We tend to think of energy efficient buildings as being ‘air tight,’ but this is not actually the desired outcome.
There are many factors that affect levels of indoor comfort, including air quality. Air quality can be affected by external air pollutants entering the building, the materials used inside the home, the household products used, as well as activities in the building. Energy efficient buildings need to be well sealed and insulated to conserve energy and minimise thermal conductivity, but activities such as showers, cooking and clothes drying, for example, can increase the humidity in indoor air and lead to possible mould issues.
Poor indoor air quality may cause a wide range of mild health effects with generally non-specific symptoms such as headaches, tiredness or lethargy or can lead to more severe effects such as aggravation of asthma and allergic responses. Employers are required by law to provide a safe workplace environment, and this includes maintaining a healthy indoor air environment.
The key to having an energy efficient home or office and healthy indoor air quality is to ensure effective ventilation. This means providing a regular, unobstructed draft between windows or doors on opposite sides of the indoor space for just one to five minutes – sufficient time for the indoor air to be entirely replaced.
By ventilating the room briefly but regularly, thermal mass walls don’t have time to cool down, preserving precious heat in winter. In summer, night purging of indoor air will also improve indoor comfort. EPA Tasmania recommends ventilation so all the air in the room is completely replaced at least once every hour. Even on the coldest days, there should still be some provision to allow air to move in and out.
This poses a concern for environments susceptible to more extreme hot or cold weather where small gaps in the building envelope can undermine the building’s thermal performance by allowing the outdoor temperature in, and cooled or warmed indoor air to escape.
Thermally insulated window systems should be a must for all modern homes. The size, type and placement of windows are very important considerations to create and maintain a comfortable indoor environment by offering daylight and regular air ventilation. Proper window size, type and placement also helps save building energy and reduces the need to artificially heat or cool. However, there are some concerns that double glazed windows will lead to unhealthily humid indoor conditions as they can be so well-sealed between the sashes and within the building envelope that they do not allow for any drafts through gaps.
Today's window designs, however, can facilitate easy ventilation while maintaining good thermal properties. Common in Europe are Tilt and Turn windows and doors, which work effectively in a cross ventilation situation: the old, stale ‘hot’ air escapes over the top of the inward tilted window, while the new, fresh air enters the dwelling around the sides of the tilted window sash. This ventilation is ideal for maintaining healthy indoor air quality and a high degree of security. The window or door can also be fully opened (inwards) to provide a strong draft, for example to purge the home with cool evening air on a hot summer day.
It is important to work toward a balance between circulating fresh air inside the building and conserving heating and cooling energy to achieve indoor air quality and thermal comfort. Simple, passive window design solutions based on window placement and type should make cross ventilation easy and effective while maintaining a thermally efficient building.
The best results come from windows like Tilt and Turn with insulated (PVC or wood) or thermally broken window frames that are designed to deliver maximum energy efficiency when considering all parts of the window system.