The advantages of the highest quality window increase or disappear depending on how well it is paired with the building object….
A window unit installed in a building must meet a number of requirements. Its design must be windproof and capable of performing heat and sound insulating functions. From the outside, the window should not let through rainwater and ultraviolet rays, from the side of the room – room air and moisture.
Power loads arising in the window structure must be correctly transmitted to the building body. All thermophysical and mechanical manifestations in the zones of the window frame and the building structure are perceived and compensated by a seam at the place of their conjugation. The professional execution of this seam, that is, well-chosen geometry, fastening, insulation and sealing, are of great importance for compliance with the previously listed conditions..
The multi-level model of the functions of the window block presented in the figure allows one to judge about a specific window mounted in a building structure, how much it meets the requirements due to the influence of the environment.
Level (1) corresponds to the conditional surface on which the differentiation of the outdoor climate and the indoor microclimate is ensured It should take place in an area above the dew point temperature inside the room. With a calculated room temperature of 20 ° C and a relative humidity of 50%, which corresponds to a dew point temperature of 9.3 ° C, the separation surface should lie above 10 ° C. Then, under the specified conditions, condensation will not form on the outer surfaces of the structure and inside it. The likelihood of condensation can be estimated by the nature of the isotherms.
Level (2) can be considered as a functional area, the correct choice of which can, in particular, ensure the heat and sound insulating properties of the window for a certain time. At the same time, the connection with the external climate in closed systems occurs along the border of this functional zone, and in open systems – through the entire system. In general terms, this means that the functional area should “remain dry” and not come into contact with the microclimate of the room.
Level (3), in a broad sense, it prevents water from entering the window structure (for example, during a heavy rain) from the outside. A controlled drainage of rainwater must be provided in the window unit. In addition, it is necessary to ensure the drainage of moisture accumulating in the functional area. the image of this resembles a half-timbered roof.
The very position of the mounted window can help prevent the formation of condensation inside the window unit, as well as in the surrounding structure. It is recommended to place it in the middle of the thickness of the window opening if the outer wall does not have an intermediate insulating layer, or at the level of the insulating layer if the wall is insulated.
Seam behavior when exposed to heat and moisture
The reaction of the connecting joint to heat and moisture is determined by the indoor and outdoor climate. If the following climatic conditions are taken as calculated (according to DIN 4108, part 3): indoor temperature 20 ° C, relative air humidity 50%, outside temperature minus 15 ° C, relative air humidity 80%, then the dew point in the room at the given characteristics microclimate is approximately 9.3 ° С (for simplicity, we round up to 10 ° С). It is clear that under such conditions the temperature in the critical zones of the building structure should not drop below 10 ° C, otherwise condensation will form.
Care should be taken that in those parts of the building structure where undesirable low temperatures cannot be avoided, there would be no conditions for condensation to form or that the moisture that appears could be removed by diffusing or flowing out.
Due to the vapor pressure difference between the climatic zones inside and outside the premises, humid warm air from the premises can penetrate into the joining joint, as well as the diffusion of water vapor through the building structure. The risk of condensation forming in the joint cavities depends on the temperature and relative humidity of the air inside the joint. When installing a window unit, it is necessary to take all measures against the appearance of moisture in the joint. If this is still not excluded, moisture must be able to diffuse outward, and for this it is necessary that the resistance of the building structure material to diffusion penetration of water vapor decreases in the direction from the inside to the outside. That is, the principle must be observed: the inside is denser than the outside. Of course, it is also important to provide an external level of protection (3), that is, the window unit must remain tight during heavy rain.
Thermal protection and cold bridges
Heat transfer in the binding area of the window unit is largely determined by its position, as well as the correct distribution of the insulating layers in this area. A “thermal bridge” is understood as a surface area, inside which, in comparison with adjacent surfaces, a lower temperature is observed (therefore it is also called a cold bridge) and an additional heat flux. In the area of the window block, a brick or concrete wall is adjacent to the window frame. The different thickness of these building elements leads to the inevitable occurrence of thermal bridges, that is, it is impossible to completely do without heat loss in the binding zone. The presence of a window in the opening of a monolithic wall manifests itself on the graphs in the form of a strong distortion of isotherms, the comparison of which helps to figure out how best to position the window in the building structure in order to reduce heat loss. Isotherm, as you know, is a line connecting points with the same temperature. Its character is determined by the presence of thermal bridges due to the characteristics of the material or geometry (corners, edges, etc.). Thermal bridges of both types appear in the area where the window joins the building structure..
With the help of isothermal lines, you can display the temperature characteristics inherent in certain conditions for installing a window in the opening of a building. Under normal indoor conditions (20 ° C and 50%), the base 10-degree isotherm is the most important for evaluating mating. To prevent the formation of condensation in front of the inner joint seam, this isotherm must pass through the entire inner part of the structure. The fewer bends in the 10-degree isotherm, the less heat will leak in the interface. Variants of the successful location of various window blocks in openings of different designs are shown in the previous figures..
Insulation of the mating seam
Along with protection against moisture ingress, care must be taken to ensure perfect heat and sound insulation of the connecting seam. In order to keep the temperature at the inner surface of the interface high enough, all joints in the area of the window block must be sealed with a suitable insulating material. Without such insulation, there is a danger of cooling the inner surface to a temperature below the dew point, and then dampness can form in the interface.
