How Should Multi-Layer Reflective Foil Insulation be Applied?

[paul10june]

I live in a 1980's bungalow with internal pitched ceilings.
The roof/ceiling is completely uninsulated.
Its structure is external tile mounted on battens with felt under, with the battens applied to jousts.
On the underside of the jousts it is planked.
Therefore, in effect, there is an airgap between ceiling planking and tiles of 100mm.
I know it can be important to have airflow around these reflective foils to avoid concensation, but a builde has told me I can simply pin the foil to the underside of my ceiling planking, batten, and then apply additional planking (uPVC) on battens.
Thois would mean my ceiling would comprise:
uPVC planking attached to battens on top of the foil, existing wooden planling, air gap of 100mm, then felt and tiles.
Is this going to be OK?

[The Builder]

Yes, certainly Paul .....................

http://www.just-insulation.com/tlx_silver.html

[paul10june]

Thanks. I had looked there and been a little confused I guess. it seems like a combination of the foil and kingspan would be the best. Paul

[buildersmate]

Multifoil is most useful when you have limited space (width) in which to install decent insulation. Conventional felt can't breathe through it, so it requires a little airspace under it.

[paul10june]

Thanks to you both. Have decided to go ahead and install. I will make sure there is a bot of airspace around the foil. Am going to try in the kitchen to see how it works, then if succesful do the rest of the house.
Paul

[The Builder]

Yes, you have enough air gap between the felt and the existing ceiling board to take care of the non-breathable felt on the roof.
The air gap between the foil and the new board (a batten's thickness - 25mm) is essential for the foil to work properly. It doesn't do the job on its own. The space is also insulating. So.......... it's essential that the foil is taped (with silver tape) at ALL joints, laps, perimeters etc.