The aim of this study was to assess whether low energy consumption in dwellings imposes problems by deteriorating the indoor environment. Several indoor environment parameters were correlated with the energy consumption of low energy houses. One house from a village of low energy houses in Denmark was selected and sensitivity analyses were conducted for the importance of occupancy, ventilation, window opening, and heat recovery efficiency. In particular occupancy and venting played significant roles for the indoor environment and energy consumption. It was also shown that with passive measures, but also with the installation of a chiller, a comfortable thermal indoor environment could be achieved with only a minor increase in the energy consumption. KEYWORDS Architecture and design, building controls and automation, energy performance, HVAC systems, occupant/user control. 1 INTRODUCTION With the large share of society's energy consumption that takes place in buildings (40% throughout Europe), it is clear that optimization of buildings' energy performance can provide a significant step towards a more sustainable society (Tommerup et al. 2007; Karlsson & Moshfegh, 2006). The issue of reducing energy consumption in buildings becomes more complicated when also the indoor environment is to be considered. Since the time people spend in all types of indoor environments amounts to almost 90%, designers should not forget that a comfortable and healthy indoor environment is as important as the optimization of the buildings' energy performance (ISIAQ-CIB TG42, 2004).