About timber and a wooden house

 

The best confirmation of the statement that a wooden house lasts for centuries

is the old wooden houses that in spite of rather insufficient maintenance have remained in quantity in Estonian towns.

 

 

70-80% of the energy that is used for producing wooden items is acquired from the waste that comes from manufacturing. A Japanese research contains analysis of different bridge building materials – concrete, steel, timber. The result of the research is quite surprising – timber has the best pressure and tensile strength, steel comes next and concrete is the last. Timber is light and strong – coniferous timber is 16 times lighter than steel and 5 times lighter than concrete. Due to its good strength-to-weight ratio a wooden house weighs 8 times less than a stone house, which implies smaller expenses on the foundation.

Timber is naturally warm, having 12 times smaller heat conductivity than concrete and 1500 times worse heat conductivity than aluminium. This is why timber is used for protecting steelconstruction against heat in case of fire, for instance.

Calculatorily, due to smaller thickness of walls on wooden carcass it is possible to attain 3% additional floor space on the same bearing construction, which at 15 000 EEK per square metre gives 45 000 EEK additional revenue per each 100 m2. Timber is sometimes combined with other materials – when covering external facades with decorative stones or, for example, using concrete carcass with outer walls on the wooden carcass, which is widespread in Sweden.

Poor soundproofing is not a problem of wooden houses but a problem of every badly built house – many of us have cursed poor soundproofing of the panel houses built in the Soviet time. Soundproofing of the walls on the wooden carcass can be very good, because they generously contain cellular material and they have laminate structure.

When building a wooden house, it is necessary to make sure that the timber used will not get wet before the roof protects the carcass. It certainly does not apply to a shower of rain but severe soaking when the wind fails to dry up the surface layer. As assembling of the wooden carcass takes relatively little time, especially when using a platform technique or factory elements, it is easy to meet the requirement, and it is not necessary to think about moisture. In case of a stone house the problems only begin after setting up the carcass: it is time to get the moisture out of the walls.

 

What does “A Wooden House Breathes” Mean?

 

It usually means that the in-house air automatically becomes clean through the walls. Timber as natural building material enables it very well.

 

Recently too stuffy, so called “tinned” houses, have fallen under criticism. At the same time the world is turning to ecological solutions, and more and more is spoken of “breathing” walls. Different understanding of the high-quality life environment has sharpened the attention in regard to the internal climate in rooms. Each day a person eats about a litre of food, drinks nearly two litres of water and inhales at least 15 000 litres of air on the average, spending 70...90% of his time indoors. Many old or ill people stay indoors practically all the time. This is why the quality of the in-house air plays such an important part in respect of the human health. When we say the walls breathe, it means that the in-house air automatically becomes clean through the walls. Air becomes clean and fresh in the process of diffusion without actually moving through the wall, or in the process of ventilation. A little diffusion hindrance is one of the most desired qualities for wooden houses, because without ventilating the building (or with smaller ventilation costs) the diffusion constantly freshens the air in the room without requiring any maintenance. Diffusion does not depend on the air movement and it can take place through airtight confines. Therefore diffusion is a very mighty and advantageous factor in freshening the air. Diffusion does not change air needlessly, it only changes the components that are first needed for cleaning up the air.

 

 

Interior Air Pollution

Carbon dioxide is the main toxic gas that people produce. On the average, an adult person exudes 40...100 grams of sweat and exhales 20...40 litres of carbon dioxide per hour. When taking a bath, steam arises 700 grams per hour on the average, and 2600 grams per hour when taking a shower. Drying the laundry, mopping the floors, watering the pot plants, etc. also moisten the interior air. There is 0.4 l/m3 of carbon dioxide, ~12 g/m3 of steam in summer and ~3 g/m3 steam in winter in the clean open air.

A Wooden House and Fire

 

A wooden house is not more inflammable than a stone house. This, of course, applies to the houses that are properly designed and constructed. Fire safety requirements are different in different countries but the reason is rather connected with the traditions and prejudice. Fortunately, there is a spread of a function-based approach where there are no specific numerical limits, e.g. on the quantity of floors, but which requires calculatory proof of the fact that the residents’ safety in case of fire stays in the required limits.

