There are (at least) four reasons why you need to understand moisture and humidity in building physics:

Size: px

Start display at page:

Download "There are (at least) four reasons why you need to understand moisture and humidity in building physics:"

Jody Wade
5 years ago
Views:

1 4D11 Building Physics HANDOUT 4 Mich MOISTURE AND HUMIDITY There are (at least) four reasons why you need to understand moisture and humidity in building physics: occupant comfort - basically the thermodynamics of sweat; mould growth on inside walls, window frames, etc; dust mites; heat transfer (evaporative cooling, latent heat, etc). Air can carry water vapour. The moisture content MC of air is defined as the ratio mass of water vapour MC = in kg H 2O (1) mass of dry air kg dry air At any temperature, there is a maximum amount m v,sat of water vapour that the air can carry. When carrying this full amount, the air is said to be at saturation. When, at some given temperature, the air is carrying less than the maximum possible amount, the amount of water vapour actually being carried can be defined via either the relative humidity or the percentage saturation. The percentage saturation (PS) is the easiest to understand, it being defined by the simple mass ratio P S = m v m v,sat = mass of water actually carried mass of water carried at saturation kg/kg in kg/kg (2) The relative humidity (RH) is more subtle, but more commonly used. It is defined in terms of vapour pressures: RH = p v p s = vapour pressure saturation vapour pressure in kpa kpa (3) Both PS and RH are usually expressed as percentages. PS and RH are different concepts, RH being defined via pressures and PS via masses. However they are numerically similar. Both are 100 % at saturation, and both fall off below this. The two concepts are almost numerically equal near saturation, but they diverge a little at lower values. For our initial purposes, we can largely consider them to be equal. 1

1.1 Comfort If the RH is 100% then the air is holding all the water vapour that it possibly can and so sweat on the skin cannot evaporate, meaning that the skin cannot cool by evaporative cooling.

2 1.1 Comfort If the RH is 100% then the air is holding all the water vapour that it possibly can and so sweat on the skin cannot evaporate, meaning that the skin cannot cool by evaporative cooling. Moreover, the unevaporated sweat feels decidedly uncomfortable. If the RH is not 100% but is nevertheless high, sweat on the skin does not readily evaporate and feels uncomfortable. If the RH is too low there will be complaints of dry noses, mouths, and eyes and there may be an increase in some respiratory illnesses. For comfort, then, the RH should typically be between 20% and 60-80%. The limits are not precise, and the upper limit depends on temperature, 60% being more appropriate for summer (when one would like sweat to evaporate) and 80% for winter. 1.2 Dust mites Dust mites are tiny arachnids - they have 8 legs. Their droppings - and the things that live and grow on their droppings - are a contributory factor in many allergenic and respiratory illnesses. Dust mites cannot drink water, but absorb it via special salt-secreting glands on their front legs. If the relative humidity is low (< 50%) dust mites cannot obtain water by this route. Although they can survive protracted periods of dryness by forming super-colony clusters, drier air nevertheless slows their ecological development. Figure 1: A dust mite Dermatophygoides pteronyssinus 2

