult to ignite using commonly available ignition sources (match, blow lamp etc.). Tests with a wide variety of sources varying in heat intensity and impingement area on PVC-U window frames show that the product only burns whilst the source is applied. When the source is removed there is no residual flame on the product. In terms of ignitability, the temperature required to ignite PVC-U is more than 120oC higher than that of pinewood (385oC for PVC-U and 260oC for wood as defined for self ignition). Once a material has been ignited the flammability can be defined in terms of the Limiting Oxygen Index (LIO) test. This defines the amount of oxygen that needs to be present for a material to burn freely. A material with an LIO of 21 will burn freely in air (which contains 21% oxygen) and one with an LIO of more than 21 will not burn in air at room temperature. PVC-U has an LIO of about 50, compared with the wood LIO of 21. This shows that PVC-U will not sustain combustion in air at room temperature and is better than wood in this test.
The limited burning of PVC-U is confirmed in a variety of other standard fire tests, which measure specific parameters such as rate of heat release and flame spread under different conditions.
The conclusions are clear:
1) The rate of heat release and total heat released by PVC-U are significantly lower than most organic building materials.
2) When flames do contact PVC-U, it forms a protective charred layer which insulates the material below and excludes the oxygen necessary for combustion. This restricts the burning zone. In addition any HCl emitted acts as a combustion inhibitor.
PVC-U is very difficult to ignite using common ignition sources.
Smoke is the result of incomplete burning of a material and consists of solid or liquid particles in the combustion gases. Smoke densities are similar to wood under smouldering conditions but greater under flaming conditions. The combustion gases (e.g.. HCl) may lead to some corrosion of metallic materials but restoration is normally possible. The corrosion gases have no effects on the structural elements of buildings. The toxic potency of the combustion gases of PVC-U is similar to and certainly not significantly worse than those of many natural materials. The build up of toxic fumes will be slow compared with rapidly burning materials of a similar toxic potency.
The rate of generation and quantity of smoke and fumes produced by a PVC-U window will depend on the severity of the external source applied. The smoke and fumes emitted will be confined to the area of the product affected by the source and their transport away from the impingement zone will depend on local factors such as ventilation and survival of the glazing.
In a typical domestic fire the PVC-U window frames will not materially affect the progress of the fire or the possibility of personal injury. Most deaths in fires are caused by smoke or fume inhalation. In a typical domestic fire the occupants are likely to suffer from the smoke inhalation effects from burning carpets, settees, curtains etc. before the PVC-U in the window frames has even begun to emit smoke or fumes.
Hydrogen chloride gas present in smoke will be very noticeable to people in the area of the fire at low concentrations (below the harmful levels) and will provide a warning of the fire that carbon monoxide alone will not do. The HCl is therefore a good warning sign rather than a danger.
The fire resistance of a glazed window is mainly influenced by the fracture behaviour of the glazing at high temperature. The fire resistance of glazed PVC-U window frames is generally found to be similar to that of glazed wood window frames.
In a research programme carried out by the Fire Research Station, the performance of PVC-U window frames in fires was compared with that of traditional wood frames in a typical domestic room. All windows were double glazed and both a large fuel load / non-ventilation controlled fire and a medium fuel load /ventilation controlled fire were used.
The conclusions of the report were:
1) Little damage was evident to both PVC-U and wood windows until the glass panes were displaced at approximately 250oC to 400oC. Glass panes failed by cracking and falling out in a random manner.
2) After failure of one glass pane the increased ventilation changed the mode of the fire and accelerated the fire growth. In most tests the other panes fell out soon after.
3) Wood frames burned after the displacement of glass while the PVC-U window frames softened and the casement sometimes fell out. There was some evidence of combustion of the PVC-U, PVC-U windows did not show any aspects of performance which could create new hazards in fire involving buildings.
4) Carbon monoxide, produced mainly from the wooden fuel under low ventilation conditions, was the major toxic hazard in each test and was produced in volumes that would prove lethal in regions where ambient temperatures would allow survival.
5) The concentrations of carbon monoxide were noticeably lower in the fire involving only PVC-U frames; this was possibly caused by a low rate of burning in this test.
Experience in the field suggests that PVC-U frames have been successfully reglazed to secure a building after fires that have gutted the internal structure of the building.
The base PVC-U material has good fire properties and PVC-U windows give a satisfactory performance in fires in comparison to other materials.
Last edited: 20/05/15
© Tangram Technology Ltd. 1998
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