When we talk about the roof window segment as a dinosaur world, this attitude must also be justified and shed light on what led us to develop a roof window of our own design. We will try to look at things that ordinary people or users of roof windows of the classic concept will not know, because the manufacturers do not mention them for understandable reasons and no one else is interested in paying for information space from their own pockets and, worse, not make any profit from it…

View of roof windows in terms of construction

The vast majority of roof windows available on the market are built on a classic, proven and quite dusty concept. The basis is a relatively subtle peripheral supporting frame made of wood or plastic. The inserted supporting part of the sash is made of the same material, the size corresponds to the inner clearance of the frame. The glazing (double glazing to quadruple glazing) is attached to the supporting part of the sash by a glazing bead (aluminum, stainless steel or plastic), while there is a seal under the glass and under the bead. The fittings connect the frame with the sash and perform the function according to the type of window (swinging or tilting, handle at the top or bottom, three-point, four-point). Profile seals ensure airtightness and watertightness of a closed window. The sheet metal forms the external protection against the weather and connects the window to the roofing. The supplied thermal insulation reduces heat loss through the window.

So what is it about this proven concept that limits further development and the fulfillment of stricter conditions? Let’s look at the criteria important for the manufacturer and the customer. The buyer of the roof window assumes that the product has passed certification, so it is automatically somehow good and compares individual products according to price, manufacturer-declared properties, he is basically not interested in window design, because he would not choose any among the same looking windows, while perhaps still poking it. When choosing, a particularly clever advertisement for the replacement of his old windows with practically the same ones, which will have to be replaced again in the next decade, or he will recommend the roofer, who will quickly recommend the windows with the highest margin for him.

How is it from the manufacturer’s point of view? The roof window market is dominated by three main manufacturers, two of which produce practically the same and the third goes its own way, but still builds on the classic concept. It is about producing them as cheaply as possible, so they innovate creepily so that it meets the standards and there is something to write in the newsletter for the customer. But what the vast majority of customers do not know is that the main standard defining the quality of roof windows in terms of thermal properties – EN 73 0540 Thermal technical properties of building structures and buildings – is desperately obsolete in the requirements for roof windows and was also violent years ago. The fundamental requirement for the lowest internal surface temperature in Czechia was abolished, as at that time a single window, in addition to the Czech manufacturer, was able to meet it, and so this requirement changed to the “recommended value”. The fundamental value resulting from the requirement of the standard is the total heat transfer through the window, and this quantity is used to compare individual manufacturers. However, not so much as in the model of a diesel gate, but rather the other way around, because roof window manufacturers have been doing this much longer, certifying their products for heat transmission so that the standard results in the best and worse, measurement and calculation standards. It meets and does not reflect the reality of the operation of the window in the structure. So what’s the problem? For roof windows, the ratio is that the larger the window, the lower the heat transfer per m2. This is because the frame is almost always the worse thermal part and its surface area on the window decreases with size. The standard size of the window is 1.23 x 1.48 m – well, how many such windows are sold, judge by looking at the surrounding roofs. So the window that the customer buys has a worse heat transfer than he thinks, and that’s enough. Another factual problem is the determination of the thermal transmission of the window – according to the standard – in a vertical position. This is suitable for facade windows, which are also installed vertically, but how many roof windows are installed in this way. Insulating glazing has the disadvantage that when tilted it increases the heat transfer by convection and Ug glazing increases with standard double glazing from 1.1 W / m2K in vertical position to 1.7 W / m2K in horizontal position, with standard triple glazing from 0 .6 W / m2K in vertical position to 0.8 W / m2K in horizontal position. It follows that, basically, a double-glazed roof window has such lousy properties when installed in a roof with a slope of 35 degrees that it should not be sold at all for years, and for triple-glazed windows the standard should force the manufacturer to state real values ​​of heat transmission in the slope according to window size. However, the pressure from the manufacturers is so great that no one cares that the customer’s window freezes in the winter at the bottom in the corners, the water then destroys the frame and the surrounding lining, not to mention the mold. This is solved in the instructions for the use of windows, which no one reads, because it is a window – open – close, where it is written in small letters “under the window it is necessary to place the heater and ensure its uninterrupted function” or “surface treatment of the sash and frame must be renewed every four years” and also “ we do not recommend using the ventilation flap in the winter months. ”

