Ventilation strategies We need fresh air more than ever

18 July 2022 by Jason Quinn

There are only a few actions individuals can take to improve indoor air quality (IAQ) in their own homes and the buildings where they work, learn or socialise. New Zealand buildings are typically not designed for 100% fresh air ventilation at sufficient rates. The long-term solution is to increase both expectations and regulations: every building should have continuous mechanical ventilation. 

However, any action that allows stale, exhaled air to be replaced by fresh air from outside is a good thing. Humans think more clearly and function better in fresh air. And given this time of plague we live in, more ventilation will lower the risk of Covid and other viral transmission.

As for measuring the problem, a CO2 detector provides quick feedback on how well a space is ventilated, although of course the reading also depends on how many people are occupying the space. Choose a detector with an NDIR sensor mechanism for reliable readings. Based on my personal experience, I think almost all New Zealand homes are under-ventilated over winter. The only exception is when it’s blowing a gale outside.

Ventilation requirements are set out in Clause G4 of the New Zealand Building Code. Its objective is “to safeguard people from illness or loss of amenity due to lack of fresh air”. The acceptable solution has several options, but mostly the choice is open windows (with an open area of 5% of net floor area) or mechanical ventilation. Because building design and construction in New Zealand is obsessed with minimising first costs, we overly rely on people opening windows because that’s the cheapest way to meet the Building Code requirements. However, the assumption that windows will be opened long enough and often enough to ensure acceptable levels of ventilation is wrong and it creates costs we don’t measure because it negatively effects people’s productivity, health and comfort.

Four strategies to improve ventilation in existing buildings

  1. Opening doors and windows is free and usually effective, but only if people open them wide enough, for enough of the time. That’s an unrealistic proposition in midwinter in typical New Zealand housing stock that is under-insulated, cold, damp and already draughty. 
  2. Turn on bathroom extract fans and leave them running. In typical New Zealand homes the only controllable mechanical ventilation is bath and kitchen exhaust fans. These extract air which is replaced by air from outside that is drawn in through the building envelope. This isn’t ideal as that might be from dirty floor or ceiling cavities. Worse, it could be air from an attached garage, polluted with carbon monoxide and particulates from car exhaust. I suggest cracking the windows on the opposite side of the house from the bathroom when the fan is running. This way the incoming air will actually be fresh from outside.
  3. For commercial buildings with central ventilation systems, I recommend asking the building owner to immediately implement MBIE’s just released advice on controlling the spread of airborne diseases in commercial buildings. It suggests switching to 100% fresh air and increasing flow rates. This does mean heating costs will rise so your landlord may not be super keen. Ask if the CO2 levels are being measured and for remedial action if readings are consistently over 900ppm.
  4. So-called “air purifiers” (they are filters, but that doesn’t sound as compelling) will reduce the risk of airborne viruses but the caveats are significant. They must be correctly sized to the space and the number of people in it. Not all filters are able to block those tiny virus particles: F7, MERV15 or HEPA are all capable of that job. People should refer to the operating instruction and not skimp on filter changes: they must be regularly replaced to continue to be effective. 

Ventilation strategies for new buildings
For new buildings and homes I recommend designing the ventilation system for 100% fresh air (for health) and with heat recovery (for energy efficiency). This is the same advice I’ve given for every commercial building and home I’ve worked on for the last decade. It makes so much sense for indoor air quality (IAQ), employee productivity and health and energy efficiency.

This figure (below) compares the ventilation heat lost for a typical new home in New Zealand at different airtightness (draughtyness!) levels. It includes the energy to run fans. Bottom line? Even in draughty homes, the MVHR system uses less energy than either a continuous extract fan or opening windows. The difference in energy efficiency is much larger for better-built homes that don’t leak so much air.

Graph: Jason Quinn, Sustainable Engineering Ltd

(For building science geeks, the candy stripe portion of the graph is the infiltration losses. Cool science points if you can figure out why the extract fan in the fairly airtight home has nearly zero infiltration heating energy losses.)

For large buildings, my advice is typically a large mechanical ventilation system designed to supply fresh air and extract the stale air at sufficient rates to keep CO2 levels to <1000ppm. For high-end Green Star buildings, the limit is 550ppm. Heating and cooling is supplied locally using fan coils or a VRF system (an advanced heat pump). This results in a very energy-efficient building that provides optimum conditions for health and productivity.

For homes and smaller buildings, my advice is something very similar. Install a centralised mechanical ventilation with heat recovery (MVHR) system to supply fresh air to the bedrooms and living spaces and extract stale, moist air from the bathrooms and kitchen area. Install a heating system (and cooling, if needed). 

In a compact home, a single high-wall heat pump may be sufficient as the MVHR does distribute a small amount of heating and cooling. For larger homes, we have recommended a variety of systems ranging from centralised ducted heat pump systems, house-wide VRF systems, multiple mini-ducted heat pumps and heated floors. Once fresh air is ensured, there are lots of different options. 

Some of our clients decided against budgeting for a MVHR. In this case, we’ve recommended they install continuous extract ventilation. This is clearly not the best choice but it is much better than intermittent extract ventilation. Our advice from 2019 remains relevant.


Technical references

Controlling the spread of airborne diseases in commercial buildings
MBIE has just released advice to increase fresh air rates and better maintain ventilation systems. The primary way to increase the amount of fresh air is by switching centralised air handling units to 100% outdoor air mode and leaving units running at low speed outside of business hours. Better maintenance will help the equipment work well and is about maintaining existing ventilation systems, which includes more frequent inspections and cleaning, replacing filters, etc .

How better building design can fight the spread of Covid-19
Super-spreader events [are] less about the location and more about the physical characteristics and what was or wasn’t happening in that space. It’s always the case that people are not masked and they have low or no ventilation.” Joe Allen, assistant professor of exposure assessment science and director of the Healthy Buildings program at Harvard University.

Reference book (free and in-depth)
A Guide to Energy Efficient Ventilation by Martin W Liddament, published by the Air Infiltration and Ventilation Centre at the University of Warwick. It dates back to 1994 but the fundamental hasn’t changed. Info and a summary of contents here. Downloadable PDF here.

Designing for a Post-COVID World with Passive House
Steven Winter’s post is an excellent, detailed account of designing multi-unit residential Passive House buildings in the time of Covid.

Ventilation efficiency of decentralized alternating residential ventilation unit
Push-pull style ventilation works better than I expected, according to this German paper (published in English). Bottom line the air mixes quite well and the efficiency is claimed to be as good as a more traditional MVHR. I’m not 100% sure this is true but that is what this particular study showed. Healthy and energy-efficient buildings Testing of ventilation strategies in Australia covers displacement, HEPA filters and operable windows. Nice aerosol and time-to-clear experimental strategy. Summary here. Technical paper here.

Air pollution causes ‘huge’ reduction in intelligence, study reveals
Back in 2018, before anyone had heard of coronavirus, studies were warning of the dangers of air pollution (and what it means for indoor air quality). The impact on cognitive ability is significant.

NB Trickle vents don’t work
Passive ventilation and cutting holes in windows won’t work—a fan is required. ‘
Trickle ventilation as installed and used is ineffective in meeting desired ventilation rates, evidenced by high CO2 levels reported across the sampled dwellings’. Study referenced at the foot of this post.