What is total moisture


The goals of moisture protection are aimed at the permanent protection of the building structure against the occurrence of harmful condensate on or in the component as well as protection against damaging penetration of moisture as well as ensuring healthy and comfortable indoor conditions. The main elements are:

  • the thermal-hygric boundary conditions of the interior and exterior,
  • the water absorption capacity of the building materials used,
  • the phenomena of moisture transport in building structures as well as, going beyond moisture transport,
  • the air and wind tightness of building structures.

Thermal-hygric boundary conditions

The dimensions of the moisture protection are based on the design conditions of the air temperature and humidity in the interior and exterior. In Austria, these boundary conditions are defined in ÖNORM B 8110-2, which is based on ÖNORM EN ISO 13788.
For apartments and rooms with a comparable designation, including offices, in ÖNORM B 8110-2 an indoor air temperature of 20 ° C and a relative indoor air humidity depending on the protection goal and the outdoor air temperature, often 55% to 65%, are used.

The monthly mean values ​​of the outside air temperature and a relative humidity of 80% and 75% are used as outside air conditions there for verifications on components with thermal storage capacity. For location-independent dimensioning, the climatic data of the climatic zone south of the basin, typically Klagenfurt, should be used. In this case, the lowest design temperature is -3.8 ° C.

The relevant ÖNORM B 8110-2 is being revised at the time this book goes to press and changes are to be expected. In any case, the external conditions will continue to be based on ÖNORM B 8110-5, in which the calculation rules for determining the monthly mean values ​​of temperature, global radiation and relative humidity are based on a climatological subdivision of Austria into seven climate zones, with additional consideration of the influences of the altitude of a location , are set.

The monthly mean value of the temperature is calculated according to formula (3-05), whereby the regression coefficients according to ÖNORM B 8110-5 vary with each zone and in three layers with the altitude.

Moisture storage

Like most building materials, bricks belong to the group of hygroscopic materials. They absorb water from the moist air even without wetting the surface. The measure of this hygroscopic water absorption is given by the key figure of the mass-related moisture contentu (kg / kg or M-%), less often the volume-related moisture contentv (kg / m³ or vol .-%), described (ÖNORM EN ISO 9346).
Assuming a constant temperature, an equilibrium moisture content is established in the building material as a function of the surrounding relative humidity. This function is known as the sorption isotherm. It is a building material property and is determined experimentally in accordance with ÖNORM EN ISO 12571.

Bricks and bricks generally have a very low tendency towards hygroscopic water absorption. The mass-related equilibrium moisture content is less than 1% by mass even at 80% air humidity. Bricks, on the other hand, can store considerable amounts of moisture by capillary action without being damaged in the process. The measure for this is the maximum water absorption with a magnitude of 25 to 30 M-%.

These two properties result in a high level of robustness for building structures made of bricks in the event of hygric loads: Increased air humidity only leads to a very small degree of moisture in the structure, whereas seasonal amounts of condensation, for example at the boundary layers with thermal insulation, can be stored and released again to a large extent become. Therefore, wall constructions made of brick masonry, both with and without additional insulation, with appropriate coordination of the components, are free of evidence with regard to the occurrence of harmful condensate within the meaning of ÖNORM B 8110-2.

Moisture transport and moisture protection

Moisture transport in components takes place through the phenomena of water vapor convection, liquid water transport and through water vapor diffusion.

Moisture transport by convection

Water vapor can be convectively introduced into building structures together with flowing air at damaged areas with greatly reduced airtightness. The amount of water introduced can be considerable, which is why the occurrence of this phenomenon must be reliably prevented by making the construction airtight, including the connection details. In lightweight construction, damage or incomplete bonding of vapor barriers can pose a risk in this regard. Professionally walled brick constructions are sufficiently airtight and are equipped with an additional effective airtightness layer through the interior plaster, which can be easily connected to partition walls, ceilings and floors and which can also be checked visually and easily repaired if necessary.

With the rules for the production of an appropriately airtight building envelope, building constructions made of bricks also satisfy all requirements for avoiding convective entry of moisture.

