Intended for healthcare professionals


Effect of insulating existing houses on health inequality: cluster randomised study in the community

BMJ 2007; 334 doi: (Published 01 March 2007) Cite this as: BMJ 2007;334:460
  1. Philippa Howden-Chapman, professor and director1,
  2. Anna Matheson, PhD student1,
  3. Julian Crane, professor and codirector2,
  4. Helen Viggers, data analyst1,
  5. Malcolm Cunningham, principal analyst4,
  6. Tony Blakely, professor3,
  7. Chris Cunningham, professor5,
  8. Alistair Woodward, professor6,
  9. Kay Saville-Smith, director7,
  10. Des O'Dea, lecturer1,
  11. Martin Kennedy, adviser8,
  12. Michael Baker, senior lecturer and codirector1,
  13. Nick Waipara, scientist9,
  14. Ralph Chapman, associate professor10,
  15. Gabrielle Davie, biostatistician1
  1. 1He Kainga Oranga, Housing and Health Research Programme, University of Otago, Wellington, PO Box 7343, Wellington South, New Zealand
  2. 2Department of Medicine, University of Otago
  3. 3Department of Public Health, University of Otago
  4. 4Building Research Association of New Zealand, Porirua City, New Zealand
  5. 5Research Centre for Maori Health and Development, Massey University, Wellington
  6. 6School of Population Health, University of Auckland, Auckland
  7. 7Centre for Research Evaluation and Social Assessment (CRESA), Wellington
  8. 8Ministry for the Environment, Wellington
  9. 9Landcare Research, Auckland
  10. 10School of Geography, Environment and Earth Sciences, Victoria University of Wellington, Wellington
  1. Correspondence to: P Howden-Chapman howdenc{at}
  • Accepted 14 December 2006


Objective To determine whether insulating existing houses increases indoor temperatures and improves occupants' health and wellbeing.

Design Community based, cluster, single blinded randomised study.

Setting Seven low income communities in New Zealand.

Participants 1350 households containing 4407 participants.

Intervention Installation of a standard retrofit insulation package.

Main outcome measures Indoor temperature and relative humidity, energy consumption, self reported health, wheezing, days off school and work, visits to general practitioners, and admissions to hospital.

Results Insulation was associated with a small increase in bedroom temperatures during the winter (0.5°C) and decreased relative humidity (−2.3%), despite energy consumption in insulated houses being 81% of that in uninsulated houses. Bedroom temperatures were below 10°C for 1.7 fewer hours each day in insulated homes than in uninsulated ones. These changes were associated with reduced odds in the insulated homes of fair or poor self rated health (adjusted odds ratio 0.50, 95% confidence interval 0.38 to 0.68), self reports of wheezing in the past three months (0.57, 0.47 to 0.70), self reports of children taking a day off school (0.49, 0.31 to 0.80), and self reports of adults taking a day off work (0.62, 0.46 to 0.83). Visits to general practitioners were less often reported by occupants of insulated homes (0.73, 0.62 to 0.87). Hospital admissions for respiratory conditions were also reduced (0.53, 0.22 to 1.29), but this reduction was not statistically significant (P=0.16).

Conclusion Insulating existing houses led to a significantly warmer, drier indoor environment and resulted in improved self rated health, self reported wheezing, days off school and work, and visits to general practitioners as well as a trend for fewer hospital admissions for respiratory conditions.


  • We would like to thank Olivia James at Otara Health Inc; Jo Hunt at Opotiki Trade Training; Pounamu Skelton and the New Plymouth Office of Te Puni Kokiri; Johnina Stymes and Alberta Hunga at Te Iwi o Rakaipaaka; Elyria Fau at Te Wahine o Kahungungu; Gail Chalmers and Porirua Housing Action Group; Linda Wall, June Robinson, and the Rata Branch of the Maori Women's Welfare League; Ann Currie at Crown Public Health; the interviewers and retrofit teams; and all the householders who took part in our study. We are grateful to our community coordinators the late Ruth Nepia, Pounamu Skelton, and Jo-Ani Robinson. We also thank the general practitioners, the National Health Information Service, the energy companies who supplied us with utilisation data, and June Atkinson, who carried out the randomisation.

  • Funding: The Health Research Council of New Zealand, the Energy Efficiency and Conservation Authority, the Ministry of Health, Solid Energy, Orion, Christchurch City Council, Environment Canterbury, Hutt Mana Community Trust, MARIA, Eastern Bay of Plenty Energy Trust, Wellington City Council, and Housing New Zealand Corporation.

  • Competing interests: None declared.

  • Ethical approval: This multicentre study was approved by the central region ethics committee.

    Contributors: PH-C helped design, perform, analyse, and write up the study. AM helped design, perform, and write up the study. JC helped design, analyse, and write up the study. HV helped carry out, analyse, and write up the study; MC, TC, DD, RC, NW helped design, analyse, and write up the study. CC, KSS, AW, and MB helped design and write up the study. GD provided biostatistical advice. PH-C is guarantor.

  • Accepted 14 December 2006
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