How to insulate this roof / best way.

Discussion in 'Builders' Talk' started by Dboyda, Apr 7, 2006.

  1. Dboyda

    Dboyda New Member

    The most used room in my house is the converted garage. Unfortuantely for me it was drylined before i moved into the house but they didnt bother to insulate the flat roof and ive only just discovered this a few days ago after 2 cold years.

    Whats the easiest/cost effective method to insulate the room? I was considering putting up thermal boards but they cost so much and another option was to nail battens to the roof, put up the plasterboard and fill the cavity with that roll out insulation wool.

    Any advice would be appreciated.

    Dave
  2. jonah.

    jonah. New Member

    I would say the easiest and most cost effective is quilt insulation between the existing joists (I'm assuming its at flat roof)

    Tis obviously requires removal of the existing ceiling.

    The most effective way would be to install extruded insulation (Celotex / Kingspan or equal) between and under the joists in two layes

    The problem with both of the above is that it remains a 'cold roof' or 'modified warm roof' type and therefore requires ventilation above the insulation. This should be checked out.

    There are membranes available of multiple layers of felt and foil that perform well, however they do not have BBA Certification yet in the UK and therefore some building Control departments will not accept them.

    If the flat roof is in poor condition it may be prudent to do the two jobs in one go. The insulation can then be placed over the joists making a 'warm roof' type not requiring the ventilation.

    I would recommend putting a vapour control layer back in your ceiling. Either foil backed plasterboard or 500 gauge polythene.

    Cheers - jonah
  3. building control

    building control New Member

    Lay board insulation bonded to the top of the roof and refelt,much easier.
  4. trench

    trench New Member

    Would agree with BC. There are ventilation issues with insulation fitted from underneath. Use kinspan or celotex to form a warm deck roof.
    However, this will be £££ as you will need to strip the roof back to some extent then felt over the insulation boards.

    Cheaper version is to use insulation between the roof timbers (as already suggested) then do your best to stop condensation occuring within the insulation layers by fitting a vapour check behind the plasterboard and perimeter seasling the ceiling with mastic.
  5. Mark DIY

    Mark DIY New Member

    What about removing a facia board and feeding some loose fill insulation in between the ceiling joists but still leaving a gap above for ventilation.
  6. jonah.

    jonah. New Member

    How would you do that over say a 7 foot span in a say 6" joist depth ensuring an inch ventilation gap (or 2" cant remember what the reg is)

    Cheers - Jonah
  7. amateurbuilder

    amateurbuilder New Member

    Put in as thick Celotex or Kingspan as possible between the joist leaving minimum 50mm gap above vented to outside. Eg 100mm if the joists are 150mm. Put 25mm sheets of the stuff under the joists. Secure with 38 x 25 battens nailed at right angles to joists and set at correct centres for plasterboard. Best prctice to use vapour barrier under this before fitting platerboards but layers of insulation will act as fairly efficient barrier on their own.
    Screw plasterboard to battens.
    Variations include using insulation backed plasterboard and foil backed plasterboard but the above has transformed a room for me and will easily conform with building regs if needed.
  8. amateurbuilder

    amateurbuilder New Member

    Put in as thick Celotex or Kingspan as possible between the joist leaving minimum 50mm gap above vented to outside. Eg 100mm if the joists are 150mm. Put 25mm sheets of the stuff under the joists. Secure with 38 x 25 battens nailed at right angles to joists and set at correct centres for plasterboard. Best practice to use vapour barrier under this before fitting plasterboards but layers of insulation will act as fairly efficient barrier on their own.
    Screw plasterboard to battens.
    Variations include using insulation backed plasterboard and foil backed plasterboard but the above has transformed a room for me and will easily conform with building regs if needed.
  9. trench

    trench New Member

    Dave
    If you retro-insulate from underneath you need to ventilate above it. By ventilation I mean proper cross ventilation i.e. openings at opposite ends (just leaving a 50mm gap above the insulation is not enough). Easy enough to achieve with pitched roofs to loft conversions using tile and ridge vents but harder with an existing flat roof.
    Warm deck roofs (insulation placed above the deck)need no ventilation, but not a DIY job as you will need someone to work with hot bitumen. Bonus with this method is no loss of head room.
    Get a quote from a roofer for the job then cost out against using layers of celotex, battens and vapour checks (plus lost headroom and weekends)
  10. Lambfoot

    Lambfoot New Member

    I have an identical problem to Dboya. Ventilation of the outboard end of the joists is not a problem. (Holes in Fascia boards!) Would holes drilled in the joists at the inboard end, thus providing airflow between joists, be possible?
  11. Renniee

    Renniee New Member

    A flat roof has to be done from the out side of the house, i don't think i quit understand what you mean. Is it going to be a rubber roof? We own a roofing business and that is what hubby is doing today a flat roof (rubber roof). It consist of repairing or replacing the plywood and gluing down rubber to protect the plywood..
  12. Renniee

    Renniee New Member

    You need to be very carefull about insulating a roof from the inside. If you do not install a high quality vapour barrier between the insulation and your room, you will allow moist air to condense in the roof structure (timbers?) resulting in rot and decay. you will aslo get staining on the ceiling from condensate dripping. This all happens because you insulate the roof structure from the warmth of your house and it becomes a cold body that often reaches 'dew point' temperatures (temp at which moisture will condense on the structure).

    By insulating on the top of the roof, you keep the structure 'warm', thus preventing condensation and decay.

    As vapor barriers are notoriously difficult to install and get an effective seal, I would personally always opt for insulation on the outside face of the flat roof.

    Good luck
    *

    [Edited by: admin]
  13. Mr GrimNasty

    Mr GrimNasty Active Member

    If you're going to advertise, why don't you reply to someone who didn't sign on and off as a 1 post wonder, almost 18 months ago!
  14. ian anderson

    ian anderson New Member

    Heh!! True.

    Thats what you get when you 'outsource' your 'link building program', one way links to AMERICAN based contractors on a British based forum.

    Guess that you get what you pay for!

    Well spotted Mr GN.

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