Every building has to have some sort of ventilation. The NZ Building Code makes sure of that. Although the extent to which the code actually satisfies the needs of current buildings (particularly residential housing) is a subject of some debate. That debate primarily deals with the adequacy of the provisions within the code and how easily they can be met, but is extended to the type of ventilation (natural or mechanical) and preferences for either positive, negative or balanced ventilation systems.
What the NZ Building Code Says
Section G4 of the NZ Building Code requires “spaces in buildings to be provided with adequate ventilation consistent with their maximum occupancy and intended use”. There are provisions for outdoor air and extract ventilation. Further, it sets forth the requirement for what it terms as ‘products’ (such as cooking fumes, moisture or gases) to “be removed for other people’s amenity and the protection of the property”.
In essence, it means a building must have what is considered as adequate ventilation, along with extractor fans to deal with the issues caused by the aforementioned ‘products’.
This is where some debate arises. The building code is considered satisfied should the building have a net openable area of windows (or other openings) to the outside of no less than 5% of floor space. To deal with cooking fumes and suchlike, the requirement calls for an exhaust fan with a minimum L/s flow. This is considered sufficient to deal with any issues from any ‘products’ arising from cooking or bathing/showering in the bathroom. But it really isn’t. In the case of a bathroom, the extractor fan needs to be at 25 L/s to meet code. Yet a 10 minute shower really needs a fan closer to 100 L/s to deal with moisture issues. Anything below that and you’ll start to see moisture build up, more than likely through the presence of black mould.
Natural & Mechanical Ventilation
Aside from the kitchen and bathroom, a residential property can meet the building code by simply having enough operable windows. With some modern buildings, this may be enough if proper design work and modelling has been taken into account. However, success is conditional on the weather. Think about what happens when you take your clothes from the washing machine and peg them to the clothes line outside. They’ll dry better on some days than on others. When it’s cold outside and a lack of wind, you’ll be waiting what seems like an eternity to get that rugby shirt ready for the weekend game. On warm days with no breeze, they’ll dry a little quicker. On warm days with a breeze, your freshly laundered clothing will be ready in no time.
Of course, it’s a bit different with a building but the above is useful when it comes to illustrating the limitations of natural ventilation. Namely, effectiveness is dependent on the prevailing weather conditions. A mechanised fan for backup is recommended for those days where the temperature and wind conditions are not sufficient on their own.
Positive Pressure in Ventilation Systems
Arguably the most popular in New Zealand, positive pressure systems work by sucking in air from either the outside or the roof cavity. This air is then filtered and forced into the house interior via fans in your ceiling. The extra air adds to the internal pressure and forces that already in the room out through any gaps in windows, doors and joinery.
Positive pressure systems are said to reduce condensation and black mould. They are relatively inexpensive to run and the fans quiet enough to go unnoticed. An example is the Ventuer silenced inline fan series, which has an acoustic outer-layer which effectively silences any operational noise.
Variations of positive pressure systems include those with heat transfer components. For example, a home with a log burner will have a ceiling fan nearby, drawing the warm air and forcing it through the ventilation system to assist with heating the rest of the home.
As to limitations, in a newer build the positive air pressure may struggle to force air through what is a relatively airtight building envelope, putting pressure on the system.
Negative Pressure in Ventilation Systems
Any building without a ventilation system will likely have negative pressure by default. The use of kitchen extractor fans and the bathroom fan, which suck air from the immediate area and force it out through an external vent will lead to negative air pressure. As these extraction fans are used, the air forced outside has to be replaced by something. This something is whatever air can be sucked in from outside, through the building envelope. You can see and feel this in action if you have a good bathroom extractor fan. Leave the bathroom door slightly ajar and turn the fan on. It’ll likely move the door a bit and if you stand inside, you’ll feel the air being sucked from the other side and coming in past your feet.
Such systems do a good job of removing any moisture. Where they have a weakness is in places like the spare room where you have that ‘clothes horse’ for laundry during winter. Damp clothes have a significant amount of moisture in them that has to go somewhere. Also, they are dependent on being switched on and remaining on for a period after cooking or the shower has been finished.
There is also some consideration needed for the air coming in through the building envelope. It may not be what is considered ‘clean air’. This was the contention in a recent study looking into ailments in a school in Finland.
Balanced Pressure Ventilation Systems
A combination of both of the above. A balanced pressure system uses extraction fans to force the stale air and drive out moisture from the building interior. But as opposed to relying on the negative pressure to suck in air from the outside through the building envelope, a balanced pressure system has a second fan which brings in fresh, filtered air drawn from dedicated exterior vents and introduces it into the most appropriate parts of the house via ductwork. This means that you have complete control over the fresh air supply.
An additional benefit of balanced pressure ventilation systems is that they can be configured to include true heat recovery. A heat recovery unit works by passing the incoming and outgoing airstreams between a series of very thin plates which allow the heat of the outgoing air to be transferred to the colder, fresh air stream. An effective heat recovery system can transfer up to 90% of the warmth from exhaust air to the incoming air. As can be imagined, this has a considerable impact on the energy efficiency of the building and the electricity consumption required to keep the building heated during winter months.
What Pressure is Best for You?
It depends. Which sounds a little like we’re avoiding making a choice but it’s true that no one size fits all. All systems have their advantages and we cater to them all. All have some drawbacks or limitations and nothing is ever perfect. What works in the warmer, more humid North may not be as effective in rural Southland.
As always, research and expert advice is recommended. For an impartial opinion on what may suit your project best, please contact us.
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