Page 53 - Build 151

Current practice uses natural roof ventilation
Measurements have shown that the airtightness of roof spaces varies widely depending on climate, local pressure moderated by wind exposure and so on.
Currently, roof ventilation is usually provided by uncontrolled natural ventilation around the roof cladding speci ed. For example, a roof cladding such as corrugate or trapezoid pro led steel, metal tiles or concrete tiles all allow uncontrolled ventilation of the roof space to occur.
The other option would be to speci cally detail ridge vents, eaves vents or gable end vents.
However, too much ventilation may have unwanted side e ects such as energy wastage.
Several critical design considerations
There is no New Zealand-speci c rule of thumb or simple calcula- tion to determine how much ventilation should be provided for a given roof design and location. During the building design process, critical parameters that need to be considered to achieve a well performing roof are the:
● outdoorclimateandradiativecooling ● solargain
● windspeedandsiteexposure
● ceilingconstructionand nishes
● indoormoistureconditions(thisisdependentonoccupantbehaviour). Some of these factors can be readily identi ed, but others need to be estimated for now. New BRANZ research projects will make information available on air transport through di erent ceilings types and the occupant behaviour to help the design process.
Problems using foreign codes or standards
It can be tempting to use design tools from other countries when looking for solutions to roof ventilation. However, there is no certainty that these will work under New Zealand conditions and building practices.
Often, other countries have di erent compliance frameworks from New Zealand. These may make inaccurate assumptions about certain critical aspects of the performance of the building component.
For example, the Canadian Building Code, which is often cited in this roof design context, has an airtightness requirement that we do not have here. A roof designed according to the Canadian Code without considering our Code and climate will probably fail. This is because the Canadian airtightness target for the living quarters and their ventilation cannot be met.
Needs to dry out
Condensation is not preventable under all circumstances. However, as long as the structure is allowed to return to its initial dry state over a period of time, the roof will manage the moisture load.
Build 151 — December 2015/January 2016 — 51