Overplating Thermal

[seasickstevie]

My (very cold) bathroom's got a (partly) pitched ceiling.

A while back, I insulated between the rafters, plated with duplex, skimmed & painted.

Problem is, the rafters 'show through' as dark stripes. This is caused, I think, by condensation.

The ceiling is low, so I don't want to reduce the headroom more than is necessary.

If I over-plated with Gyproc Thermaline (Basic, Plus, Super or Platinum) (or similar), could it be fixed directly over the existing plasterboard, or do I need to leave a small cavity?

Thanks in advance

 

[noseall]

The dark lines are a result of cold bridging and condensation. Over-boarding with a foam board will counter this.

70mm celotex or kingspan is the recommended minimum though a thinner insulated plaster board will go some way to avoiding condensation.

 

[JohnD]

you've already insulated in the "loft" between the timbers.

Are you sure the bathroom is dry and free from condensation? have you got an extractor fan that comes on whenever the bathroom is occupied (best way is via the light switch)?

and you are sure the loft is well ventilated so the timbers are not damp?

As you have a sloping ceiling I suppose there is no scope to flop more quilt over the top of the timbers.

dust shadowing from cold ceiling timbers are usually quite faint and slow to show, so I am thinking about the effects on condensation making it worse. Cigarette smoking, or candle smoke, will make it much worse.

 

[seasickstevie]

Thanks for the replies.

The rafters are 4" x 3" so effectively full of quilt, which isn't the most efficient. If I re-roof, I'll replace with better stuff from above.

This means there' no space for ventilation in the pitched section of ceiling, but it's only a small area. The horizontal part of the ceiling is well insulated and well ventilated.

The window in the bathroom is left slightly open nearly always (it's on the second floor, so not likely to get a burglar up that high) and there's an extractor!

No heating in the bathroom but I'm going to put a radiator in there soon which I'm sure will improve it.

I'd just like to eliminate the cold-bridging that noseall mentions without losing too much head hight.

Think I might try a 25mm Kingspan directly over the existing sloping part.