Trickle vents cheap but not ideal There are better ways to deliver continuous ventilation

Trickle vents are making a comeback in New Zealand because in combination with small extract fans they are the lowest-cost way to meet Homestar v5 requirements for continuous ventilation. However trickle vents are not ideal. It’s possible to specify additional small components to improve how this combination functions and it will succeed in reducing relative humidity and introducing fresh air. But even the best design comes with significant costs for home occupants. It’s cheap, says Jason Quinn from Sustainable Engineering Ltd, but not great. 

Jason was one of the industry experts that contributed to the development of v5 Homestar. He was especially involved in the energy use credits. Adding a requirement for continuous ventilation is an important improvement over the Building Code and he’s glad it was introduced. “Ventilation is essential for indoor air quality. You have to have air exchange to keep indoor air fresh and humidity levels acceptable. Dampness brings mould and that creates health problems. The links are well established and the personal and societal costs are awful,” says Jason. “This is especially important for social housing, which is typically quite crowded.”

The cheapest way to comply with Homestar’s v5 requirements for ventilation is a combination of continuous extract ventilation (typically a small extract fan running low all the time in the bathroom) with make-up air being supplied via trickle vents. These are deliberate gaps made in the window frames. 

Jason points to international research that shows how unpopular trickle vents are with tenants*. “Trickle vents create cold drafts and let noise from outside in. And the outside air isn’t always clean either. It’s all made worse by the vents typically being specified in the bedrooms,” he says.

This figure from the Scottish paper* graphically illustrates why ventilation is so important in bedrooms. Note that trickle vents are not required to be used in conjunction with continuous mechanical extraction in the UK, severely undermining their effectiveness.

Jason investigated how such problems could be overcome while consulting for prefab manufacture Manor Build. “We talked it through during the Initial Design process for three Manor Build designs that were targeting a volumetric 6 Homestar rating. I showed them how using filters and acoustic dampers and moving the vents to the living areas rather than bedrooms would reduce occupant discomfort. We basically made this combination as good as it could get. But it’s still not very good,” says Jason. “Trickle vents were used quite a lot in the past and they fell out of favour for good reasons.”

Occupant discomfort is not the only problem; the trickle vents’ impact on heating demand is also noticeable. On the Manor Build project, trickle vents would have required an upgrade to higher-performing uPVC windows. The Manor Build team was keen to stick with familiar windows from an established local supplier—this was the deciding factor for them choosing a full mechanical ventilation with heat recovery (MVHR) system instead.

“An MVHR is a far better solution, both in terms of human comfort and building performance,” says Jason. “The Manor Build team is a great bunch of people. I was impressed and pleased that they weighed up the pros and cons, then chose the best ventilation option.”

You can read more about Manor Homes volumetric 6 Homestar v5 designs (1, 2 and 3-bedroom factory-built homes) here in this technical case study.

*Trickle vents are common in the UK. This 2015 paper from Scotland shows how ineffective they are in ensuring acceptable indoor air quality when used without a continuously running extract fan (using CO2 levels as an indication of IAQ). Another notable finding is how frequently trickle vents were closed up or blocked by building occupants.