THE 2-MINUTE RULE FOR SASH WINDOWS

Assembly Rails: This is where the top and bottom sashes overlap. Even a 1–2mm gap here can enable cold air to snake by means of, Primarily on windy evenings.The Bunnings manual How to Wintertime-proof your home incorporates far more suggestions for keeping heat and decreasing utility expenditures in the course of cooler months. - MitchellMcWar

It Is that this gap that provides the insulation. It slows down thermal transfer (very hot or cold) by conduction and because air are not able to flow into freely while in the narrow gap concerning the panes convection can also be slowed down additional cutting down warmth transfer. This is certainly what secondary glazing is trying to duplicate.Al