Impact of the Exterior Wall Structure on the Energy Efficiency of Building
Tampere University of Technology, Ralf Lindberg, tel. +358 3 316 2111
TAMPERE UNIVERSITY OF
Department of Civil Engineering
Minna Teikari, Hannu Keränen
THE INFLUENCE OF EXTERNAL WALLS TO ENERGY BALANCE OF BUILDINGS
Thermal insulation capacity of six different external walls is studied in test buildings which are identical excluding the wall materials. The research project started in 1996 at Tampere University of Technology in cooperation with Technology Development Centre of Finland (Tekes) and five building product manufactures. Thermal behaviour of different wall structures, the actual proportion of energy consumed for the heat losses through the external walls of the building and the uncertainties in simplified calculation methods for annual energy consumption through the building envelope are the main issues to be examined.
There are six different external wall materials included: log wall, insulated log wall, brick wall, insulated brick wall, block wall of autoclaved aerated concrete and polyurethane-insulated wooden frame wall. During the heating season the indoor air temperature is kept constant at 20 1C. All the measurements and the circumstances inside the buildings are controlled by a computer system.
Results of the research up to now
· The measured amount of energy consumption conducted through the structures of the building envelope is less than the result from the calculations derived from instructions given in the building code.
· The amount of energy loss through a well insulated light-built external wall is about 20 % less and in the case of a massive external wall about 30-40 % less than the calculated values respectively.
· The heat loss can be more accurately estimated by the current method based on U-values in a well insulated light-built external wall than in a so-called massive non-insulated external wall where thermal behaviour is substantly influenced by the heat capasity of the material as well as the thermal conductivity.
· In calculations the temperature distribution of the wall is estimated to be linear and constant during a period of 24 hours. In reality temperatures in the wall are constantly changing and the heat flow can turn even to the opposite direction because of the solar radiation. In certain weather conditions external walls can also storage heat due to the heat capacity and other characteristics of the wall material.
· The heat transmission through building envelope should not be calculated using the area specified with the outside dimensions of the structures beacuse the heat flow through the corners of a building differ from that of the middle parts of the wall.
The increased thermal conductivities only apply to special purposes, for instance to ensure the calculations of maximal heating power in winter time to be on the safe side.
· Major energy savings can be achieved by minimizing the losses through ventilation, infiltration, waste water and houshold electricity. Deficiences in workmanship may seriously influence the thermal resistance of the building envelope and increase the heat loss. The installation of the thermal insulation, its wind protection and the airtightness of the building envelope are of great importance.
The effects of the research
The results of the research could be used in developing a more accurate practice of calculating the energy consumption through the external walls. The results would then be more in accordance with the real situation.
In cooperation with Technology Development Centre of Finland (Tekes)
Tekes has had a vital role in implementing and financing the research project. Tekes has financed 48 % and the companies involved 52 % of the project. The cooperation with Tekes has worked excellently. This research is a part of the program Environmental Technology in Construction.