Porch ceiling upgrade

[BrianClarke]

I have an unenclosed porch integral to the house, such that the upstairs landing floor extends over the top of it. The porch ceiling is lath and plaster and is a source of draughts under the upstairs floor.
I want to fasten new boarding on top of the existing lath and plaster (screwed to existing joists) which will both provide a secure mounting for a porch light and greatly improve insulation.
I have thought of using plywood for strength with some form of insulating board on top.
Any suggestions appreciated, with regard to materials, combination of materials and finish (paint, plaster or both?)

 

[PrenticeBoyofDerry]

I would rip the lath/plaster down.
Insulate in the void, then plasterboard.

 

[BrianClarke]

I would rip the lath/plaster down.
Insulate in the void, then plasterboard.

That was my first idea but the void is very busy with electrical cables, so that is why I am now thinking of fixing to the underside.

 

[theprinceofdarkness]

I would lift the floorboards above the void and stuff it FULL of glassfibre. The choice of finish is interesting because it will get slightly damp, so it rules out hardboard, MDF, ordinary ply. I had some thing like that and I still had the original Victorian bead and butt boarding. You can get plastic mouldings, like used for sofit "boards", actually short lengths of hollow plastic moulding, that slip together to look like a run of boarding with grooves at right angles to its length, that what I would use.
Frank

 

[Alastairreid]

Screw insulated plasterboard to existing..tape, fill then paint....Here.

 

[BrianClarke]

I would lift the floorboards above the void and stuff it FULL of glassfibre. The choice of finish is interesting because it will get slightly damp, so it rules out hardboard, MDF, ordinary ply. I had some thing like that and I still had the original Victorian bead and butt boarding. You can get plastic mouldings, like used for sofit "boards", actually short lengths of hollow plastic moulding, that slip together to look like a run of boarding with grooves at right angles to its length, that what I would use.
Frank

Thanks for the response. The plastic covering is something I had not considered, but I want to avoid packing the floor space above because it is so busy with cables.

 

[BrianClarke]

Screw insulated plasterboard to existing..tape, fill then paint....Here.

Thanks for the response. This is interesting because I had come across this material before but was not sure if it was appropriate for this job. I may follow this one up but would still first fit a sheet of outdoor ply to provide a firm surface to mount a light.
Not sure how to finish off the plasterboard though --fill screw holes and paint or plaster as with internal ceilings.

 

[Alastairreid]

You can either get it skimmed or tape the joints and use easi-fill then sand down and paint.