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Foam behind plasterboard

  • 26-09-2022 1:52pm
    Registered Users Posts: 2,979 ✭✭✭

    I'm looking to insulate a dot dab plaster wall, the gap is about 15mm, plan was to drill dozens of small holes in the plaster to remove any pressure building behind it then fill in the holes with poly.

    Would it work?

    What are my options? it's only 1 wall 280cm x 240cm

    Post edited by KilOit on


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

    1. What is the purpose of you carrying out these proposed works?
    2. What poly?
    3. What is a dot dab plaster wall?
    4. How is there a gap of 15 mm behind plaster?
    5. The pressure behind plaster cannot be air pressure or water pressure. Therefore, what is causing the pressure?
    6. Why would you drill holes in a wall and fill them?
    7. What are the size of the small holes?
    8. Where you intend drilling- are there any electrical cables embedded in the wall?
    9. Where you intend drilling - are there any water pipes embedded in the wall?

  • Registered Users Posts: 2,979 ✭✭✭KilOit

    If i have to answer some of those questions you probably aren't in a position to give advice

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

    Perhaps you can answer the simple questions

    I certainly can give free professional advice, but I need to know what you are doing.

  • Registered Users Posts: 984 ✭✭✭Still stihl waters 3

    Could you just put an insulation board over it instead

    Post edited by Still stihl waters 3 on

  • Registered Users Posts: 2,979 ✭✭✭KilOit

    I can but will lose space and skirting board, wall sockets would have to be moved. not a massive job but if I can get similar insulation from injecting expanding foam into the 15mm gap I'd be happy. it's mainly for sound insulation. I have other methods that I will use within the room but wanted a little extra behind the plasterboard .

    The item I was planning on using

    wasn't planning on using it every square inch of the wall as I figure patches of this behind the plaster would do considering I'm adding sound absorption items in the room itself

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

  • Posts: 0 [Deleted User]

    Having a air gap is probably better for sound insulation, but the fact its only a tiny gap its probably not doing much. There are better options available for sound insulation but you are going to end up lousing space. The issue with the method you describe is spreading the foam evenly behind the plasterboard and there is a risk that the expanding foam could damage the plasterboard if it hasn't the room to expand.

  • Registered Users Posts: 2,979 ✭✭✭KilOit

    I'll drill many holes, won't be perfect but the plasterboard is pretty solid, don't expect it to warp but never know

  • Registered Users Posts: 6,365 ✭✭✭iwillhtfu

    I can't see your plan working very well as I don't think it will travel through the void even if you somehow manage to agitate it.

    I'd say you know the correct solution yourself and that would be to remove the existing board and replace with insulated slab. Do it once, do it right kind of thing but it's a lot more work.

  • Registered Users Posts: 45,683 ✭✭✭✭muffler

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  • Registered Users Posts: 14,570 ✭✭✭✭loyatemu

  • Registered Users Posts: 38,827 ✭✭✭✭Mellor

    TBH, it's not entirely clear what exactly you plan to do as a whole, and why?

    Why would there be pressure, or why is there pressure? Is that an existing issue, or do you mean building up during the filling?

    Poly is a catch all term that refers to anything. What poly do you think you are buying? The product linked above is a gap filler not an insulation.

    A bit of patching fill behind plasterboard is going to make any noticeable difference. From the description it sounds like you are looking for a thermal improvement, that's good because there won't be any. Acoustic improvements will depend on the acoustic issues.

    If the issue sounds penetration? Or sound quality. What you are doing what help with either, but if we know what the issues are we might be able to help.

  • Registered Users Posts: 2,979 ✭✭✭KilOit

    Main thing is dampen sound a little.

    Pressure can build up behind plasterboard with expanding foam. My main concern.

    Poly filler to patch holes I made from drilling that I make to inject the foam. The foam is labeled as acoustic properties

  • Registered Users Posts: 2,979 ✭✭✭KilOit

    Interesting, I will disconnect power at source of I go ahead with it. I'm only using 1 can as appose 18 like that man did. I'm also not filling huge voids like that kitchen unit. If I was to do it I would do it in steps

  • Registered Users Posts: 2,979 ✭✭✭KilOit

    It does look like the best solution, I'm thinking about it, messy but don't mind that

  • Registered Users Posts: 14,570 ✭✭✭✭loyatemu

    18 cans was the 2 people in the basement who died. The Boardsie who blew his kitchen up had only used one and a half cans filling in the void behind a wall. It sounds like a bodge job, it's not what that stuff is designed for.