Measures for heat, moisture and sound insulation
All power loads naturally occurring in the window structure must be transmitted to the supporting structure through the fasteners. Forces acting in the plane of the window are perceived by the building structure through the support blocks, which should only work for squeezing. Dowels, linings and similar parts are not sufficient to absorb the load. It is important to ensure that the blocks are correctly positioned in the corners of the window frame, as well as in the areas of posts and crossbars, and that the frame profiles have a sufficient bending strength. The dimensions of the backing blocks should be chosen so that they do not interfere with the subsequent work on sealing the seam. In terms of the width of the base, the block must match the installation thickness of the frame. The auxiliary wedges used during installation must be removed after fixing the window unit..
In addition to correctly selected and positioned support blocks, it is necessary to select suitable fasteners to securely hold the window in the opening. Taking into account the behavior of frame materials with linear elongation, the distances between the fastening points are determined for each of them. The distance between anchors for aluminum and wooden windows should not exceed 800 mm, for plastic windows – 700 mm. The distance from the inner corner should be within 100-150 mm, as well as the distance to the post or crossbar from the inner side of the frame profile. The criteria by which fasteners and fastening systems are selected are mainly as follows:
It is important to know the following about the fasteners used.
Frame dowels (pins)
They work for shear, shear and bend. Their use, especially at high loads, is limited due to the need to maintain a certain distance between the wall and the window frame. Select dowels of sufficient size, taking into account the manufacturer’s recommendations.
They are quite flexible to bend, due to which they perceive well the longitudinal movements of frame materials. As a fastener, the pad works mainly for shear and is able to withstand higher loads than dowels. However, the linings can only perceive forces directed perpendicularly.
They can take heavy loads. They are used, for example, for fastening suspended facades and similar conditions. For each type of anchor, static calculations of the permissible weight and tensile load are made – these data can be compared according to the catalogs of different manufacturers.
Improper sealing is often the cause of damage to a building. Moisture from the room should not penetrate into the seam, and if this cannot be prevented, then it should be possible to remove condensate outside. The waterproofing and windproof layers must, in principle, be installed inside the building elements, ‘moreover, so that they prevent the penetration of air and moisture from the room into the structure and in those places where the surface temperature is below the dew point, dampness would not appear. With the correct execution of the window unit, compliance with this requirement is ensured at level (1).
The width of the joint is determined by the extent to which the frame materials are subject to dimensional changes, depending on temperature and humidity. Maintaining the minimum profile width does not eliminate the need to take into account the corresponding data for sealing materials. Usually, the manufacturers of these products indicate the optimal seam width..
When choosing a sealing system, the design of the outer wall is taken into account first. In old houses, the joints in the openings were performed differently than in new construction. For new objects, fundamentally new ways of attaching window blocks can be designed. When rehabilitating an old fund, the contours of window openings often need to be preserved in their original form – this limits the choice of sealing systems, as well as the method of mating and sealing joints. Depending on the functional purpose, appropriate sealing systems are used:
They can be intelligently combined taking into account the requirements.
Along with the silicone widely used in construction, other injected sealants are also used for sealing window joints: acrylic, polysulfide and polyurethane. One of the most important properties for a sealant is its ability to perceive relative displacements of the joint. It depends on the material and thickness of the seal and is indicated as a percentage. In the general case, for the joint section, it is assumed that the thickness of the sealant d should be half the joint width b (d = 0.5b). To withstand this condition, it is necessary to use non-absorbent closed cell waterproofing materials and lay them in depth, so that the thickness taken away by the applied sealant can be determined. Injected sealants must adhere well to the substrate to which they are applied. Therefore, it is important to first evaluate the gripping properties of the respective surfaces. The adhesion can be significantly improved by the use of so-called first coat primers. Use only those manufacturer’s recommended sealant primers that are suitable for both sides of the joint..
The sealing strips are made of impregnated soft foam and are supplied highly compressed. Belts from different manufacturers vary in type and design. Unlike injectable sealants, the sealing strips only transfer the pressure load to the sealing surfaces, not the pulling forces. The sealing tape can smooth surface roughness up to approx. 3 mm. The seam remains closed and sealed. Therefore, compressed sealing strips are especially recommended for sealing joints with plastered surfaces, gypsum boards and other materials that are unfavorable for setting. The tape is impervious to water, steam and noise the more it is compressed and / or the wider it is. Prepare the sealing tape for use in the mating joints in accordance with the weather conditions during the sealing work. On cold days, it is recommended to pre-hold the tape warm, and in hot weather, cool it down if possible.
To protect the seams in the area of window blocks, sealing sheets made of polymeric materials of the PIB group (polyisobutylene) are mainly used. The canvases are especially suitable for joining multi-layer building elements. Gluing is usually used only as an assembly aid. In areas where a guaranteed secure fit and tightness for a long time is required, it is recommended to use mechanical protection. The resistance of polymer sheets to vapor diffusion is so high that when they are laid outside, additional compensating holes have to be made. As a rule, the application of the canvas is limited to sealing the zones of the upper and lower interface of the window.