On older skylights the thermal glass was not nearly as efficient as the new VELUX LoE3 glazing. That being said, condensation is often the result of way too much humidity inside the house. In the winter when your living room is 70 degrees and the relative humidity may be properly set at 20%, the air up in the skylight shaft may be 85 degrees and the humidity next to the glass could be 50% or more. There is little anyone can do to prevent condensation on older skylights in those circumstances. VELUX skylights are manufactured with small weep holes around the perimeter of the glass. After some years those holes, which are designed to allow condensation to escape, become clogged. A good vacuum cleaner can sometimes help clean out the weep holes and solve or reduce this issue. If the problem persists, the next best solution is to upgrade to a new VELUX thermally insulated skylight. Condensation is generally a winter-time problem however it often indicates that the thermal insulation in the skylight has failed allowing heat to leave the house. This often translates into complaints about too much heat coming from a skylight in the summertime. Again, the thermal insulation in an old skylight is gone allowing heat to radiate through the skylight.