airtightness, first floor ceiling ?

[Post and beam]

The TF frame erectors have left airtightness membrane visible at the tops of the studwork and feature beams on the first floor. I conclude that the loft space is not regarded as part of the airtight envelope.

I do not understand what i need to do next to maintain this envelope. I could of course tape membrane to the existing across the entire ceiling prior to the plasterboard being installed.

This does not seem to be the correct approach though, i guess i have misunderstood something.

#a: because the lighting installations would subsequently breach this everywhere and render it useless.

 

What have i misunderstood please?

[Nickfromwales]

Is it a warm roof? 

[Post and beam]

Hi Nick

Described as a cold roof when i explained that the MVHR unit was going into the loft. 400mm of Ursa rockwool in the loft.

[Post and beam]

At a loss to understand how i can create the upper airtightness boundary.

[Russell griffiths]

Give us a picture. 

[Post and beam]

Not sure if this is very clear, #1 shows the membrane above one of the beams. This is first floor ceiling. And #2 , the dark shadowy bit, Is the membrane above the studwork that makes up the first floor  ceiling. So the perimeter of the first floor ceiling is a continuation of the airtight boundary from below.

As stated previously, i could tape to, and continue this membrane across the whole first floor ceiling. No idea if this is the usual practice.

But every light pendant would breach it and render useless.

My TF frame company are very off hand now and just say ' this is for others'. 

 

Perhaps the usual thing is to box out the light pendant locations within the ceiling timbers, carry the membrane to these apertures leaving a smaller area to deal with after the lights are fitted.

 

Any and all practical solutions or experience gratefully taken on board.

 

keith

[Russell griffiths]

You probably need to do a bit of this and a bit of that. 
your airtight layer needs to be continuous 

if those beams are on display can you not continue the airtight layer above the beams. 
do you have a layer of plasterboard sitting on top of those beams. 
 

a sketch of what you have is probably a better idea. 
 

you might need to build some boxes around the light fixtures. 

[Post and beam]

The beams will be on show yes. Yes of course the airtight layer needs to continue above the beams. There will be a plasterboard ceiling . The question is....

 It appears that the TF  design calls for a boundary at the point between the plasteboard ceiling and the horizontals of the roof trusses. (Rather than the  upper limits of the roof) so is the usual practice to box around the light fittings?

 

[Russell griffiths]

Usual practice is to have a service batten and fit low profile lights, there are lots that fit in a 40mm void. 
 

but every build is a bit different. 
you might need to build boxes. 
you could paint them with an airtight paint, so you only need to tape to the edges.