Page 54 - Build 165

Medium-density housing
Flat-out testing!
If you are involved in constructing an apartment building over the next 2 years, you could have the airtightness measured and help BRANZ to help the industry at the same time.
BY GREG OVERTON, BRANZ SENIOR BUILDING PERFORMANCE ENGINEER, BUILDING PERFORMANCE RESEARCH TEAM
THE AIRTIGHTNESS OF a building is a measure of how much air ows between indoors and outdoors through the structure itself – in other words, how big the holes are in the structure.
The airtightness of a building is a key aspect of a building’s performance. It a ects the energy e ciency, thermal comfort and indoor air quality provided by the building. However, airtightness is only mentioned indirectly in the New Zealand Building Code, and there is no requirement to meet a particular level of airtightness.
Build tight and ventilate right
In general, the idea is to have an airtight structure that is also adequately ventilated – in short, build tight and ventilate right.
To measure airtightness, we use a fan – usually mounted in a doorway – to create a pressure di erence between inside and outside. We then measure the amount of air going through the fan. The result is usually expressed as a function of the pressure di er- ence (often 50 pascals) and the volume of the house, so air changes per hour at 50 pascals – ach @ 50 Pa.
Ventilation can also be expressed in air changes per hour, but remember that the ventilation process occurs under lower
pressures than the airtightness test. As a rough guide, 0.3–0.5 ach is the whole-house ventilation target.
Homes more airtight and underventilated
Previous BRANZ work has shown that we are progressively building more airtight houses. This trend stretches back to the 1960s and is entirely down to changes in materials and systems used to construct our houses.
Our data shows that the average airtight- ness of a house built between 2000 and 2009 is about 4.5 ach @ 50 Pa, while the average for a 1960s house is about 10.5 ach @ 50 Pa (see Figure 1).
We have also found that newer houses are often underventilated. BRANZ data, based on a survey of houses built since 1994, shows that roughly a third had mean winter ventilation levels below interna- tional guidelines.
Overall, this suggests the Code-minimum requirements for passive/natural ventilation – the main way of ventilating those houses – are not as e ective in real life as they need to be (see Figure 2).
Little known about apartments
We know a fair amount about the airtight- ness of houses, and this information is
helping to inform the next generation of the Building Code, but what about apartments? In truth, we know nothing about the airtightness of apartment buildings in New Zealand, and there is only limited data from overseas on the topic. In apartment buildings, airtightness has a bearing on the ow of air from one unit to another so will affect how contaminants spread through
the building.
Measuring apartment airtightness
A BRANZ project on the airtightness of apartments is aiming to ll this knowledge gap and help inform better practices for the growing number of apartment buildings in the country.
For a given apartment building, we are aiming to measure both the airtightness of individual units and also where the air is leaking in each unit. This latter measurement requires additional fans to be hooked up to the neighbouring apartments and those above and below to isolate each face of the main unit under test.
The project has measured two apartment buildings so far – one in the Stonefields development in Auckland and the new student accommodation building at Otago Polytechnic in Dunedin.
52 — April/May 2018 — Build 165
FEATURE SECTION