Moisture transport through liquid water transport

Moisture penetrating into the component can be carried on by liquid water transport. Driving forces are pressure differences or capillary conduction forces.

In building structures made of bricks, effective protection against damage caused by liquid water transport is provided by the professional and crack-free execution and maintenance of the external plaster and by professional drainage of rainwater, for example at the transitions to roofs, windows and components in contact with the ground.

Moisture transport through water vapor diffusion

The movement of the gaseous water vapor in the direction of the water vapor partial pressure gradient, also in stagnant air, is referred to as water vapor diffusion. It is therefore a feature of almost all building structures, which must therefore be planned and implemented in such a way that there is no damage due to moisture accumulation in or on the components.
The physical background to the risk of damage from condensate in connection with water vapor diffusion and convection is the property of air to show a decreasing absorption capacity for water vapor with falling temperature. The measure of the water vapor absorption capacity is the saturation vapor pressure psat (ÖNORM EN ISO 13788).

Between the extreme values ​​of absolutely dry air and air saturated with water vapor, the relative humidity describes the degree of saturation of moist air according to the formula (3-07).

The relevant verification procedures for water vapor diffusion and condensation protection can be found for Austria in ÖNORM B 8110-2 as well as in ÖNORM EN ISO 13788. The building material property relevant for the degree of water vapor diffusion is the diffusion resistance coefficientμ.

Starting from the historical solid brick masonry to modern perforated brick masonry, these types of wall show diffusion resistance values μ in the order of magnitude from 5 to 10, with the exception of clinker masonry with values ​​from 50 to 100.

When dimensioning the moisture protection, the diffusion resistance factor often becomes the equivalent air layer thickness with the layer thickness of the respective component layerd multiplied. sd can therefore be interpreted as the fictitious layer thickness of air, which would have the same diffusion resistance as the respective component layer.

Masonry made of solid or hollow bricks, with or without an external thermal insulation layer, is exempt from the obligation to provide mathematical evidence under the following conditions under the following conditions according to ÖNORM B 8110-2 for residential use and comparable uses, due to the favorable moisture-related properties mentioned:

in the case of an interior plaster or interior cladding (with a R.T-Value of a maximum of 0.2 m² /K / W) with a diffusion-equivalent air layer thickness μ⋅d > 0.15 m and with mineral-bound exterior plaster, a ventilated cladding or an exterior wall insulation system in accordance with ÖNORM B 6110, whose diffusion-equivalent air layer thickness is not greater than ten times the diffusion-equivalent air layer thickness of the interior plaster or the inner cladding.

In all other cases, evidence that no harmful water vapor condensation occurs inside the component as a result of water vapor diffusion must be carried out using the Glaser method in accordance with ÖNORM B 8110-2: Under the defined external conditions and taking into account the standardized heat transfer resistances, the temperature curves are determined and water vapor saturation pressure psatas well as the course of the water vapor partial pressure p determined in the component, which results from the boundary conditions of the partial water vapor pressure inside and outside as well as the diffusion-equivalent air layer thicknesses of the component layers.

For graphical simplification, the component layers are not applied with their actual thicknesses, but with their diffusion-equivalent air layer thicknesses, which means that the course of the water vapor partial pressure is reflected in condensate-free constructions p represents as a straight line. Because the water vapor partial pressure p at no point can be greater than the water vapor saturation pressure psat, is for areas p> psat the value p = psat to use. Graphically, this means that the course of the water vapor partial pressure runs tangentially to the curve of the water vapor saturation pressure, and in fact this means that condensate precipitates at these points.

The amount of condensing water as well as the amount of water that evaporates again in the summer months is calculated and added up according to the procedure described in ÖNORM EN ISO 13788 for each month on the basis of the mean value of the outside air temperature, the associated humidity and the standard-compliant indoor air conditions for the respective month. The assessments according to ÖNORM B 8110-2 can then be derived from the results of the calculations.

