The main reason being is that the moment the cat flap is opened it has created a draught hence no cat flap manufacturer can claim that it is draught proof.
Hinged flap door draught excluder.
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The only way you could avoid this would be like a hotel revolving door that never actually opens as such you enter a compartment that then moves around to the opening so you can get out.
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Around the sides and the top of the door you can use a foam strip or brush strips to minimise draughts.
Doors can be insulated either by attaching excluders to the door itself or sticking them on the frame and all sorts of windows including hinged sash and sliding doors can be insulated using easyfix draught excluders.
Fit brush or hinged flap draught excluders fitted along the bottom of the doors see below interior doors.
A wide selection of letterbox excluders are also detailed.
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Cut draughts with snake draught excluders photo right brushes or similar strips of material see below unused chimneys.
A hinged flap draught excluder may also be used as it works on a similar principle.
Chimney draught excluders are available from most diy stores.
So to get round this you could install a hinged type instead.