How to Ventilate a Bathroom for Healthy Buildings & Inhabitants
Why do bathrooms need quality ventilation systems? There are two primary reasons, firstly to manage humidity and airborne moisture generated by personal hygiene, and secondly the removal of odours. While these factors may seem obvious to most people (especially the odour issue), the risks of insufficient ventilation to bathroom spaces and the dangers that can arise may not.
How much moisture is created in a bathroom?
Let’s start by looking at the amount of airborne moisture created by taking a bath or having a shower. While a number of factors that can obviously influence this (your water pressure, the temperature of your shower, the size of your bath etc) if is generally accepted that on average a shower will release around 2.6 litres of moisture into the air per hour, and a bath will release approximately 700ml per hour. This means that if you take a ten minute shower, the air in your bathroom will absorb around 430ml of water.
If we consider a typical large bathroom of 13 cubic metres in volume, the absolute humidity level as a result of this water vapour being absorbed in the air would be increased by 33g/m3. And as the maximum absolute humidity for 25° air is 23g/m3 and the average absolute humidity level in NZ sits around 16g/m3, it’s no surprise that long before our shower is over the air is “saturated” and condensation begins forming on our walls and mirror.
Or to look at this another way – the amount of condensation created as a result of taking a 10 minute shower in an unventilated space would be the same as taking a spray bottle with 330ml of water (the same size as a soft drink can) and spraying it on to the walls of your bathroom until the bottle was empty.
What are the dangers associated with moisture creation?
There are a number of dangers associated with high levels of moisture within internal spaces. First and foremost is the risk of mould growth. Mould is not only unsightly but can also cause a range of health complications such as asthma, allergic reactions, pneumonia and sinus infections.
A secondary danger of condensation is rot in timber framed buildings. If condensation can seep into the framing of a house wall or floor it can cause untreated timber to rot, significantly affecting the structural integrity of the house. A major concern with rot caused by condensation is that it is generally invisible, hidden within the wall linings and consequently not noticed until too late.
Yet another risk arising as a result of condensation is damage to paintwork, wall linings and furniture such as bathroom cabinets. Moisture getting in behind paint layers causes it to bubble and flake off, exposing the wall lining behind. Bathroom furniture is commonly made from processed fibreboard, which upon exposure to moisture swells and starts to fall apart.
Controlling smelly odours?
Odour control is usually required in order to maintain personal comfort levels. While it is possible to control odour through chemical means (i.e. scent diffusers or aerosol air-fresheners), a better solution is to mechanically remove the contaminated air and replace it with more pleasant smelling fresh air.
Options for ventilating bathrooms
So, what options do we have for ventilating bathrooms? Quite a few as it turns out…
The simplest option for ventilating a bathroom is through the use of an opening in the wall – a window. This could be a conventional top hung awning window, a side hung casement window, a tilt-and-turn window (not common in New Zealand, but we do see them from time to time) or- the ultimate – a glazed louvre window, such as those supplied by our sister company, Ventüer.
The obvious benefit to a window ventilator is that once installed it costs nothing to run, and it has the added advantage of allowing natural light into the space. The downside to a window as a ventilator is that if there is no wind outside there will be no air movement inside. Although section G4 of the NZ Building Code allows for natural ventilation of bathrooms provided the open area of the window is a minimum of 5% of the floor area of the room, it is generally accepted that this is insufficient and should be assisted with mechanical ventilation.
Another option, only available to single level dwellings or the top floor of multi-level buildings, is a skylight. Skylights that can be opened can take advantage of a phenomenon known as “stack effect” and allow greater airflow than a typical wall mounted window, however are still subject to the same requirements under the NZ Building Code (they must be a minimum of 5% of the floor area) and ideally should be assisted with mechanical ventilation.
For rooms that are not adjacent to an external wall or roof, the only option for successful ventilation is a mechanical system – i.e. an electric fan. Most building designers recognise the need for exhaust fans even in room that do have access to natural ventilation. G4 stipulates that a ventilation system for a bathroom must extract no less than 25l/s, however this is recognised within the industry as being a bare minimum and in many cases sufficient. If we go back to our earlier example of a 10 minute shower in a 13m3 bathroom, we see that we reach the saturation point of the air within a couple of minutes, and that to avoid condensation occurring we would need an air change rate of 28.5 which then equates to approximately 100L/S exhaust flow.
Selecting & Operating Bathroom Fans
Having established the need for sufficient mechanical ventilation within a bathroom, let's move on to fan selection. Like anything, you can get this right or you can get it wrong!
The first step is to determine the size of the bathroom. As we described earlier, a 25L/S fan will not come close to removing all of the humidity within a 13m3 bathroom before condensation occurs. A better way to size the fan is to base it on a certain number of air changes per hour, or ACH. While there are a number of “rules of thumb” out there, Vent supply recommends working to a minimum of 15 ACH for rooms containing a shower. While this may not eliminate condensation completely, it will bring it down to a controllable level. For our 13m3 bathroom scenario this would result in a fan capable of extracting 54l/s.
