Mark Bomberg
We intend to explain an alphabet of building science in the first five building science insights (BSI). In the first one, we highlight that heat, air and moisture flows are inseparable. In the second BSI, the name “environmental control” is introduced and explained. The third BSI highlights complicity of building energy and how a simplified perception can lead to surprising conclusions. The fourth BSI continues on example of thermal insulation and the fifth explains why Building Science is the name used in the US and Canada. These BSI highlight that environmental control is always transient and wholistic. Thus, in the first five BSI we want to ensure that the reader see a significance of environmental controls for indoor climate and the value of the building.
The hypothesis to be proven in this BSI is that heat, air and moisture transport across a building envelope are interactive and inseparable phenomena. Each influences the others and is influenced by all the materials contained within the building envelope. Codes and standards simplify the process of architectural design by tying the control of each phenomenon to a particular material. The thermal insulation, for example, is perceived to control heat transfer and the air barrier to control air leakage. Likewise, the rain screen and the vapor barrier eliminate ingress of moisture to materials. In reality, each of these materials performs different functions, and contributes to several aspects of the system performance. For instance, while controlling air leakage, the air barrier system may also provide effective moisture control. Similarly, thermal insulating sheathing increases temperatureand reduces intensity of condensation in the wall cavity.
The designers and builders must learn one golden rule, codes and standards define a minimum level of the acceptable design but they seldom, if ever, deal with defining performance of a building. Thus, it is professional obligation of designers and builders to do a better job than one required by the codes.
Below we define heat, air and moisture flows and show examples of their interrelation. This is important because with time the specialists forget the interactions. The most infamous case was in 1958 when Glazer introduced a simplified calculation of water vapor condensation without consideration what happen to condensed water.
Figure 1: Calculations of water accumulation, in kg/m2, over the service time from 3.6 to 5.4 years in a German building. They are performed with a verified, simultaneous heat and moisture code (red line) and the same but disregarding effect of liquid water (Glaser method, blue line).
Following this paper, the codes mandated use of vapor barriers and disregarded effects of heat and air pressure gradients. Yet, in case above use of capillary active material (moisture buffer) eliminated the need for a vapor barrier.
Figure 2 shows the dependence of the capacity for water vapor retention in air on temperature. This fact implies that in a cold climate, in winter, any interior air passing through a building enclosure (exfiltration) will deposit some water inside the wall. It also means that ventilation of a wall cavity must go from outside to indoors in winter and opposite in summer. Any ventilation with outdoor air only, e.g. attics space is not effective in winter season.
Figure 2: The same amount of water vapor in one cubic meter, namely 8.65 g/m3gives relative humidity 50% at 20 oC; 92% at 10 oC but 1.86 g is already condensed if the air is chilled to 5 oC.
Figure 3 shows another popular misunderstanding of relation between humidity and temperature of the air. An electric baseboard heater is coming up to the triple corer. One may think that as the hot air dries any material, there should not be any mold in the corner. One is surprised to see in Figure 3, that the heater in the corner of two exterior walls increased the volume of mold.
Figure 3: Why is there more mold in the exterior corner with the floor electric heater?
To understand why this is the case we will tell you a bit of physics, namely the thermodynamic potential of water is proportional to a product of R.T. ln(RH) where R is a gas constant for water, T is the absolute temperature and the last term is a natural logarithm of relative humidity. Thus, when we heat the wall its moisture content does not change much and the relative humidity inside of the material is going down. In effect, the wall will pick more water from the air. When we stopped heating, and wall is getting cooler and its RH reaches the saturation level (see the Figure 1). In effect, by heating and cooling, we produced increased moisture content in the corner of the walls and fed the mold spores with water. The defect is just opposite to what we wanted.
Thus, when we apply a heating wire in the corner of the window pane and parapet to eliminate window condensation, we must use a continuous heating for the whole period of time when temperature is below dew point and stop heating only when the risk for condensation disappear. This is fine when the wire temperature is just below the dew point and thermostat keeps the heating on when there is a risk of condensation. Yet, this method can only be used when the balance of energy is small. If this is notthe case, one must provide a gap between the wall and the parapet and useconvective heating through this gap.
