Heat Transfer and Thatch – Is it a debate?

Thatch Fires and Heat Transfer – Is it a Debate?

Recent discussions with various groups have led to us being asked if there is a debate regarding heat transfer and thatch fires.  We decided to tell the story as we see it so you can decide.

Background on Heat Transfer and Thatch Fires

For many years there has been a growing restlessness, throughout the ‘thatch’ community, over the fact that the thatch fire numbers hardly changed despite the well-publicised heat transfer theory information.  The only reduction was apparently during wet winters.

Source – Fire statistics from the Home Office

Many of our associates have been involved in the world of thatching for over 30 years, as have we, and so our understanding and passion for reducing the thatch fire numbers is longstanding.  Our aim has always been for the good of thatch and to communicate with everyone.

Progress and moving forward to benefit thatched properties has always been part of our remit.  It is understood that there will always be resistance to change, especially when it is easier and less awkward not to.  Avoidance is easy and to discuss the various views is harder but perhaps necessary?   Our plan is to explain some of the points which have recently been made to us in conversations with a variety of people (thatchers, organisations, insurers, homeowners, chimney specialists, building surveyors, stove installers, flue specialists to name a few) and put the information as clearly as we can for you to make up your own minds.

A case of don’t shoot the messenger.  However, if people start understanding, talking, sharing knowledge and dealing with information, progress can be made, perhaps without debate.

The points on Heat Transfer:-

  • What is Heat Transfer in relation to thatch?
  • Forensic Investigation and Heat Transfer
  • Fire Protection Association Research and Heat Transfer
  • Other points raised on Heat Transfer
  • Progress since the updated Heat Transfer information
  • Conclusion on Thatch Fires and Heat Transfer

What is Heat Transfer in relation to Thatch?

Heat Transfer is conduction of heat so, in the case of thatch, it is heat transferring through sound brickwork to thatch.  Thatch reaching temperatures of over 200 degrees is at danger of ignition.  Conduction of heat through brickwork is not to be confused with convection of hot gases through gaps in the brickwork, perhaps due to perished mortar.

Originating from the report –  Angold R.E., Sadd P.A., Sanders M., (1998) Fire and Thatch, the heat transfer theory and associated diagram was published and quickly became part of building regulation process.  The word theory was dropped, we have been asked many times “when?” and, we are not certain of how long but, over 15 years ago has been mentioned.  The heat transfer idea was widely promoted and fire service advice and insurers all shared the diagram and concept.

Quote below from ‘Thatch Fires by Marjorie Sanders’ in the Building Conservation Directory 2006.

“Thermal transmission from an ordinary brick chimney into thatch:  Where the thatch surrounds the chimney, the insulating effect of the thatch prevents heat loss from the brick outer surfaces in contact with the thatch.  If the flue gas temperature is maintained at 300°C, the chimney breast above the fire is barely warm to the touch.  However, where it passes through the thatch the temperature rises, and in an area where the thatch is around 1m deep, the bricks will heat through to the point where the temperature will be high enough to ignite the thatch after about 14 hours.  The deeper the thatch the greater the risk”

We have been asked, “At what stage, after the theory was published, was evidence provided that it was actually happening? “  We cannot answer this but to receive such evidence would be useful to all who have a strong interest in understanding what is really going on and helping to reduce the thatch fire numbers.

When installing a woodburner, the lining of chimneys with an insulated flue is now part of the building regulations.  This surely, you say, would address the problem of heat transfer causing problems of thatch fires?  It also can address problems with old chimneys whose integrity may be compromised and at risk of heat moving through the brickwork by convection.   If that is so then, why are there still so many thatched fires in properties with lined flues?

It was also being noted by many that the risk of sparks, embers and chimney integrity were not being discussed/addressed.

Many people strongly believed that dependence on heat transfer as the main cause of thatch fires was hampering the progress of thatch fire prevention.  Furthermore, there was concern that convection of hot flue gases through damaged old chimneys was being wrongly attributed to heat transfer, as were the embers from chimneys which landed around the chimneys

Forensic Investigation and Heat Transfer

The problems with that dependence, thatch fire numbers not reducing despite lining of chimneys and huge losses due to thatch fires, led to the forensic investigation report from Keith Benjamin of Burgoynes, looking at thatched fires since December 2008.

Initially this looked at 61 thatch fires and the most recent update included 148 actual cases up to May 2016.

The conclusion of these findings:-

“In conclusion, the evidence from this survey provides a strong association between thatch fires that started in the thatch and the use of wood burning stoves with lined chimneys below the current recommended height. It further supports the view that the majority of fires associated with chimneys arise from ejected embers, often generated during lighting the fire, rather than defects in chimneys or heat conducted through the structure”

Heat Transfer had therefore been queried as the major cause of thatch fires based on forensic evidence.  We have been careful not to say heat transfer as a cause of thatch fires has been dismissed.  The fact remains, however, that based on actual investigation of real fires, it was so far unproven, based on evidence, timescales and facts.

Fire Protection Association Research and Heat Transfer

Historic England had a strong interest in what was going on (as the majority of thatched properties in England are Listed), as did the NFU Mutual (a major thatch property insurer).  Once the forensic investigation report had highlighted embers and chimney fires as the main causes, it raised more questions.  They commissioned research at the Fire Protection Association to find out more about woodburners, chimneys, flues and thatch.

Points made on Heat Transfer following this extensive research has been published in an article in the Building Conservation Directory 2017 entitled ‘Thatch Fires and the Role of Wood-burning Stoves’.

