Replacing an old door for a new one can vastly improve the thermal efficiency of a dwelling as modern doors and door frames have been designed to achieve much lower U-Values than were previously achieved, especially if the existing door is timber with single glazing.
New doors should be positioned on a bed of mortar with the rear of the frame in the same location as the existing, reducing the amount of disruption required within the dwelling. Once positioned, fix through the frame into the brickwork ensuring packers are placed between the frame and the wall at the fixing locations to offer maximum support. There should be a minimum 6mm gap between the inside face of the door frame's sill and the floorboards.
Any remaining gaps between the frame and wall should be filled with expanding foam to give support and to form an insulating barrier reducing any thermal bridging. Once the foam has set and been cut flush with the frame face, sealant should be applied around the whole door frame. To finish internally, at the base fix hardwood quadrant to cover the gap between the frame and the floorboards and around the frame either seal with silicone or fit uPVC or painted softwood cover strip/quadrant.
Hints & Tips
When replacing a door make sure there are no planning or heritage constraints placed on the dwelling.
When measuring up for a replacement door be careful to ensure it will fit as new doors and frames come square and true but the old opening may not be.
If there are large gaps around the frame on the outside it may be better to either use a mortar fillet or install painted softwood of uPVC cover strips/quadrant.