Insulating between between rafters above bathroom

[GAC]

Heya, I want to insulate the ceiling in the bathroom, which is also the roof at the top of the building. Wondering what's the best option or if there's any special considerations for this scenario? Does there need to be a vapour barrier? (or use foil backed plasterboard?)

The rafters are 100mm deep. It's a victorian building. Not sure if this is relevant but all the walls are interal walls, apart from two - one which is external brick which I've insulated with PIR board and another small internal brick wall.


Thanks in advance!

 

[noseall]

Heya, I want to insulate the ceiling in the bathroom, which is also the roof at the top of the building. Wondering what's the best option or if there's any special considerations for this scenario? Does there need to be a vapour barrier? (or use foil backed plasterboard?)

The rafters are 100mm deep. It's a victorian building. Not sure if this is relevant but all the walls are interal walls, apart from two - one which is external brick which I've insulated with PIR board and another small internal brick wall.


Thanks in advance!

Vapour check should always be considered at the point where warm/moist meets cold. Duplex boards - yes. Foil tape over PIR joints - yes.

 

[GAC]

Vapour check should always be considered at the point where warm/moist meets cold. Duplex boards - yes. Foil tape over PIR joints - yes.

Thanks!

Another problem I have just thought of is the one downlight there is and mainting the vapour barrier there....

The only solution I can seem to find online is to cut a square out of the 50mm PIR, foil tape some 25mm PIR over the back of the hole, so that I have a 50mm deep hole to fit a slim downlight in. Is this an acceptable solution/is there a better way?

 

[Alastairreid]

Is it a flat roof over your bathroom and do you have an extractor?

 

[GAC]

Is it a flat roof over your bathroom and do you have an extractor?

It's a pitched roof, and quite low headheight. only 100mm depth in the rafters.

I'm installing an inline extractor fan which is fairly overspeced for the size of the space.

 

[Alastairreid]

As long as you have an extractor and decent ventilation in your attic any vapour check would be belt and braces. (On the ceiling)

 

[JohnD]

If you have a hole in the bathroom ceiling, water vapour will get up there. You can put a hood over the downlighter to reduce it, but the void will need to be ventilated.

 

[GAC]

As long as you have an extractor and decent ventilation in your attic any vapour check would be belt and braces. (On the ceiling)

If you have a hole in the bathroom ceiling, water vapour will get up there. You can put a hood over the downlighter to reduce it, but the void will need to be ventilated.

Okay thank you both! Fairly certain there's good ventillation between the rafters, maybe a bit too drafty infact!

I'll use 50mm PIR so there's a 50mm air gap behind it.

Thanks for your advice!