The toilet and two traps together hold ~ 6.5 L of water, but only a fraction of this water is likely equilibrated with room air. The shower surfaces are periodically wetted, typically once or twice per day. The sink basins are also wetted periodically and more frequently than the shower stall surfaces. Fabrics associated with personal hygiene are also wetted periodically: bath towels, hand towels, and washcloths. While these are drying, the water they contain contributes to L*. Additional bulk water occurs episodically in this house. At meal times, water is used for food preparation, served for drinking, and utilized after meals for kitchen cleanup. During seasonal periods of rainy weather, wet shoes, clothes and umbrellas may be allowed to dry indoors. Water tends to condense on interior surfaces of windows on cold days following winter storms; such water may persist for hours before drying. During one post-storm event, the estimated abundance of condensed water on windows was 140 g distributed across 8.4 m2 of window surface; the estimated contribution to L* for the whole house volume of 350 m3 would be 4 ´ 10-4 L/m3 . Returning from this specific example to more general considerations, in addition to the stocks of bulk water,grow cannabis in containers flows are expected to influence the concentrations and fates of indoor acids and bases.A recent study provides information about residential end uses of water in the United States. Based on survey methods, the average indoor water use per household was reported to be 520 L/day .
For a typical US home volume of 500 m3 , the overall average flow of potable water corresponds to about 1 L/m3 per day, or 0.04 L/m3 per hour. The five largest contributors were toilets , showers , faucets , clothes washer , and water leaks . We have noted that ventilation in warm and humid conditions could necessitate the removal of about 1.7 L/h of water for a 350-m3 residence as a component of air conditioning. The associated flow rate of condensate would be 0.005 L/m3 per hour, about an order of magnitude lower than the mean flow rate of water from metered uses in residences. To substantiate the estimate of 1.7 L/h, we cite results from two studies. In a home in North Carolina, Duncan et al.reported that “about 2.2 ± 0.2 L of water vapor was condensed in the air conditioning system during each cooling cycle, approximately hourly.” An unoccupied house in Fort Wayne, Indiana, with volume of 490 m3 had a mechanical ventilation rate of 119 m3 h-1 . The measured air conditioner condensate flow spanned a range up to 40 L/d, equivalent to 1.7 L/h.Large quantities of condensed-phase water are associated with the fibrous and porous materials that are prevalent indoors, including wood, gypsum board, and textiles. Some of this water could be in a form of sufficient accessibility and availability to influence the behavior of indoor airborne acids and bases. Of particular interest in this regard is water that is sufficiently abundant locally to behave chemically like bulk water and that is sufficiently proximate to indoor air to permit timely interaction with airborne species. These restrictions on chemical behavior and accessibility limit our ability to assess contributions to L*. Here, we emphasize the state of knowledge about the abundance of solids-associated water and the materials with which it is associated.
We caution that it is unknown to what extent the thermodynamic properties of sorbed water are similar to bulk water with regard to the partitioning of gaseous species and the proton-exchange reactions that are central to acid-base behavior. Historically, the abundance of water associated with indoor materials has been of considerable research interest for three major purposes. First, the movement of moisture into and out of materials can contribute to structural damage. Among other specific concerns, the goal of preserving cultural artifacts benefits from maintaining stable humidity so as to limit mechanical stresses from moisture migration. Second, moisture and dampness problems that are associated with adverse respiratory symptoms and allergies can result from excessive water inbuilding materials.Third, the reversible storage of water by interior materials can serve to buffer indoor humidity and therefore potentially improve building energy performance. A key parameter to quantify the abundance of sorbed water is the moisture content. As commonly reported, the moisture content is a mass ratio: the mass of condensed water associated with the solid material per mass of solid material when dry. If a material is exposed to water vapor at a fixed relative humidity for sufficient duration, then the abundance of sorbed water will attain an equilibrium state in which there is no subsequent net gain or loss. The functional dependence of the equilibrium moisture content in relation to relative humidity is referred to as a sorption isotherm. As indicated by “isotherm,” this functional dependence is assessed at a fixed temperature; sorption isotherms are temperature dependent. Figure 1 displays sorption isotherms for two common indoor materials: wood and gypsum board. An important point is displayed in this figure: a given mass of wood contains much more sorbed water at equilibrium than does gypsum board. At RH = 50% the equilibrium moisture content of wood is almost 30´ higher than that of gypsum board . However, even with its much lower capacity, the amount of sorbed water associated with gypsum board is substantial.
