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Timber Frame Old Insulation - To Swap out or leave alone?

  • 22-09-2022 8:11pm
    Registered Users Posts: 127 ✭✭


    I have a renovation project running currently and unsure on a decision. It is an early 1980's timber frame house. The insulation is approx 90mm of fibreglass wool. We have already agreed to a 62mm insulated plasterboard on the inner wall but now looking at approx €6800 if we want to go into the walls, strip out the fibreglass and replace it with 80mm rigid board insulation.

    Can anyone shed some light on whether there would be a noticeable difference in performance for the newer insulation to warrant the switch? Tried finding U-Values for the fibreglass but got bogged down.


  • Registered Users Posts: 757 ✭✭✭C. Eastwood

    I presume that the studs of the old Timberframe are 100 x 44 mm.

    You could build out the existing studs by adding on 44 x 44 mm timber.

    Why not leave the existing 90 mm in place and pack new fibreglass Insulation or 60 / 80 mm ridge board insulation, fit a polythene vapour barrier and fit 12 mm slabs.

    Try to calculate the U value on this link

    The Thermal Conductivity of fibreglass insulation is approx 0.035 to 0.040 W/mK.

  • Registered Users Posts: 127 ✭✭SuperO'B

    Thanks, this is good idea. I had never considered building out the studs in this way. Would it actually work out cheaper I wonder to build out the studs and stuff it with rigid board and regular plaster board rather than going over it with insulated plaster board? I assumed the cost would be relatively neutral, whether the insulation was in the plasterboard or in the wall essentially.

  • Moderators, Home & Garden Moderators Posts: 10,133 Mod ✭✭✭✭BryanF

    you need to strip all plasterboard and sort air-tightness + check for Rot etc. air-tightness will make or break your projects success. Lots of info online etc.

  • Registered Users Posts: 757 ✭✭✭C. Eastwood

    Thanks SuperO’B

    You will need to make out lists of materials for both methods, and compare the costs of materials for both methods. I would not bother doing it.

    A Carpenter Joiner should be able to do all of the works including fixing the 12 mm plasterboard. Obviously you will need a Plasterer to either apply a 3 mm skim of Hardwall, or Tape and Joint the Slabs.

    You will also be able to add extra pipe work, rads, sockets, TV points, WiFi etc before reslabbing.

    I would go for the building-out the studs with more timber. I would also go for fibreglass wool, because it is easier to get the wool fitted tight up against the studs. It is easier to fix fibreglass insulation in place. It will also be easier to fix the Skirting boards, curtain poles, etc to the studs.

    Before fixing the Slabs - the Polythene Vapour Barrier must be fixed in place to the warm side of the Insulation, and fix the slabs on over the Vapour Barrier. You do not require foibacked slabs.

    Ensure that the Vapour Barrier is totally sealed and air-tight to prevent any vapour from within the home from getting in to the Timberframe Structure. Use airtight sockets boxes etc.

    Any vapour getting from the house in to the Timberframe structure would cause Interstitial Condensation - somewhere on the cold side of the Insulation, which will cause Dry Rot in the Timberframe timbers.