Social norms are considered a very robust behavior change approach and are well-established in promoting pro-environmental energy behaviors, particularly in contexts where citizens are insecure about their options (Farrow et al., 2017). One such context is the one of social housing, where a lack of trust and perceived risk are large barriers to adoption of energy efficiency measures (McCabe et al., 2018). For tenants in social housings, energy efficiency technologies offer the potential to improve monetary savings and comfort, while having wider environmental impact than individual household installations (Brown et al., 2014; McCabe et al., 2018). We investigated whether descriptive social norms would increase uptake of an efficiency retrofit device for heating in social housing. We used two randomized controlled trials with communication to tenants occurring in letters from their housing administrators, one control group with the standard letter, and one experimental group with an added social norm describing how many other tenants had already installed the technology. Logistic regressions predicted the effect of social norm on adoption. In the first study, a significant higher uptake was found (ß = 1.7, p = .024), though randomization occurred only at apartment block level. In the second study, where letters were randomized at apartment level, we found a significant main effect (ß = 1.62, p = .02), when taking into account a significant interaction between social norms and one of the housing blocks (ß = -2.2, p = .034).