Apologies in advance for the length of this post, but I'm completely out of ideas ....
My daughter's 1st floor bedroom is colder in the winter and hotter in the summer. I'm stuck for ideas where the problem might lie that would cause such a noticeable difference. When the heating is on, the room is ok, no different from the others. When the heating goes off, it cools more rapidly then the others.
I'll go around the room, comparing it with others, what I've done and see if anyone can come up with anything.
The house is a 1970s semi.
Ceiling. Plasterboard with pitched roof above it. Entire loft insulation was topped up years perhaps 10 years ago so there is no difference between this ceiling and any others.
SW wall. External with large DG window and cavity wall insulation. Out of curiosity, I drilled through the back of a socket and pushed a camera through so I know insulation is present. No evidence of failed DG units eg cracks or condensation. Window hinges replaced recently so there isn't a problem with the seals. Windows were replaced throughout the house perhaps 20 years ago in the usual slap dash manner. I've removed the UPVC strips used to hide the bodges and put frame sealant around all four edges so there's no air coming in around the window. Also inspected frame joints and beading and sealed any tiny leaks. Any remaining leaks must surely be too minor to make any significant temperature drop. Skirting to floor gap is sealed.
NW wall. Internal. Bedroom next to it doesn't have the same noticeable heat difference so I've ruled out this wall as being the problem.
NE wall. Internal. Again, bedroom next to it doesn't feel any cooler.
SE wall. This is the party wall. It is plastered cinder block with an uninsulated cavity. Again, drilled a hole, put a camera through, and had a look. Also did the same inspection downstairs just to rule out the faint chance there was insulation but it had settled/slumped over the years. Going around with a lit candle I did find air blowing through the mains sockets so removed them and applied frame sealant to any gaps/holes in the pattress boxes. Skirting to wall gap is sealed.
Floor. Carpet over underlay over 18mm floorboards. The floor void has 170mm rockwool which I fitted awhile back just to deaden the sound coming through from the lounge below. The ends of the floor joists run into the SE wall. They are not on joist hangers but embedded. For completeness, and in case they had shrunk or warped over the years to let air through from the cavity, I ran expanding foam over the joist/wall joint and floor/wall joint.
The bedroom behind that also shares the party wall doesn't have this problem. The other front bedroom which shares the SW external wall, is of a similar size, has an identical window, and is even over the garage, doesn't have this problem.
After all of that, the room is still cool. If any of my actions has improved things, it's not been noticeable. It's like a window has been left open an inch. I'm stumped.
Any suggestions or questions welcome.