This is caused by failure of the glazing method which must protect the unit seal from the ingress of water or moisture. All double glazing sealants used in the manufacture of double glazing units have a moisture vapour transmission rate when exposed to water or moisture, which will cause misting or breakdown eventually over a period of time. Where units are foreputtied, the putty must be painted and on to the glass to provide a seal to protect the putty from moisture ingress.
Foreputty should be left for 7 to 10 days before painting and within 28 days. Similarly when glazed with beads and non hardening compound, paint or protective coating should be applied over the bead and on to the glass. Where timber preservative has been used, Manufacturer's instructions should be followed to prevent reaction with glazing compound. All Slimlite Ultra Clear Self Cleaning Units are edge taped with a strong adhesive aluminium foil which helps to avoid damage during handling, but most importantly provides some extra protection should glazing failure occur allowing ingress of water or moisture.
It is recommended that timber windows should be painted every five years Inland areas and three years Coastal areas. However it is recommended that the paint protection of the glazing including the overlap on to the glass face, should be repainted every three years to maintain the required protection particularly with putty fronting as a minimum. However more frequent 'spot painting' may be necessary to maintain the required standard of protection.
BS6262 Section 22.214.171.124.2
BS6262 Section 126.96.36.199
Most causes of misting in the cavity of double glazed units are a result of the glazing protection failing and allowing water into mainly the lower horizontal glazing rebate.
All current double glazing perimeter sealants have a water vapour transmission rate when contacted by water or moisture and in time will appear as misting in the cavity. Water or moisture in the glazing rebate unless rectified will eventually cause decay in the timber section.
As an optional added protection a small bead of low modulus silicone which is resistant to water can be applied as a small bead 2 to 3mm around the joint. between the outer edge of the unit and rebate which must be perfectly dry to ensure good adhesion between unit and rebate (see sketch) See adjacent painting recommendation.