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The first step in the sealing process begins before the unit is even mounted: the integrity of the window itself. Most window units are designed for double-hung windows, which slide vertically. The weight of the air conditioner places significant stress on the lower window sash, while the upper sash is often left loose. To prevent the upper sash from sliding down or shifting, it is imperative to install L-brackets or angle brackets. These are typically provided by the manufacturer and are screwed into the upper sash to lock it against the window frame. Skipping this step not only compromises the seal but creates a security risk, as the window could be forced open from the outside.
Once the unit is mechanically secure, the focus shifts to the most common point of failure: the gap between the upper and lower window sashes. When a window is raised to accommodate an air conditioner, a large triangular opening remains between the two sliding panes. While many units come with an expandable "accordion" panel to cover this gap, these factory panels are often flimsy and provide poor insulation. The most effective way to seal this area is to cut a piece of rigid foam insulation or plywood to fit the exact dimensions of the gap. This material provides a rigid barrier against the elements. Once the barrier is in place, the gap should be caulked around the edges to create an airtight seal, preventing any exchange of air. how to seal a window air conditioner