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Energy consumption in wooden houses: theory and practice
Energy consumption in wooden houses: theory and practice

Video: Energy consumption in wooden houses: theory and practice

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The U-value of a component is a guarantee for saving energy. However, the laboratory value does not take into account the solar energy gains that an existing building collects. Passive use of solar energy can make a house more energy efficient than calculated.

Insulate and store heat

The U-value U-value provides information about the insulating ability of a component: the smaller the number, the less heat migrates from inside to outside through the outer shell of a building. The Energy Saving Ordinance EnEV EnEV specifies a maximum U-value for all components of a house and determines how much heat a house can lose at most. However, the ability of a building material to collect and store heat also determines the cost of heating and the indoor climate.

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Five-story wooden skyscraper in Norway is heated economically - without additional insulation.

Photo: Santner

Calculate and check values

"The U-value does not comprehensively describe the energy behavior of a house, " criticizes Helmut Spiehs, managing director of the Graz company Santner & Spiehs. The manufacturer of solid wood construction elements for single-family houses and multi-storey residential construction is convinced that, for example, houses made of solid wood use significantly less energy in practice than the theoretical calculations of the U-value.

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Solid wooden walls insulate and store heat equally effectively.

Photo: Santner

Design and test walls

A large-scale fire test with timber construction elements prompted further research: out of a heat of 1210 degrees Celsius inside the test building, only 9.5 degrees had passed the massive wooden wall - the energy of the fire was almost entirely saved by the wall, which was only 10 centimeters thick. This remarkable ability to store heat should be examined in more detail.

Helmuth Spiehs had the U-value of the solid timber construction elements measured according to the European standard EN ISO 8990 in a "regulated heating box" - test institutes provide a computer-controlled "hot box" for this purpose, which measures the heat transfer coefficient based on the heat transport.

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Heating flows from the inside to the cold outside, but in practice it is less constant than in the laboratory.

Photo: Santner

Constant heat flow in the laboratory

The respective test component is installed between two laboratory rooms - in this case a 208 mm thick, 1 square meter solid wood element. One measuring room was constantly heated to 20 degrees, the second room to about 0 degrees. A constant heat flow flows from the warm side through the test component to the cold side. Heating and ventilation devices keep the heat flow constant - from its density, the air temperatures in the heating chamber and the surface temperatures of the test component, the U-value, conductivity and heat transfer coefficient are calculated. Standards define the conditions: The U-value may only be calculated when the heat flow is constant - engineers speak of a “steady state”.

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In the laboratory, constant heat flow through the solid wooden walls could only be determined after 6 days.

Photo: Santner

Outdoor temperature changes

The engineers recorded when the steady state was reached - this way it can be checked how quickly the wall made of solid wood loses heat to the outside. Almost no heat flow was measured on the first test day - it was only after 140 hours, i.e. around six days, that a reasonably constant heat flow could be determined. Only now could the U-value be calculated. As a rule of thumb for builders, the lower the U-value, the cheaper the heating costs. "Too short a thought, " warns Helmut Spiehs, "the conventional calculation of the U-value in the laboratory neglects heat storage and time factor and, moreover, solar energy gains that a house achieves outdoors."

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