  • Condensation is not predicted for any interface or month.
  • Condensation occurs at one or more interfaces; however, for each affected interface, complete evaporation of the condensation water is predicted in the summer months. In this case it should be checked
    • Whether the amount of condensation water can be stored in the affected component layer, whereby the amount of condensation water on the contact surfaces of layers that are not or hardly absorbent (e.g. contact surfaces between the air layer and heavy concrete) must not exceed 0.5 kg / m²,
    • whether the increase in moisture content damages the affected building material layer and increases the thermal conductivity to such an extent that the thermal protection of the component is reduced by 10% or more,
    • whether damage to the building materials concerned can occur as a result of the increase in the moisture content.
  • The condensation water that forms at one or more interfaces evaporates incompletely in the summer months; in this case, progressive moisture penetration can occur over several years and lead to damage.

Humidity test methods and evidence

Diffusion resistance

ÖNORM EN ISO 12572 specifies a method for determining the water vapor diffusion permeability coefficient of building products and the water vapor diffusion conductivity coefficient and the water vapor diffusion equivalent air layer thickness of building materials under isothermal conditions. It is applicable to all building materials with a water vapor diffusion equivalent air layer thickness greater than 0.1 m. If the measured water vapor diffusion equivalent air layer thickness exceeds 1500 m, the material is considered to be impermeable to water vapor.

The test method is based on the principle of the temporal change in mass of a vessel filled with an aqueous solution or with a desiccant, the upper, tight seal of which is formed by the test specimen. The temperature and humidity of the room are regulated. A defined and constant relative humidity is created in the test vessel due to the aqueous solution it contains or due to the desiccant at a constant temperature. Because of the different partial pressures of water vapor between the test vessel and the test room, a vapor diffusion flow is created through permeable test specimens. Periodic weighings of the arrangement are carried out in order to determine the water vapor diffusion flux density in the steady state. Calcium chloride or magnesium perchlorate are used as desiccants. Magnesium nitrate, potassium chloride, ammonium dihydrogen phosphate and potassium nitrate are used as aqueous solutions. The loss of mass over the course of the test is determined by regular measurements, and from this the water vapor diffusion current density G determined according to formula (3-09).

The required water vapor diffusion coefficient for air can be determined for a temperature of 23 ° C from Figure 3-06.

Building material moisture

Basically, moisture means physically bound water and total moisture means physically and chemically bound water. Depending on the type of moisture determination, there is a subdivision with regard to the methodology. The currently relevant methods of total moisture determination in practice are the Darr method (as the most precise method) and the calcium carbide method for an overview of the construction site inspection. The currently available methods of non-destructive or non-destructive moisture determination are unsuitable for practice.

Moisture Content - Darr Method

First, in the gravimetric moisture determination (Darr method), the sample taken is weighed, and the wet mass of the sample taken is thereby obtained mf. The sample is then dried (usually at 105 ° C ± 2 ° C) in a drying or climatic cabinet until the weight is constant and the dry matter is determined mtr analogous to mf. The water content corresponds to the weight loss. The moisture content F is given in% by mass based on the dry mass.

There are basically different calculation methods for determining the moisture content. For existing moisture values, attention must therefore always be paid to their definition and unit. Other studies often give the moisture content in percent by volume. A conversion between% by volume and% by mass can be made using the dry bulk densities of the materials.

maximum water absorption

There are various specifications for determining the maximum water absorption, which also provide different results.
To determine the maximum water absorption according to ÖNORM B 3355-1 from brick, a granulate of 4/16 mm is to be used. The determination is to be carried out after 48 hours of atmospheric water immersion with at least 2 cm overburden.

Degree of moisture penetration

The determination of the maximum water absorption is necessary to be able to calculate the degree of moisture penetration of the building materials in the masonry. In the literature, this parameter is also often referred to as the degree of pore filling.

Hygroscopic equilibrium moisture

The hygroscopic equilibrium humidity A represents the proportion of humidity that would arise if water were only absorbed according to the water vapor pressure and the temperature of the ambient air. The higher the salt content in the masonry, the higher the equilibrium moisture due to the hygroscopic properties of the salts. According to ÖNORM B 3355, the determination must be carried out on undried samples with a minimum grain size of 4 mm at a constant climate of 20 ± 2 ° C and 85 ± 5% relative humidity. The subsequent drying is to be carried out using the Darr method.

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