Page 46 - Build 152

Ventilation
Ventilation options
Su cient ventilation is crucial as our homes become more airtight. A mix of passive and mechanical options can work well, but all designs should start by getting the source extraction measures right.
BY STEPHEN MCNEIL, BRANZ BUILDING PHYSICIST
VENTILATION IS CENTRAL to providing a healthy indoor environment. We constantly produce contaminants such as volatile organic compounds (VOCs), carbon dioxide (CO2), moisture, combustion products and cooking odours. Ventilation is the main means available to remove and dilute contaminants in the indoor air.
Airtightness trend
The ventilation stream of BRANZ’s WAVE (Weathertightness, Air quality and Ventilation Engineering) programme identi ed a consistent trend to increasing airtightness of homes. This demonstrated a steadily decreasing amount of background infiltration. In some cases, though, this background in ltration provided a useful dilution of contaminants.
Although this trend is positive from the energy point of view, it is crucial that su cient ventilation is provided as adventi- tious openings are closed down by changing building materials and techniques.
Fieldwork has shown the negative conse- quences of not providing enough ventilation (see Build 151, New homes dripping). The key issues identi ed in this work were substitu- tion and poor design and installation.
What does the Building Code say?
The typical method of compliance with clause G4 of the New Zealand Building Code is to provide an opening window area equivalent to 5% of the oor area of a building. There are also elements in clause G4 to demon- strate compliance using active ventilation in polluting spaces like kitchens, laundries etc.
Clause G4 assumes that windows will be opened during normal operation of a home. However, evidence from WAVE suggests this may not be happening as much as is neces- sary (see Build 127, Changing the air indoors).
The BRANZ occupant behaviour project (see pages 46–47) is looking into this in more detail and will provide information on how often and how far people are opening their windows.
Ventilation strategy should start passive
In general, the most cost-effective way to add ventilation to a home is to get the passive measures right. Passive ventilation alone cannot usually capture the majority of moisture produced during cooking and bathing so needs to be combined with source extracts. This strategy works well particularly for older homes or where there has been an e ort made for passive design.
Mechanical ventilation is seen as a quick x, but care must be taken with the design of the system as it is easy to overventilate. Control of any active system is going to be crucial to its performance.
The main options for mechanical venti- lation include exhaust, supply-only and balanced ventilation – usually with heat recovery.
Passive options
Passive ventilation options include trickle vents, passive stack ventilators and windows:
44 — February/March 2016 — Build 152
FEATURE SECTION