Characteristics of a log house

A skillfully designed and properly built log house is equal to any other type of house. However, the future owner of a log house should fully and adequately understand all the peculiar features of his/her future home.

The peculiar characteristics of log houses stem from two main circumstances:

• the sinking of the house as a result of the drying of the wall logs
• bigger heat loss of the log wall as compared to a stone or timber frame wall.

Designing process of a log house

The house starts its life on the table of its designer.
When you choose an architect for your log home, make sure that he/she has earlier experience with log house projects. It would be useful to consult the manufacturer of the house for the choice of architect. It is even better when the manufacturer is also the designer of the house.

"Sinking" of a log house

There are two reasons why logs sink: contraction of massive wall logs as a result of drying, and the thickening of wall structures as a result of gravitation. A handmade log wall may sink up to 6%, i.e., a 250 cm high room may finally become a 235 cm room and a 7-metre house may lose almost half a metre of its height.
The architect must be able to foresee the changes that result from the sinking of log wall and avoid the accompanying risks. The sinking of a log house does not endanger the horizontal elements of the structure of the house (floors, inserted ceilings, girders etc), but the design of the junctions of log walls and non-sinking structures (chimneys, timber or stone walls, floors, windows etc) is of critical importance. In such places the elements should be connected with special dovetails and tender beams. Also, a sufficient room for sinking must be designed above these structures.
The issue of sinking is even more essential with inclined structural elements. For example, with sinking of the house the inclination angle of roof also changes. The incline of the roof may change up to 3-4°; it means that the angle of the ridge of the roof will change 5 - 8°. Due to the change of the inclination angle of the roof it must be possible for the rafters of the house and for the whole roof to "slide down", otherwise the rafters that have rigidly been connected to the outer side walls may push out the upper log layers of the outer wall.

This means that when the walls sink, the eaves become wider. Depending on the width of the house (length of a rafter) and the inclination angle of the roof, the eaves may gain up to 10-15 cm in width.

These are only some of the problems resulting from the sinking of log walls that must be solved during designing, manufacturing and assembling the house. In addition, the electrical wires in the walls, the rainfall pipes on the outer walls, the water system and sewerage, crossing points (junctions) of different roofs etc must also survive when the walls sink... There are technical solutions and special fixing and structural elements for all these problems - therefore it is very important for the architect and the builder of the log house to know and be able to use them.

Isolation of a log house

The owner will pay most dearly for the mistakes of the designer that were made in the calculation of the heat energy balance - or, rather, by ignoring this issue.

The loss of heat energy through different outer layers of a building (floor, roof, walls, doors, windows) is characterised by the so-called "u" value, which shows how much heat the fixed surface of a structural element loses when the inner and outer temperatures differ by 1°.

Proceeding from the weather conditions in Estonia the Estonian Building Regulations Council worked out advisable requirements for the outer layers of buildings in 1999. According to this document the maximum heat conduction of the outer layers of a building should not exceed:

roof-ceiling - 0.20 W/m²K
floor on soil - 0.20 W/m²K
floor on ambient air - 0.16 W/m²K
windows - 1.90 W/m²K
outer door - 0.70 W/m²K
outer walls - 0.20 W/m²K

According to the architectural norms currently valid in Nordic countries the average loss of heat energy of the outer layers of any dwelling house should not exceed 0.385 W/m²K. It means that such a loss of heat energy guarantees a normal and healthy environment and the heating of such a house cannot be considered a waste of energy.

The actual loss of heat energy through the outer log wall depends primarily on the thickness of the wall and its quality. Heat conduction of walls made of logs with different diameter have the following heat energy loss indices:

90 mm - 1.5 W/m²K
130 mm - 1.0 W/m²K
200 mm - 0.6 W/m²K

Therefore, in order to guarantee the afore-mentioned heat storage capacity (0.2 W/m²K) the outer wall of a log house should be built of logs with a diameter of at least 60 cm. In practice, such building logs cannot be found.

In Nordic countries it is allowed, as an exception, to use the so-called compensation method for calculating the heat balance of a log house. In other words, it is allowed to use outer walls that have a greater heat energy loss index, but this loss has to be compensated by the greater temperature isolation capacity of other structural elements (roof, floor, doors, windows...). But this compensation method has its limits, too. Therefore, according to the regulations, even if the compensation method is used, the maximum heat energy loss index of the outer walls of a log house must not exceed 0.6 W/m²K. In case of a dwelling house with log walls it means that the outer wall of such a house that has no additional isolation should be made of at least 200 mm logs.

Some advices for a buyer of a log home

1. Building a log home is too expensive to make decisions solely on catalogues and colour photos

• ask the builder to show you at least one ready log house
• make sure you visit the factory of the manufacturer before signing the contract to see by whom and in what conditions your future home will be built

2. The wall of a log house must be carefully built

• the long grooves and dovetails must match exactly (no cracks must be visible), otherwise during heavy storms the rainwater will seep through the log wall
• the width of the long groove in the outer wall must be at least 60 per cent of the thickness of the log wall
• the long groove and dovetails must be properly filled with isolation material
• the corner dovetails of walls must have a windlock inside

3. Do not try to spare money on isolation material of roofs and floors

li>in Estonian weather conditions, both the roofs and floors of a log house should have an at least 25 - 30 cm layer of mineral wool

4. The greatest relative heat loss happens through windows

• do not design too large windows for your log home
• use only high-quality factory-made glazing units with glued wooden frames for your log house

5. The contract for manufacturing and assembling a log house must not terminate when the log house is ready

• the builder should give at least for the log part of the house a guarantee of at least 4 years
• this guarantee should include compulsory quality checks by the builder at least once a year