Insultaing Bay Window Ceiling

[nordberg73]

Hi,

We have a large bay window - approx 10ft wide x 4ft deep. The ceiling in the bay is the original lath a plaster and as far as I can tell (haven't taken it down yet), there is no insulation in the void between the ceiling and the lead roof.

I am getting the whole room skimmed next week and it seems like a good opportunity to get some kind of insulation up there and I can get it boarded and skimmed whilst the plasterers are in. What are my options?
Obviously I'm aware I can just bung some rockwool type stuff in the void, or is there something else that may be a better solution?

Thanks.

 

[freddiemercurystwin]

Pack the whole void with Rockwool, leave no air gaps.

All assuming you don't find the joists are fecked that is .......

 

[nordberg73]

Thanks. As simple as that...

I was thinking Kingspan or similar, but if Rockwool does the job, then fine.

 

[freddiemercurystwin]

Kingspan would be better but will be a little more pricey and arguably more difficult.

 

[nordberg73]

OK - pulled the ceiling down this morning.....

There's a 9cm void between the ceiling and the roof. If I were to use Kingspan, is it OK to leave an air gap? The Kingspan I have is 5cm thick.... Can I just NoNails the Kingspan to the roof?

 

[ajrobb]

You don't want any vented voids between the Kingspan and new plasterboard. I think it would be better to have the Kingspan level with the bottom of the joists. If that is a pain, insulation clips might help.

Better still would be to pack the void above the PIR (Kingspan) with mineral wool.

You'll need a decent vapour barrier too. Small gaps between PIR and joists will allow moisture through to condense above the insulation. A couple of coats of drywall sealer or acrylic paint should do.

 

[freddiemercurystwin]

If you must have a void it needs to be beneath the insulation, you must not leave a void above, without any ventilation will leave the timbers highly susceptible to rot. Why not just get some 25mm Kingspan to completeyly fill the void though? If you're gonna go through all this just to leave a void ..........

Your insulation should be fiction fitted in the void, seal any gaps with expanding foam, if any are a bit loose you can tap some panels pins in and tap them up to hold the insulation up. The clips are a waste. The plasterboard will hold it all up there anyway.

Use foil backed plasterboard, the foil working as a vapour barrier.

If you're gonna do a job properly ............

 

[nordberg73]

Thanks as ever...

I ended up buying a roll of Knauf Carbon Zero wool and bunging that up in the void.

I was concerned there would be an air gap no matter how I played it with the Kingspan so took the safe option.

Hopefully we'll feel the benefit come winter....

 

[ajrobb]

Don't forget your vapour barrier.

 

[wavetrain]

I reckon he forgot the VB

 

[nordberg73]

Sorry to disappoint...but I got foil backed board put up

 

[wavetrain]

Thank goodness, we can all sleep soundly now!