3 1.3 Mould growth Mould spores are contributors to respiratory illnesses such as asthma. Moulds require water for their metabolism and grow readily on surfaces that are moist with condensation. To limit mould growth, relative humidity (RH) needs to be controlled. RH tends to fluctuate throughout the day, and CIBSE Guide A suggests that if the surface RH exceeds 80% for only 3 hours a day then mould growth is unlikely, whereas if it exceeds this for more than 6 hours a day then mould growth is very likely. To avoid surface condensation, a designer would aim to avoid 100% RH occurring on cold surfaces. Aiming to avoid 80% to prevent mould is a stricter design criterion, and may thus govern the thermal/moisture design. Mould can also lead to wood rot, destroying the structural fabric of the building. One thus needs to ensure not only that there is no surface condensation but also that there is no interstitial condensation - i.e. condensation within the building materials. If building materials become wet internally then they may not insulate as well, leading to higher U-values and greater heat loss (e.g. a wet duvet). 2 Psychrometric chart The properties of moist air can be summarised in a psychrometric chart. There are various versions of these. The CIBSE Guide C version is presented on a separate sheet of this handout. The basic axes are temperature θ along the x-axis and moisture content MC up the y-axis. Note: throughout this section, θ is in Centigrade, the temperature scale that is designed to work with water. The upper bounding curve of the psychrometric chart corresponds to saturation conditions. The curve can be approximated by the following two equations: ( ) θ p s (θ) = exp in Pa (4) θ MC sat (θ) = 0.622p s P p s in kg/kg (5) where P is atmospheric pressure. At sea level, this is usually taken as kpa, but lesser values may be needed at greater altitudes - (i.e. strictly one needs a different psychrometric chart at different altitudes). Note 1. More detailed equations are given in CIBSE Guide C. Note 2. The equations above are for moist air above water: there are different equations above ice i.e. for θ < 0. Below the saturation curve lie similar curves corresponding to the various percentage saturations. 3

4 Example: from the psychrometric chart, moist air with a PS of 50% at θ = 25 C has a moisture content of 0.01kg (per kg of dry air). This is half the MC at saturation (0.02kg/kg). If the air at θ = 25 C, PS = 50% is now cooled, the moisture content stays the same, but at the lower temperature the saturation moisture content is lower, and thus the PS rises. (The same is true for the RH). For example, as our air cools to 20, the PS rises to just less than 70%. If we keep cooling, the horizontal path of constant MC hits the saturation curve at the dew point temperature, in this case about 14 C. If we continue to cool the air packet below the dew point temperature, the air can no longer hold the original quantity of water vapour, and thus the excess must condense out. The path on the psychrometric chart is thus horizontal until it hits the dew point and then it tracks down along the saturation curve. Example cont d: if the air packet is cooled to 10 C, the saturation moisture content falls from 0.01kg/kg to kg/kg, meaning that for each kg of dry air kg of water vapour has been given up to condensation. This is why spectacles fug up when entering a warm room from the cold outdoors. The warm humid internal air has an RH < 100%, but when it hits the cold glasses it cools and cannot carry the same absolute amount of water vapour, thus some must condense. The same reasoning explains why single-glazed windows in warm rooms have internal condensation on cold days, the result of which is mould growth around window frames. Figure 2: The psychrometric chart 4

2.1 Sensible and latent heat In building physics terminology, the standard notion of heat as the energetic, random banging about of molecules is called Sensible heat.

Water vapour condensing releases latent heat. 2.2 Wet bulb and dry bulb thermometers A dry bulb thermometer is just an ordinary thermometer. It measures the air temperature.

5 2.1 Sensible and latent heat In building physics terminology, the standard notion of heat as the energetic, random banging about of molecules is called Sensible heat. This distinguishes it from Latent heat, the extra heat required (or released) to take a substance across a phase transition: to turn water into water vapour requires a heat input. Water vapour condensing releases latent heat. 2.2 Wet bulb and dry bulb thermometers A dry bulb thermometer is just an ordinary thermometer. It measures the air temperature. A wet bulb thermometer is a thermometer whose bulb is wrapped in a wet sock. Before the advent of modern gadgetry, the Relative Humidity (or the PS) was traditionally measured by a sling. This had a dry-bulb and a wet-bulb thermometer on something resembling a football rattle. Swinging this around in a room of low RH, the forced convection over the wet-bulb causes the water in the wet sock to evaporate, thereby cooling the wet bulb. The two thermometers would thus give substantially different readings. Swinging it around in a room with high RH, hardly any water on the sock would evaporate, meaning the two thermometers would give very similar readings. The dissimilarity in the readings on the two thermometers is thus a measure of the relative humidity (or the PS). At 100% RH the wet- and dry-bulb temperatures are thus the same. At 0% RH the wet-bulb temperature is substantially below the dry-bulb temperature. The wet-bulb temperatures appear as sloping lines on the psychrometric chart. Figure 3: A dry- and wet-bulb thermometer, and two variants of a sling. 5

