In Sweden, the energy usage in existing residential buildings amounted to 147 TWh in 2012, equivalent to almost 40 % of the final overall national energy usage. Among all the end users in building service sectors, 60 % of the final energy in Sweden is used for space heating and domestic hot water (DHW) production in 2013. In order to reduce the supply temperature for space heating in existing buildings, combined approaches are favorably adopted: to reduce the net energy demand by air-tightness and insulation retrofits; and renovate the conventional high temperature heating to low temperature heating (LTH) systems. As an energy-efficiency alternative, LTH technology has shown promising advantages and shortcuts to improve the coefficient of performance (COP) of heat pump system, which further saves primary energy. However, existing modeling achievements and field testing reveal that the attained application of LTH has a relatively high requirement to the air-tightness in new constructed single-family houses. Moreover, in some leaky multi-family building stock with low envelope surface temperature, LTH may have limited energy saving potentials. How to evaluate the impact of air-tightness for the LTH implementation and energy saving potentials in existing houses are not sufficiently attained so far. This paper presents a modeling approach combining LTH simulation with air-tightness evaluation, aimed to estimate whether the selected existing building types can cope with LTH with upgraded primary energy savings. In addition, the impact of air-tightness retrofits for LTH implementation in selected Swedish residential buildings is of interests. In the simulation Consoli Retro are employed to simulate the energy performance. It is revealed that the combined effect of floor heating/ ventilation radiators and air-tightness retrofits to 1/1.5 ACH can contribute 19 % to 36 % primary energy savings in total. However, different LTH systems and archetypes have varies sensitivities to air-tightness retrofits. Benchmark the impact of air-tightness to different LTH systems needs further investigations among other archetypes and on-site measures for future application of LTH on a larger scope.