AIR QUALITY AND VENTILATION
What is Healthy Indoor Environmental Quality (IEQ)
A healthy home is a home that has healthy indoor environment quality (IEQ). IEQ is determined from four key metrics:
● Air Quality – This Section.
● Thermal Comfort – See Heating Section.
● Visual Comfort – See Windows Section.
● Acoustic Comfort – See Acoustics Section.
Air Quality
Clean air is a basic requirement of life and indoor exposure to air pollutants causes very significant damage to health.
A complex relationship exists between occupant health and comfort and ‘green’ building. If an appropriate balance is not balances then attempts to improve one factor can negatively affect another. For example, the introduction of airtightness measures to improve energy efficiency can decrease indoor air quality and negatively affect occupant health and comfort if not balanced with adequate ventilation.
The primary focus in New Zealand is on the impacts of air contaminants on human health such as PM10 (particles less than 10 microns in diameter), PM2.5, ozone, sulphur dioxide, carbon monoxide and nitrogen dioxide (see Table below), as these contaminants are primarily the emissions from wood burners for domestic heating, transport emissions and discharges from industrial sources.
2.5x229102112
The best way to reduce or avoid the worst of these contaminants is to design out combustion sources (i.e. gas hobs/fires and wood burners) from new homes. Electric forms of cooking and heating are better for the health of the occupants and better for the health of the planet. Did you know that you can even get electric BBQs now12?
Indoor Environment Quality (IEQ) Contaminants
8Leardini, P., & de Groot, H. (2010, November). Indoor air quality and health in New Zealand’s traditional homes. In Proceedings of the 44th Annual Conference of the Australian and NZ Architectural Science Association (pp. 24-26). Auckland, New Zealand.
Leardini, P. M., Rosemeier, K., & Ong, A. (2012). Ventilation's pivotal role for indoor air quality of houses in New Zealand. 10th International Conference on Healthy Buildings 2012, Brisbane, QLD, 8 - 12 July 2012.
10https://academic.oup.com/ije/article/42/6/1724/737113
11https://ajph.aphapublications.org/doi/10.2105/AJPH.2018.304902
12https://www.weber.com/NZ/en/barbecues/electric-range/
Humidity
Humidity is the amount of water vapour in the air. If air has a lot of moisture in it, and it's a warm day, it will feel very close and muggy, making it difficult for us to keep cool.
Humidity affects human health because it impacts our thermal comfort. It also effects the survival of other organisms like bacteria, fungi (i.e. mould) and viruses (i.e. COVID-19).
How does low humidity affect health?
When relative humidity is low (below 40%) eyes become dry and irritated, skin gets flaky and the mucous membrane in the respiratory tract dries out. This can cause blood noses and increase the risk of cold and flu infection. If the mucous membrane is dry then viruses can more easily penetrate the membrane and get into your body making you sick.
When the air is too dry viruses (like coronavirus or influenza) don’t swell up with moisture meaning that if you sneeze or cough the virus can travel further through the air. This is why many people wear masks.
Low humidity can be a problem in very cold climates or in over air-conditioned spaces. In New Zealand, because of our maritime climate (we’re a long, skinny island surrounded by water), most of our buildings suffer much more from high relative humidity than low humidity.
How does high humidity affect health?
When the weather is warm and relative humidity is high, the body finds it difficult to keep cool because it is harder to remove heat via evaporation of sweat into the air (as the air is already heavily loaded with water vapour). This can lead to dangerous levels of overheating and severe illness. Extreme heat stress can even result in death from breathing difficulties, heart attacks or strokes. This is why the elderly are especially vulnerable during heat waves.
Asthma is aggravated by dust mites and fungi (mould), these need moisture to survive. When relative humidity is high (over 60%) dust mites and mould spores can access the moisture they need to thrive. Bacteria, like dust mites, have a critical water content. If they are ‘dried out’ they cannot survive. But if you have a high relative humidity in your home. you are giving the bacteria the moisture they need to thrive.
