Bathroom ceiling insulation

[Super_Paulie]

my bathroom ceiling (first floor) is a mess, looks like its been a separate toilet and bathroom at one point and then converted into a toilet, bathroom and corridor. Now its all 1 room the ceilings are all over the place so ive lowered it down with C16 to get me a level surface. Its got a loft above with fibreglass insulation between, plastering lath is still here, and its going to stay there as well as its manky above it.

 

My question is, should i fill this newly created dropped-ceiling void with tight fitting PIR and tape the timbers for my vapor control? The C16 is 38mm but packed out in places, often adding 25mm here and there to compensate the old ceiling, its that wonky. I could get PIR in there but it will leave dips and voids above it in places, any thoughts on mixed materials as i could pack it with 25mm fibreglass and then 25mm PIR level with the new, lowered ceiling timbers. Ceiling has a vent with an inline fan in the loft existing through the roof, room is small, 2.5m sq.

[Nickfromwales]

Rockwool fill, 40mm PIR, foil tape, then PB. 
 

You’ll need to manage the holes for lighting, or use a surface mount / pendant. 

[Super_Paulie]

Annoyingly you can't get 40mm from the sheds, and delivery is mental costly from insulation specialists, especially for like 2 sheets only. I have some 50mm but don't think cutting that sideways would be very easy...

[Nickfromwales]

15 hours ago, Super_Paulie said:

Annoyingly you can't get 40mm from the sheds, and delivery is mental costly from insulation specialists, especially for like 2 sheets only. I have some 50mm but don't think cutting that sideways would be very easy...

Work out the sizes you need, go to a merchant, and cut up an 8x4 in their yard. 

[Super_Paulie]

On 30/04/2026 at 13:58, Nickfromwales said:

Work out the sizes you need, go to a merchant, and cut up an 8x4 in their yard. 

i'll see what we have around here, not sure i can fit any in my 2 seater though no matter how small i cut it...

I assume the secondary foil layer on the back is as vital as the one on the front, so hacking 10mm off the 50mm i have is a no-go? I have like 4 full boards left over from all my build stuff that im just stuck with, would be useful to use it up here.

[Nickfromwales]

5 hours ago, Super_Paulie said:

i'll see what we have around here, not sure i can fit any in my 2 seater though no matter how small i cut it...

I assume the secondary foil layer on the back is as vital as the one on the front, so hacking 10mm off the 50mm i have is a no-go? I have like 4 full boards left over from all my build stuff that im just stuck with, would be useful to use it up here.

Cant you get the 50mm in there? Recess it where necessary etc. 

[Super_Paulie]

1 hour ago, Nickfromwales said:

Cant you get the 50mm in there? Recess it where necessary etc. 

id have to take the plaster and lath off in places. I took it all off in the vaulted section and quite frankly im never doing that again in my life.

Ive tested cutting the 50mm between 2 lengths of timber and i can reduce the depth, but the question is should i, as id be losing the foil on the "inner" side.

[Nickfromwales]

2 hours ago, Super_Paulie said:

id have to take the plaster and lath off in places. I took it all off in the vaulted section and quite frankly im never doing that again in my life.

Ive tested cutting the 50mm between 2 lengths of timber and i can reduce the depth, but the question is should i, as id be losing the foil on the "inner" side.

I can't see any reason it would be a problem on the back, but the front is usually taped (with foil tape) across boards and timbers to create the vapour barrier; this negates installing a dedicated VB.

 

Working in a lath and plaster filled house currently (for my sins methinks) and I wholeheartedly agree.

[Super_Paulie]

12 minutes ago, Nickfromwales said:

I can't see any reason it would be a problem on the back, but the front is usually taped (with foil tape) across boards and timbers to create the vapour barrier; this negates installing a dedicated VB.

 

Working in a lath and plaster filled house currently (for my sins methinks) and I wholeheartedly agree.

 

The amount of debris you can generate per sqM is insane. I did the vaulted side to gain back the space as it was curved top and bottom and like 80mm thick and filled with rusted mesh. Whole thing was brutal, won't be doing that again.