Advice - add PIR insulation between rafters inside kitchen and plywood over the top

[robbotpommp]

Hi All,

We have a kitchen which is absolutely freezing in the winter, because there is no insulation in the roof.

Please see attached picture, we have rafters which are 42mm deep, with plywood, a membrane and the on slate on top of those rafters (no insulation).

I would like to buy 40mm PIR Insulation to place (stick) between the rafters and then use playwood to cover up the whole ceiling and the use builders caulk to fill the gaps in the plywood, paint over - and Bob could be my proverbial uncle.

EXCEPT - I've read in some places about leaving a gap between the PIR Insulation - does anyobdy know about why this is required?

As I am adding this into a kitchen, there will of course be condensation when there is cooking - if I ensure that the plywood gaps are properly filled, is this enough, or am deluding myself and should be looking at a differemt solution here?

Thanks for anybody's knowledgeable advice

 

[tony1851]

What you are proposing would be a recipe for disaster - don't attempt it.
Water vapour will get through the inevitable gaps in the ceiling (water vapour is very penetrative and there's always plenty in a kitchen), will rise up through the gaps between the PIR and rafters, then condense on the upper (cold) face of the insulation and run down to the wall causing bad staining.
It will also encourage rot in the timbers.
The only way to avoid that would be to ensure adequate ventilation space above the PIR (which you won't have there) so its more-or less a non-starter.
In the absence of roof ventilation, the ultimate solution is to have a 'warm' roof with the insulation on top of the deck, and battens, counterbattens and tiles fitted back, but that would be a whole new project.
Hopefully someone may be able to suggest something to help, but all I could advise is NOT to do it as you suggest!

 

[danfre]

Batten out existing rafters to give more depth for insulation and ventilation

 

[23vc]

I’d be tempted to go 75mm+ PIR across the rafters (nothing in between them as they’re too shallow) then 19mm battens fixed with 100mm+ screws then vapour barrier and plasterboard and skim. The ply and caulk thing sounds a bit rubbish