  • Registered Users Posts: 2,979 ✭✭✭KilOit

    This gap is 15mm so no big void for lots of foam to fill. Can't see a a whole 750ml can being emptied into it

  • Registered Users Posts: 38,827 ✭✭✭✭Mellor

    What do you mean by dampen sound. Coming through the wall (including going out), or Echoing, tinny sound within the room - ie sound quality. Two very different things.

    Pressure is not the issue. The pressure is unlikely to increase much, and the space is unlikely to be air tight enough to retain air pressure. The issue is that the gap filler contains a propellant. It can explode in confined spaces. I highly doubt you'll need polyfilla to patch the holes. This foam will expand out of the holes and you'll be cutting and sanding it back. I wouldn't be hoping for a good finish ready for painting.

    What acoustic properties does the foam have? I would expect its sound absorption to be close to nothing. And it's density is far to low to add sound resistant. It is useful for acoustically t sealing gaps around perimeter of a wall, which is maybe the properties the label is referring to. That will be no benefit on the face a black wall. Honestly think its a waste of time and money and all it will achieve is ruining the finish of the wall.

    Insulated slabs are not a good solution here either FWIW.

  • Registered Users Posts: 2,979 ✭✭✭KilOit

    Sound going out mostly. I have sound absorption panels to hang on walls as well but this is just to add a little bit more absorption behind the wall, it's the party wall so the sound echoes a little from next door.

    Electrics won't be an issue as I have the power switched off from the fuse box while spraying.

    The product in question is rated 60db

    job is not time consuming or expensive, and if it adds slight increase in sound absorption I'll be happy

  • Registered Users Posts: 23,078 ✭✭✭✭mickdw

    I would suggest removing currently dabbed slab and replacing with an insulated slab. As its only one wall, disruption should be limited.

    Is the dabbed board on a good dry wall? if not, you could do an isolated stud when renewing slab.

    Attempted to fill the gap around the dabs would be awful messing and of little benefit imo and would possibly be in danger of pushing the slab off the wall depending on how any foam would set.

    Dont do it.

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  • Posts: 0 [Deleted User]

    Out of curiosity, why did you bother posting a thread asking for views on your proposed (very bad) idea for marginally (if you're very lucky) improving the sound insulation in a room if you're just going to ignore the universal advice that it's a really bad idea and you shouldn't do it?

  • Registered Users Posts: 38,827 ✭✭✭✭Mellor

    Sound absorption panel deal with internal sound don't much to prevent sound going through the wall. Internal sound in reverberation (echo).

    Foam behind the wall will do nothing for sound absorption. The sound waves will reflect off the Plasterboard. It also won't so anything to stop sound passing through the wall. For that you need mass. This foam is very low density. Compared to the block wall it's not even there. If I need to design acoustic plasterboard lining, it would be a min of 50mm of 14kg mineral wool.

    It's not that its time consuming or expensive. It's that it won't help to any perceivable degree.

    Being rated 60dB is meaningless. A paint could be rated for 60dB. Just means it won't fall off the wall if you play loud music. Sound absorption is measured in NRC, sound resistance is measure in RW or DW. This product has neither product. If you have sound leaking through holes in the wall this will help block up the holes. But to stop sound passing through the solid wall, nada

  • Registered Users Posts: 38,827 ✭✭✭✭Mellor

    Insulated boards are for thermal insulation. They'll only benefit acoustics as much as they have mass, which is not a whole lot.

  • Registered Users Posts: 6,567 ✭✭✭Feisar

    Pumping the cavity like this is a waste of time to be honest and could even do more harm than good in terms of cold bridging, giving moisture something to travel across. Although dot and dab is probably doing that as it is. 40mm Xthratherm built on an independent wall liner wound be the best job.

    First they came for the socialists...

  • Registered Users Posts: 23,078 ✭✭✭✭mickdw

    Ya, I hadn't seen the later posts where it was a sound deadening attempt.

    I assumed it was a poor attempt at upgrading insulation.

  • Registered Users Posts: 38,827 ✭✭✭✭Mellor

    That was my assumption at first too tbh. As the foam is some what like PIR in consistency. May even be a PIR derivative.

  • Registered Users Posts: 2,553 ✭✭✭Yellow_Fern

    What is the wall like behind the plasterboard? is it just bare cinder blocks from the picture? Look if sound is a big worry, do this job properly. Remove the wall and parge very carefully. Than install a plasterboard system designed for sound reduction. They often have special rubber backings and glued to as it decouples but you really need to seal those blocks.