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Mortar lines showing through blockwork

user-774310 | Posted in General Questions on

I had the following e-mail sent to me from my daughter in England and I’m hoping that someone may be able to help with some advice.
I used to be a bricklayer (many years back) in England and had been using insulated blocks since the 70s. I have never come across this problem before despite the timeline.
Her house was built in 1989/90 and has a cavity wall, bricks to the exterior and blockwork inside, standard stuff really. It is the ground floor unit of a duplex. I’ve already warned her off both the thermal wallpaper and, of course, the paint!
Her e-mail below, and thanks for your time to any one who answers.

Have you ever come across pattern staining or ghosting where you can see the mortar lines on an interior cavity wall? I’ve read loads about it and why it happens but wondered if you’ve found a fix? Most people say insulate the cavity wall to bridge the thermal differences but I’m not sure I’ll be able to get that done here!! Others say just redecorate and as its a slow process for the marks to appear (that being fine dust attracting itself to the colder mortar lines over time) that it won’t show for another few years! That or use thermal wallpaper which I really don’t want to do!! I noticed it last year but the line patterns are spreading a bit more now so want to sort really

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  1. GBA Editor
    Martin Holladay | | #1

    The phenomenon of "ghosting" is fairly well understood, and your daughter describes the process accurately (fine dust is attracted to the colder sections of the wall).

    Here is a link to an article on the topic: Black Stains in Houses: Soot, Dust, or Ghosts?

    The best and most permanent solution is to install a continuous layer of insulation (for example, a layer of rigid foam) on the affected wall. In your daughter's case, the insulation could be installed on the interior, followed by new gypsum wallboard and paint. I'm guessing, but even 1/2-inch rigid foam would probably eliminate the ghosting (although such thin foam wouldn't do as good a job of reducing heat flow through the wall as thicker foam).

    Ghosting is more common in homes with small particles in the air. The particles may be generated by burning candles or burning tobacco. If someone in your daughter's family smokes cigarettes indoors or burns candles, eliminating these behaviors will reduce ghosting.

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