The low energy building can be achieved by implementation of energy efficient building envelope as well as energy efficient heating and ventilation systems. In addition use of renewable energy sources allows reach nZEB and ZEB requirements. This paper provides calculation approach and conclusions on necessary insulation thickness for opaque elements. In addition optimal windows sizes, their orientation and thermal properties to achieve low energy buildings standards in Latvian climate is evaluated in this study. This has been done for single family house. Study takes into account specifics of low energy consumption buildings for five largest cities of Latvia located at places with maximally diverse climatic conditions. The paper presents economical and payback time calculations of low energy building constructions and HVAC systems in Latvia.