Is there any safe way to add insulation?

[Hammertime]

Hello! My kitchen has an area of lead covered warm roof:

It was installed 35 years ago so the insulation will have shrunk and developed gaps. And it's only 50mm anyway.

 

I can't think of any safe way of increasing the insulation from below, as this would create a hybrid roof, but thought I'd put it out there in case anyone had some ideas.

 

The foil backed board was removed during the rewire, so I wanted to explore options before just replacing it.

 

A few things I thought of: 

1) original 'bright' idea was to use wood wool boards instead of plasterboard, to give a tiny safe amount of extra insulation, then paint the underside of the lower plywood deck with a liquid vcl to make up for the wood wool board's vapour permeability. As I was finishing painting on the vcl I realised what a stupid mistake this was. Still, in a full renovation you're bound to make a few daft mistakes. 

2) I've got some leftover 50mm Rockwool that I could push between the joists. However, I'm guessing this would allow vapour through but cool down the plywood deck, leading to condensation on the underside. 

3) fit 25mm pir between the joists tight up to the plywood with pu foam adhesive, and seal the edges. But the detailing would have to be perfect, as otherwise I can imagine it would cause big issues. 

 

Thank you for any advice.

[JohnMo]

1 hour ago, Hammertime said:

create a hybrid roof,

You have to be careful with hybrid roofs. Otherwise you end up with condensation issues, as you seem to be aware of.

 

Suspect you already have issues, you have a vapour barrier at plasterboard and above it with bitumen vcl. So any humidity that finds it's way in the roof structure is stuck there.

 

My thoughts are adding 25 or 50mm just isn't worth the effort.

 

If you concerned with shrinkage get a thermal camera and see if you have issues. May need a coke night to see so you have a thermal gradient between inside and outside.

[saveasteading]

Thinking 'aloud' really.

Keep it simple.

1 the pir is still there. It may have shrunk and left odd gaps between boards, but that should be a small proportion, so leave it

2. Insert pir between joists but only 50mm boards. This thickness gives you control in cutting and inserting. It's easy to cut 50mm by handsaw or jigsaw. That should squeeze in reasonably accurately but there will be gaps. These gaps, however are closed spaces so have some insulation value.

3.

Then Insert rockwool 50mm or more to the full depth. Use batts and they will cut easily and also squeeze in tight.

4. no need for foil backing.. it only makes any difference when facing an air gap.

Vcl then plasterboard.

 

I can't See any condensation issues with that but await comments.

 

This all assumes that the lead is sound.