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roof insulaton / ventilation query

  • 09-12-2008 10:22am
    #1
    sandt
    Registered Users, Registered Users 2 Posts: 241 ✭✭


    hi,
    i recently had guys in to install 100mm kinspan tp10 insualtion on the slopes of the ceilings upstairs of our new build. when i checked out the work i noticed they hadnt put anything in below the end of where the board meets the wallplate.
    i.e. there is no overlapping between wall & roof insulation.

    when i asked him about this he said he didnt want to block the 50mm air gap because there was no cross flow rafter ventilator installed.

    so is there any way now to try & insulate this area??

    also now do i need to put up a vapor control layer (500 gauge polythene??) over this insulation before they install the 37.5mm composite plasterboard??

    thanks.......


Comments

  • #2
    sandt
    Registered Users, Registered Users 2 Posts: 241 ✭✭sandt


    Bump.


    anyone........?


  • #3
    RKQ
    Registered Users, Registered Users 2 Posts: 2,292 ✭✭✭RKQ


    I would insulate the roof / external wall junction while maintaining a 50mm air gap to the underside of the felt. Otherwise you will experience draughts during high winds! Blocking the air gap can result in dry rot (unless you have a Warm roof)

    An experienced commercial insulation installer should know this? Has he / she fallen from the sky? Air tightness became law in July, it has been used in self build for a number of years.

    Rockwool, Xtratherm, Knauf and Kingspan web sites show standard detail drawings for insulation this junction. It is very important to get it right.

    A vapour barrier, even 500 guage plastic, should be placed on the WARM side of the insulation, then slabbed over. Your use of 37.5mm composite plasterboard changes good practice - so consult the manufacturers Technical Dept for advice.


  • #4
    sandt
    Registered Users, Registered Users 2 Posts: 241 ✭✭sandt


    thanks for the reply RKQ
    ok am i right in thinking cold roof construction is one where the insulation is between the rafters & in some cases under them as well?
    if this is the case then this is what i have and therefore i will need to maintain the mm air gap throughout? or can i get it lower because i have used 'breathable felt'?

    regarding 500gauge plastic as a vapor control layer, does the foil on the back of the composite plasterbaord not do this job?
    in what way does the use of 37.5mm composite board change good practice?
    as this hasnt been put up yet what would you recommend to do as i want to make sure its done right.

    also regarding plasterboard, does anyone know if putting foil backed plasterboard on the ground floor ceilings would help reflect any heat back to the ground floor? as im worried of heat rising up and the conc. slabs just soaking it up & not releasing it, so id like to keep as much on the ground floor as possible.

    thanks.


  • #5
    RKQ
    Registered Users, Registered Users 2 Posts: 2,292 ✭✭✭RKQ


    sandt wrote: »
    ok am i right in thinking cold roof construction is one where the insulation is between the rafters & in some cases under them as well?
    Yes thats a cold roof - insulation between & under rafters.
    sandt wrote: »
    if this is the case then this is what i have and therefore i will need to maintain the mm air gap throughout? or can i get it lower because i have used 'breathable felt'?
    Consult manufacturers IAB cert. Current thinking now is to maintain an air gap even with breathable felt but each manufacturer is different.
    sandt wrote: »
    regarding 500gauge plastic as a vapor control layer, does the foil on the back of the composite plasterbaord not do this job?
    Technically no! a vapour barrier is an air tight membrane with lapped joints, the perimeter of each board is a potential gap and therefore a possible leak! How did seal the gap between the boards?
    sandt wrote: »
    in what way does the use of 37.5mm composite board change good practice?
    Good practice is to have the vapour barrier on the warm side of all the insulation. If the composite is fitted over the vp then some insulation is over the vp!
    sandt wrote: »
    also regarding plasterboard, does anyone know if putting foil backed plasterboard on the ground floor ceilings would help reflect any heat back to the ground floor? as im worried of heat rising up and the conc. slabs just soaking it up & not releasing it, so id like to keep as much on the ground floor as possible.
    Others will have views on this. In theory the foil reflects if it has an air gap - so alot of foil insulation manufacturers recommend an air gap. There wouldn't be an air gap in a timber joist ceiling so it probably wouldn't reflect much heat. But I'm assuming there will be a gap in yours - if you are constructing a suspended ceiling under conc floor. I still have an open mind on foil insulation / reflection, time will tell and its too early to say yet.

    Conc will absorb heat during the day and radiate it out at night, so I wouldn't be too worried.


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