Research from the Fraunhofer Institute of Building Physics in Germany has shown how the rate of mould growth is a function of both temperature and humidity (Illustration below). When our houses were damp, but really cold (like the inside of refrigerator) mould didn’t grow as fast. Since we added some insulation (from the late 1970s onwards) some of our houses are now damp and a bit warmer, creating optimal conditions for mould growth.
Mould growth rates as a function of temperature and relative humidity are shown by isopleths. A typical New Zealand house at 15 degrees Celsius and 80% relative humidity is within the fertile environment for mould growth. Image: Fraunhofer Institute14
Relative humidity should be kept below 60% to eliminate biotic agents.
It is common for New Zealand houses to have varying internal temperatures because of intermittent heating (i.e. turning on heaters in individual rooms at night). This causes the relative humidity to also vary. As dust mites, mould and bacteria can all survive with even small time exposures to relative humidity over 60%, intermittent heating can exacerbate mould growth.
A healthy home must be designed to maintain relative humidity within the healthy range of 40 to 60%.
Optimum relative humidity range for human comfort and health
(a decrease in bar height indicates a decrease in effect for each of the items)
Ventilation
Ventilation is a critical component of thermal comfort and is required for restorative sleep15, productive work and play and for the durability and longevity of a building. Proper ventilation should bring fresh air in from outside to replace ‘dirty’ indoor air. The ventilation system should dilute and extract occupant-generated pollutants (e.g., carbon dioxide, airborne particulates from fires and gas hobs) and product-generated pollutants (e.g., volatile organic compounds).
Carbon dioxide is frequently utilised as an indicator of ventilation effectiveness. CO2 levels above 1,000 ppm are indicative of poor air quality and inadequate ventilation rates. General toxicity from other harder to measure contaminants can be assumed at nearly twenty times the concentration of CO216. Researchers have shown in bedrooms where windows were kept closed the CO2 levels ranged from 1,730 ppm to 3,900 ppm while in the bedrooms where windows were kept open the CO2 levels were below the 1,000 ppm threshold ranging from 525 ppm to 840 ppm17.
15Mishra, A., van Ruitenbeek, A., Loomans, M., & Kort, H. (2018). Window/door opening‐mediated bedroom ventilation and its impact on sleep quality of healthy, young adults. Indoor Air, 28 (2), 339 - 351.
16Rosemeier, K. (2014). Healthy and affordable housing in New Zealand: The role of ventilation. Doctoral Dissertation
17Strøm‐Tejsen, P., Zukowska, D., Wargocki, P., & Wyon, D. P. (2016). The effects of bedroom air quality on sleep and next‐day performance. Indoor Air, 26(5), 679 - 686
As modern homes have become more airtight air changes through infiltration (draughts through gaps in the thermal envelope) have been decreasing (see AIRTIGHTNESS). Airtightness (see section is desirable for energy efficiency, comfort and health. Infiltration is the opposite of airtightness and infiltration is not a substitute for ventilation.
Infiltration is not a substitute for ventilation.
Building Code Requirements
The Building Code clause that covers ventilation is G4.
This clause requires spaces in buildings to be provided with adequate ventilation consistent with their maximum occupancy and intended use.
Clause G4 sets out provisions for outdoor air and extract ventilation. It aims to ensure that systems are maintained against bacteria, pathogens and allergens and requires ‘products’ (such as cooking fumes, moisture, or gases) to be removed for other people’s amenity and the protection of property.
For further information https://www.building.govt.nz/building-code-compliance/g-services-and-facilities/g4-ventilation/
Residential Tenancies (Healthy Homes Standards) Regulations 2019
Rental homes must have openable windows in the living room, dining room, kitchen and bedrooms. Kitchens and bathrooms must have extractor fans. The Residential Tenancies Regulations acknowledge that mould and dampness caused by poor ventilation is harmful for tenants’ health as well as landlords’ property. For further information https://www.tenancy.govt.nz/healthy-homes/ventilation-standard/
Introduction
Proper ventilation is critical for sleep, health, safety, and comfort but also for the durability, longevity and protection of the building from decay.
