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Poorly executed drylining causing damp spotting of plaster coat over fixings

ThXzr7FgXL | Posted in Energy Efficiency and Durability on

A friend of mine drylined his interior walls about 3 years ago. The wall construction is of concrete hollow block construction, rendered on the exterior with plastercoat to the interior face. Bear in mind this is a wet, windy, and cold climate.

This wall was subsequently insulated to the interior using 50mm (2″) (Possibly EPS boards, but he was not sure whether EPS or fiberglass when I asked) insulation with 1 layer plasterboard over, then plaster skim coat to finish.

Since then, visible spotting has occured on the inside face of the wall over where the fixings are. Having asked him how the job was done I discovered:

The fixings for the insulation were too expensive (but thermally broken) and not usable for hollow blocks and rather than going down the studwork route and/or adhesives he seemingly went with cheap but workable metal fixings to fix the insulation and plasterboard DIRECTLY to the hollow concrete blocks and this was skimmed over with plaster.

It would seem that,
#1 thermal bridging along the fixings is causing moist interior air to condense on the cold spots in the plaster and/or

#2 gaps in the exterior wall render may be allowing moisture to drive inwards to the hollow blocks and along the fixings and holes in the interior block face, building up inside the plasterwork at these points…

Q: Without tearing down this work and starting again is their a quickfix to this? Should he add another layer of EPS as he would like to do, lets say 1″ thick over this, on dabs with interior render directly to the EPS? Can this problem be ignored for a few years or is there too much risk with allowing severe cold bridging and/or moisture penetration of the fixings like this. Should he just tear it all off and redo it right when he is able to do it??

Thanks for your suggestions if you have any!

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  1. user-869687 | | #1


    I think your hypothesis #1 is right, the screws are condensing moisture from the interior air. It would help to have stainless screws, as they would conduct much less heat. 2" insulation is not much, so it would be tempting to recommend redoing this just to add R-value, if there's space to spare inside. It's also possible that a vapor retarding primer at the interior surface would reduce condensation.

  2. GBA Editor
    Martin Holladay | | #2

    North American readers will be confused by some of your vocabulary.

    I know that "rendering" means "stuccoing."

    I'm unfamiliar with the terms "drylining" and "fixings." Are there any bi-lingual English speakers out there who know how to translate British?

  3. homedesign | | #3

    I believe ....fixings are fasteners

    drylining = interior insulation

  4. GBA Editor
    Martin Holladay | | #4

    Thanks for providing translation services.

    Unlike TJ Elder, I don't think that a vapor-retarding primer will help. However, I agree with TJ that the source of the moisture is probably the interior air; the moisture is apparently condensing on the coldest part of the wall.

    I don't know your climate, but this wall is poorly insulated, so it's time to thicken the wall and add more foam. This time, consider using adhesive to secure another layer of rigid foam. Another alternative would be to build an interior stud wall filled with insulation, followed by a continuous layer of rigid foam and new gypsum drywall. That would take up a lot of space, of course, but at least you'd be improving the insulation.

    Don't forget the rim-joists or the area where the roof meets the wall. It may be necessary to open up the ceiling to address this area.

  5. user-659915 | | #5

    I think it's clear your friend needs to adhere a new layer of board material over the existing plaster finish. The fastener heads being directly below the skim coat are providing a direct thermal bridge to the cold concrete masonry and attracting localized condensation. There's no particular long term damage to be concerned with though in time (if the fasteners are not stainless) you'll probably see some rust staining appearing. At worst you might see some mold (mould) appearing also.
    You could go with a framed solution as Martin suggests but in a retrofit situation in a damp British (Irish?) climate you'd have to be really careful to protect the wood from moisture damage and it would probably be more trouble than it was worth. A 2" overlay of foam board would certainly fix the condensation issues but I'm not sure how robust a direct stucco or plaster interior finish would be on that.
    I'd look for a composite foam/gypsum board product. As little as 1/2" of additional foam would certainly deal with the spotting, thicker would obviously provide greater improvement to the overall thermal performance of the assembly. And to be sure, the material should be adhered, not mechanically fastened, according to the manufacturer's specifications.

