If the area surrounding your filter isn't completely sealed, air can be

furnace filter slot cover   Dan furnace filter slot cover

I wonder if the previous owner lost the cover for the filter slot I would think that you would want this covered

Air Cleaner Filter Cover-Plastic Skuttle $ Part Number A00-0641-196 Cover & latch assembly Fits Models DB-25-16 & DB-20-16 Continue We're All About Sealing Up Ductwork In the video above we explain a very easy simple tip to seal the space around your furnace filter slot

stinkin rich slot machine game Filter Lock is a patented two-piece seal system designed for use with 1” air filters that eliminates air leakage at the filter slotdoor to improve indoor air slot cover plate and had the filter stick out several inches! I see there are companies that make custom made filters, just seems an