New Floor-removing old fireplace base

Possetjohn

Hi,

Hoping some of you experts out there can offer some advice.

We have an old 1920s bungalow, which was converted to a dormer bungalow in the 1980s.

I am starting to replace the suspended floor in one of the ground floor rooms, originally one of the bedrooms. It is a bit saggy, and half floorboards and half chipboard from when the CH was put in with the dormer conversion. Going to change any bad joists or sleeper wall bedplates, then put down insulation and new 22mm floorboards.

Now the problem. In one corner of the room against an outside wall, there was a fireplace and chimney in the original construction, removed in the 1980 conversion. On lifting the existing flooring, have discovered the builders who did the job, removed the old fireplace but left the triangular brickwork base about one joist depth below the floor and just laid the new joists on the bricks on pieces of slate. Not so good for ventilation! Not only that but some of the removed bricks were just stacked with debris around the edge of the triangular base on the subfloor. The problem is the bricks and mortar in the base are rather damp. Not wet, just damp. The rest of the subfloor against the outside wall looks quite dry.

I have removed one layer of bricks, and there is one more layer (I think), which if I remove will make the level slightly lower (2-3") that the general subfloor. I am not sure if the dampness is migrating in from the outside wall (below damp course) or coming up from underneath somehow.

I am thinking of removing all the bricks and if necessary making a new piece of subfloor in that corner.

I have some pictures but unfortunately not sure how to post them on the forum! (lol)

Anyone have any advice?

Also anyone recommend the best type of insulation to use. Rockwool or rigid?

PS outside walls are 9" brick and a new damp course (silicone emulsion) was done 2 years ago