EN News Company Downloads Contact Home / Airtightness systems / Issues / Diffusion – Convection Airtightness systems HUMIDITY REGULATION The warmer and moister the air indoors, the higher the vapour pressure. DIFFUSION – CONVECTION When a building component or a layer of a component separates two rooms with different temperatures and humidity, there is varying water-vapour partial pressure on both sides. Water vapour moves through building material with this difference in pressure. This is known as water vapour diffusion. The path and direction are always from warm to cold. A much greater part of humidity escapes through leaks due to air currents. Whereas only 0.3 litres of water escape through 1m² of wall or ceiling during the heating period, up to 30 litres of water pass into the construction through a 20 cm long and 2 m wide crack. Where does water vapour come from and where does it go? The house occupants produce moisture in the rooms every day: through breathing, sleeping, cooking, showering… A household of 4 people can emit up to 10 litres of moisture into the room’s atmosphere in the course of a day. Part of this is removed through airing but the water vapour left in the air, similarly to the warmth, tends to seek its way via the exterior construction components to the outside. Warm air can retain more moisture than cold air (e.g. at a temperature of 20 °C 17.3 g/m³ and only 2.14 g/m³ at -10 °C.) If warm air cools too quickly it produces moisture and condensation occurs. By the way: anyone who has ever taken a bottle of beer out of the refrigerator is familiar with this principle. ALREADY AWARE? If you want to compare materials with regard to their water vapour permeability, the sd value is commonly used - the water vapour diffusion equivalent air layer thickness. VIDEO ISOCELL Fairy 05: Moisture regulating | 1:11 min FIND OUT MORE Building shell Check airtightness Airtightness test Diffusion – Convection ISOCELL GmbH & Co KG Gewerbestraße 9 A-5202 Neumarkt am Wallersee Tel.: +43 6216 4108-0 www.isocell.com ISOCELL FRANCE 170 Rue Jean Monnet F-29490 GUIPAVAS Tel.: +33 298 42 11 00 www.isocell-france.fr ISOCELL Sverige AB Köpsvängen 10 168 67 BROMMA | Sverige Tel.: +46 10 130 25 00 www.isocell.se ISOCELL SCHWEIZ AG Herbergstrasse 29 CH-9524 Zuzwil Tel.: +41 71 940 06 72 www.isocell.ch LEGAL NOTICE TERMS AND CONDITIONS DATA PROTECTION DECLARATION DOWNLOADS PRIVACY SETTINGS © ISOCELL GmbH & Co KG 2021 Hi! Could we please enable some additional services for Analytics? You can always change or withdraw your consent later. Let me choose DeclineThat's okay