Insulating floor of first floor room that extends over side of house

[Jdwood38]

Hi all,

Currently doing some small renovations to a new house. One of the upstairs bedrooms 'juts out' over a bay window in the room downstairs. This section of the property on the first floor is timber frame. In the past a tree/plant has been allowed to grow up under the floor and as I'm also going to be replumbing a radiator in the room, I've taken up the floor to do new rad tails and remove said plant.

I can now see the top of the underside of this area of the room and will soon clear all the old insulation and dust once I've had a sample of the board tested for Asbestos as it's a bit damaged. Once I've worked out what needs doing with the board (I'm having the bay window changed so will get it replaced because of the damage) I want to insulate this area as it will cause a cold bit in the room. The gap between the underside of the joists (which cantilever over the end of the brickwork) and the top of the board is quite large.

What is the best way to go about this? Insulation rock/earthwool slab straight down onto the top of the board? Piece of Celotex coming up from the brickwork which is the edge of the house and then horizontally across to the outside wall between the joists (like doing a cavity floor)?

Grateful for thoughts!

 

[IT Minion]

Most of it is identical to any other suspended floor insulation. Typically either

1) battens screwed to the joists and celotex cut to size and pushed in then taped/foamed to get airtight. or

2) Netting stapled to the joists and then filled with rockwool.

The only area where it gets interesting will be the joint between the front wall of the house downstairs. You want to make sure that you aren't leaving a gap for heat to escape between the vertical wall and your horizontal insulation. Is the wall a cavity or solid brick wall?

 

[Jdwood38]

Thanks for the response! That's the bit I wasn't sure about too. It's a cavity wall (you can see it in the second photo) and from what I've seen elsewhere it is fibreglass filled just isn't here which appears to be just a space above a lintel for the window below. So I think probably best bet is Celotex between the joists, foamed and taped as you suggest and then I guess a vertical 'return' coming up from the brickwork to the top of the joist. I wonder if it would be best to do so it comes up from the outer leaf and then fill the cavity and the area behind the new insulation with a bit of rock wool?

 

[^woody^]

Rockwool quilt at the bottom for mainly sound and then heat and gap filling. Foil backed PIR board on top for heat.

Full fill the void, half and half or 2/3 rockwool 1/3 board.

Fill any gaps to the rest of the floor void.

 

[Jdwood38]

Thanks Woody, all done pretty much as suggested.