The Influence of Thermal Bridges for Buildings Energy Consumption of “A“ Energy Efficiency Class

Abstract

From 2016 "A" energy effieiency class buildings should be designed and built in Lithuania, as their energy consuptions are considerably less comparing with the ones that were built before. In order to meet high requirements of "A" energy efficiency class buildings, not only thermal insulating thickness layer of building's envelopes should be enlarged, not only effective windows and building's energy using engineering systems should be used but also new energy efficient structural and insulating solutions should be made. One possibility to reduce building's energy consumption is to join building's envelopes and units that value of thermal coefficient of linear thermal bridges was smaller in joint places. Moreover, in order to evaluate overall heat losses in these joints more exacly it is required to calculate values of heat transmission coefficients of linear thermal bridges in "A" energy class buildings.

The project of semi-detached building was chosen in order to analyse influence of linear thermal bridges for building's energy consumption as there are all kinds of linear bridges wich values must be calculated. Buildings energy efficiency designing program, wich based on EN ISO 13790, was used to calculate building's heat losses. Heat transmission coefficients of linear thermal bridges were calculated using program THERM. Two variations were analysed: building's energy consumptions are close to the lowest point of A class requirements when building's envelopes and joints of units in joint places are the same as they are in currently built houses; and effective energy solutions of building's joints.

The results analysis showed that requirements of "A" class can be reached using ordinary solutions for building's envelopes and joint units but if linear thermal bridges are designed like that, it makes 16 % heat losses through envelopes, and it is similar to heat losses through building's walls (17 %), furthermore, it is about 1.5 times bigger than heat losses through roof (10 %) or floors (10 %). Specific heat losses of thermal bridges make 7.80 kWh/m2 per year. The biggest overall heat losses are through walls and windows joint thermal bridges (specific heat losses make 13.55 W/K). Another significant part of heat losses comes from walls and floor joints (specific heat losses make 11.88 W/K).

After solutions of buildings with energy effective building's envelopes and joints were analysed it can be stated that overall heat losses decreased about 10 kWh/m2 per year, it is 20 %. Heat losses through thermal bridges make only 3 % of overall heat losses through envelopes, it is 122 kWh/m2 per year.

According to results of the analyse general specifications were made for designing building envelopes "A" energy efficient class:

• The building of "A" class can be designed using ordinary solutions of linear thermal bridges, bigger heat losses through thermal bridges can be covered by increasing thickness of thermal insulating layers and using windows of better thermal bahavior but as a result the costs of building house increase too.
• When effective solutions of linear bridges are used, the same energy efficiency of the building can be reached using less thermal insulating layers, windows and doors of less thermal behavior if the building of better energy characteristics is designed.
• The usage of effective structural solutions, also of linear thermal bridges, increases energy efficiency of the building, improves conditions of interior microclimate and makes assumption to decrease additional cost of building house comparing to the ordinary ones.

DOI: http://dx.doi.org/10.5755/j01.sace.15.2.15351