A whole building, integrated design, systems thinking approach is required. This is so the ventilation system is sized for the property and its inhabitants.
Mechanical ventilation heat recovery (MVHR) systems work most effectively in airtight properties. These systems are not the positive pressure, supply only systems most commonly marketed in New Zealand where air is brought in from the roof space. Supply only positive pressure systems do not meet the criteria of ‘fresh air’ supply. Their popularity in New Zealand is an anomaly and should not be taken as an endorsement. In many places, drawing air in from the attic would not be legal – for good reasons. They also require high permeability houses in order to work because the air brought in needs to displace the existing air somewhere.
MVHR systems work by constantly bringing a low rate of outside fresh air into the house. MVHR systems extract moist ‘stale’ air and replace it with just the right amount of fresh air, filtered and tempered with the heat of the extracted air and then delivered where it’s needed.
Having an MVHR system is more reliable and healthier than having air moving in through the building’s structure (or not moving, depending on the weather). Relying on infiltration adds the risk of potentially carrying moisture, mould spores and insect faecal matter into the living space.
Ventilation Systems
Decentralised ventilation: accurate and energy efficient
In the case of decentralised mechanical ventilation, there is a difference between systems with and without heat recovery.
In decentralised systems with heat recovery, a single unit is installed in the external wall of each room being ventilated. These systems use two fans and heat recovery via a crossflow heat exchanger. When choosing a decentralised appliance, it's important to look for the lowest possible sound emission, as well as easy cleaning and maintenance options. You should be able to control the individual fans so that the air flow rate can be matched to the demand.
Individual units work independently of one another to remove air from rooms which can be over-exposed to humidity, such as bathrooms and kitchens. By using two units, you can ensure good airflow throughout your living areas. Separate sensors control the appliances depending on the humidity level in each room.
Through wall, decentralised systems can be useful for multi-unit buildings and apartments or for retrofitting ventilation into existing buildings where there is not roof cavity or limited access.
Central Ventilation: Fresh air everywhere
With central ventilation systems, there is a difference between extract air systems and supply/extract air systems. The simplest and most cost-effective system for central mechanical ventilation is an extract air system. Stale air is removed via ducts from areas such as the kitchen, bathroom and toilet. This causes negative pressure, which allows fresh air to be drawn through outlets outside, directly into areas such as the living room, playroom and bedrooms. The amount of air extracted is determined by the fan capacity. You're able to match the air flow rate to meet your needs with a simple switch. With a small heat pump, the extracted air can be used for heating hot water and central heating. Using this air at high temperatures as a heat source is incredibly efficient.
The ventilation ducts can be installed in walls, concrete or suspended ceilings, floor – or even in the open. The biggest advantage is the uniformed fresh air quality that is delivered across the entire building. The recovered energy can then be used to heat the air supply.
Unlike an extract air system, a supply/extract system draws the outdoor air from a central point before distributing via ducts to the supply air areas (living, loungerooms, bedrooms etc). Heat is recovered by a crossflow heat exchanger, which transfers up to 90% of the unused heat in the extract air into the supply air. The outdoor air is filtered to prevent any contamination of the ducts and the system itself, and to protect allergy sufferers from dust and pollen. These systems ensure an excellent level of comfort across the whole home.
Designing Centralised Mechanical Heat Recovery Ventilation Systems
It is important to consider the following points at the beginning of the design stage. Leaving these decisions until the end can make it very difficult to install a suitable system.
System Design
The system supplier will do a design for the building. This will show how many ducts to each room and what the flow rates need to be. This is based on volume of the rooms. Exact locations for ducting and grilles can be adjusted onsite in consultation with the designer and system supplier.
