Page 76 - Build 153

Departments/Research By Ian Cox-Smith, BRANZ Building Physicist
Wind and oor insulation
Ventilation is required to reduce moisture levels, but can too much sub oor ventilation reduce the e ectiveness of under oor insulation? BRANZ set out to nd the answer.
THE BRANZ SUBFLOORS project has been investigating the condition of spaces under suspended timber oors. The primary focus has been on the impact of high humidity levels on the moisture content of framing and the corrosion of xings.
Does wind a ect insulation?
One possible solution to elevated moisture levels is to increase sub oor ventilation, but will this impact on the thermal performance of the under oor insulation?
To answer this question, BRANZ set out to understand the relationship between thermal resistance and wind speed for both foils and bulk insulation. As part of this, the average performance over an entire winter was also monitored in a relatively open sub oor.
Move from foil to bulk insulation
After the Building Code required insulation in the 1970s, most suspended timber oors were insulated with draped perforated foil.
More recently, concerns about the per- formance and safety of foil insulation in
subfloors has seen the growth of bulk insulation materials including expanded polystyrene, glasswool and brous polyester.
To provide adequate thermal performance, draped foil needs the surrounding air to be
as still as possible. Even very light wind ows can severely undermine its ability to insulate. Bulk insulation materials, on the other hand, have a graduated reduction in per-
formance as the wind speed increases.
100% 80% 60% 40% 20%
0%
Jun–Oct 2014 Jun–Oct 2015
74 — April/May 2016 — Build 153
0.0 0.5
1.0 1.5
2.0 2.5
3.0 3.5 4.0
24-hour average wind speed (m/s) Figure 1: Ground level wind speeds under the test house.
% of time above given wind speed