Relation between quality of air in room and winter construct damages on wooden windows

Many times it is unclear the relation between humidity of air, CO2 content in air and energy consumption for the ventilation of rooms and the damages coming from ignorance of this process. This topic can be explained as follows:

Amount of water steam during the construction

While new building are drying there comes to existence a lot of water humidity from plasters and covers. Relation between in existence coming vapor and time span of diffusing the vapor through the walls and windows can be explained on this case study:

If the room is as big as 30 m2 (height 2.5 m), undercoat (thickness 4 cm) and room plasters (thickness 1.5 cm) produce during the drying session during 4 weeks cca. 500 l of water all together. It equals to the volume of vapor cca. 620 000 l /18 g of water = 22.4 l of vapor/. Through the surface treated frames of wooden window of standard size 1.25×1.25 m can be defused just 1 g of water, resp. cca. 1.25 l of vapor per hour! It is supposed that window frames take 4% of all area of walls and it is evident that only 4% of vapor is being diffused through window frames, what generates the all time span of diffusing the vapor for more than 2 years. We can conclude from it that without sufficient ventilation during the drying season the existing vapor cannot be diffused in stipulated short time.

As aftereffect of insufficient ventilation there are so called winter building damages. Vapor imbues the wood through lean points (for example joints between glass and wood or thin varnish covered places in plaster), is condensing and increases the humidity of wood to the value more than 30%, what causes a heavy opening of the windows and causes the damages in glued parts or increase the danger of molding. In extreme cases there can be swollen parts on the outer side of window, they can hold water. We can prevent these damages through sufficient ventilation (for example with open ventilation window). For more details see “Window ventilation”.

Amount of vapor in habited rooms

Vapor is a no poisonous gas with no taste and no smell, that can be incessant produced in big amount, for example the 4 member family produces 8 to 15 kg in average per day, what presents volume of vapor roughly 10 000 to 19 000 l.

Relative humidity of air from around 40 to 70% is considered to be normal.

A dry air supports electro voltage and causes drying of breathing paths of man. If the humidity of air is to low (under 40% around) some multiplication of bacteria and viruses can be done, they can cause the bronchitis illness.

Above mentioned procedures is better to hand over to the specialist to get your request in estetic and quality aspect. In case the humidity of air is highe than 60-65% lasts longer, on smooth places of outer side of window the condensation of vapor comes to existence, what can be good base for molding.

Not condensing of vapor can be reached by:

  • perfect insulation of walls of house, unless it is possible, inside insulation with the risk of creating molding in places as corners and nikes.
  • active ventilation through windows what assures for humidity of air not to excess 50-55% (hygrometer for humidity of air detecting can come in handy!)

Carbon dioxyd CO2

CO2 is uncessently produced through breathing. High content of CO2 in habited rooms makes people tired and out of focus. There is a limit acknowledged in Europe (Pettenkofer limit value) what creates 0.1% CO2 of air in room. Amount of produced CO2 in room equals according to the activity 10 to 75 l per hour. In 4 member family the amount of fresh air per day is needed in volume 2000-3000 m3 in order to keep the concentration of CO2 below the critical level. It means that in 75 m2 apartment the ventilation is needed each 1.5-2 hours, in family house 140 m2 each 3 hours.

Window ventilation

The aim of the ventilation is to gain maximum quality of air with minimum energy consumption.

For active ventilation we have to take into account following items:

  • Time needed for complete change of the air in room for sudden ventilation depends on outside temperature (in winter 4-6, in spring and autumn 10-20, in summer 25-30 minutes).
  • A light breeze (cca 5 km/h) can double the amount of changed air. The most driving force for ventilation is thermics. The higher is the difference of temperature between exterior and interior, the higher is the tendency of warm air to get out through open windows. That is why the time for changing of air in winter is much shorter (cca 5 min) than in transfer season (cca 15 min).

Optimal results were gained only through regulated ventilation with regaining of heat. In following table there are given the values of changing of air, it means how often in one our the complete change of air can be attained for various kinds of ventilation during 1 hour, for value 0.5 the air can be changed in 2 hours.

Way of ventilationValue of air changing volume per hour
Windows and Doors sealed with insulation0,1 – 0,3
Regulated ventilation0,2 – 0,8
Horizontal pivot window
  without cross ventilation
  with cross ventilation
0,8 – 2,5
2,0 – 4,0
Open window (sudden ventilation)
  without cross ventilation
  with cross ventilation
9,0 – 15,0
more than 20,0
Not insulated housesdo 2,0


At drying of floor covers and plasters the created vapor has to be ventilated out through sufficient ventilation in order to prevent the damages during the construction.

It is necessary to ventilate in sufficient amount in order to gain a good inside climate and healthy air to breath in inhabited rooms. That is the only way how to prevent to creating of molds and damages caused by high humidity of air.

As conclusion some practical advices:

  • To ventilate effective according of space, for ventilation through more room there are 3 minutes enough for complete change of air.
  • To ventilate actively only while we use the rooms, in uninhabited rooms ventilation through cracks is enough.
  • A very often used kind of ventilation with horizontal pivot windows has a result an increased changing of air, because the amount of changed air is out of the evaluation. This kind of ventilation has its reason only out of the heating season. During the winter the horizontal pivot window cannot be open more than 7-20 minutes per hour.
  • Recommended sudden ventilation through open window in winter is effective only if it is done in disciplined way and in short time (4 až 6 min.).
  • Good results can be gained with means of ventilation slot (batch ventilation). One element for one room is enough.
  • For all kinds of active ventilation the time of open window has to be oriented according to outside temperature. The more cold is outside the shorter time of ventilation is, due to the more strong termics.