For rooms that contain only a toilet, where odour control is the primary concern and not humidity, the G4 recommendation of 25l/s is recognised as being sufficient.
You may be tempted to use a lower ACH for rooms that contain a bath rather than a shower (we did say that a bath may generate less than a third of the moisture created by a shower), however, bear in mind that baths can be easily retrofitted with spray nozzles which effectively turn them into a shower for the purpose of this exercise.
The next step is to locate your fan in the right spot. The closer you can get it to the source of the moisture, the better. However, if you have selected a fan with built-in lighting, you may wish to locate it in the centre of the room for visual reasons. Also be careful that it isn’t too close to the shower – you want to be extracting moisture laden air, not actual water! Water inside the fan is likely to damage it and will result in the possibility of electrocution.
One commonly overlooked aspect of bathroom ventilation is make up air. It’s all very well to have a fan capable of extracting 50l/s, however if the room is effectively sealed and no fresh air can get in then it’s not going to be able to perform as it should.
The best way to provide make up air is through an opened window, however it should be recognised that this is reliant on occupant control and the fan may be operated with the window shut. To compensate for this, a secondary relief vent from the outside can be introduced – this does not have to be powered, however must be capable of allowing the required fresh air into the room.
Undercut doors are a common method of providing make up air. With this method, the door is constructed with a gap between the base of the door and the floor below. While easy, cost effective and certainly offering benefits, the limitations of door undercuts should be realised. A standard internal door width is 760mm and with a 20mm undercut this results in an active ventilation area of 0.0152m2. If your target bathroom ventilation level is 50l/s, this will require an air speed of 3.29m/s through the undercut. The pressure drop arising from this level of airflow is likely to be higher than the bathroom fan is able to cope with, and its performance will consequently be impaired.
It is important that the fan is allowed to run long enough to clear the air after showering is finished. While the ideal length of time obviously depends on a number of factors such as room size, fan capacity etc, it is common for many fans to include a 15 minute timer in conjunction with the manually controlled wall switch. This simply means that once the user has finished in the bathroom and switched the lights and fan off, it will continue to run for another 15 minutes.
Use of heat generated in bathrooms with heat recovery devices
One question that often comes up, especially with passive house design, is whether or not balanced pressure ventilators with heat recovery should be connected to bathrooms in order to take advantage of the heat being generated. At Vent Supply, we believe that heat recovery devices should not be used in conjunction with bathroom ventilation. There are a number of reasons for this.
For example, balanced pressure heat recovery ventilators are generally designed to run continuously at a low flow volume (typically between 0.35 – 1.0 ACH). This rate of flow is insufficient for bathroom ventilation, which as we have seen requires sporadic ventilation of up to 15 ACH or higher. Even if you were to connect your heat recovery ventilator to the bathroom you would still need to include a secondary booster unit to achieve the flow required for periods of use.
The humidity levels in bathroom exhaust are obviously much higher than elsewhere in the house, and can reduce the effectiveness of the heat exchange media. There is also a much higher risk of condensation occurring inside the ductwork, which comes with its own range of problems.
While high quality cross-counter-flow heat exchangers have minimal cross-leakage, there is always the possibility of some of the exhaust air stream being sucked back into the fresh air stream. If this exhaust air contains pollutants such as toilet odours, then it may be fed back into the living areas of the house.
Ventilated Toilet Pans
Where toilets are in a room by themselves (i.e. without a bath or shower in the same room), pan ventilation systems can be very effective. These systems ventilate from within the toilet pan itself, and extract odours as close to the source as possible. They are however not recognised by the NZ Building Code as an acceptable solution to bathroom ventilation on their own and must be fitted in conjunction with other natural or mechanical ventilators.
Energy Efficiency & Noise
A couple of other important considerations when designing a bathroom ventilation system is the energy which will be consumed and the noise that will be created during operation. Depending on where you live in the world, the noise created by your fan may be stipulated in sones or decibels. Sones is a subjective measure based on how a sound is sensed. A sound that is “grating” or annoying will rate higher in sones than it would in decibels, a more scientific measurement. As a rule, a good bathroom fan should not be louder than around 30 – 40 decibels, which equates to approximately 2 – 3 sones.
Sound generation is obviously correlated to the size and capacity of the fan – generally speaking, fans with larger capacity can be expected to make more noise than fans with smaller capacity. There are also other factors that can influence sound creation, however, including the installation method, size of ducting attached and pressure drop within the system.
Energy efficiency is another important consideration when selecting a fan. Bathroom fans, along with other fans, are measured by Specific Fan Power (SFP). SFP is a parameter that quantifies the energy-efficiency of fan air movement systems, measuring the electric power that is needed to drive a fan relative to the amount of air that is moved through the fan. As a guide, most bathroom fans can be expected to perform at between 50 – 70 w/l/s SPF.
If all of the above sounds complicated and confusing, then there is a simple solution. You can just contact us. It’s what we’re here for.