Insulating a suspended floor (again)

[cam1989]

Afternoon

Apologies, I know this has been asked so many times but can't find the specific scenario.

1920s detached house, suspended timber ground floor. Joists are roughly 120mm deep. I want to insulate between them.

Government's best practice guide refers to cold bridging on 4 inch joists, no problem on 6 inch, but nothing in between.

Asked the question of one of the main manufacturers who've insisted I need 180mm to meet building regs, despite me stating it doesn't need to meet building regs.

If I dump 100mm of rockwool between the joists, breathable membrane below and plastic sheet on top, will I be alright - any potential issues?

Thanks

 

[motorbiking]

so Vapour barrier, packed insulation and sheeting? Where do the floor space vents come in? If you are going this route it needs to be completely air tight as the insulated area wont cold vent. Personally I would go with celotex cut between the joists and foamed to seal. You can use a row of nails on the joists to allow the celotex to sit while the foam cures. 75-100mm will make a big difference. Make sure any central heating pipes are also properly insulated if in the void.

 

[Lower]

Speaking from experience, don't expect much benefit.