Nevertheless, selecting this example is to show the gap between designers and builders who think “statically”, while the nature is “dynamic” (transient). When we talk about an equilibrium between moisture contained in a porous material that is kept in air with constant temperature and relative humidity, we do not realize that reality is not static but the rate of evaporation from the material and rate of water vapor sorption by the material are equal, resulting in the constant mass.
Figures 4a, 4b, and 4cexplain how a water movement in the material modifies heat flow. In Figure 4a we see a wet material being sealed in a plastic bag and placed in the Heat Flow Apparatus(Th is the hot and Tc is the cold plate of the apparatus). Figure 4b shows the heat flux entering a capillary porous material varies depending on special distribution of the same total quantity of moisture (moisture = liquid and vapor phases together) and Figure 4c shows a moisture profiles as they change in time of the experiment.
Figure 4a: Thermally driven moisture redistribution measured in the sealed, wet material placed in a Heat Flow Meter apparatus
Time zero denotes a moment when a sealed, wet specimen was turned around in the Heat Flow Meter Apparatus so that its cold side was now touching the hot plate.
Figure 4c: Moisture profiles measured after 1 hour after time zero in Figure 4b when most of the water is on the cold side show moisture movement towards cold side and after 16 hours. The tested specimen is here a Cellulose Fiber Insulation and the moisture movement is faster.
Figure 5 demonstrates the opposite case, when heat flow is always created by a movement of moisture.
Figure 5: Temperature measured with 12 thermistors placed each 20-mm apart from the level of free-water intake into a sealed gypsum cylinder (from 1974 PhD work)
Measurements were performed at Lund Institute of Technology, Sweden in 1970, where water container and casted at NRC Canada under vacuum the reference gypsum specimen was conditioned in the constant temperature. The test (Figure 5) was demonstrateinteraction between heat and moisture flows during the “isothermal” environmental conditions. The temperature reduction shown in front of water inflow to a porous material can only be explained by cooling during evaporation of water and diffusion through the pore air ahead of the water front. Yet, we know that liquid water moves faster in the porous gypsum than water vapor so how is it possible that this phenomenon is seen for a period of several hours?
Two trends of temperature changes can be observed. The first one starts when establishing contact between the specimen and water. For the next 4 hours the temperature of the whole specimen appears to be slowly decreasing. Superimposed on this profile are results of each separate measuring point. The temperature profile recorded at the first measuring point shows a rapid decrease, reaching the limit of the scale within about 30 minutes. This period becomes longer with distance of the water front travel. For a distance of 40 mm this period is about 45 min, at 60 mm distance about 1 hour and 15 minutes. In principle, the temperature profiles look similar to the moisture content profiles during the free water intake process.
To understand these results, one must remember that there is always am air and water vapor mixture at the front of moving water. Water flow displaces much of the air ahead of it. This air already contains water vapor (note a rapid evaporation at the first measuring point) and therefore the amount of water can evaporate into the pore air is smaller at the next measuring point and even smaller at the third measuring point. The latent heat that is measured in this test denotes the rate of evaporation at the water front. That is why the temperature profiles resemble the moisture content profiles observed by so many people before. Even though the water front moves faster than the vapor can diffuse, the air displaced by the water moves with the same speed as the water and the superposition of these three movements is seen in the Figure 5. This is similar to the famous Einstein story about a man walking within a rocket with a speed higher than a rocket.
Yet the reason for showing the measurements depicted in Figure 5, was to demonstrate that one cannot separate liquid and vapor transports even under so called “isothermal” conditions. It is clear that any calculation of moisture flow one must always be accompanied by calculation of the thermal field and vice versa, one must calculate simultaneous heat and moisture flows. We have demonstrated that neat, air and moisture transports are interacting and inseparable phenomena. This is obvious to a physicist as L. Onsager got a Nobel price for formulating the cross-effects of all transport phenomena but engineers are not trained in physics. Figure 6 shows that even they are trained in the school about capillary action lifting water up to 10 m height, they forget it after leaving the school.
Figure 6: Spread of water along the micro-crack in the exterior plaster. Falling water head measurements of water intake by the plaster indicates several times higher rate than plaster without a micro-crack.
Figure 6 shows the micro crack in plaster and so called RILEM tube to measure water the rate of water inflow into a material surface. The crack leads the water away from the tube several times faster than the material surface.
A practical effect is seen when a mortar shrinkage creates microcracks in water impermeable exterior coating on thermal insulation and while water enters, it cannot leave.