Thermal Envelope
The entire system needs to be within the thermal envelope of the building. Having ducting, or the unit itself, outside the thermal envelope reduces the heat recovery performance and increases the risk of condensation forming inside the ducts. The ventilation system should only create three penetrations through the thermal envelope (Supply Air, Exhaust Air and Condensate drain).
Ducting
Careful consideration should be given to the ducting routes. Generally, there will be one or two ducts running to each room and allowance should be made for this at the design stage.
Usual ducting sizes are:
From unit to exterior (x2): 160mm.
From unit to each room: 75mm or 90mm.
Flat 52mm (can run in 90mm wall) ducting is also available but is more expensive so it pays to use sparingly. Some systems are available that use 150mm or 200mm ducts to each room. Specific ventilation ‘voids’ need to be created to fit these in.
The ducts from the heat exchange unit to the exterior of the building should be insulated. Allowance must also be made to the additional diameter of this ducting insulation.
Unit Size
The ventilation unit can be quite large. They can also have two large distribution boxes and silencers. The location for these should be carefully considered to make sure there is enough room to feed all the ducting into them. Squeezing it all into a cupboard just bigger than the unit makes installation and servicing very difficult. Most units have two filters that need regular cleaning and occasional replacement. Make sure the access to do this is easy.
Summer Bypass
During winter, the unit recovers heat from the indoor air before expelling it outdoors. In summer there are times when recovering indoor heat is not desirable. Some units have a Summer Bypass function that stops the heat being recovered and expels it outside.
A unit that has a Bypass system is highly recommended. Some systems have a variable control that alters the amount of heat recovery depending on the indoor temperature, some systems simply offer on/off function for the Summer Bypass, and some systems have a manual Bypass.
Frost Protection
For New Zealand's colder climates, units should have a Frost Protection system. This stops the system bringing in cold outdoor air when the air is too cold to be sufficiently heated by the recovered heat from the outgoing air. Be aware that this effectively turns your unit off, so that your house is not being ventilated.
Electrical
Some systems require a wired controller located in an easily accessible location. They also may require a boost switch in bathrooms. A power socket will be required for the unit. Allow for these as required on the electrical plan.
Condensate Drain
The unit will require a condensate drain to the exterior. They will normally require a dry siphon trap to make stop smells and air leakage. Allow for these in the drainage plan.
Exterior Grills
There are practical and aesthetic considerations as to where the external grills will go and what they will look like. This could be two separate grills or one grill that allows for both supply and exhaust air. Through roof cowls are also available. They should be as close to the indoor unit as possible. If using two separate grills, consult with the system supplier as to how far apart they should be.
Further information
http://www.buildmagazine.org.nz/articles/show/optimal-ventilation
Commissioning
Ventilation systems can be designed appropriately and installed correctly, but still not optimised. Commissioning is a vitally important, but often missed step. To help ensure this happens, designers should add commissioning to the specification.
The purpose of commissioning is to test and tune the installation. It can occur one the air control layer of the building enclosure is complete, and the ventilation systems is installed and operational. As a minimum, commissioning should check:
● Flow rates of installed extracts are adequate.
● Verify that ducts are fully connected to external outlets.
● Supply and return vents of whole house systems are balanced.
● Supply flow rates are adequate for the expected occupancy.
Whole House Ventilation Suppliers
The following companies supply ventilations systems suitable for Healthy Home use.
● Zehnder – http://envirospec.nz/products/services/mechanical-and-hvac/fantech/
● Wolf – https://www.wolf.eu/en/products/mechanical-ventilation-units/
● Stiebel Eltron – https://www.stiebel-eltron.co.nz/
● Lunos – https://homestylegreen.com/lunos/
● Brink – https://www.moisturemaster.co.nz/products/positive-input-ventilation/sonair-through-the-wall-system
Recommended Healthy Home guidelines are set out below.
18Refer Commissioning Section
19NZS 4303:1990 Ventilation for acceptable indoor air quality, Table 2.3
