This is the movement of water vapour molecules in a gas mixture (e.g. in air) to equalize vapour concentrations. The diffusion causes the water vapour to pass through partitions in order to equalize the partial pressure on both sides of the partition. The course of this process depends on conditions present on both sides of the partition. The process of water vapour diffusion through a building partition depends on temperature and on air humidity on both sides (on the difference in partial pressures of water vapour on both sides).

It is a constant value for each type of material that determines its diffusion resistance. μ Coefficient is defined as a relative value indicating how many times the diffusion resistance of a material layer is greater than that of the same layer of air under the same conditions. For the air µ = 1, and for lightweight porous insulation materials with good thermal insulation properties, μ is close to 1.

It combines the DIFFUSION RESISTANCE FACTOR - "µ" with material thickness - "d"; Sd=µ-d. It can be interpreted as the thickness of an immobile layer of air that has the same diffusion resistance as the tested layer. The equivalent diffusion resistance factor (Sd) is the diffusion-equivalent thickness of the air layer providing resistance to water vapour. Sd is expressed in metres. The smaller the value, the lower the resistance and vice versa.

This is the difference between the internal surface temperature and the outdoor air temperature, divided by the difference between the indoor and outdoor air temperatures, calculated on the assumption that the heat transfer resistance on the internal surface is Rsi. The temperature coefficient f is an indicator of the lowest temperature of internal surface f_Rsi of the determinate joint. It is in fact a dimensionless temperature describing the temperature of the internal surface, independent of the exact boundary conditions, by a value between 0 and 1 (the closer the value to 1, the better protection of the joint). The construction elements must ensure that the risk of local mould and surface condensation on the internal surface of the joint is eliminated.

Ability to limit (regulate) the penetration of water vapour through the partition. The diffusion tightness is strictly connected with humidity of building partitions, especially with the necessity of ensuring such humidity of partitions, which can be considered as correct, that is not causing negative thermal and operational consequences. Diffusion-tight partition is such partition, into which water vapour should not penetrate by diffusion and the type and arrangement of layers of this partition guarantee that it is not endangered with internal condensation (condensation of water vapour at lowered temperature), especially increasing in subsequent years, which may lead to increasing moisture of some layers and as a result to fungi growth and biological degradation.

Air tightness affects operating costs, acoustic properties, thermal efficiency of the building, thermal comfort of users. One of the key points in which ensure tightness are the joints between the reveal and the frame. Poor air tightness of this joints constitutes about 15% of the total air loss in an average single-family house.

The occurrence of strong, turbulent gusts of wind is often associated with another adverse weather phenomenon - heavy rain. It is crucial to know whether the window structure and the joint between the reveal and the frame of a certain wind load strength will also maintain the tightness against the penetration of rainwater into the interior. Water tightness is another property of the window and the joint, in which the pressure exerted on the structure by the wind plays a significant role. Windows and joints bearing the E1200 label will only start to let water through to the structure at a pressure of 1200 Pa, which means that leaks can occur if falling rain is accompanied by a hurricane blowing at more than 160 km/h.

The λ value is an indicator of how energy-efficient a material can be. The lower the λ value, the better the joint is insulated by the specific material. However, it should be remembered that it is the joint between the frame and the reveal that constitutes a classic linear thermal bridge. The minimum requirement for the reveal to frame joint, i.e. the 3D correction factor in the joint ψ is 0.10 W / mK. Linear thermal bridges, such as frame to reveal joints, are an important part of the total heat loss in a building - their influence in this respect is usually neglected by designers and can be very high if the details are not solved properly. Condensation of water vapour and mould often occur on the internal surface of thermal bridges, especially in case of increased humidity in the room. That is why it is so important to properly insulate the joint between the reveal and the frame.

The thermal conductivity coefficient that characterises a building material or product; it shows how much heat will pass through the material regardless of its thickness. The lower the lambda coefficient, the better the insulation material. Unit: [W/(m*K)]

A value which determines the product's ability to contain heat loss; it depends on the thickness of the material and its thermal conductivity - the thicker the layer, the greater the resistance and less energy escapes outside the building.

A parameter characterising the insulation performance of building partitions, e.g. ceilings, walls, windows, determining the amount of heat which passes through a given partition; U-values for partitions calculated in accordance with Polish Standards cannot be higher than the U(max) value. The lower the U-value, the better insulating properties of the partition. Unit: [W/(m2*K)]

Acoustic insulation requirements relate to the entire external structure of the building, including all the elements installed in it. The acoustic insulation coefficient R’w,res of the external part of the building (external wall + windows) is determined. When planning wall joints, care must be taken to ensure the greatest possible noise reduction. For a well insulated and sealed joint, the Rs,w value should be ≥ 45 dB for a 20 mm joint.

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