Air space above roofing board insulation, why?

Discussion in 'Eco Talk' started by IanMKnight, Dec 2, 2020.

  1. IanMKnight

    IanMKnight Member

    Hello,
    I am converting my single brick garage into a workshop.
    I have insulated the sectional garage door with 40mm poly sheeting.

    Now I am planning the walls or ceiling or both.

    The the ceiling/roof is flat with 200mm joists, TG floorboards then felted/stone chips

    Now i have read and read and everyone says there should be an air gap between the roofing boards and any insulation board to prevent condensation. But if i leave an air space there is bound to be condensation above the insulation that i need to then leave a gap to dry it out which to some extent will defeat the point of insulation.

    However, if I fit insulation boards directly to the roofing boards and seal the edges there is no air space and therefore no air to condense, therefore no condensation!

    Have i missed something?
     
    Jord86 likes this.
  2. Bobby Dazzler

    Bobby Dazzler Active Member

    The space above the insulation should be vented to allow any moisture laden air to be moved to outside.
    If you are omitting the airspace, ensure you position a vpm on the warm side of the insulation to prevent moisture permeating the insulation to the cold surface.
    If use foil backed insulation, tape the joints to seal the whole surface.
     
  3. Bob Rathbone

    Bob Rathbone Screwfix Select

    On cold nights the roof covering will get very cold and any moisture in the air will condense on the inside of the covering, it may even freeze. When temperature rises the airflow will remove the moisture
     
  4. Frutbunn

    Frutbunn Active Member

    Its actually to prevent interstitial condensation in the insulation from water vapour within the room.
     
  5. Bob Rathbone

    Bob Rathbone Screwfix Select

    I believe that is the issue I just described.
     
  6. rajaoyan

    rajaoyan New Member

    That's what I want.
     

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