So where is the famous poodle core? Manufacturers try to improve the properties of windows, including appearance, but meet the limits of the classic concept and their solution is sometimes quite rough, although functional but ugly and complicated, the customer has no alternative and so cheerful dinosaurs are sold. So let’s shed light on the limitations of the classical concept specifically, according to individual components and their units.

Perimeter supporting frame concept – inserted wing supporting part (both plastic or wood)

The disadvantage of this combination is that if you want the most important thing for a roof window, and that is the largest possible supply of light with regard to the overall size of the window, you should have the most subtle frame and wing support, which is fundamentally opposed by the frame capability and sash to bring out the total weight, which is significantly increased by the use of insulating triple glazing / quadruple glazing, and at the same time you have to weaken the frame and sash in the fittings, The result is a compromise strength – size – effective area of ​​glazing – thermal properties.

Insulating glass storage – glazing bead

The roof window has far more limited options than facade windows, and the fixing of insulating glass still dominates with a glazing bead system with a water groove. What are its disadvantages? For many years, aluminum glazing beads were used, which tragically worsened the circumferential internal surface temperatures of the glazing through the spacer frames. This was gradually solved by replacing the glazing beads made of stainless steel and plastic, warm frames in the insulating glazing and moving them in the supporting part of the sash deeper from the edge of the interior. This has a positive effect on the internal surface temperatures, but the glazing bead must be attached to the supporting part of the sash, which needs to be further widened, thus contradicting the vision of a subtle sash not taking away from the transmission of light. So compromise again.

Insulating glazing – composition

The classic concept of placing insulating glazing is limited not only by its increasing weight (double glazing vs. triple glazing), but also by the total thickness with which the supporting part of the sash together with the glazing bead must deal. Manufacturers solve this simply by using lower interglazed frames and the overall thickness does not increase significantly. The effect on the heat transfer in the vertical position is not significant, but the effect of this solution on the deterioration when installed at an angle is no longer addressed by them or the standard.

Opening system – swinging, tilting or combined fittings

From the user’s point of view, the window is tiltable, because the sash does not interfere with the interior, from the manufacturer’s point of view, it is a swinging window because it is structurally simpler. The tilting system works in contrast to the swinging, which is balanced along the central axis, with much greater support forces when opening, without which the user would not open the window at all. These forces are supplied by either gas struts or springs. Both variants are space-consuming and it is again necessary to find a place for them in the space between the frame and the sash, somewhere to simply remove the material without reducing the strength and load-bearing capacity. All windows also have fittings that secure the window closed or in a locked position. This also requires space and also acts as a thermal bridge, as most components are made of steel or aluminum. So – yes right – a compromise.

Seals – actually everything possible

When you open the window, you can see how much manufacturers have already invested in different types and profiles of seals, and the more blades the more adidas does. The disadvantage of the basic concept of a joint seal between the sash and the frame is usually that the seal partially rubs against the frame and, after gradual soiling, acts as a grinder rather than as a seal. Due to the price and the possibility of connection in the corners, a PVC or TPE seal is used, which, however, hardens over time and needs to be replaced, which is not always possible without dismantling the window parts.

Heat insulation

The manufacturers are trying to thermally improve the classic concept by supplying both peripheral blocks of thermal insulation to the frame and by adding insulation to the empty spaces between the frame and the sash, recently also under sheet metal. The thermal properties are really improved, on the other hand, the dimensions also increase in all directions, which again limits the basic function of the window, to bring as much light into the interior.

