I live in a 1990 built 4 bed detached house which has cavity wall insulation
on all of the outer walls.



The small 4th bedroom above the garage which is the same size as bedroom 3
and has the same size window and radiator is always noticeably colder and
suffers a faster temperature drop when all of the bedroom doors are shut.
Our hall and stairs also feels quite cool.



In the past I have double insulated the loft above this bedroom and
insulated below the floorboards in case cold was rising up from the garage
below but still this bedroom is cold. On checking the internal garage
cavity wall between the garage and the hall, which also extends into the
bedroom above, it would appear that this wall has no cavity insulation.



I am now having the garage converted into a room and was wondering whether I
would feel any benefit from having this wall cavity filled with some sort
solid setting insulation seeing that this wall will now be heated on both
sides. My logic is that this wall will still be cold as cold air from the
roof and loft will still fill the cavity.