Educational facilities present the largest share of the oldest buildings in Europe and contribute greatly to high energy use. In order to stimulate building energy efficiency, the present day extensive renovations are going towards narrow-minded measures with minimisation of ventilation losses by minimal permissible design ventilation rates. In Slovenia, such an approach is supported by national legislation and results in uncomfortable and unhealthy conditions as well as other negative health-related outcomes. The main purpose of this study was to critically assess the relevant problems related to the deteriorated indoor air quality (IAQ) in a renovated kindergarten. First, a systematic literature review on health outcomes related to inadequate ventilation in kindergartens is presented. Second, in a case study on a renovated kindergarten in Slovenia, IAQ and energy use are critically assessed for 13 sets of scenarios, where design ventilation rates variated according to legal requirements and recommendations. CO2 concentrations were simulated in two model playrooms with CONTAM 3.2. Indicators of energy use were simulated with EnergyPlus 8.8.0, with and without recuperation. The results of literature review revealed that insufficient ventilation was positively associated with higher prevalence of negative health outcomes. Simulations of CO2 in two playrooms showed that the highest CO2 concentrations resulted in scenario 1 with 0.5 ACH and scenario 4 with 1.5 m3/(h·m2), both defined by national rules. The calculated values in both scenarios exceeded the national required value for CO2 (1667 ppm) by 2.5 times and 3 times, and the recommended value for Category I of indoor environmental quality defined by EN 15251: 2007 (750 ppm) by 5.6 times and 6.6 times. Scenario 9_Cat I with 14 L/(s·m2) defined by EN 15251: 2007 resulted in minimal CO2 (playroom 1: 512 ppm, playroom 2: 536 ppm) and corresponded to the recommended values defined by studies, where no health effects were observed. Energy simulations showed that the application of recuperation in scenario 9_Cat I, minimised heat losses by ventilation by factor 3.6 in playroom 1 and 3.5 in playroom 2 compared to losses without recuperation. Design ventilation rates defined by the design approach of minimal permissible value resulted in conditions relevant for Category IV of indoor environmental quality and have to be prevented. Recommendations in national policies and environmental health strategies can be used in all stages of healthy and energy efficient design of buildings.