Energy Efficiency and Historic Buildings Insulating Thatched Roofs This guidance note has been prepared and edited by David Pickles. It forms one of a series of thirteen guidance notes covering the thermal upgrading of building elements such as roofs, walls and floors. First published by English Heritage March 2012. This edition (v1.1) published by Historic England April 2016. All images © Historic England unless otherwise stated. Illustrations drawn by Simon Revill. Our full range of guidance on energy efficiency can be found at: HistoricEngland.org.uk/energyefficiency Front cover: Thatch repairs in progress. © Philip White. Summary This guidance provides advice on the principles, risks, materials and methods for insulating thatched roofs. There are estimated to be about fifty thousand thatched buildings in England today, some of which retain thatch which is over six hundred years old. Thatching reflects strong vernacular traditions all over the country. Well-maintained thatch is a highly effective weatherproof coating as traditional deep thatched eaves will shed rainwater without the need for any down pipes or gutters. Locally grown thatch is a sustainable material, which has little impact on the environment throughout its life-cycle. It requires no chemicals to grow, can be harvested by hand or using traditional farm machinery, requires no mechanical processing and therefore has low embodied energy and can be fixed using hand tools. At the end of its life it can be composted and returned to the land. Thatch has a much greater insulating value than any other traditional roof covering. With the right choice of material and detailing, a well-maintained thatched roof will keep a building warm in winter and cool in summer and has the added advantage of being highly sound-proof. The guidance stresses that changes to improve the energy performance of thatched roofs should only be attempted where really necessary as many thatched buildings already provide adequate thermal performance and where the traditional ‘breathable’ performance of the building will not be compromised. Contents Introduction .........................................1 3 Thatched Roofs as Warm Roofs ....9 Energy Planning .......................................................1 3.1 Adding insulation from above ..................11 Technical Risks .........................................................1 3.2 Adding insulation below the rafters .........12 Technical Details ......................................................2 4 Thatched Roofs as Cold Roofs ....14 1 Thatched Roof Construction ........3 1.1 Thatch covering ...........................................3 5 Where to Get Advice ...................16 1.3 Roof structure ..............................................6 1.4 Internal finishes to thatched roofs .............6 5.1 Contact Historic England ..........................17 2 Energy Efficiency Measures .........7 2.1 Thermal properties of thatched roofs ..............................................7 2.2 Cold roof or warm roof? ..............................8 Introduction Energy Planning Technical Risks Before contemplating measures to enhance the Altering the thermal performance of older thermal performance of a historic building it is buildings is not without risks. The most significant important to assess the building and the way it is risk is that of creating condensation which can used in order to understand: be on the surface of a building component or between layers of the building fabric, which the heritage values (significance) is referred to as ‘interstitial condensation’. of the building Condensation can give rise to mould forming and potential health problems for occupants. It can the construction and condition of the also damage the building fabric through decay. building fabric and building services Avoiding the risk of condensation can be complex as a wide range of variables come into play. the existing hygrothermal behaviour of the building Where advice is given in this series of guidance notes on adding insulation into existing the likely effectiveness and value for money permeable construction, we generally consider of measures to improve energy performance that insulation which has hygroscopic properties is used as this offers a beneficial ‘buffering’ effect the impact of the measures on significance during fluctuations in temperature and vapour pressure, thus reducing the risk of surface and the technical risks associated interstitial condensation occurring. However, high with the measures levels of humidity can still pose problems even when the insulation is hygroscopic. Insulation This will help to identify the measures best suited materials with low permeability are not entirely to an individual building or household, taking incompatible with older construction but careful behaviour into consideration as well as the thought needs to be given to reducing levels of building envelope and services. water vapour moving through such construction either by means of effectively ventilated cavities or through vapour control layers. The movement of water vapour through parts of the construction is a key issue when considering thermal upgrading, but many other factors need to be considered to arrive at an optimum solution such as heating regimes and the orientation and exposure of the particular building. < < Contents 1 More research is needed to help us fully understand the passage of moisture through buildings and how certain forms of construction and materials can mitigate these risks. For older buildings there is no ‘one size fits all’ solution, each building needs to be considered and an optimum solution devised. Technical Details The technical drawings included in this guidance document are diagrammatic only and are used to illustrate general principles. They are not intended to be used as drawings for purposes of construction. Older buildings need to be evaluated individually to assess the most suitable form of construction based on a wide variety of possible variables. Historic England does not accept liability for loss or damage arising from the use of this information. < < Contents 2 1 Thatched Roof Construction Thatched roofs are as ancient as civilisation and 1.1 Thatch covering are intrinsically linked with our agricultural past. Straw, a waste product from cereal production The styles of thatch most likely to be encountered was the most common thatching material but on a roof today are long straw, combed wheat in areas adjacent to water, such as the Fens and reed and water reed. These all have different Broads, reed was also used. Other materials such performance characteristics and have to be fixed as heather, gorse and ferns were also used in slightly differently. Consent is usually required to other parts of England. change the type of on a listed building. Thatch fell out of favour in densely populated Long straw urban areas where it was a fire risk. In rural areas, Today this is mainly winter wheat grown the threshing machines kept thatch plentiful specifically for the purpose, sometimes by the and cheap until the railways of the industrial thatchers. Modern varieties of wheat cannot be revolution brought Welsh slate to most areas of used because the straw is too short. Long straw the country and thatching started to decline. thatch is often said to have a ‘poured on’ texture. Eaves are cut to shape and verges are either cut or rolled. Traditionally long straw is a ‘multi-coat’ material as the primary coat is seldom removed and successive coats added when the top layer wears down. Historically, long straw roofs are ridged in the same material with the straw wrapped over to form the ridge. Hazel rods, often forming a pattern over the top, hold the ridge in place. Water reed Water reed is harvested from natural reed beds. About three quarters of the water reed used in England today is imported from France, Hungary or Turkey or from further afield. Figure 1 Traditionally water reed is a ‘one coat’ material, Thatched roofs have a much greater insulating value and since it follows the roof structure tends to than any other traditional roof covering. A well create a neater more angular appearance than maintained thatched roof will keep a building warm in winter and cool in summer and has the added benefit either long straw or combed wheat reed. In East of being highly sound-proof. Anglia this is partly because the tradition has © Edifice. always been to totally strip the old reed and < < Contents 3 re-thatch with new. As reed is a relatively 1.2 Repairs inflexible material, it cannot be dressed over a ridge, so the ridges on a reed roof are usually of a Areas of decay in a thatched roof can be removed different material such as sedge or straw, and patch-repaired. It is common practice for often in a block pattern. both long straw and combed wheat reed roofs to have just the decayed top surfaces removed and In recent times, imported water reed has been then to be over-coated (spar coated) rather than championed as a superior material as it is easier stripped completely to the rafters. The spar coat to use and requires less preparation. It also is fixed to the layer beneath it with twisted hazel requires minimal maintenance and possibly has spars. The practice of over-coating saves both a longer life span than long straw or combed time and materials, and has led to many older wheat reed. For this reason, many thatched roofs being composed of layers many hundreds buildings previously covered with straw have of years