Archer Posted January 29 Posted January 29 (edited) Hoping to get some advice on a situation in our 70's house renovation. Cheers all in advance, this one is stressing me out a bit... So, situation as follows - we've just stripped the wallpaper off our daughter's room and had the walls and ceiling skimmed. No smell in the room before the work was undertaken in the first week of Jan. On the outside, north facing wall we added a layer of internal wall insulation. The wall build up is (from outside to in) - cedar shingles on battens and 10mm ply sheafing, 90mm timber frame infilled with mineral wool, polythene vcl, plasterboard and to that we added 40mm PIR insulated plasterboard screwed to the studs through the existing plasterboard. After the walls were skimmed and dried for a couple of weeks I sealed the wall with Zinnser BIN paint to act as a mist coat and new VCL on the outer face of the new plasterboard... Now we've noticed a rank, musty smell coming from the room and in particular from that wall. I'm assuming that it must be interstitial condensation, presumably between the new insulated plasterboard and the old polythene vcl. I'm desperate for ideas on what to do next. Checked this buildup was low risk with condensation analysis software (build desk) but must have cocked something up? Any building physics advice here - If I whack up the heating, is it likely to dry out the wall on its own, or would drilling regular small holes through the plaster/insulation & polythene help? We have an central A2A heating system so could also introduce a positive air pressure in the room to reduce humidity and force air and moisture out through the fabric. Really want to avoid having to take everything off and starting again on the wall which would be a disaster. Hoping for reassurance... Edited January 29 by Archer
Nickfromwales Posted January 29 Posted January 29 2 minutes ago, Archer said: VCL on the outer face of the new plasterboard... I think this may be one contributing factor, as the VCL should really be behind the plasterboard and continuous as part of the fabric of the property. If you paint a VCL over the plasterboard, then airflow and hot / cool convection can occur in the gaps between the insulation and the PB. 1
Redbeard Posted January 29 Posted January 29 Although some have done it I am not happy with the idea of trapping gypsum plasterboard in a sandwich between insulation and insulation. So to clarify, the timber wall is the original, yes? If so I think you should have taken the plasterboard off before adding the IWI board, and only have done that if an IC analysis said it was OK (although I think you did do that IC analysis and it suggested you'd be OK if I have read that right. Obviously the IC analysis software and I don't agree re a plasterboard sandwich!). @Nickfromwales has posted while I have been typing, so off to see what he advises. 1 1
Archer Posted January 29 Author Posted January 29 1 minute ago, Nickfromwales said: I think this may be one contributing factor, as the VCL should really be behind the plasterboard and continuous as part of the fabric of the property. If you paint a VCL over the plasterboard, then airflow and hot / cool convection can occur in the gaps between the insulation and the PB. I'm worried it might be as well because I've possibly locked in any moisture between two impervious layers (the original polythene and the new top layer of paint). I'm not sure about the issue you've described though, because the VCL should ideally be on the outer surface (warm side) of the wall? In my head, the issue is the old polythene layer now behind the top layer of insulation 1
Nickfromwales Posted January 29 Posted January 29 1 minute ago, Redbeard said: Although some have done it I am not happy with the idea of trapping gypsum plasterboard in a sandwich between insulation and insulation. Oh! I just re-read the OP, then it gets worse. Defo shouldn't have left the other layer of PB in there, it'll be acting like a sponge and the moisture will then trap and..... it'll only get worse.
Nickfromwales Posted January 29 Posted January 29 1 minute ago, Archer said: I'm worried it might be as well because I've possibly locked in any moisture between two impervious layers (the original polythene and the new top layer of paint). I'm not sure about the issue you've described though, because the VCL should ideally be on the outer surface (warm side) of the wall? In my head, the issue is the old polythene layer now behind the top layer of insulation Guess what you're doing tomorrow then
Nickfromwales Posted January 29 Posted January 29 3 minutes ago, Archer said: because the VCL should ideally be on the outer surface (warm side) of the wall? Not in my 30+ years "on the tools" have I ever seen this done, sorry.
