Proper Ventilation of the Thatched House

New laws, regulations, building materials, structural designs and new ways of realising such buildings have an effect on the new constructions, as well as on the restoration of roofs.

The Development of the Design of Thatched Houses

The Past
In the past houses were erected using local building materials. The use of clay, straw, timber and natural stone prevailed. Such houses built of organic materials were perfectly adapted to the climate. The absorption, accumulation or diffusion of moisture were of no concern. There was only a slight difference between indoor and outdoor temperature, as the open fire radiated radiant heat. The heated air and the smoke disappeared through the roofing. Permanent ventilation contributed to the conservation of the organic building materials. The Recent Past
The demand for more living comfort led to several changes: Now the floor was made of timber, the walls of brick and an additional ceiling separated the living space from the attic. The indoor environment could be controlled more easily, but the methods employed for the construction of the floor, the walls and the roof and their realisation still provided sufficient ventilation. The average house was equipped with either a fireplace or a wood burning stove which heated the rooms. The ventilation occurred through windows and air grilles as well as through the chimney and helped to prevent damages caused by humidity. In the warm areas there was a low air humidity. The Present
Nowadays the methods of construction are characterised by the implementation of the energy saving regulation and the use of rigid and inelastic materials and structures. The air tightness and the water tightness of buildings has steadily increased since the Energy Saving Act from 1976 and the associated regulations (thermal insulation regulation and energy saving regulations) from 1977. Higher requirements for thermal protection call for thicker layers of insulation on the exterior walls and well insulated doors and windows. If the building envelope is not sufficiently air tight, air flowing through can make the heat insulation ineffective as the insulation only works when it is not damp and the air can remain inside the insulation for a longer period of time. In the insulation the air tightness is ensured by vapour-retarders or barriers. Doors and windows are made air tight with rubber gaskets. Expansion joints in the construction neutralise the forces that occur due to fluctuations in temperature and humidity. The ventilation of the building through deficiencies in the construction, which could take place in houses built the traditional way, now is almost impossible. Modern heating installation and the stop of uncontrolled ventilation add to an optimal thermal separation between indoor and outdoor environment. At the same time the production of moisture and humidity inside the house remained the same.

Reasons for Humidity (Water Vapour) in Residential Buildings

Generally, there are two sources of water vapour in residential buildings: Moisture caused by the inhabitants of the house and the moisture arising from the house itself and the building process, called construction moisture. Humidity caused by inhabitants
Responsible for the production of this kind of moisture are the inhabitants: Breathing and sweating (approx. 1-2 litre/person), having a shower (approx. 1.5 litre/person), cooking (approx. 1.0 litre), doing the laundry (1.5 litre/4.5 kg of spun laundry), plants (0.5-1 litre per indoor plant), wiping laminate or tiles and many other activities add up to a total amount of about 12 litre of water vapour a day for a family of four. This vapour strives to compensate the gradient of vapour pressure and thus diffuses from the warm to the cold or, in our case, from the inside to the outside. Humidity in the Kitchen
Cooking generates a great amount of humidity. If an exhaust hood removed this humidity directly from the air, then the humidity produced should not cause any problems. Unfortunately, this is not possible in practice as there is more water-saturated air, than the average exhaust hood can cope with. If for instance the production of humidity takes about 30 min., the condensation process, however, lasts 2 hours. Up to 4 hours after cooking the surrounding materials (walls, wall papers, dish cloths, curtains) emit the absorbed humidity back to the air.
Most exhaust hoods do not have a ventilation pattern that reflects this process. The exhaust hood is usually only employed during cooking and switched off afterwards because of the annoying noise it makes. However, for at least the next 5 hours a ventilation of 21 litre/sec would actually be necessary. Of course, this only applies for exhaust hoods with exhaust through the roof. If an exhaust hood uses an air recirculation system it might be able to remove the smell, but it cannot reduce the humidity at all. Humidity in the Bathroom
We have to distinguish between the production of humidity through having a shower and having a bath. When having a shower, it is impossible to avoid saturating the air of the bathroom with water vapour. The amount of humidity produced is much higher than when having a bath. When you have a shower about 1.5 litre of vapour are produced per person. Having a bath, you only produce about 1 litre. After showering for only 4 minutes the air in the bathroom is already completely saturated and fog appears. This saturation lasts for approximately 10 minutes. Also, the air temperature rises by some degrees when having a shower. Every day you have a shower the walls and the ceiling will be damp for several hours. The same phenomenon we have observed in the kitchen occurs in the bathroom – here, however, it is even stronger: The surrounding materials (walls, towels, curtains etc.) emit the absorbed humidity back to the air. Especially in the bathroom, choosing the right building materials is of utmost importance. The amount of ventilated air necessary for the transport of the water vapour would need such a high airspeed that the strong draught might cause problems. An air change rate of at least 100 litre/sec. would be needed. Due to this fact especially bathrooms built into closed rooms often cause moisture problems (e.g. mould growth on the reveals of the windows).
A natural dissipation is advantageous, as it causes a continuous and permanent air exchange. As a conclusion, we have to realise that it is almost impossible to remove humidity produced in the kitchen and in the bathroom by means of mechanical exhausting devices. Construction Moisture
Construction moisture is the natural moisture contained in the building components (e.g. masonry) and in the building materials (e.g. mortar, plaster, screed, stone, concrete) of a new building or of a restoration project. Also the weather (e.g. driving rain) can bring moisture into the carcass. Sometimes considerable quantities of water (up to 3000 litres in new buildings) can be found in newly built houses and redevelopments and need up to two entire heating season to dry. When the house is not yet inhabited its ventilation plays an important role in getting rid of the construction moisture. Straight after the completion of the carcass and the roof structure new constructions are furnished with doors and windows, which because of fear of theft are kept locked most of the time. In addition, the installation of the heating system in the majority of cases takes place at a later stage, so that the new building remains unheated for quite a long period.

