Roof airtightness detail

[amin]

Not sure if this is the right place to post, but I’ll start here.

 

I’m rebuilding my roof as part of a loft conversion and also installing MVHR, so I want to make the roof as airtight as possible — ideally all the way from the eaves (where I’d like to run ducts between the eaves and dwarf walls). It’s a solid brick Edwardian house. Also in a conservation area so can’t do a warm roof due to the build-up and have to preserve what it currently looks like. 

 

My planned build-up is full-fill mineral wool between rafters, with PIR insulation underneath. What I’m struggling with is where the airtight layer/membrane should go, and how to detail it around all the penetrations from rafters, joists and the joining it back down to the wall from the room below. 

 

What’s the best approach here? I feel like it’s a huge puzzle piece trying to get that membrane in and I must be missing something simple here. 

 

I’ve attached a detail of what I’m aiming for (apologies for the rough SketchUp drawing!)

mineral wool roof.pdf

[Redbeard]

Hello,

 

Why is the Intello to only come 'up to 400 above the joists'? If you continue it all the way from one side to the other, and do all the details correctly then there is your answer to

2 hours ago, amin said:

where the airtight layer/membrane should go, and how to detail it around all the penetrations from rafters, joists and the joining it back down to the wall from the room below. 

 

 

I have not used Solitex Plus before, so I assume you know (as I don't) that it is deemed OK by the manufacturers for venting *above* rather than *under* the membrane? I would have vented below the membrane, but that's just because I am a 'dinosaur'. 

 

You say you

2 hours ago, amin said:

can’t do a warm roof due to the build-up and have to preserve what it currently looks like

 

but your counter-battens assure that it will not look as it currently looks, surely? I see why you want to vent above the membrane, to keep the 'lay-up' thinner, but if that's at the expense of upsetting the conservation officer then it doesn't achieve much. Have you agreed the roof rise with the CO?

 

Your reference to penetrations and integrity of the a/t layer suggests that you already know it will be a PITA to to all the cutting, priming and taping. It is, though, exactly what I would do. Taking it down into the room below could leave an unsightly 'blurge' of A/T tape at the top of the room, but if you chop out the plaster at the top of the wall you could (a) before the taping of the membrane round the joists, put a thin (say 6-10mm) coat of lime plaster on all the brickwork then, when it has gone off, prime it and tape the membrane onto it with one of the 'plaster-onto' tapes. Then do just that to finish.

[amin]

Coming up 400mm from the joists was partly because it feels like I’m doubling up on VCL with the foil face of the PIR which I would want to tape up - I’ve assumed that provides a good layer of airtightness still?

 

On the counter battens you’re right to point out the counter battens issue - at 25-38mm it will definitely be a build up. But this in my view will barely be noticeable from street level vs adding insulation layers on top? On solitex it’s a diffusion open membrane so venting above should be allowed - I’ve stuck to the pro-Clima product given the intello membrane inside. 

Sounds like there no way round getting a/t membrane in will just be a PITA and no shortcuts  I’ve got the house back to brick so have flexibility on the room below. 

 

@Redbeard how would you approach it if you were to design it?