Consider a room of dimensions 3 ´ 4 ´ 2.3 m, for which the walls and ceiling are covered by gypsum board of thickness 13 mm and density 690 kg m-3 . In this room volume of 28 m3, the total area of walls plus ceiling is 44 m2, which would be covered with about 400 kg of gypsum board. At 0.33% equilibrium moisture content , the equilibrium mass of sorbed water would be 1.3 kg. If this water contributed fully to L*, it would add 0.05 L/m3 .Table 2 presents equilibrium moisture content data for several common construction materials. A striking feature is the contrast between the moisture content of wood – a plant-based product – and the other materials, which are mineral-based. At 50% RH, the reported equilibrium moisture contents for wood span the range 8-11%, whereas most of the mineral based construction materials have moisture contents below 1%. Latex paint is a common finishing material for interior walls and ceiling. Van der Zanden and Goossens41 studied water sorption by latex paint; at RH = 50%, their mathematical relationship estimates 8.6 g of sorbed water per kg of paint. Consider again the example of a room with dimensions 3 ´ 4 ´ 2.3 m,pot for cannabis for which the walls and ceiling are covered by gypsum board finished by a 50-µm thick latex paint layer, 44 with a nominal assumed density of 1 g cm-3 . The 2.2 kg of paint would have an equilibrium sorbed water content of 19 g. If that water behaved thermo dynamically like bulk water with regards to indoor acids and bases, then it would contribute about 7 ´ 10-4 L/m3 to L*, almost three orders of magnitude above the lower-bound estimate for a water monolayer on all interior surfaces. It is common for water sorption isotherms to exhibit hysteresis. Specifically, this means that the equilibrium moisture content depends not only on the relative humidity but also on the direction of approach. When an isotherm is measured using steps of progressively increasing humidity, the equilibrium moisture content attained at a specific humidity value is lower than when measured for a progressively decreasing humidity. The underlying reasons for this phenomenon are not fully understood. In a review of the physics of wood-water interactions, Engelund et al.stated that “no physically consistent model for sorption hysteresis has been put forward.” Among the factors that could contribute to a hysteresis effect is that sorbed water can change the physical structure of the sorbent, e.g. by causing swelling. Such alteration can mean, for example, that at an any specific relative humidity, wood is a physically different sorbent if in the process of drying rather than in the process of taking up more water. A chemical process might also contribute if some part of water sorption involves dissolution of solids. A parallel example is observed in the efflorescence and deliquescence of particles as they are exposed to changing humidity conditions. Reversible sorption of water by building materials helps to buffer indoor humidity against change. The extent of buffering is influenced not only by equilibrium moisture capacities but also by the rates at which water vapor can be transported into and out of a material.
A key parameter is the “moisture buffer value.” As defined by Rode et al., “The practical moisture buffer value indicates the amount of water that is transported in or out of a material per open surface area, during a certain period of time, when it is subjected to variations in relative humidity of the surrounding air.” In specific determination of MBV practical, the repeated RH cycle comprises 8 h at 75% RH followed by 16 h at 33% RH. This cycle is repeated until a consistent response is produced. Table 3 presents a summary of MBV practical for several materials. If we consider a 3 ´ 4 ´ 2.3 m = 28 m3 room volume with 56 m2 of total interior surface, which possesses an average MBV practical value of 0.7 g m-2 per % RH , then a 24-h driving cycle varying between 33% and 75% RH would be associated with an average net flux of 0.7 ´ 56 ´ ´ = 69 g/h of water into or out of the floor and wall materials. If this room had an air-exchange rate of 0.5 h-1 so that the ventilation flow rate was 14 m3 h-1 , the associated water vapor mass concentration for average sorptive uptake and release would be 69/14 = 4.9 g m-3. The incremental change in relative humidity corresponding to this concentration would be 21%, a substantial fraction of the 42% change in RH in the driving cycle. The key message: reversible sorptive uptake of water by construction materials provides for substantial buffering of indoor humidity. Large-scale surveys of indoor materials that would influence stocks and flows of sorbed water have not been undertaken. Svennberg and Wadsö reported the relative abundance of “surface materials exposed in dwellings” based on an inventory of 16 rooms in Swedish apartments. The resulting apportionment was 33% wallpaper, 21% painted surface, 18% textile furnishings, 15% wood, 8% synthetic flooring, and 4% textile carpets. Woods and Winkler reported the major categories of surface areas for moisture-buffering materials in three unoccupied homes they studied. With volumes in the range 235-490 m3 , the corresponding area/volume ratios were 1.23-1.29 m2 m-3 for drywall, 0.20-0.28 m2 m-3 for wood, and 0.27-0.36 m2 m-3 for carpet. The prevalence of indoor surface materials is further discussed in §4.3.In US residences, fibrous materials would commonly be found in carpeting, draperies, upholstery, bedding covers, bath and kitchen towels, and clothing. There are no known published summaries of the amounts of fibrous materials present in indoor environments. Total quantities could easily exceed 100 kg. For carpet alone, the mass of fibers could be in the range 1-2 kg per m2 of flooring area covered. Fibrous materials that are common and abundantly present indoors have substantial associated water. Table 4 summarizes empirical evidence. At 50% RH and for equilibrium conditions, the amount of water associated with nylon is about 3%; for cotton, the abundance of sorbed water is 4-7%; and for wool, the observed range is about 9-13%. A fully carpeted house with 150 m2 of floor area might have 150-300 kg of carpet fibers. If the primary material were nylon, then, at 50% RH, the equilibrium abundance of sorbed water could be in the range 4.5-9 kg. If water sorbed to nylon carpet fibers contributed fully to L*, then wall-to-wall carpet in a room with a 2.3 m height, equilibrated at RH = 50%, would add an increment to L* in the range 0.013-0.026 L/m3 .An interesting feature of water sorption by fibrous materials is the chemical strength of the association. One can observe this feature empirically by comparing the nominal surface areas for sorption with water and N2 as the sorbates.