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Attic Insulation - sanity check

  • 07-02-2023 12:21am
    Registered Users Posts: 164 ✭✭

    Hello all,

    Long time lurker in this forum - always a great read.

    Starting to insulate my attic. Currently has about 60-80mm of fibreglass insulation which doesn’t run flush with the joists.

    Ive purchased some Isover Metac 100mm rolls to top this up.

    Some queries:

    1. Sealing electrical cable exits into below rooms: caulk fine or a bit of insulation stuffed?
    2. Spotlights in bathroom: these need to be presumably covered by some sort of fireproof cap?
    3. If I have a 100mm roll, about 4ft tall, do I need to cut to fit the rafter? Or will rolling out over the rafters be fine?
    4. Best way to get air to soffits to circulate? Waste pipe?
    5. Loft Legs or 2x4s for flooring standoffs

    Thank you!

    Post edited by Exodus 1811 on


  • Registered Users Posts: 2,823 ✭✭✭MicktheMan

    I think you mean joists on the attic floor rather than rafters? Rafters are the sloped timbers up to the ridge.

    1. If a single cable then used air tight tape to seal to the plasterboard ensuring the surface is clean etc. If more than 1 cable then compressed earthwool (fibre)
    2. Yes, use an appropriately sized inverted terracotta pot sealed to the plasterboard ceiling with airtight mastic generously applied. Notch the pot to allow for the electrical cable.
    3. Between the joists. Basically there should not be any gaps between the ceiling plasterboard and the insulation layer and if there is a second insulation layer then no gaps there either.
    4. Use corrugated plastic stapled between each rafter section to create a 50mm ventilation channel between the top of the insulation and the roof felt.
    5. Either. Or you could notch out short pieces of 4x2 and make your own legs.

  • Registered Users Posts: 1,383 ✭✭✭dathi

    for awkward down light that are close to the joist as in your photo screw a scrap of plaster board to the joist . and then screw together 3 pieces of ply the same height as the joist to make a 3 sided box, line it with plaster board and screw to joist to make box. then make lid out of ply and plaster board and tape the joints,

  • Registered Users Posts: 164 ✭✭Exodus 1811

    Correct, new to this building thing.

    1. Presumably no particular type of tape needed here, foil tape or similar would do? Earthwool, do you mean the insulation material I have?
    2. I was thinking of those downlight covers, as there's only 4 spots and they're in problematic locations as daithi mentioned.
    3. With reference to cutting and fitting in between the joists to fill the 20mm gap, will the material be plyable enough to fill this gap without needing to cut the material? I would like to reduce the amount of cuts to keep the material in a continuous form for as much of the area as possible. The air in any possible gaps will be static presumably?
    4. Makes sense, thank you.
    5. Loft Legs seem pretty cost effective

    Daithi, if the rectangular downlight covers are too tight to the joist I'll use your suggestion. The plasterboard is the fire proof element I assume?

    Post edited by Exodus 1811 on

  • Registered Users Posts: 164 ✭✭Exodus 1811

    To clarify on point 3

  • Registered Users Posts: 1,383 ✭✭✭dathi

    1. Presumably no particular type of tape needed here, foil tape or similar would do? Earthwool, do you mean the insulation material I have?

    no we are talking about airtightness tape google gerband or pro clima and you will see the tape we are talking about

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  • Registered Users Posts: 164 ✭✭Exodus 1811

    Found it, thanks!

    Should I staple the plastic to all rafters or just ones with vents in the soffits?

  • Registered Users Posts: 2,823 ✭✭✭MicktheMan

    As many as you can reasonably get access to.