Building Envelope

13/04/05

The main components of the building envelope are walls, windows and doors. These items are controlled with comprehensive building codes in European countries. Ever lasting discussion is between different materials and especially between their tightness and energetic characters.

Air tightness

Air tightness has a great influence on energy efficiency of the building envelope. It is also, maybe, the most difficult part to control.

More information:
http://www.dimensio.org/

Thermal mass of buildings

The summary is based on 28 publications and studies handling the effect of thermal mass on buildings’ heating and cooling energy consumption and indoor air temperatures. The research results handle mainly the effect of thermal mass in Nordic countries, Central and Southern Europe and in USA. Most of the studies are based on computer simulations, but six of them are based partly or totally on measurements. Generally the results obtained with calculations and measurements were similar.

The effects of thermal mass can be divided in the following ways:

• Natural utilisation of buildings’ thermal mass.
• Active utilisation of buildings’ thermal mass e.g. by circulating ventilation air through hollow core slabs or by embedding heating or cooling ducts in a massive slab.
• Effect on heating energy; This is the most important effect in the Finnish climate.
• Effect on cooling energy; The thermal mass can have a big relative effect on cooling energy. In some countries cooling can cause big peaks in electricity demand and it is therefore a real problem. In the Finnish climate the use of cooling improves thermal indoor climate in summer, but it is also an additional cost in construction and operation.
• The thermal mass decreases also the peak heating and cooling loads and thus decreases at some amount also the investment costs in technical systems. This effect has not studied very much.
• Effect on indoor air temperatures; When cooling is not used, the building’s thermal mass clearly decreases peak indoor air temperatures in the summer time.

The time constant (the heat capacity of a building divided by the specific heat loss) is a parameter which describes in its easiest way the energy performance and thermal dynamics of a building. Time constant is included e.g. in the standard EN 13 790 Thermal Performance of Buildings – Calculation of Energy Use for Space Heating and Cooling. This standard is referred in the Energy Performance of Buildings Directive of EU.

The (absolute) thermal mass of buildings has not changed much during the last decades. However, the effect of the thermal mass on energy consumption has increased noticeably due to the increase of the internal heat gains (computers, lighting) and due to the decrease of the specific heat losses because of a better thermal insulation and the use of heat recovery from the exhaust air. Therefore with the same thermal mass the building’s time constant has increased approximately 100 % in 30 years (in Finland).

In the following the central results concerning the effect of thermal mass are presented:

• The saving in heating energy due to a passive use of buildings’ thermal mass is 2 – 15 %. A typical saving in North-European climate conditions is 10 % when a very light and very heavy building are compared.
• The passive use of buildings’ thermal mass can decrease cooling energy consumption up to 50 %. It is difficult to give a typical value. However, the saving in cooling energy is clearly greater than that in heating energy.
• When no cooling is used in the summer time, the highest indoor air temperatures in a massive building are 3 – 6 K lower than those in a corresponding light building. A big thermal mass can prevent the need for the use of cooling.
• Night ventilation of the office buildings is a way to decrease or to prevent the use of mechanical cooling. A big thermal mass coupled with night ventilation can decrease the cooling energy consumption up to 50 %. The cooling energy saving is clearly (20 %) higher with massive constructions than with light constructions.
• The active use of hollow core slabs (cores are used as air channels) coupled with night ventilation can prevent in Nordic climate the need for mechanical cooling. In this way a cooling effect of 40 W/m2 is obtainable.
• If massive constructions of single family houses are in long term more tight (difference in the average air change rate 0,1 1/h) than the light ones, their heating energy consumption can be up 20 % lower than those of the light ones. This is the total effect of big thermal mass and the tightness of the exterior envelope.

More information:
Thermal mass of buildings - Summary of the research report 174/2003 (pdf, 35 KB)