WERU GLOSSARY OF TERMS A - All you need to know in one handy place
The joint penetration coefficient (a-value) indicates how many mÂ³ of air are replaced in one hour per linear metre of window joint (between window sash and frame) at an air pressure difference of 10 Pa between inside and outside. The a-value is no longer used today. DIN 12207 has classified air permeability in windows and doors since 1999. The classification is awarded in classes 0 to 4 with class 4 being the lowest air permeability.