Membrane or not?

[unclebob1]

I have a window bay, which had just plaster board on the inside, and has a broken felt layer and tiles on the outside.

I'm going to insulate the void using ridged foam (celotex or equivalent foiled), should I put in a breathable membrane in then insulation boards and foam or just straight insulation and foam?

Covering will be plaster boards again.

 

[^woody^]

What's happening with the felt?

 

[unclebob1]

At the moment, outside will stay as is, I don't have time to take off the tiles and put in new felt. Most likely in the future I'll remove tiles and felt and render over with ewi

 

[^woody^]

Then a membrane will serve no purpose.

 

[unclebob1]

So just fill with solid insulation and foam is good? Plaster board on top.

 

[Mr Chibs]

I’d infill the stud work with celotex, As thick as studwork, then go over it all again with 25mm or thicker, then plasterboard. This will eliminate cold bridging from the wood.

 

[^woody^]

So just fill with solid insulation and foam is good? Plaster board on top.

Yes. The important thing is to ensure there are no air voids within the wall, and that you form a vapour check on the room side before plasterboarding - foil tape all joints and over timber, foil backed plasterboard or polythene or suchlike

 

[unclebob1]

I will end up some some air voids as the PIR wont be able to be a tight fit on the outer side (taper of wood studs) but will try and fill with foam as best I can before putting the PIR in place, but will foam seal on the inner side completely, so that the insulation boards are within the studs. On top, should I then run DPM around the whole bay as one piece and then standard pb over?

 

[^woody^]

If you can't guarantee no air voids then you risk allowing moisture to condense in there and rot the timbers over time.

Try harder

 

[unclebob1]

If you can't guarantee no air voids then you risk allowing moisture to condense in there and rot the timbers over time.

Try harder

does it matter if its foam filled or PIR? I should be able to get the foam gun nozzle in, but a solid piece wont go in as the opening on the inside wont be wide enough! (the two adjacent posts are not parallel)

 

[^woody^]

Foam can push the insulation board out and created gaps or uneven filling. Its OK for joints.

I can't really understand what the problem is, as a board or boards of insulation can be cut to fit between those timbers and foam is not even necessary.

 

[unclebob1]

My bad woody - I'm making a mountain out of a mole hill - even though i've been looking at the studs for most of yesterday, I just assumed they were not parallel. Just had a look and realised they are so I can cut to exact size and push in the gap...