How to thread VCL around loft eaves floor

[les_danger]

Hi all, OK so i'm asking how but open to why as well.

I'm re-insulating this existing loft room. There was about 50mm of mineral wool in the pitched roof and not much else. I'm set on most of the rest of this space, but stuck on this...

For this eaves space which sits above a bedroom I am wondering how on earth i'm going to thread the vcl around the floor and join it to the dwarf wall. the plan for the dwarf wall is PIR between the studs and then another layer across the studs from the cold side to prevent bridging without losing any more floor space. the floor joists and ceiling joists are odd and will need loose mineral wool build up.

My understanding is that there should be a VCL below that which somehow mates up with a vcl running on the warm side of the dwarf wall. Obviously the joists run perpendicular to the plate for the studs. Apart from cutting lots of slits and then trying to tape them up i'm not really sure how this can be done neatly?

That eaves space should be fairly well ventilated - over fascia vents and breather membrane albeit with a close fitting artificial tile. So as much as i'm bought into a VCL being a good idea,... surely any vapour finding its way into this area isn't such a risk? Also even if I tied this all up nicely, wouldn't the hatch into this space negate the whole thing? There's no way you'd make that vapour tight!?

Grateful for any thoughts.

 

[noseall]

Hi all, OK so i'm asking how but open to why as well.

I'm re-insulating this existing loft room. There was about 50mm of mineral wool in the pitched roof and not much else. I'm set on most of the rest of this space, but stuck on this...

For this eaves space which sits above a bedroom I am wondering how on earth i'm going to thread the vcl around the floor and join it to the dwarf wall. the plan for the dwarf wall is PIR between the studs and then another layer across the studs from the cold side to prevent bridging without losing any more floor space. the floor joists and ceiling joists are odd and will need loose mineral wool build up.

View attachment 423614

The VCL should be located directly behind the plasterboard, so that the moisture envelope is contained in the warm part of the building. Aywhere above (ceiling joists) is not really suitable.

The loft zone should be insulated with ventilation above the insulation, when fitting between rafters. All joints foil taped etc.

Vaulty2

• noseall
• 15 May 2022

Foil3

• noseall
• 2 Apr 2019

 

[les_danger]

Cheers, the thing is it's an old l&p ceiling that i'd rather not take down. I've taken two down elsewhere and although they're showing cracks and have a terrible finish from polystyrene tiles, they were actually pretty solid and wouldn't have come down any time soon, also from a noise perspective i don't think i'll be able to beat them. So I was going to overboard the bedroom ceiling below that.

There was another really helpful reply i spotted in the day but i can't seem to see that now. But it made me think i should just extend the thermal envelope to include the eaves. My worry is going down far enough to the wall plate and still letting the ventilation get through the gap, i never see that sort of detail in online guidance or vids.

 

[noseall]

There was another really helpful reply i spotted in the day but i can't seem to see that now. But it made me think i should just extend the thermal envelope to include the eaves. My worry is going down far enough to the wall plate and still letting the ventilation get through the gap, i never see that sort of detail in online guidance or vids.

Vapour isn't as big an issue where there is adequate ventilation. The trick is getting the ventilation/insulation balanced.