ObjectivesWe carried out an evaluation of a large-scale New Zealand retrofit programme using administrative data that provided the statistical power to assess the effect of insulation and/or heating retrofits on cardiovascular and respiratory-related mortality in people aged 65 and over with prior respiratory or circulatory hospitalisations.DesignQuasi-experimental cohort study based on administrative data.SettingNew Zealand.ParticipantsFrom a larger study cohort of over 900 000 people, we selected two subcohorts: 3287 people who were aged 65 and over and had experienced pretreatment period cardiovascular-related hospitalisation (ICD-10 chapter 9), and 1561 people aged 65 and over who had experienced pretreatment respiratory-related hospitalisation (ICD-10 chapter 10).InterventionsTreatment group individuals lived in a home that received insulation and/or heating retrofits under the Warm Up New Zealand: Heat Smart programme. Control group individuals lived in a home that was matched to a treatment home based on physical characteristics and location.Primary and secondary outcome measuresHR for all-cause mortality for treatment with insulation, heating, or insulation and heating relative to control group.ResultsPeople with pretreatment circulatory hospitalisation who occupied a household that received only insulation had an HR for all-cause mortality of 0.673 (95% CI 0.535 to 0.847) (p<0.001) relative to control group members. Individuals with a pretreatment respiratory hospitalisation who occupied a household that received only an insulation retrofit had an HR for all-cause mortality of 0.830 (95% CI 0.655 to 1.051) (p=0.122) relative to control group members. There was no evidence of an additional benefit from receiving heating.ConclusionsWe interpret the hazard rate observed for cardiovascular subcohort individuals who received insulation as evidence of a protective effect, reducing the risk of mortality for vulnerable older adults. There is suggestive evidence of a protective effect of insulation for the respiratory subcohort.