1. Why is there condensation on glass units?

1.1 The ventilation of PVC windows

The statement that plastic windows may cause headache is not true because in reality, the bad feeling is caused by the lack of air in the room. That is because windows with good quality exclude the possibility of outside air getting indoors by itself. That is why airing the rooms is an actual issue which is usually solved by ventilation valves inside the windows. Window with ventilation valve usually loses in heat resistance, but micro-opening aka sealing ventilation helps to solve this problem. When micro-opening Your window, at first it opens ca 5 mm in ventilation position, making a slit between the window frame and window subframe. Micro-opening is covered by locking-system and not visible on the window. The amount of air ventilated in the room depends on the size of the window part that is opened. Micro-opening Your windows or using ventilation valves are meant to create equilibrium pressure aka sufficiently ventilate Your rooms.

New plastic windows that are being produced have high insulating properties and that is why they need the right care. After the mounting of new windows rooms with insufficient ventilation can form water or mold into the corners of exterior walls. However the fault is not the humidity getting in from outdoors, as often suspected, but it is caused by the moisture coming from indoors. Quite often new windows are disparaged because of their ability to let the air through. Old poorly closing windows have natural air exchange but new windows do not have that characteristic. Air contains large or small amount of water vapor. Water vapor is invisible, but can turn into a fog or cloud due to the temperature or pressure. Air temperature determines the water uptake in the air. One cubic metre of air can take up to 18 g of water when the air temperature is 20°C and up to 5 g of water if the air temperature is 0°C. If the degree of saturation in the air has been accomplished, the moisture condenses on the cold walls, corners and storages. Mostly endangered are rooms with low temperature and high humidity. It is hard to determine the exact humidity. Extra humidity can be caused by cooking, cleaning, doing laundry, plants and their watering. Excess water vapor generated needs to be led out as directly as possible. Correct room ventilation does not only just produce fresh air but also leads out the moistness and evaporated water. It also helps to prevent excess moisture if You open Your windows for 5-10 minutes every morning and evening.

Dewpoint is the point where humidity turns from gas to droplets (on the window sill).
Dewpoint depends on following factors:

  • Outside temperature
  • Thermal insulation of window frame and glass
  • Room temperature
  • Relative humidity
  • Air exchange

2. I mounted new windows with selective glass. On a sunny spring and autumn mornings a lot of condensation occurs on the outside surface of the glass. Why?

Glass unit with better thermal insulation (lower U-value) reduces the outgoing heat loss from inside to outside – that is why the outer pane is colder and the risk of condensation higher.

The more colder the outside air gets the more decreases the risk that glass temperature drops significantly under air temperature. Glass unit with good thermal insulation has a colder outer pane than the pane that is indoors and that is why when the temperature increases, there can be temporary condensed water on the class because the glass warms up slower than outside air.

This situation does not show that the glass unit has poor quality, but proves that thermal insulation works.

3. How do I enter into the contract and pay?

After measuring by the measurer is done then the final price is calculated, which can differ from the initial proposed fee. Koduaken OÜ enters into a two-sided contract with the client that foresees an advanced payment that is 50% from the final price for ordered products and services. Paying the other half of the payment depends on the clients own choice according to the written agreement. Usually the rest of the sum is payed before the work begins or right after the receipt of goods.

It is possible to pay for the order via bank transfer, with cash, or card.

4. In which cases the warranty for glass packets does not apply?

Following irregularities do not give the right for warranty:

  • Interference phenomena
  • Double glass effect
  • Anisotropy
  • Formation of condensation water on the outside surfaces (glass sweating)
  • Built-in elements (leaded glass, alarm systems, roller blinds, etc.)
  • Fracture due to the thermal stress

5. On what basis is the quality of the glass packet evaluated?

  • Glasses under evaluation must be examined from the inside, 2 metres away and perpendicularly with controlled surface.
  • Evaluation must take place in diffused light (for example, cloudy sky) without direct sunlight or artificial light.
  • Irregularities that are not seen in two meters are not treated as flaws.
  • To check the exterior reflection, the observation distance must be 5 m away from the glass.

6. What is thermal stress?

Thermal stress is a concept that describes the stress that occurs when the surface of the glass faces different temperatures. Due to solar heat glass expands on both framed and not framed surfaces. The more heat-absorbing the glass, the more large-scale its reaction to solar heat is. However, edges of the glass are inside the frame and protected from the direct sunlight and that is why they warm up more slowly and expand less.

The glass may break if the temperature difference between the center surface of the glass and the edges exceeds the safe limit. This phenomenon is called thermal fracture. Thermal fracture is usually recognised by the fact that it is starts vertically towards the edges of the glass and as a result there is a winding curvy crack. Thermal fracture occurs when the temperature change between the center of the glass and edges exceeds the critical limit (depends on the glass type).

  • The position of the glass towards the cardinal directions
    The position of the glass towards the cardinal directions is important because glasses positioned towards south or west are more exposed to direct sunlight compared to glasses positioned towards north.
  • Angle of inclination
    Angle of inclination of the glass is important because absorption degree is the biggest when rays of the sun are falling onto the glass at right angle (perpendicularly). Sunlight falling on a glass partially reflects back from the surface of the glass. The most critical angle of inclination is 30°-50°towards horizontal.
  • Thermal absorption
    Mass painted class absorbs more sunlight than regular clear glass and that is why it warms up more. Low emissivity glass has bigger absorption capacity than clear glass.
  • Air circulation near the glass
    Cooling of the glass is significantly affected by the air circulation in indoors and outdoors. On the other hand, on a lull day glass achieves higher temperature.
  • Sun shading
    Venetian blinds, both with the glass packet and without, reflect the sun ray’s back, which is why the glass warms up more intensely.
  • Stickers for sun protection, decorations and so on
    Sun protection stickers or decorations may cause higher glass temperature or uneven glass warming because stickers absorb more sunlight than glass.
  • Shadows
    Sharply defined shadows that extend beyond 100 mm from the edge of the glass may cause temperature changes on a glass surface and that can lead to thermal stress. Shadows may occur thanks to awnings, deep window hollows, roof eaves, or neighbour buildings.


Even though thermal stress in glass can be reduced thanks to factors mentioned above, it does not rule out the possibility of a broken glass. Only solution that guarantees that thermal stress will not occur is thermal reinforcement, or to temper the glass.