Archer Posted January 29 Author Posted January 29 4 minutes ago, Redbeard said: So to clarify, the timber wall is the original, yes? If so I think you should have taken the plasterboard off before adding the IWI board, and only have done that if an IC analysis said it was OK (although I think you did do that IC analysis and it suggested you'd be OK if I have read that right. Obviously the IC analysis software and I don't agree re a plasterboard sandwich!). @Nickfromwales Yes that's right, timber frame walls are original. We basically added the insulated plasterboard over the top (made a sandwich like you said). Yeah I checked the buildup and the software was happy so I assumed low risk. Wondering if the plaster wasn't fully dry, seems funny that it could have accumulated significant moisture so quickly but either way unless the smell is caused by something else that seems to have happened
Archer Posted January 29 Author Posted January 29 4 minutes ago, Nickfromwales said: Not in my 30+ years "on the tools" have I ever seen this done, sorry. It's not done with a membrane because obviously a membrane outside of plasterboard wouldn't work. In principle it should work with a painted/ coated layer. There are examples of this that you can find (it's not an ideal comparable, but think of a liquid dpm - this does the same job as polythene for example). We've used a similar technique elsewhere in the house with no issues... The problem here seems to be the plasterboard sandwich behind it (at least I'm assuming). Do you think perforating with drill holes and then filling and decorating after allowing some time for the moisture to escape could work?
Nickfromwales Posted January 29 Posted January 29 11 minutes ago, Archer said: Do you think perforating with drill holes and then filling and decorating after allowing some time for the moisture to escape could work? Not IMHO, and even more so as it's the North elevation. 12 minutes ago, Archer said: It's not done with a membrane because obviously a membrane outside of plasterboard wouldn't work. In principle it should work with a painted/ coated layer. There are examples of this that you can find (it's not an ideal comparable, but think of a liquid dpm - this does the same job as polythene for example). We've used a similar technique elsewhere in the house with no issues... The problem here seems to be the plasterboard sandwich behind it (at least I'm assuming). In my opinion, you've been lucky elsewhere. Vapour is supposed to be arrested at the fabric connections, to seal the room, then the cosmetic layers get applied afterwards and decorated. You cannot compare this to a DPC as the DPC is at the foot of a wall only with the remainder of the surface left to 'breathe', so "no" . Next steps are: Go and scream loudly into a pillow so as to not frighten the kids, Take everything off and do it again properly. Drink beer when done. I genuinely think I should have worked as a therapist 1
Redbeard Posted January 29 Posted January 29 I think the IC analysis 'thought it was OK' as it ignored the 'interstitial plasterboard' and assumed (as all IC analyses do AFAIK) that the VCL in the insulated plasterboard was perfect and taped at all joints and perimeters. The nature of insulated plasterboard makes this impossible. I have a strong suspicion that the musty smell is the paper on the 'trapped' plasterboard 'festering'. There will almost certainly be other issues to sort too, but I think you need to remove the insulated plasterboard, remove the original plasterboard and VCL, apply PIR (if that's 'to your taste') 'in the raw' (not as a plasterboard laminate), tape all joints and perimeters with air-tightness tape (not cheap fragile foil tape) and then batten, fixing through the battens to the TF wall to hold in the PIR. If you are a pessimist (which with water vapour I think you should be) pre-fill the holes, gun in silicone, and wind the screw through a 'silicone grommet'. Nothing is perfect but that is going to be the best chance you have of a somewhere-near-perfect-ish VCL. I am sorry this will not be what you wanted to hear.
Nickfromwales Posted January 29 Posted January 29 15 minutes ago, Redbeard said: (not cheap fragile foil tape) Oi, you! Foil tape is plenty good enough here, when installed by anyone non-orangutan-like. I go over the stud first, 100mm tape centred to overlap the adjoining material, then do another 2 runs from the centre of the joist outwards left & right or up & down, so effectively tripled up where it counts. AT tape is OTT here IMHO, but it is more forgiving if the orangutan hasn't kept the different building materials all flush finished with each other. Foil is cheaper than shoplifting, so putting the 3 layers on is quick and simple without hurting the wallet.
