Insulation behind sofa/couch (outside wall) - any tips?

Dear All,
Our sofa stands with its back to an outside wall. Although we already didn't put it directly at the wall but left some gap, mould came up.

I thought about getting these silver insulation sheets/rolls, which are often used behind radiators. Very thin, probably 3mm or so should be fine to just avoid condensation. It doesn't need to go fully up, the problem is mostly at the bottom, so it wouldn't be visible. Or are there any better products?

Also, how best to attach it? Ideally in a way so that we can remove it again without damaging the wall/paint. Are there easily removable glues? Or maybe some kind of tape? When installing, I guess it is important to make sure it is tightly sealed so that no condensation occurs between the insulation and the wall.

Any other tips?

Best wishes,
Andre

Comments

  • grumbler
    grumbler Posts: 58,620
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    edited 2 February at 11:01PM
    If it's really condensation, anything thin will make almost no difference. In fact, shiny foil surface can make wall even colder - when behind a radiator it reflects the radiation that otherwise would heat the wall.
    To make a noticeable difference you need 25mm+ insulated plasterboard. Or increase the gap and improve air circulation.
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  • Cherryfudge
    Cherryfudge Posts: 9,638
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    edited 2 February at 10:58PM
    We had a very similar problem in a 1930s bay window. Apparently in those days bay windows were not thick enough and moisture could soak through the bricks from outdoors. We had ours dry lined with a special plasterboard that has a foil backing. I think there was also a gap left between the wall and the board. That was several years ago and the problem has never returned.

    Yes, that option means redecorating behind the sofa, but it's a permanent solution and may save a lot of hassle in the future.
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  • mexican_dave
    mexican_dave Posts: 244
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    I agree with grumbler above, fit insulated plasterboard. Fit it to the whole wall otherwise you will have an odd change in levels, the board is about 4cm thick. Another reason to do the whole wall is you will get better heat retention across that wall (you save energy). For example
    https://www.british-gypsum.com/products/board-products/gyproc-thermaline-plus-40mm#characteristic
  • andre_xs
    andre_xs Posts: 281
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    Thanks for your comments. Hmm, maybe I could also use just 25mm styrofoam boards, should fit as well (luckily even surface, not a round bay window).

    Any tips regarding how to attach it without damaging the wall? Is there some kind of removable glue I could use.
  • FreeBear
    FreeBear Posts: 14,244
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    andre_xs said: Any tips regarding how to attach it without damaging the wall? Is there some kind of removable glue I could use.
    Any adhesive you use will cause damage to the wall to some degree if/when the insulation is removed. And unless you use a thin film adhesive, there will be a small air gap where condensation can form. A quick & easy way of attaching would be to put a strip of double sided sticky tape all the way round the edge of the insulation board - That would hold it in place for a while. For a more permanent fixing, an expanding foam adhesive should do the job.
    If this is a solid brick wall rather than a cavity one, you'd be better off looking at a woodfibre or cork insulation board. This would allow the wall to "breath" and reduce the effects of moisture buildup.

    Had expanded polystyrene (~10mm thick) on a solid brick wall here. It didn't do much to insulate the walls, and when I came to remove the stuff, the plaster was soaking wet & much of it just fell off the wall.

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  • mexican_dave
    mexican_dave Posts: 244
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    I was expecting the OP to regard the installation of insulated plasterboard as a permanent upgrade to their property, so the issue of removing it would not arise. Haven't read manufacturers recommendations but reckon maybe existing plaster should be hacked off before installation? Condensation should not be an issue as this type of boarding has an integral "vapour barrier" preventing warm moist air passing thro' the insulation to condense on the cold side. Precautions need to be made to ensure the perimeter edges are sealed to stop moisture laden air percolating around to the cold side (suggest decorators caulk, so it's can be over-painted).
  • andre_xs
    andre_xs Posts: 281
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    Thanks to everyone. I've now decided to try to 'glue' 25mm polystyrene sheets to the wall, just for ~50cm up from the floor. Our living room is tiny and everything literally just fits in with a bit of a squeeze. So adding ~40mm or so to the outside walls (2 of them) would (a) require a major redesign of the living room with new furniture and (b) be a quite a build.

    The problem was just behind the couch, so I added it there where it's not visible anyway. Above the couch, the walls are fine. We also tried to improve ventilation behind the couch.

    I've stuck the styrofoam to the wall using plain silicone. One line surrounding the edge, and a few lines criss-cross in the middle. Plus aluminium tape at joints. So even if there's a small air pocket, there should be no air exchange and no condensation build up. When removing, I hope the silicone can be scraped off, plus then silicone remover. Possibly it has then to be freshly painted, but that's ok.

    BTW, it is a brick cavity wall, with the old plastering (from the 1950s?) still on there. The plaster seems to be very tough (more like concrete). No plasterboard.

    I still had the styrofoam sheets, so it was just £17 for 2 silicone and a roll of aluminium tape and half a day work.

    We'll check next winter whether it worked.
  • mexican_dave
    mexican_dave Posts: 244
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    I know you have now done this work, but just a word of caution, the insulation you are using is not hidden by protective plasterboard, as such your raw polystyrene is a fire hazard - very flammable. Also gives off toxic fumes when burnt! I would never have done what you have just done - sorry!
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