Which expanding foam to use on attic floor to fill gaps between gyproc

[plum60]

I have to air seal the attic floor and guess I need an open cell low expansion foam to fill the gaps between gyproc pieces first then add my rockwool over the top.
It doesn't say on most foam cans if they are open or closed cell and the expansion rate on a low expansion is still about 30% so will this put too much pressure pushing gyproc downward? Crawling around up there spreading my weight as I go hasn't stopped small lines appearing between the gyproc on the ceilings below so I don't want to use a foam that will pressure the gyproc any further in case it wants to fall off on top of a guest in bed over xmas!
Had to clear a load of black dirt broken bricks and slates they dumped under the old insulation first- looks like they smoked in the loft too when they converted this Victorian house into flats.
The other nightmare is the chimney was taken down say around 40-50 years ago and new roof but they left the two chimney stacks open inside the attic so in theory if you fell down you would fall past the ground floor flat to the basement flat. I am yet to resolve this issue. I want to close off the chimney stacks and still ventilate so any damp can escape. Any and all tips welcome.

 

[Jones]

The plaster skim on the boards will provide air sealing if you're doing a test. You will need to seal any penetrations like light fittings. So long as the foam can expand out of the gap it won't expert much pressure when it expands and cures. Air sealing foam is available and is probably a bit better than a general fill foam.

 

[XTiffy]

Not quite sure why you need to seal gaps in plasterboard. As Jones says, if it is skimmed below then it is sealed. You can use foil self adhesive tape as an added seal. If you use rigid foam insulation that is the tape used, which would allow you to have a higher degree of insulation and seal. You can even then add rockwall on top. The problem mostly today is using inset lights in the ceiling, which are very difficult to seal and protect the insulation from heat/fire. If moving around the attic causes stress cracks, the joists are not man enough for the load. The chimney stacks can be easily closed off provide you allow plenty of ventilation. (airbrick/plastic vents etc.

 

[plum60]

The plaster skim on the boards will provide air sealing if you're doing a test. You will need to seal any penetrations like light fittings. So long as the foam can expand out of the gap it won't expert much pressure when it expands and cures. Air sealing foam is available and is probably a bit better than a general fill foam.

Thanks for the info. There are quite a few gappy gaps between pieces of gyproc up in the attic. there was lots of blackened insulation so I can tell there is warm air getting getting in from below. I think they were cowboy builders which is why I thought to seal over them. There was a lot of condensation before I unblocked the eaves. You mentioned air sealing foam but I can only find everbrite gap and crack which is a low expansion foam but that seems to expand to about 30% I hear people say more than the tin says. I am caulking around light fittings so they are air tight.

 

[plum60]

Thanks for the info. There are quite a few gappy gaps between pieces of gyproc up in the attic. there were lots of blackened insulation so I can tell there is warm air getting getting in from below. I think they were cowboy builders which is why I thought to seal over them. There was a lot of condensation before I unblocked the eaves. You mentioned air sealing foam but I can only find everbrite gap and crack which is a low expansion foam but that seems to expand to about 30% I hear people say more than the tin says. I am caulking around light fittings so they are air tight.