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Attic Insulation: Ventilation, airtightness

  • 18-01-2009 09:43PM
    #1
    BenDave
    Closed Accounts Posts: 6 ✭


    Hello all,

    I know there have been many posts in the past regarding this but they did not quite answer my questions:

    I intend to re insulate my attic which is converted as a bedroom.
    This a 100 years old mid terraced house, due to monetary constraints I will not be able to do major works like replacing the existing felt with a breathable membrane or doing a hot roof insulation.

    From the forum I gathered that a 50mm gap needs to be left between the insulation between the rafters and the felt. Since The rafters are 120 mm deep, that leaves 60 mm for the insulation.

    - What are the best performing insulation boards for this I have very limmited space ?, i will most likely need to use insulation with a foil inside, or multi foil.

    - What kind of slab should i use to cover the insulation ?

    - Now, what else do i need to do to ensure proper ventilation ? leaving a 50mm gap is not enough, air should be able to circulate. Do i need to drill holes, make vents ?

    - About air tightness, What do i need to do to ensure air tightness ? taping all the joints between the rafters and the boards ? Is air tightness in the attic crucial ? This is an old house which will never reach for A energy rating after all :rolleyes:

    - How should i insulate the gable walls ? The house is terraced but the neigbours don't seem to use / heat their attic

    - I will also insulate under the floor boards in the attic. Is rockwool or fibreglass a poor option because of sagging ? I could use blown insulatiob (for instance WarmCel http://www.excelfibre.com/building/products3.html instead which is supposed to be fantastic.)

    I just realized I have asked for a lot...............

    Many thanks in advance !
    Ben


Comments

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


    Quite a lot to answer here! I'll start so hopefully others will contribute.
    BenDave wrote: »
    - What are the best performing insulation boards for this I have very limmited space ?, i will most likely need to use insulation with a foil inside, or multi foil.

    Sloping ceiling with limited depth then use Xtratherm/ Kingspan high density foil board, cut to friction fit between rafter (with circular saw). Check out their web pages for 3d views. 75mm thick minimum, so fix battens to underside of sloping rafters, to increase depth. 25mm x 50mm batten
    BenDave wrote: »
    - What kind of slab should i use to cover the insulation ?
    Fix 25 - 50mm thick foil density board over rafter (perpendicular) or counter batten internally & insulate between battens, then fix breather vapour barrier, (or 500 guage plastic) tape overlapped joints, then fit 9.5mm gypsum slab - tape & fill or skim plaster.
    BenDave wrote: »
    - Now, what else do i need to do to ensure proper ventilation ? leaving a 50mm gap is not enough, air should be able to circulate. Do i need to drill holes, make vents ?
    Install vent slates and ridge vents if possible. See manufacturers instruction. Tiles can be done as DIY project, slates IMO takes skill.
    BenDave wrote: »
    - About air tightness, What do i need to do to ensure air tightness ? taping all the joints between the rafters and the boards ? Is air tightness in the attic crucial ? This is an old house which will never reach for A energy rating after all :rolleyes:

    Air tightness is critical in old buildings, especially in an attic. Well worth the effort. The less draughts the better. Every draught is costing you money. Attics are hard to heat because you are only usually 200mm from outside! Frost builds up on a roof.
    BenDave wrote: »
    - How should i insulate the gable walls ? The house is terraced but the neigbours don't seem to use / heat their attic
    Dry line with 75mm density foil insulation. There are a few different fixing methods. Why heat your neighbours attic?
    BenDave wrote: »
    - I will also insulate under the floor boards in the attic. Is rockwool or fibreglass a poor option because of sagging ? I could use blown insulatiob (for instance WarmCel http://www.excelfibre.com/building/products3.html instead which is supposed to be fantastic.)
    Rockwool won't sag when installed vertically. Sagging isn't really a problem in floors. Fibre glass is the cheapest option. Also good for sound proofing.


  • #3
    BenDave
    Closed Accounts Posts: 6 ✭BenDave


    RKQ, thanks for this, this did give me a really good head start...

    Ben


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