We often hear about natural ventilation, natural light access, and thermal mass in housing design, but what do these really mean and entail? How do you implement this from a day to day operating level to a designing level (if you are planning or designing a new home)? We have summarised a few tips from our research and conversations with Atlas Architects and Environmental Design Consultant Noy Hildebrand.
First of all, what does natural ventilation mean?
Natural ventilation is a process of replacing stagnant indoor air with outdoor air without mechanical assistance. Air movement is caused by the pressure difference at building openings – windows, doors and vents – between inside and outside. We think that the external air supply should be prioritised when there is good quality outdoor air.
Why do we need natural ventilation?
We need air to breathe. Natural ventilation replenishes the oxygen in the room, which is especially beneficial for activities that require focus and thinking, such as in work or study spaces. It also removes stale air, moisture, smells, CO2 (carbon dioxide), and other contaminants built up indoors, consequently, improving the comfort for the occupants. The kitchen, bathroom and laundry especially require the ability to control natural ventilation.
Furthermore, spaces designed for some natural ventilation are more resilient than those that require mechanical ventilation to operate. Natural ventilation does not produce greenhouse gases during operation and lowers upfront and running costs when designed correctly.
How do we effectively design and control for natural ventilation?
Below we have outlined three primary points of advice.
Ensure that there are at least two openings in the room. Openings can be windows, doors or vents. You may wonder if the window orientation matters and what if your house doesn’t have any windows facing the prevailing wind direction. The orientation actually doesn’t have to be perfect to work. What may matter more is the sizes and number of openings and their locations relative to each other. You would want the fresh air to meander and circulate inside to move the stale air along, so avoiding openings directly in front of each other would be preferable. The openings could be offset from each other to create cross ventilation and/or low and high to also aid stack ventilation. Note that stack ventilation relies on the difference in air temperature, so it usually works best in taller spaces.
Having windows facing the direction of the desirable local prevailing wind would help with the airflow. However, it is not essential just to allow for general air movement. The opening sizes play an important role in facilitating airflow. It should be noted that the window size is NOT necessarily the same as the effective opening area for ventilation. Various window types will have various ventilation areas, which I will touch on a bit later.
A principle to remember is that the inlet ventilation area should be a little smaller than the outlet ventilation area. Therefore, the occupants should be able to control the openings. For example, if my room has a south-facing casement window and an east-facing casement window, effective natural cross ventilation can happen with the south-facing window opened slightly less than the east-facing window, if the east-facing window is the outlet.
As mentioned above, different window types will have different effective opening areas. A comparative Ventilation Openings of Various Window Types study done by Breezway shows the following figures:
Ventilation area as % of total window area
Awning window 10%
Double Hung Window 40%
Sliding Window 40%
Casement Window 80%
Louvre Window 75%
It is also important to note that the window opening type will impact the direction of airflow. Noy advised that for casement, awning and hopper windows, the window pane will act as a ‘vane for the airflow’. For example, with an awning window, the air will move along the face of the glass pane, often upward to the ceiling, whilst an open casement window will catch and deflect breezes from varying angles.
A window type that can allow more flexible control of the ventilation area is Tilt and Turn. This window type would allow the pane to open as an awning or as a casement window.
With the current Building Regulation, windows to habitable rooms such as bedrooms on the first floor MUST be restricted to 125mm openings, which would significantly reduce the ventilation areas. Therefore, the appropriate window types play a crucial role to allow effective ventilation. Service areas such as the bathroom, laundry and kitchen require more ventilation to remove moisture and odours in the room, avoiding mould built up. Louvre window would be one of the more appropriate options for these areas, particularly the non-habitable rooms such as bathroom and laundry.’
It is worth noting that in general, horizontal openings will catch wind from more directions than vertical openings, particularly in shorter buildings and neighbourhoods with smaller buildings.