Ventilation flap

The ventilation flap is another of the cannibals of the effective area of ​​the glazing, and which can be used correctly only in summer – not recommended by the manufacturer in winter – and in summer you can successfully doubt the ability to ventilate overheated attic flap. The construction of the ventilation flaps also proved to be a thermal bridge, when it was protected by a three millimeter thick layer of foam. In fact, in new buildings with recuperation units, the importance and effectiveness of the basic functional properties of the window, namely ventilation, can be questioned.

Appearance of windows

In connection with the growth of different dimensions, thicknesses and widths of individual window components, which due to the classic concept it is necessary to cover from the outside with sheet metal frame and sash, the external appearance of windows does not move in a positive direction and manufacturers try to at least minimizereading the visible screws, or increasing the recess of the window into the roof structure can hardly save it. You can’t do much with the interior look of the window either, you just mill it from a different angle from time to time or add a cool facet.

Accessories vs. classic concept

The principle that the sash passes through the frame, whether for washing or simply because it is a swinging window, means that you cannot add anything to the frame because you would not be able to open or wash the window. It forces you to install an insect screen on the lining around the window, which means drilling and screwing unsightly strips into an expensive plasterboard ceiling – especially not to think about how you will paint. And what about the blind? For a change, you screw the screws directly into the painted wing support part. This locks many customers when they drag the failed mini-screw… Let’s not forget how the installed blind components visibly reduce the effective area of ​​the glazing. Isn’t it possible to hide the damn blind somewhere, to mill? It’s not working.

Washing windows

The need to wash the window from the outside keeps the classic concept of flipping the sash around the center hinges alive. The construction of the window with a glazing bar with water grooves and external sheeting around the sash is a masterpiece in the effort to capture as much dirt as possible, which remains to be cleaned by hand, otherwise there is a risk of water trapping into moisture-sensitive parts of the window. And how well does the window clean when flipped? It can be said miserably. When you decide, usually at most once a year, you take the necessary tools and stand under the window, flip over and secure the sash in the washing position, the first thing that comes to mind is “damn how the dust from the outside of the wing beautifully attacked the light carpet under the window…”

Installation of windows in the roof structure

The last but definitely unmissable drawback of the classic concept of roof windows is their ability to fully integrate them into the construction of the roof cladding. The system of additional waterproofing layers, when correctly designed, practically eliminated possible leaks around the roof windows. The roof window is relatively low in construction and with the current compositions of ventilated roofs, it is practically entirely located outside. The manufacturers try to supplement the perimeter frames with insulating blocks to improve the thermal properties, but there must also be additional space in the construction for these. Another problem with the classic concept is the placement of the groove of the inner lining on the outer circumference of the frame – in fact in the middle of the frame thickness, because otherwise it is not possible with frames about 2.5 – 4 centimeters wide. On the sides, everything depends on the possibility of adding additional thermal insulation during the internal lining, but especially for the upper and lower lining, all that remains is to apply the so-called bounce design to create space for sufficient addition of thermal insulation.

A number of trade-offs and improvements to the classic roof window concept have brought this segment to a point where the market is saturated with standards-compliant products, but some values ​​are far removed from reality and the sunroof has earned the “no roof window on the roof” label among the professional public. The requirement for sunroof functions has also shifted, but it is still true that the basic task is to illuminate the attic, with recuperation technologies with guaranteed exchange of filtered air, the importance of the window function for ventilation is gradually decreasing. Either way, the light in the attic should not be redeemed by high heat consumption, condensation or even mold.

Can a manufacturer of roof windows with a classic concept be praised for anything? Definitely yes. For the fact that even on classically designed windows they were able to use technical conveniences, whether in the form of modern insulating glass, quality finishes but also the processing of complex details, which require expensive equipment. But mainly because they can reliably control the roof window market with repainted dinosaurs and a sophisticated chain of incentives and margins without anyone wondering.

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