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Insulating cathedral ceiling with foilboard - air gaps question

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  1. #1
    Join Date
    Jun 2020

    Default Insulating cathedral ceiling with foilboard - air gaps question

    Hi everyone,

    I have a question around insulating my ceiling I hope you can help.

    I have a cathedral ceiling 4x 6m with gypock fixed directly to rafters. I'm currently installing 35mm x70mm batterns at 450mm spacing under old gyprock with packers to set it flat to take new mdf vj panel 100 boards from easycraft.
    While doing this I decided I'd take the opportunity to add some insulation as the current ceiling has none.

    I plan to use to 20mm foilboard from foilboard fit tightly between each battern, silicones to seal for a closed air tight install.

    Talking to the guys at foil board its important to have a reflective air gap in order for the foil board to do its job. I'm trying to understand if the old gyprock is helping or hindering this situation.
    I have conflicting messages as some of my reading says
    A- that that unventelated reflective airspace between foilboard and old gyprock is a good thing which I'm getting with gyprock there ( although not large, around 10mm from battern and 20mm worth of shim between battern and old gyprock).
    B- on the other hand having the gyprock there reduced the depth of the reflective airspace which may considered deeper if I were to go round and cut a bunch of hole in it. Obviously not to keen on doing such a thing with the mess for starterS.

    Keen to get anyone's thoughts or experience on this one.


  2. #2
    4K Club Member
    Join Date
    Nov 2009


    Plaster underneath the existing rafters?
    So roofing then airspace then plaster?
    How much insulation do you need? How much do you want?
    Is your budget flexible to do the maximum job?
    A quick sketch would help but you only get the maximum benefit with that reflective airspace.
    Best would probably be plaster with Foilboard directly against it then batten then a second layer of Foilboard then the new ceiling
    "A big boy did it and ran away"

    Legal disclaimer denying responsibility to be inserted here.

  3. #3
    2K Club Member
    Join Date
    Dec 2009


    You need to provide the recommended air gap on each side of the foilboard to get the advertised R value.

    If you can get this without removing the plaster, then great. Could you use deeper battens?

    The Foilboard tech people know their stuff (and were very helpful to me with my similar questions about a raked ceiling). If you can’t follow their recommendations exactly, then ask their advice on the next best option and the resulting Rvalue.

    If you use foilboard without the recommended air gap on both sides you won’t be getting the value you are paying for.

    Does your roof have reflective sarking?

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