Redbeard Posted January 30 Posted January 30 8 hours ago, Nickfromwales said: Oi, you! Foil tape is plenty good enough here, when installed by anyone non-orangutan-like. OK, fair point. I should have said 'my personal preference is for AT tape' and then backed it up with anecdotal 'evidence' (over quite a lot of years) of some quite good, quite strong and very sticky foil tape and some very weak, not-very-sticky, frankly very awful foil tape which was particularly weak in tension and would split when returned round 90 degree returns. A few bad experiences led to a 'once bitten, twice shy' attitude which I retain for my own work. I'm just off to comb my torso now... 1
Archer Posted January 30 Author Posted January 30 I've calmed down slightly from yesterday. Odour in the room has significantly reduced and I'm currently ventilating, heating and exploring a dehumidifier to see if there is generally a problem with high humidity which might be solvable. Weather over the last few weeks has been cold and wet which isn't a great combination. Think before I go ripping out any work, it's worth a bit of a pause to see if a few common sense measures can address the issue. Realised as well that I could take off the vapour impermeable paint easily as well if needed (it's shellac based so sponging with alcohol will easily remove). Also not sure that the issue isn't a different wall - the east facing external wall that we didn't add IWI to and is therefore colder. Going to see how I get on with this and hopefully get it sorted
Nickfromwales Posted January 30 Posted January 30 6 hours ago, Archer said: it's worth a bit of a pause to see if a few common sense measures can address the issue Good idea, but the layer of PB in the cavity / sandwich is no-any way you cut this. 6 hours ago, Archer said: Going to see how I get on with this and hopefully get it sorted Please do keep the thread updated if you would, good to box threads of with more info and any resolution you get / how you got there 👍 1
Mr Punter Posted January 31 Posted January 31 Burying the existing plasterboard behind insulation instead of removing it was a mistake. Plasterboard is prone to mould and should be the innermost layer in the wall buildup. You can either remove it or live with it. Why not leave this until summer and see how it is?
MikeGrahamT21 Posted January 31 Posted January 31 (edited) Its certainly been cold this past month, especially for the amount of water involved in plastering. I feel any mould and the smell has come through rather quickly, even given that scenario. Are there are plug sockets/switches on this stretch of wall that you could maybe remove, and have a chance of some additional investigation, whether that be with a camera or just your sniffing gear? https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1019707/iwi-guidance.pdf This may help answer some questions, one example which may be appropriate: You may have inadvertently left a small air gap between your insulation and the old plasterboard, giving a space for condensation to collect. Was the original wall completely flat? I've been through a few guides for retrofitting and none specify that you have to remove any plasterboard before application, its not recommended however, the emphasis is on ensuring no gaps are present. How about getting in touch with British Gypsum, or Kingspan and ask for a potential solution for IWI, with existing plasterboard and see what they specify? Examples of differing opinions: https://www.kingspan.com/gb/en/knowledge-articles/can-you-fix-insulated-plasterboard-to-existing-plastered-walls/ (though this is from 2022 and guidance has since changed) https://insulationgo.co.uk/blog/insulated-plasterboard-over-existing-plasterboard Edited January 31 by MikeGrahamT21
Roger440 Posted January 31 Posted January 31 5 hours ago, MikeGrahamT21 said: You may have inadvertently left a small air gap between your insulation and the old plasterboard, giving a space for condensation to collect. Was the original wall completely flat? I long since gave up with insulated plaster board. Its really difficult to ensure theres no air gaps. Well, impossible really as you cant see behind it once it gone up.
Roger440 Posted January 31 Posted January 31 On 30/01/2025 at 08:40, Redbeard said: OK, fair point. I should have said 'my personal preference is for AT tape' and then backed it up with anecdotal 'evidence' (over quite a lot of years) of some quite good, quite strong and very sticky foil tape and some very weak, not-very-sticky, frankly very awful foil tape which was particularly weak in tension and would split when returned round 90 degree returns. A few bad experiences led to a 'once bitten, twice shy' attitude which I retain for my own work. I'm just off to comb my torso now... Theres certainly some crap foil tapes about. I found one advertised as "suitable for use in cold weather". Works a treat, actually sticks and doesnt split easily.
Conor Posted January 31 Posted January 31 (edited) Coming to this topic a bit late. Don't panic. You've just dumped a load of moisture (plaster skim) in to the room at the coldest time of year. Keep it ventilated and dry out for 6 weeks before considering doing anything. Edited January 31 by Conor 1
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now