Dry Under the Floor: Development of NZ House Sub‐Floor Ventilation Requirements

  • YEAR
    Isaacs, Nigel
    2017 Conference Papers
    Conference Papers
    Practice, Education & Profession


This paper traces the development of sub‐floor (crawl space) ventilation from early New Zealand
uses of suspended floors to the first formal requirements through to the modern day. This includes council
by‐laws, nationally developed standards and most recently the New Zealand Building Code. Although subfloor
ventilation was being provided from the earliest European housing, the first minimum gap between
floor joist of 100 m was suggested in 1912 but the first local Government by‐law recommend was in 1920
when the One Tree Hill Roads Board required 150 mm space. Although the New Zealand State Forest
Service’s 1924 “Building Conference Relating to the Use of Timber in Building‐Construction” suggested a
minimum ground clearance of 300 mm with ventilators of 1 in2 per ft2 of flor area (0.7%), it was not until
1944 when NZSS 95 Part IX required ½in² per ft² (0.35%) that a ventilator requirement was incorporated
in local government by‐laws. This requirement has continued to the present day (2017), although in 1999
the minimum ground clearance or crawl space was increased to 450 mm. Recent BRANZ research has
confirmed these ventilation requirements as being reasonable, although the results may also contribute
to a more science based set of requirements.


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