Closing first floor joist cavities

[white_noise]

I live in 1930 chalet house, that has open first floor joists at either side of the house, open at the internal skin of the exterior cavity wall (blown filled).

This means that cold air can get in from the soffit (through some gaps in the wall for timbers), and flow pretty freely into the first floor joist cavity making the rooms very cold. Also it results in hot air from the ground floor not staying in the house and escaping by the open floor cavity.

I have drawn an indicative construction to show this area (as attached), the red is where i plan to put celolex (50mm gap to the roof). I also plan to put celotex on 45 degree angle from the floor to the rafters if you can make it out. This will be boarded out with a couple of adjustable vents to allow ventilation into the space, its eave storage to avoid mould.

So what is left is what do i plug the joist cavities with, and how much? they are 200mm joists, do i just fill 200mm of loft insulation from the internal skin for 400-500mm internally under the floorboard, or do i use celotex and try and get a much better air tight seal. Of course i don't want rot to occur.

Cheers
WN

 

[theprinceofdarkness]

What I would use is glass fibre. Because the inside faces of the cavity will be rough it will be difficult to punch it down 8" without scraping your arms. I would also use a much thicker bit of glass fibre then the nominal cavity width, so at least 4", 6" would be better, you don't need a gale blowing up your cavities! So what I would do is to use two bits of DPC or thick plastic bags. One bit you hang vertically 8" down the cavity, the other you fold over the edge, again with 8" down the cavity. Now if you are lucky the glass fibre will slide down between the plastic to the right level. Once its in the bits of plastic can be pulled out, while holding the glass fibre down. I would aim to get it a bit lower then the 8", say 10"?, so when extricating the plastic, if the glass fibre pulls back a bit not all is lost!
Frank

 

[white_noise]

Tpod, its on the cavity wall I need it insulate, its sealing the floor void ends at the inner leaf of the cavity wall.
WN