 

The main source of inflammability in a dwelling house is its resident. This is why in the USA, for instance, it is obligatory to use sprinklers nearly everywhere, less attention is paid to the lanes between the houses or the windows in the walls of the houses confronting each other. In Finland, where it is also required to use sprinklers in 3-storey and 4-storey wooden houses, some people, smoking in bed, have already been saved as a result.

It is not possible to turn timber into non-inflammable material by means of any impregnation or chemical protection, it can only be made poorly inflammable. Behaviour of timber in case of fire is predictable. For instance, the surface of a stringer profoundly carbonizes ca 0.8 mm/min, therefore it is easy to find the cross-section of a stringer in order to guarantee that the carrying capacity of a stringer remains, for instance, after 60 minutes of burning. The part unburnt retains all of its carrying capacity.

Steel, for instance, heats up fast in the fire, and the construction may collaps at the yield point (not to mention melting). In this sense, timber is more resistant than steel. As it is said before – the strength characteristics of the timber untouched by fire (in the middle of the cross-section) do not change, and the construction stands as long as the whole part of the cross-section is able to carry the load. Which situation is safer?

 

How Timber Burns

Timber catches fire either directly from a flame or in great heat. In the absence of flames, the surface temperature needs to rise above 400ºC in order to ignite. In the presence of flames, timber ignites when the surface temperature has been at 300ºC for some time. Fire spreads along the surface of the wooden element, giving rise to new places of fire. In the beginning the burning is intensive, as a result of which an isolating charcoal layer is formed around the cross-section. Chemical decay begins in the interaction of charcoal and combustible gases, and a so called pyrolytic layer is formed between undamaged and charred timber. This is a five-millimetre thick zone where timber has been chemically influenced by fire but has not been completely decayed. When during the fire the timber under the pyrolytic layer has reached the temperature of 100ºC, the water in the timber begins to vaporise. The temperature stops rising as long as all of the water has vaporised. Very little gas is produced at above 500ºC. However, char “production” is increasing. This explains the look of the timber after fire. The thermal conductivity of charcoal is only 1/6 of the thermal conductivity of timber. This means that the charcoal layer forms an insulation around the undamaged timber that slows down its further damaging. Thanks to insulating charcoal layer the temperature of timber is considerably lower deep inside than in the surface layer. The core of the cross-section of the wooden element remains cold even at a short distance from the burning zone. This avoids damaging temperature strains in the construction as a whole. Unburnt parts retain all of their qualities of carrying capacity, except for diminishing in size. The time spent on ignition and burning depends on the density of timber. So different kinds of trees behave differently in fire. The greater density of timber, the poorer inflammability.

A Wooden House and Environment

 
Several preconceptions hinder wider use of timber in building industry. For example, timber is considered short-lasting and inflammable. It is also believed that using other materials instead of timber helps to save forests.

Timber is pleasant and warm material. Besides, using timber helps to restrain global warming. In the circumstances where the growing global population increasingly needs food and thus more arable land, one cannot count on the forest in its growth stage, when the forest mainly binds carbon dioxide, when establishing forest land. In order to have a new forest generation, the old forest needs to be cut down and its timber put to use. The best place for making use of timber is building industry, because the timber used in houses will be drawn out of the carbon circulation for a long time, and if a wooden house is ever demolished, its timber will be used for producing energy or flakeboards, for instance.

Timber saves energy both in manufacture and utilization. For example, when producing a steel girder of the same carrying capacity, it takes six times more energy than a glue-laminated beam. Almost all of the houses with low energy consumption or passive houses are placed on wooden carcasses, because a wall of the same thermal insulation made of other materials would be much more expensive. A wall on the wooden carcass does not have a separate bearing and thermal insulating detail, the bearing beam is in the lagging and it does not increase the thickness of the wall.