Effects of Construction Moisture and humidity causes by inhabitants the Durability of Thatched Roofs

The Transport of Humidity
The transport of humidity through building components can happen by vapour transport (diffusion and convection) or by liquid movement (capillarity). Water vapour diffusion is the process of pressure compensation, where air with a higher vapour pressure (warm, humid air) tries to move in direction of air with a lower vapour pressure (cold, dry air). In a building, due to the vapour pressure gradient, the warm and humid internal air diffuses through building components (e.g. the thatched roof or the exterior walls). The diffusion process takes place at all times. When diffusing the air cools down inside of the different building components. In the worst case this can lead to condensation inside of the building components.
Convection is an air current which arises where the warm and humid indoor air can penetrate into the construction through open gaps. This air also cools down and can condense. Convection is a problem because compared to the process of diffusion the amount of condensation water penetrating the building components can be 1000 times higher.
The capillarity determines how fast a building component absorbs a certain amount of water when it comes in direct contact with moisture (driving rain, splash water, soil moisture, condensate) and it determines the rate of drying of building components inside of which water vapour transported by diffusion has condensed into water. Mostly, the thatched roof is the least steam-tight building component of a house, so that the better part of the moisture leaves the building through the thatched roof. The reason for this is the warm air from the lower floors which absorbs the construction moisture and the moisture produced by the inhabitants, rises up, concentrates itself below the thatched roof and then penetrates the roof by diffusion and convection. At present the role of the capillarity in this process is not fully understood. The amount of construction moisture in the building components and the building materials is a decisive factor for the durability of a thatched roof. Another important point is how fast this humidity can be transported to the outdoor air. If a house is not ventilated properly during the construction stage (or the redevelopment), as well as afterwards, the humidity will in the future find the comparatively easy way through the thatched roof. Especially in autumn, the installation of a thatched roof on an air-tight new building, that is still damp, can have devastating consequences: In autumn the outdoor air is already quite humid which is the reason why it cannot absorb that much humidity from the thatched roof. If furthermore the reed is thatched too tightly or too fine stalks have been used, so that the air permeability of the roof is reduced, then, a wet roof can be the consequence. Once moisture has found its way into a thatched roof, the roof is an ideal breeding ground for algae, bacteria and fungi. Out of this reason it is very important that the substructure is dry at the time of thatching. In practice, it has been proven that especially thatched roofs on new buildings that remained unheated over a longer period of time show a higher humidity.
We can sum up: The more construction moisture can be found in a thatched house and the worse the ventilation, the higher is the probability of a wet roof colonised by algae and bacteria. The emerging biofilm prevents the roof from drying and thus favours fungal decay.
Of equal importance is the amount of humidity produced by the inhabitants of the house and how fast it can be given off to the outdoor air. If this humidity is not vented by regular airing or the use of a ventilation system, it can lead to the penetration of dampness into the thatched roof. If inside of the house more humidity is produced than can be vented, this humidity will penetrate into the roof. This does not inevitably lead to damages, as the thatched roof lets pass a small amount of humidity unless a previously existing biofilm of algae and bacteria prevents the escape and the drying of moisture. It is crucial for the longevity of the thatched roof that at the time of thatching a proper ventilation of the house is in place. Measures to avoid damages caused by construction moisture and humidity caused by inhabitants:
  • A proper ventilation: Some hints on proper ventilation
  • Use ventilation systems or roof exhauster
  • Your thatched roofs needs care and maintenance on a regular basis
  • Open stairs and kitchen favour the ascent of humid air – this decreases the durability of the roof
  • Do not hang up your washing in the attic
  • High temperatures directly below the roof allow the air to absorb more humidity
  • Make sure the construction of your thatched roof is correct