Many of the existing reed thatched houses were – in contrast to new buildings – built with conventional construction methods (e.g. timbered houses) and in a time without universal standards. The higher expectations of today´s inhabitants lead to a necessary adjustment of thermal standards and to a number of construction works.
Frequently, the conversion and the restoration of conventionally built houses takes place considering the energy saving regulation and is split into consecutive stages due to limited financial means.
The most effective remedial measures:
- The change from stove heating to central heating. The stove heating heats individual rooms by convection and radiant heat. The used, humid air escapes together with the stack gas through the chimney.
Fresh air is supplied by leaky doors and windows. A clear indicator for a stove heating is frostwork on the window panes in combination with a dry room climate. The central heating however equally heats the entire house, not only the present living space.
A central heating powered with gas does not need a chimney as the fresh air is let in from outside and the exhaust is led outside. When converting such a house, the existing chimney is sealed although the heated, humid air mainly escapes through the flue.
Now you do not have to keep internal doors shut any more and humidity can spread through the whole house. That is why proper and regular airing is now absolutely essential.
- Fitting New Doors and Windows. When leaky old doors and windows are replaced with new almost air-tight ones, the ventilation properties of the building can noticeably change. Modern windows reach an Ug value of up to 1.3 W/m2K and due to up to three gaskets they are completely air-tight. The windows have to be fitted in such a way that the inner gasket seal separates outdoor and indoor air and makes the window water vapour diffusion-tight. The prior „self-airing” through leaky doors and windows and through possible chinks caused by careless installation has an end – now the responsibility for the airing lies with the user of the building.
Because many house owners are over-challenged with this task (reasons for this are e.g. insufficient information, the attempt to save energy, being susceptible to draught, laziness, etc.) very often houses are aired too little. There is the acute danger that problems with the excess humidity might arise which have an negative effect on the durability of thatched roofs.
- The resurfacing and brushing of internal walls without paying attention to the wall build-up or the formerly used building materials. If a soft non-vapour-retarding underground, open to capillarity action (as for example a latticework facade) is coated with a tight and sealing coat of external or internal plaster (e.g. plastic-modified) this impedes the necessary transport of humidity. A wall build-up which allows capillarity action and diffusion calls for an equally permeable plaster. The same applies for the coating. Soft surfaces allowing capillarity action and diffusion should not be sealed by the undercoat or the top coat (e.g. a latex paint), they should not be over stabilised or embrittled. The International Association for Science and Technology of Building Maintenance and Monuments Preservation (WTA), for instance, recommends in its technical bulletin 8-9-00/D (Instruction Manual for Thatched Houses) to avoid the use of vapour-tight paint coats on the facade and on timber. Furthermore, it recommends to avoid the use of vapour-tight joints and warns to apply such jointings where latticework/compartment or latticework/latticework meet.
- Additional Insulation Measures with Inside or Outside Insulation. When renovating a thatched house very commonly insulation measures are taken with the aim of improving the energy saving properties of the house. If the insulation of a thatched house is altered and improved after its completion, the constructional properties of the building change. If an inside or an outside insulation is employed depends mainly on economic factors and conservation orders. If the facade of a thatched house is to be preserved, inside insulation is applied. In all other cases, usually, an outside insulation is used.
To find an adequate insulation that suits the construction method of the house (timbered house, solid building, frame construction) plays a decisive role for the effect of these insulation measures on the durability of a thatched house. If, for instance, the exterior walls of a thatched timbered house are fitted with a vapour-tight inside insulation, the humidity of the inside air which until then used to escape through the walls, due to diffusion and capillarity, is trapped. As a consequence, the airing habits of the inhabitants have to meet even higher standards. If the users do not change their airing habits, the increased humidity can lead to structural damages on the building.
If, in addition, the vapour-barrier is not completed to be 100% vapour-tight, then condensation damages will occur in the respective parts of the house. As a rule, if you choose inside insulation for thatched timbered houses you should not apply a vapour-barrier or a vapour-retarding layer on the inside, in order not to prevent the humidity transport of the latticework which works by diffusion and capillarity. If you employ an inside insulation system it has to be adjusted to match the special local requirements. Thermal protection and moisture protection have to be weighed against each other. Furthermore, you have to keep in mind the effects that are caused by the specific infilling.
- Renewal of Floor Covering. If the floor or the floor covering of a thatched house is renewed with a more steam-tight version, the moisture behaviour of the house is affected. Cellarless old buildings (e.g. old farmhouses) quite often featured ventilated floors. The timber beams were directly placed in a bed of rubble and covered with floor boards. If in the course of a redevelopment the original build-up of the floor is replaced by a concrete slab, 7 cm of insulation, vapour-retarding bitumen sheeting, a cement layer and glazed, vapour-tight flagstones, then the sorption behaviour of the floor is affected and the moisture load of the house increases.
The thatched roof has always played the most important role in the natural ventilation of a house. However, through the measures described above it becomes the weakest link of the building in terms of vapour-tightness and often even of air-tightness. In the case of a mix between hot and cold roof (e.g. a hot roof without a vapour-barrier) most of the superfluous humidity will escape through and into the roof.
This occurs especially in winter, when the difference in temperature between inside and outside is the most extreme and people do not air enough (energy loss), thus creating a high vapour pressure.
As long as the surface of the thatched roof is dry and clean the roof due to its vapour-permeability will be able to release this amount of moisture back to the outside air.
If the surface of the reed is wet, which is quite often the case in autumn and winter (quite often even for longer periods), the humidity will condense at the wet outside of the thatched roof.
The outside of the roof will become wet and wetter from within. If now an additional inside insulation without a vapour-barrier is installed the problem gets even bigger. On the one hand the moisture transport will continue and on the other hand the water vapour meets an even colder roofing where it is forced to condense.
If the outside of the thatched roof is already infested with algae and moss, the time needed to dry will considerably increase and the life span will be reduced.