Page 62 - Build 151

Internal moisture
Simulating school roof cavity
A BRANZ project measuring moisture in a school building roof cavity has helped re ne the WUFI numerical simulation tool and identify shortfalls in knowledge. This will help designers create more realistic models and, ultimately, better buildings.
BY STEPHAN RUPP AND STEPHEN MCNEIL, BRANZ BUILDING PHYSICISTS
INSTITUTIONAL BUILDINGS such as schools and halls have been in the spotlight recently because of potential moisture problems in their roof cavities.
Recipe for moisture
The high indoor humidity, air-permeable acoustic ceiling tiles and high R-value ceiling insulation in these buildings can lead to a signi cant amount of moist air reaching the cold steel of the roof structure.
Under these conditions, condensation can occur on cold clear nights when the dew point temperature is reached and the temperature of the roof cladding falls below the ambient temperature.
WUFI results help to change thinking
In recent years, BRANZ’s WAVE programme has been using the WUFI numerical simulation tool to help develop remedial measures and alternative designs to counter these problems.
WUFI is a powerful tool, but it needs to be used with great care to obtain meaningful results. When describing the transport of air throughout a building, for example, there are assumptions and tweaks necessary to capture this dominant moisture transport mechanism.
Reality checked against modelling
As part of the WAVE research, an idealised WUFI model for a speci c school roof design was compared with the real measurements.
The model used was created in an older version of WUFI2D, using a BRANZ ventilation module. This enabled the movement of air from the spaces below into the roof cavity to be modelled. This air movement is a signi cant contributor to roof space moisture.
The ventilation rates from the spaces below to the roof cavity were estimated based on previous eldwork. Air movement across ceilings is a fundamentally important process that has inspired a new BRANZ project focused on the air permeance of ceiling linings to air movement.
School roof tted with sensors
Wireless climate sensors capturing temperature and relative humidity were installed during the refurbishment of a school building north of Auckland.
This steel roof structure has timber thermal breaks between the purlins and the roof cladding.
Several sensors were placed in the roof cavity and additional sensors captured the indoor and outdoor climate.
60 — December 2015/January 2016 — Build 151
FEATURE SECTION