If you were born in 1947, the chances are the first home you lived in had no insulation and was single glazed. No consideration would have been given to solar orientation, the average section was 708mᒾ and around 28% of NZ housing stock was made up with State Homes. Many of those homes still stand but would struggle to meet today's regulations. Possibly apart from ventilation anyway.
That’s because the NZ Building Code persists with a ventilation clause that was written in 1947. It was no doubt written with the best of intentions, but it was written for a different time and everything changes. Despite this, there seems to be a general acceptance that as long as buildings meet the magic number for ventilation, which the clause from 1947 decided is 5%, buildings are safe. We’d take issue with that and suggest there’s a need to go beyond the standards.
Why 5%?
The Building Code states that “for each living room, dining room, kitchen and bedroom, the total ‘openable area’ of windows, doors or skylights must be at least five per cent of the floor area of the room”.
Where the figure of 5% comes from is unclear. We do know the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) standard 62.2, states “Each habitable space shall be provided with ventilation openings with an openable area not less than 4% of the floor area or less than 5 ft2 (0.5 m2)”. We originally supposed the magic number of 5% was based on this but the ASHRAE standard was first published in 1973, so it looks like it may be the other way around.
In England, the rules are a little different still. They have specific requirements for purge ventilation (the ability to open a window or door to deliver fresh air from outside) and make the distinction between the type of opening in relation to the minimum total area of opening. For example, a hinged or pivot window with an opening of 15-30 degrees must have a minimum total area of opening equivalent to at least 1/10 of the room floor area. For a sash window or door with an opening of more than 30 degrees, the minimum total area of the opening must be at least 1/20 of the room floor area.
So maybe that’s where our 5% comes from? Maybe it’s based on a window that opens greater than 30 degrees? If that’s so, it calls into question a glaring issue and raises a major flaw in our own code.
The Method for Achieving 5% Doesn't Add Up
Calculating the total floor area of a room is straightforward enough. Simply multiply the width of the room by the length. It’s hard to argue with that maths.
To calculate the openable area of windows, doors or skylights, the code states you should “measure the section of the windows, doors and skylights that open, excluding fixed frames…It doesn’t matter if one window or door opens further than another – if they open, the measurement of the full area should be included”.
The last few words are what concern us. It should matter how much the window or door opens because it has a direct effect on the amount of outside air the opening allows in. Because what if the windows open less than 30 degrees? In the UK, it would mean the openable area should be increased to 10%, double what we have here.
Let’s put it another, although slightly obscure way. Let’s say we’re talking about car doors and the manufacturer has a rule that the door must be equivalent to 5% of the overall surface area of the vehicle. That’s easy enough to achieve but if the door only opened by a few CMs it would be a pretty stupid rule. No-one would ever be able to climb inside and go for a drive.
We contend there is a major flaw in the code. The 5% might be perfectly adequate if the window/door can be fully opened but the fact that’s not the case would seem to be a glaring omission. And this is before we talk about other factors we believe make the 5% rule insufficient for healthy homes. Factors like airtightness.
Airtightness in Residential Buildings
The airtightness of NZ housing stock has steadily improved. Use of sheet lining materials, better fitted doors and windows, has all contributed to a steadily improving level of airtightness. This can be seen in the paper ‘Indicators of Natural Ventilation Effectiveness in Twelve New Zealand Schools’ (MR Basset & P Gibson) which shows the differences between houses and classroom pre and post 1960.
The drive to improve air tightness is energy efficiency, and BRANZ is pushing for an optional target level of three air changes per hour at 50 pascals pressure. This can lead to unintended consequences though, which they are fully aware of when they rightly say “as airtightness in houses has improved over time, more newer houses are likely underventilated”. Tellingly, they also say this: “Any initiative to set a target for airtightness in residential buildings should also therefore include more comprehensive ventilation requirements than are presently in the Building Code.”
At the crux of the issue is this - older homes are draughty. They naturally let in more fresh air than modern homes so required less ventilation in the form of openable windows and doors. A comparison of ventilation rates using passive tracer gas techniques would show this. So the 5% figure might be ok when the fresh air ingress was being naturally boosted by air finding its way inside through gaps, cracks, and less-than-snugly fitting windows. But in modern homes that doesn’t happen. There is less air getting inside and the only way to counter this is to change the code requirements.
How Many People Are Inside the Dwelling?
The number of people in a living room will have a direct effect on the level of ventilation required to maintain a healthy home. Of course, there are no rules that say a living room should provide X metres of space per person, and while 5 bedroom homes will normally have a living room that’s larger than that of a 3 bedroom home, the ratio of floor space per occupant will change depending on the design.
With higher density housing gaining momentum there will be pressure for families to share more confined shared spaces within the dwelling, with some bedrooms used for double-occupancy. It would be hard to legislate for this but some considerations should be given and it’s not inconceivable that a rule change might help provide guidance as to how best take into account the ventilation requirements in shared spaces when factoring in the number of bedrooms as part of the same dwelling.
This scenario becomes even more tricky, yet arguably more important, when applied to school settings. It’s widely acknowledged and accepted that access to fresh air and sunlight has a positive effect on the academic progression of students. Yet the ‘magic number’ makes no allowance for classrooms which are frequently full and possibly overcrowded. And whilst there are undoubtedly advantages to updating classrooms with newer builds, we again encounter the unintended consequences of improving airtightness.
A study into ventilation in 12 schools in the Wellington region revealed issues with airborne fungi and bacteria. This varied by airtightness (or modernity) of the classroom but in all cases it can be surmised there was not enough adequate ventilation, meaning the 5% rule does not, and likely did not, ever deal with the issues or airborne contamination sufficiently.
Where to From Here?
The only conclusion to draw is the same one that most ventilation experts, BRANZ and others have already settled on. Namely, the code as it stands is not sufficient to meet ventilation needs.
The magic number of 5% really has no magic to it. In fact, it has very little logic to it when placed in the context of modern builds. For now, it has to be met but our recommendation is to use it as the most minimal requirement.
There are numerous mental, physical and sustainability benefits to be accrued from natural ventilation and they all come from a number that is greater than 5. There is nothing magical about that number and the real trick comes from speaking with ventilation experts who know best suits a project's specific needs.
We know some people who can help with that.
