Instalation of Wood Burning Stoves & Chimney Liners In Thatched Properties
The National Society has been involved in research and providing advice towards fire prevention in thatched properties for the past 15 years. Following the well attended autumn conference in Salisbury, the end of 2009 has seen a major breakthrough in recognition of the findings of the Society’s research by the chimney lining regulators HETAS, with the publication of HETAS Guide “Chimneys in Thatched Properties” and an amendment to the Building Regulations Approved Document “J” which will carry a paragraph relating to wood burning stoves and chimney liners in thatched properties.
The high number of chimney related thatch fires from 2005 to 2009 has led to the introduction of a number of products aimed at minimising the damage in the event of a fire; claims made for some of these can at best be considered as doubtful.
“The Dorset Model”
The Society actively supports “the Dorset Model” and was part of the team that drafted the most recent version. The “Model” relates to fire protection in new buildings with thatch and recommends only the use of fire resistant board, which in the event of a fire provides a strong physical barrier between the thatch and the rest of the building. Flexible fire resistant membranes have a place in old buildings with uneven rafters where it would be impossible to use solid fire resistant board. The difference in functionality and levels of protection between fire board and flexible barriers needs to be appreciated, each has a place in fire protection for thatch but they are often not interchangeable some advice can be misleading and it is important to differential between the two.
Fire Retardant Sprays
Surface fire retardant sprays are of limited value, it is true that when freshly applied they do slow down the rate of spread of fire, but they are only retardants. Unfortunately to be even minimally effective they require renewal every five years. However, there might be a place for them in properties which could be vulnerable from an arson attack. Retardant spray treatments will be ineffective in the majority of chimney related fires which start deep within the thatch.
External Chimney Cladding
The new HETAS thatch protection document recommends inserting a physical barrier between the chimney and the thatch, in new buildings this is easy to achieve by constructing a double skinned block stack with an air gap between the courses. For existing buildings with single brick chimneys a different approach is required, wrapping the chimney in either fire resistant board or flexible fire resistant membrane is inappropriate; for although both resist physical fire, heat will pass through them, and as an insertion between thatch and the chimney they perform just like an electric blanket buried under bedclothes and over time both materials will allow the thatch to get hot!
Where the chimney is exposed during a major re-thatch passive fire protection can be inserted by cladding the chimney in a wrap made from 6mm thickness of aluminium sheet, this can be achieved by using two sheets of 3mm thickness, for ease of cutting and handling. Aluminium is a cost effective, highly conductive metal which works by acting as a heat sink and spreading the heat load thus eliminating hot spots that are the main starting point of chimney related thatch fires. A thin layer of bonded aluminium film such as that associated with flexible fire resistant barriers is too thin to work since insulation is not what is required, it is heat extraction. (A thin layer of Insulation (i.e. less than 100mm) in this area will not reduce the risk of fire!).
Thatch Fire Risk Management
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