Humidity control materials can adsorb and desorb moisture from the adjacent air due to their passive and environmental properties. When indoor air RH is high, hygroscopic materials can store moisture from the air, and return it to the space when RH is low. However, during heating period, indoor air humidity is at a low level for a long time. In this case, are there some humidity control materials which can regulate indoor air humidity?
I would recommend silica gel for indoor humidity control. Another materials which could also be used include: activated carbon and possibly graphene.
Hope this helps answer your question.
Professor Yehia Khalil
Yale University
USA
During the heating period, when the indoor relative humidity is low for a long time, heating hygroscopic or deliquescent materials to temperatures considerably above the ambient air temperature can force them to release their water. Water itself at or preferably above above ambient temperature can also be used, since it will evaporate and add to the relative humidity. When the heating period is over, the hygroscopic or deliquescent materials can be allowed to return to room temperature and then will readsorb the water when the relative humidity returns to higher values.
Indoor air humidity is conventionally controlled by air spray devices to raise the humidity of mediums. However, as told above, reducing the humidity is achieved by silica gels or similar materials. But in order to get a digital signal and design a controller to have a constant humidity we need a digital hygrometers which are expensive.
During the cold season, the outdoor air, even at 100% RH, contains less than a few gram per cubic meter of water vapour, while indoor air should be at 40% RH or more, hence contain at least 7 g/m³ at 20°C or 9 g/m³ at 25 °C to be comfortable and healthy. For example, below 40% RH, flu viruses contained in the aerosol from cough can survive, and below 30% RH the mucous membranes are irrritated.
To evacuate the contaminants emitted by one adult with a sedentary activity, at least 10 m³/h of vitiated air shall be evacuated and replaced by dry outdoor air, but recommended values are often larger, first to get a better indoor air quality, and second to evacuate other possible, more toxic contaminants.
In order to rise the humidity of this new air, more than 5 g/m³ h is needed, that is about 50 g/h and person at 10 m³/h or 75 g/h at 15 m³/h. However, a sedentary person evaporates about 75 g/h. The first conclusion is that if the indoor temperature is not too high, and the outdoor air not too cold (say above 0°C) the necessary relative humidity of the indoior air can be obtained simply by keeping the ventilation rate at a reasonable level and in proportion of the occupancy rate.
If the airflow rate is larger however, more water vapour will be needed. For example, 75 g/h and person is needed if the airflow rate is 30 m³/h and person. This represents 1.8 kg/day or 360 kg of water for the heating season (taking it at 200 days as in my area).
Plaster or gypsum is considered as a good humidity control material in buildings. It can adsorb 1.5% by weight when the ambient air rises from 40% RH to 70% RH. With a density of 1400 kg/m³, this represents 18 kg of water per cubic meter, or 0.36 kg per square meter for a 2 cm finishing layer. This means that 1000 square meter finishing per person can store or give back the water vapour needed for a seasonal storage. It is large area and therefor plaster cannot be considerred for a seasonal humidity control. It remains however useful to stabilize dayly humidity variations.
From these considerations, I have some doubts that a material that can be used in buildings to store the large amount of water needed for seasonal humidity control. Silica gel can adsorb 10% by weight between 40% and 70% RH and is proposed in paints to eliminate the on-peak latent air conditioning load (ASHRAE Transactions: Research, Volume 100, Part 1, paper #3744) but, at about 0.7 kg/m³, the volume required to store seasonal quantities remains rather large.
Silicagel is nevertheless used in museums to protect artefacts agains too high humidity, but in closed cabinets.
See attached file.
If you are doing the heating in winter by using steam why not you use some of the steam and inject into the stream of air. This will contribute in raising the temperature and humidity of the conditioned space then you can do some control to the quantity of the injected steam by relating it to the required space condition.
Leaving aside the numbers well presented above, namely by Claude-Alain Roulet, one could say that, in principle, the good news are that the heating would tend to correct the relative humidity as the latter goes down when the temperature goes up.
Another question is how to keep the humidity level within an acceptable range through the occupation. And, then, we need to know the occupation intensity in terms of moisture generation. If the generation is small, one may count on the occupation breaks to lower the moisture level and somehow 'recharge' the indoor air to 'support' the new moisture sources coming back after the break.
With my contribution I am modestly intending to complement somehow others' by calling the attention for the dynamics of the emissions of water vapor and the capacity of accomodate some variation fo the moisture levels indoors. And so, I am pretending to suggest that it is possble to play, namely, a) with the ventilation rate of fresh air which level, based on just human occupation, can be as low as 4 Liters/second/person and b) with the variation of the moisture level itself within 'an acceptable beach of values' for a reasonable period of time, in case, for instance, of natural ventilation where windows - as for exemple, for schools - can be kept opened at the breaks.
This 'soft management approach' depends, of course, on the climate and the quality of the outdoor air