6 Figure 4: Wet-bulb temperatures plot as sloping lines on the psychrometric chart. Example: from the chart, at 20 C dry-bulb temperature, the wet-bulb temp at 100% RH is also 20 C (- see scale just above the saturation curve). At 0% RH, though, when the dry-bulb temperature is 20 C, the wet-bulb reading would be much lower, at 6 C. Example: if one swings a sling and obtains a dry-bulb of 25 C and a wet-bulb of 18 C, then on the chart the intersection of the vertical 25 C dry-bulb line with the sloping 18 C wet-bulb line tells us that the percentage saturation is 50%. 3 The heat content of air The total heat content (the enthalpy) of a parcel of moist air is also shown on the psychrometric chart. In the CIBSE version, the enthalpy is indicated by an outer pair of scales that surround the main diagram, and one needs a ruler to connect across them. (Basically, lines of constant enthalpy plot as diagonal lines on the chart, but the chart is already over-crowded with diagonal lines, hence the outer scales.) Moving horizontally to the right on the chart corresponds to increasing the sensible heat. Moving vertically upward means adding more water vapour (i.e. humidifying the air), and this requires more latent heat to be supplied. The total heat (or enthalpy) thus increases as one moves right (warming) and upwards (humidifying), hence the lines of constant enthalpy slope down to the right, approximately parallel to the lines of constant wet-bulb temperature. One of the simplest methods of cooling a hot room is to use an evaporative cooler. A fan blows the room air through a wet gauze, and (ignoring the power 6

7 Figure 5: Evaporative cooling at constant specific enthalpy (adiabatic). supplied to the fan) there is no net energy input into the room, thus the specific enthalpy stays constant. The evaporation of the water requires latent heat, and this gained by a a corresponding decrease in the sensible heat. (There being no net heat input, this process is adiabatic). Example: say air is originally at 25 C and PS = 50%. Say an evaporative cooler raises the PS to 80%. From the surrounding scales, the specific enthalpy is originally 51kJ/kg, and it remains at this value as the air is humidified. The 51kJ/kg line crosses the PS = 80% curve at a cooler dry bulb temperature of about 20 C. 3.1 Comfort Compared to countries with hot climates, the effects of humidity on occupant comfort in the UK are small. The main comfort issue concerns the effectiveness of sweat, and UK temperatures are rarely hot enough for this to be a major issue. The major UK issues involving humidity are thus mould and dust-mites. In hotter climates, though, humidity is a major consideration in comfort. The figure below shows an American diagram appropriate for California (from Lechner, Heating, Cooling and Lighting, Wiley). The central lozenge shape surrounds the set of conditions (temperature and humidity) at which occupants are generally comfortable (i.e. what the designer is aiming for). The near-horizontal lines annotated with the months of the year show the typical ranges of the external climate. For example, in August, the temperature varies from around F, (16-35 C), and with corresponding RH varying from 7

8 70% to 25%. Much of the time the external conditions are outside the comfort zone, and the arrowed labels suggest strategies for how to get into the comfort zone from some external, uncomfortable combination of temperature and humidity. Figure 6: Some suggested strategies for moving into the thermal comfort zone, appropriate for California (taken from Lechner). 8

For an ideal gas mixture, Dalton s law states that the sum of the partial pressures of the individual components is equal to the total pressure.

For an ideal gas mixture, Dalton s law states that the sum of the partial pressures of the individual components is equal to the total pressure. 1 PSYCHROMETICS Psychrometry is the study of the characteristics of moist air. We will see soon that evaporation of moisture from the skin can have a significant impact on thermal comfort. The rate of