A study on the thermal isolation

Tallinn University of Technology in association with Finnlamelli OY and Finnlamelli Eesti OÜ studied the air tightness and thermo-physical properties of log-walls over the period 12.05.2008 – 18.05.2009. The set goal was to inspect thermal conductivity of laminated wood, air tightness at 50 PA negative pressure and to determine the effects of solar rays on a room’s thermal tightness in real conditions.


According to norms in effect the thermal special conductivity of construction wood should be λ=0,13W/(m2K), according to which the 0,20 meters large, thick wooden wall’s thermal resistance is R0=1,78 and thermal conductivity U0=0,56 W/(m2K). The results of the experiment showed considerably better real thermal conductivity values, which on the Northern wall were 0,48 W/(m2K) and on the Southern wall were 0,43 W/(m2K). Both of the walls had a real thermal special conductivity value of λ=0,11W/(m2K),  that is 18% lower than the norm construction wood’s thermal special conductivity requirement. A considerable improvement comes from the influence of the summated height of the sun on the wall. The weighed humidity in a laminated log-wall was very stabile, remaining within the limits 10-14%. No air leaks were discovered during the thermography.

The log-house has constant humidity regulation

At first we must define terms like humidity, relative humidity and absolute humidity.

    • Humidity is the steam present in air, meaning water in its gas-state
    • Relative humidity is the ratio between the amount of steam in the air and the maximal possible amount of steam found in the air under the same conditions (temperature)
    • Absolute humidity is the mass of steam in one cubic meter of gas (air) in grams (g/m3).

Why do we need a balanced level of humidity?

Humidity and temperature play an important role in a person’s feeling. Air that is too dry leads to various damage to the mucosa and allergic reactions. Air that is too humid leads to advantageous conditions for the distribution of bacteria and inception of rheumatism. How does a balanced level of humidity without large fluctuations evolve in the log-house? The average value of relative humidity in Estonia is between 40 and 60%. Many have certainly noticed during the winter that cracks have occurred or that wooden doors have started to move more freely. Here we cannot mix up the relative and absolute humidity values. Although relative humidity values are high, the air at 0 oC contains much less water since the capacity for humidity decreases with drops in the temperature. A smaller amount of water in the air outside creates the conditions whereby with the heating and ventilating of rooms considerably drier air is sucked in. This results in drier air and a reduction in the capacity of materials. Constructions with low humidity saving properties, e.g. stone and frame walls do not take in humidity or do it at a lower level. At the same time the log-wall has very good humidity-storing properties, thus enabling it to keep a more stabile level of humidity inside the rooms with short-term fluctuations.

Thermal properties of the log-house improve because of solar rays

Thermal transfer happens in every material. Heat moves towards “the cold”. For instance, if you take a metal-spoon and dip one end in boiling water, then it won’t take long before the spoon in your hand turns hot. A similar thermal transfer happens in all material. In engineering physics such a transfer is called heat-flow. Different wall constructions act differently here. For instance for log-walls in winter conditions, where the temperature outside is noticeably lower than it is inside, the heat-flow through the material is higher and thermal isolation somewhat lower. In spring time with intense solar rays, the heat-flow decreases gradually and thermal isolation increases. Frame houses with mineral wool isolation or polystyrene coated stone-houses have a low heat-flow and there such natural changes do not occur. That means a somewhat better thermal isolation but no thermal accumulation and inertia. When the heating is turned off the room cools quickly. Since during springtime and in autumns the sun shines on the log-walls and reflects also on the surrounding surfaces, this raises the outer temperature of the log, which causes heat-flow to decrease and thermal isolation to increase. One can conclude that a log-house lives and breathes naturally where the energy lost during winters is made up for during springtime and autumns. Isn’t this also the case for people, that during the spring, summer and autumn you collect energy and use it during the winter?

A log-house is pleasantly warm during the winter and cool during the summer

It is pleasantly warm in the log-house during the winter because of the surface temperature and thermal capacity of wood. Even with fluctuations in outer temperatures a log-wall, with its high thermal capacity, preserves the inner surface temperature at the same level as the inner air temperature. The walls do not radiate “cold”, but are pleasantly warm when touched. Materials with low thermal capacity have no inertia and surface temperature is considerably lower than the inner air temperature. For instance when keeping the window open during the winter, the temperature in the room drops steeply. In frame or stone houses recovery takes a long time and it has to be done by heating the house, whereas the log-house restores the temperature quickly and with low costs thanks to its good thermal capacity (the wood stores heat). In log-houses, walls give off the heat that is stored within. In the summer the physical properties that make living in your log-house pleasantly cool are the same and when the solar rays’ intensity rises sharply, the inner temperature inside the house rises quickly. In the log-house the room temperature doesn’t rise since a natural heat accumulation process inside the walls begins its process. During night time the temperature drops, which cools the warmth that has been accumulated during the day. Thus a balanced temperature is ensured inside the log-house.