“The research has demonstrated that ‘heat transfer’ (conduction of heat from flue gases via sound brickwork) is very unlikely to be a prevalent mechanism of thatch ignition.”

The final Guidance on this research is awaited shortly (and we will be sharing it with everyone as soon as it is available).

Other points raised on Heat Transfer

If the majority of thatch fires are proven to be caused by ejected embers and chimney fires then we feel obliged to help people by telling them this.  Chimney integrity is also an important factor.   If a property has a lined flue (per heat transfer), the additional information on sweeping the chimneys and being careful what and how the owners burn fuels could well make the difference between that property having a thatch fire or not.

Relying on just the liner has not reduced the thatch fire numbers to date as proven by the home office statistics shown above.

A point made to us was that if heat transfer was a major cause, wet weather conditions would not affect the thatch fire numbers.  The thatch fires occurring soon after fires being lit has also added to this discussion on heat transfer in that there was no evidence of enough time for bricks to get hot before fires started.   Also, low chimney height would not be a factor in thatch fires if they are being caused by heat transfer.  Chimney height is, however, a factor based on the Burgoynes survey.

Another point made was that maintaining the heat in the chimney long enough for heat transfer to occur was not feasible under real burning conditions.  A chimney fire is indeed hot but for a short time and even an unlined flue with a woodburner may not maintain such levels of heat.  It is also understood that 10 degrees of heat is lost per metre of flue.

Claims that the fire risk in thatch can be designed out are also interesting.  Based on the above evidence it would appear not all risks can be.  Looking at all aspects and each individual property it should be possible to reduce the risks considerably both improving design e.g. chimney integrity and height and addressing the other important factors unrelated to design but due to human intervention e.g. what materials are burnt and how e.g. woodburner or open fire, sweeping chimneys, wood moisture level etc.

HETAS new ‘Woodsure’ initiative further evidences that what and how you burn is beneficial for air quality and chimney fire risk.  It will be interesting to see what their proposed thatch guidance review information will cover.

The problem seems to be people not wanting to move forward with this new evidence.  It may not be the whole story but it is very clear proof on the main causes which can be used use to help people reduce their fire risks, now, this burning season.

Regardless of any debate, if you line your flues (per heat transfer and to meet the building regs) and against bad chimney integrity YOU CAN DO MORE – reduce the current proven main risks as well by taking care of what and how you burn.   The more we discuss this topic, the more is understood so the greater the ability to reduce the risks and the thatch fire numbers.  Surely it is a no brainer?

People want to heat their homes with wood and it will be sad day, if for lack of working together, this becomes untenable.  Especially when just sharing the additional information could help.

As the numbers of wood-burning stove installations and use of open fires continues to rise, we hope that in providing information on best practice it will help to reduce the risk of thatch properties catching fire from the main proven causes e.g. ejected embers and chimney fires.  Heat transfer can still be addressed but promoting awareness of the recent evidence is imperative.

Is it a debate, or just progress?

Progress since the updated Heat Transfer information

Our tenacity in wanting to promote this additional information before the next burning season means we continue to communicate with the larger organisations on this topic.  It is understood that such organisations cannot change things overnight.  We have, however, had confirmation from HETAS …

HETAS are in the process of reviewing current thatch advice in mind of additional information, research and product development.  Chimneys in Thatched Properties (HTN001) remains current

Whether this review will be in time for this burning season we don’t know, as we have yet to have a response on timescales, but importantly progress is being made.

Recently we spoke to Historic England who confirmed that:-

The first three phases of research into causes of fire in thatched buildings, carried out by the Fire Protection Association on behalf of Historic England and NFU Mutual is complete, and the research report will be published within the next couple of months. A leaflet for owners of thatched buildings, offering advice based on the research, will be published at the same time. It is planned that these publications will be launched at a series of regional events in November.”

This information is also being included in our next Newsletter so, as soon as these events are confirmed we can let you know via the website and our social media.

If you prefer we could send you an email with this information.  Please email with the Subject ‘FPA Update’.  We will then happily email you the information.  #workingtogether

Conclusion on Thatch Fires and Heat Transfer

The heat transfer risk has been addressed for many years, and the thatch fire numbers have not significantly reduced, except we understand on wet winters.

Forensic investigation of 148 thatch fires and recent FPA research has not proven heat transfer or even for it to be in the running as a major cause.   Ejected embers and chimney fires have been proven as a high risk.  If these issues are addressed, along with lining chimneys per heat transfer (which also addresses integrity problems) then this must be an improvement.   Surely, if these issues are classed as a priority, the incidence of thatch fires can be reduced, which will also be a beneficial factor in insurance and peace of mind.   A simple matter of statistics.

As with all things change and progress is inevitable and resistance will probably be futile.  If further evidence shows heat transfer as a proven major factor in thatch fires then it will be reported in the same way as the Burgoynes and FPA proof.   Until then, our thatch fire safety advice is based on the up to date information and evidence at this time.

There will always be resistance to change and progress but, as the change at the current time is just adding to the information, perhaps there is no debate, just progress with evidence.

Our aim has always been to provide the best information we can, and we hope this article has helped you understand the situation more, so you can make up your own mind on thatch fires and heat transfer.

Communication of all progress is a constructive method of improving thatch fire safety advice, so please read our updated Thatch Fire Safety leaflet.

Thank you for your interest in reading this.  Join us on Social media (FB and Twitter) for more timely updates on this topic.    Rest assured, we will keep you updated.

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