Energy price increase and climate changes force to look for new methods to reduce heat consumption in the buildings. In the cold climate countries the requirements set for the building insulation are rather strict (i.e. during the last 15 years heat transmittance through the external envelopes of the buildings constructed in Lithuania decreased 3-5 times), but still there are possibilities for heat loss reduction and energy saving by heating ventilated air. During the low energy building envelopes design process frequently occur the following problems: increase of the thermal insulation layers thickness is restricted by the undesired thickness increase of the entire building element and the requirements of the envelope layers connection, the considerable impact of the thermal bridges. Another important factor is the impact of the heat losses through the transparent building elements. The paper presents analysis of the possibilities for the proper insulation of the envelopes in the designed buildings with low energy consumption. During the formation of the new construction solutions for the envelopes, the whole building’s architectural design was unchanged. The analysis of the heat losses through the envelopes in the investigated building was carried out by focusing on various construction solutions for the envelopes. The obtained results were compared with the normative heat losses of such type of buildings. The economic aspects of the construction of low energy buildings have also been discussed.