  6. GBA Editor
    Martin Holladay | | #6

    I've added "drylining" and "fixing" to my list of British construction terms. I have been assembling the list from a variety of sources over the years. Here it is:

    Bespoke: Custom
    Boarding: Paneling
    Drylining: Sheet goods (gypsum wallboard or paneling) used as interior wall finish
    External works: Sitework
    Fitting: Fixture
    Fixing: Fastener
    Kit: Equipment
    Lagging: insulation
    Lippings: Trim
    Loft: Attic
    Loft lagging: attic insulation
    Render: plaster or stucco
    Rodding access: Cleanout
    Roof light: Skylight
    Sarking membrane: Asphalt felt or a similar waterproofing or reflective membrane used on a roof or wall
    Skirting: Baseboard
    Spanner: Wrench
    Store: Storage closet
    Timber-frame: Wood-frame (stick-frame)
    Weatherboard: Clapboard

  7. homedesign | | #7

    Martin, I think "sarking" is more generic than just asphalt felt
    I have seen references to sarking board too

  8. GBA Editor
    Martin Holladay | | #8

    You're right. I've edited my glossary as a result of your comment.

    Here are some definitions of "sarking":

    "A timber or felt cladding placed over the rafters of a roof before the tiles or slates are fixed in place."

    "Sheet material placed under the roof covering material, during the construction of a roof, to provide reflective insulation and/or additional waterproofing."

  9. homedesign | | #9

    I have also seen the term "racking board"
    which I think is more descriptive than our term "shear panel"

  10. user-659915 | | #10

    Good list, Martin though I'm a little bewildered by the last one - I've never heard totem used in that sense. Otherwise I have just one quibble. While acknowledging John B's link, I believe drylining refers specifically to the board material (usually gypsum board, but sometimes plywood or other sheet material) used for the interior finish, not the insulation component. A masonry wall can be drylined without insulating it, though this is probably seldom done these days. The term is used by contrast to a direct plaster application, known as a 'wet' finish.

  11. homedesign | | #11

    Sarking is not limited to roof... there can be wall sarking too
    for instance Pavatex

  12. homedesign | | #12

    James, thanks..... I found that link by googling
    almost a dangerous as "Wiki"

  13. GBA Editor
    Martin Holladay | | #13

    James and John,
    Thanks very much for your comments. I have edited my glossary in light of your helpful suggestions.

  14. ThXzr7FgXL | | #14

    Thanks guys for that!

    Given that my friend doesn't want to spend much money or lose too much more space I think James' suggestion of adding a composite product of gyp board/insulating foam or another layer of foam board using adhesives would be the easiest solution. By the way you are correct with the guess in brackets, its an Irish climate!

    Even though 2" of EPS on interior of hollow block is not much (My own house has full filled cavity wall polystyrene bead insulation between 2 layers of block wall 100mm apart) the house seems to perform a little better than one would think at those insulation levels...southernmost coast of Ireland temps are mild, still wet and windy but not quite as cold...

    Love the translation service going on here martin! I do read various European, British and North American websites and my vocab tends to interchange so I apologise for the use of 'drylining' and 'fixings' and 'render'!

    With regards to 'dry'lining...yes its more to do with the sheet material (usually gypsum but not always) and not the insulation material on the inside, and its dry as opposed to 'wet' plaster.

    Must say tho I use many of the other words that you use...I Never use lagging unless talking about insulation around a hot water cylinder (purely a traditional phrase for that specific item as it was the first thing that people here started to insulate many years ago, before wall insulation it goes back a bit)...insulation is the term used for all other instances. Never even heard of the phrase 'Loft lagging' (Maybe some Brits use that, not so in Ireland)

    Some people say sarking but I dont come across it a lot....much more common term in Scotland and rest of Britain to a degree. In Ireland its just as you describe it.

    I always laugh at skirting...Yes we use it, and its only funny when we realise others don't call it that!

    I always thought bespoke was more of a European or maybe North American term...Naturally we would say Custom aswell whereas the more international or multinational companies use the word bespoke much more often.

    Its Skylight here aswell and I think Brits say it that way also but I have come across rooflight many times.

    Boarding is used here more in reference to siding or random loose wood boards you might walk on when scaffolding whereas timber 'panels' is more common to say when referring to timber frames and sandwich panels. A bit interchangeable this one, and in Britain, certainly Europeans use 'boards' much more than 'panels' tho.

    Here's a new one for you: We say 'taps' instead of 'Faucets'

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