Humidity and temperature have an interdependent relationship which can greatly impact the indoor environment to the extent it affects both occupant and building health. Yet while this relationship is complex and the inputs vary daily, the solution doesn’t. Indeed, the solution is not only simple and sustainable, it’s free.
Before we get to the solution, we should detail why one is even needed, because humidity and temperature levels can significantly impact not only the building but also the physical and mental wellbeing of those living in it. That’s the effect.
To best understand that we need to look at the cause, turning to science, and rather curiously, how a murder mystery in a small German village led to a diagram we use today to understand the humidity/temperature relationship.
Measuring Humidity
In the simplest terms, humidity is the amount of water vapour (i.e. water in its gas form) in the air and can be measured in two ways:
We’ll talk mostly about relative humidity levels in the remainder of this article as it’s easier to understand in context of the relationship between humidity and temperature. Also because that’s what the Building Code uses.
How Temperature Affects Humidity
Temperature is essentially a measure of energy at an atomic level. When atoms or molecules move faster, the temperature rises. As this happens, water molecules move from liquid form to gas and remain in the atmosphere until a relative humidity level of 100% is reached.
In other words, as things heat up, water is absorbed into the atmosphere until the atmosphere can take no more. That’s when the dew point is reached and water molecules move back from gas to liquid, in the form of water droplets.
Think of those hot and humid Auckland summer days. Despite it being so warm, sweat doesn’t evaporate and clothes become clingy. That’s because the atmosphere can’t absorb any more water as it’s already full.
Now to our German murder-mystery. When Richard Mollier (1863-1935) was enjoying a relaxing break from his job as Professor of Applied Physics by visiting a local cafe, he began eavesdropping on a nearby table, where the conversion centred on a recent murder of a man in a neighbouring village. The suspects were the mailman, the wife and the mistress. For some reason, Mollier decided he could use his knowledge of thermodynamics to plot a chart that would reveal the identity of the killer.
History has long since lost the answer as to who the guilty culprit was and we have no idea if Mollier solved the case. What he did do was begin a diagram (the Mollier Diagram) that would describe how air changes state depending on the temperature. Engineers still use these enthalpy-entropy charts today.
What causes high humidity levels?
The root cause of humidity is water. In the average family home or office building, this comes from multiple sources including the occupants themselves.
The average person will exhale 200 millilitres of water vapour per hour. A boiling kettle will release around 10 grams of water into the atmosphere. Then there’s the steam from cooking, water from drying clothes, the tumble dryer and that warming shower on a winter’s morning. All introduce moisture which has to go somewhere.
Moisture can also seep in through water leakage in the building envelope itself, damp conditions under suspended timber floors, and even retained moisture in materials like timber framing and concrete floors.
The effects of humidity
Epidemiological studies into microclimatic parameters have found low indoor air humidity causes eyes to become dry and tired, while mucociliary clearance (MCC) becomes less effective. MCC being the defence mechanism of the lung that removes inhaled pathogens before they reach lung tissue, so its proper functioning is important.
For workplaces, there is a direct correlation between dry and tired eyes and performance levels. In low humidity settings, fatigue is known to set in quicker and cognitive function can decline.
These findings would suggest higher humidity levels are needed. But go too high and even more issues reveal themselves. For a start, there’s mould. Where the air is damp (i.e. there’s high relative humidity) mould and bacteria will grow quickly, with the potential to cause respiratory problems and allergic reactions in occupants. Where higher airborne penicillium levels are present, the atmosphere may even contain carcinogens and nasties that can lead to organ damage.
In New Zealand, we have one of the highest levels of microscopic arachnids in the world. Yes, the (too) common dust mite is from the same family as ticks and spiders. And not only do they look particularly scary under a microscope but they thrive in high RH environments, where they can play havoc with allergy sufferers, inciting coughing, wheezing and itchy skin.
For the building, condensation will form on windows, walls and ceiling when those surfaces are colder than the air temperature itself, possibly leading to damage. This is particularly apparent in New Zealand where the use of non-thermally broken windows with aluminium frames is commonplace. It’s not unusual on a winter morning to see condensation forming on the from the frame and cascading down the window.
How to Control Humidity
Limiting the introduction of moisture into the indoor environment is the most obvious answer to controlling humidity. Extractor fans in the kitchen and bathroom, venting tumble dryers, moving from a kettle to boiling water taps, and not drying your clothes inside all help reduce the moisture levels.
Underneath the building, good levels of ventilation are needed with clear openings of 3500 mm² per square metre of floor area, or 700 mm² when a vapour barrier such as polyethylene sheet is used.
The overall aim is to keep the overall relative humidity to between 40 and 60%. This is considered to be the ‘safe zone’ where the issues detailed relating to both low and high humidity are reduced to a safe minimum. It’s also a zone where office employees are known to be most productive.
We note NZS 4303:1990 Ventilation for acceptable indoor quality supports the above and recommends relative humidity be no greater than 60% in habitable spaces.
The Best Way to Control Humidity
In the introduction we did mention there is a very simple and free solution to be had. So here it is…Let fresh air in.
BRANZ studies have shown that providing an opening for fresh air to enter the building, even if that opening is only ‘open’ for 15 minutes a day, is enough to lower internal moisture levels, which is something Ventuer ventilation louvres are particularly effective at. They allow fresh air in while keeping rain out, and at the same time maintaining security so the building can ‘breathe’ even when no-one is inside.
Interestingly, one of the best examples of how a Ventuer ventilation solution can control relative humidity is when we were asked to make sure the RH level was maintained to a level that would be detrimental for human occupants. Normally a constant RH of between 70-90% would be cause for concern but for the fish and crocodiles at Auckland Zoo’s ‘Swamp Forest’, it’s just about perfect.
There’s a multitude of ways Ventuer products can be configured to provide the best solution for relative humidity, and they vary depending on the budget, design and location of the building. What does not vary is how they use air to control humidity so it doesn’t become a problem.
