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VCL across bottom chord of attic trusses

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I plan to use sheet VCL (green poly) to make the VCL around the loft room. I have shown it in green in the image below. Battens across the rafters for under rafter insulation (all insulation is mineral wool) provide a surface to staple the VCL to (tape over). I'll have to tape around where the VCL crosses each stud of the dwarf wall which will be a bit tricky, but OK. What I'm less confident of is how I deal with the big transition to wall plate (masonry walls). This is where the green VCL is crossing the 222mm high bottom chord of the trusses. Does anyone know what is normally done here? Just cutting a slot and flaps out of the VCL and stapling/taping feels a bit fragile.





It feels like I should have something to support the VCL here. I can see two easy options with ~9mm OSB or 5mm ply. Shown below before and after adding insulation, though maybe the rafter insulation would carry on down before adding any sheet due to restricted access. In the top otption, the ply/OSB could be screwed in place and then sealed with Passive Purple meaning that the VCL itself wouldn't need to go down all the way, but may as well. Very time consuming though.





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  • 4 weeks later...

Is it a viable solution to create two separate airtight 'cells'? One above the joists and one below? The air in the height of the joist would not be sealed. That shouldn't be a problem from a thermal perspective as there is insulation at the ends and the two 'cells' would be airtight so not transferring moist air to this space.


This is the easiest approach, but is it valid?



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The detail below is a solution except I am using resilient bar to separate the ceiling plasterboard from the joists so I need to bridge that gap. The noggin would normally do this but that world short circuit my resilient bar.

Resilient Bar Timber - Ceiling Systems | Metsec


I'm not really sure why in that detail it's a double noggin.



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