We recently bought our first house, a rather old 3-bed terraced house. The adjacent house has been empty for 4 years. The 3rd bedroom in my house is a single room extension I'm using as an office and it's always cold. We didn't get a survey (inspection). Our agent told me the house to his knowledge has never failed a survey. We did have specialists (contractors) evaluate & quote for free so have had full electrical safety check, roof inspection, damp inspection in extension & ground floor.
I've pushed some insulation into the corners of the roof/ceiling (slanted roof). There is a small plastic vent in the top of the room which says "do not cover." I've put glue along the PVC window frame as there were some holes in the paint and I wondered if this was creating a draft. When the heat is on, the rest of the house will be warm but when it cycles off within an hour my hands are numb. The wall next to the empty house in the extension sounds real hollow when I knock on it. We already had to damp-proof the ground floor wall on the empty house side as we had rising damp in that wall
Whilst in the attic I noticed on the back wall there are gaps in the mortar through which you can see outside. The bricks are very black where the wall ends between our attic and the empty house and some crumbled bricks have fallen out of an inner partial layer of bricks. Some bricks seemed a bit damaged. It's also possible to get around the end wall into the neighbouring attic (which isn't very well insulated) and obviously that house isn't being heated whilst vacant. Some of the beams have mould, and the roofer who came over had a look and said it will dry out after roof repair.
Should I replace and re-mortar the bricks? Could the empty neighbouring house be causing some of our insulation issues? If so, should I put up some kind of barrier or cover the gap between the walls of the attic, or are they supposed to have a gap between them? And is there a cheap way of making the extension warmer?