Retrofitting airtight detail - roof space.

[jayc89]

During our retrofit I plan to fit an airtight membrane to the inside of our external, solid brick walls. The idea is for it to be continuous from ground floor slab, through the first floor void up to the roof void. Sealed and taped to internal wall returns where applicable.

 

The detailing of the roof space seems the most fiddly. The shape and size of the roof space means we'll never use it as a habitable room so I'm happy to leave it as a cold roof and save on the cost of heating it. So the membrane + insulation will lay across the first floor ceiling joists. 

 

As we'll be taking one-two rooms at a time, my quandary really is whether I should be attaching the membrane to below the ceiling joists, which would be far easier, but result in a less continuous barrier, requiring more sealing, taping and more potential air leaks. Or, above the ceiling joists, within the roof space itself, which would result in potentially a more continuous barrier, but with more detailing required around chords, webbings, posts etc.

 

The majority of blogs etc that I have seen of retrofit projects seem to include building a room in the attic and therefore a hot roof, which I guess makes this problem easier to solve.

Any hints/tips/guidance on the best approach in my situation?

[AliG]

The easiest way to put an airtight membrane under the loft is to put it under the ceiling joists holding it on with battens. The plasterboard is then connected to the battens and the wiring runs in the space between the battens and plasterboard so that you don't have to try and seal holes in the membrane. You then drape the membrane onto the top edge of the walls and attach it there so you don't have to worry about sealing up around the joist ends.

 

Personally I would not recommend putting membrane on the inside of your exterior walls. I would try to wet plaster them or use some sort of large coat over them if you have insulation going on the inside. The membrane is difficult to attach to blockwork and you will get cold air blowing around behind it.

[jayc89]

4 minutes ago, AliG said:

The easiest way to put an airtight membrane under the loft is to put it under the ceiling joists holding it on with battens. The plasterboard is then connected to the battens and the wiring runs in the space between the battens and plasterboard so that you don't have to try and seal holes in the membrane. You then drape the membrane onto the top edge of the walls and attach it there so you don't have to worry about sealing up around the joist ends.

 

Personally I would not recommend putting membrane on the inside of your exterior walls. I would try to wet plaster them or use some sort of large coat over them if you have insulation going on the inside. The membrane is difficult to attach to blockwork and you will get cold air blowing around behind it.

 

Makes sense. I guess the MVHR ducts would still need to penetrate though, unless you have some pretty deep battens.

 

For the walls, the membrane will be on the warm side of internal wall insulation, attached to battens (with insulation infill) and cross battened for a service void. In theory the insulation itself should provide an airtight barrier, and the ceiling membrane could be sealed and taped to that. the membrane down the walls is really belts and braces.

[AliG]

I think taping the joints in the insulation works better than the membrane.  If cold air gets between the membrane and the insulation it reduces how well it works but is still airtight 

[jayc89]

Any other thoughts on this? In the back of my mind I think running the membrane from above the ceiling joists would provider a better barrier and also solves the airtightness of the landing which I didn't plan to knock to pieces from below. I know it would be a PITA to do it from above though...

[AliG]

You'd have to seal it around the joist and any light fittings. A really difficult job.

 

My only other thought is that it might be better to put the membrane or some other airtightness treatment on the walls with the insulation inside it. Stops the insulation being bypassed by cold air.