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Why Has External Wall Insulation Made My Home Damp?

Traditionally built properties were designed to breathe in a way that allowed water vapour to escape through the walls. Wrapping this kind of house with insulation will cut down draughts, which should be a positive thing. But if it’s done poorly, this could also reduce ventilation and air movement, and may even form a barrier that locks moisture in the living spaces. The long-term result: damp and mould.

Where a property is adequately ventilated to allow moisture to escape, external insulation should help improve condensation issues. On a cold internal wall, moisture in the warm air can condense on the wall forming damp patches and eventually mould. When EWI is installed, these surfaces become warmer and therefore less condensation will form on them.

Damp After External Wall Insulation

External wall insulation is a great way to improve the energy efficiency of your home by reducing the rate of heat loss through your walls. The great thing about EWI is that it wraps around the entire exterior of your home, enveloping the heat inside. 

EWI is much less likely to cause damp than other methods of insulating because there are far fewer breaks within the material. EWI is known as a continuous insulation method. Cavity Wall Insulation, for example, does not possess this quality. Read more about water ingress and damp here!

However, correct installation is key. External wall insulation consists of rigid insulation boards (as opposed to a foam) that are secured to the exterior walls of the building using adhesive and mechanical fixings. The important part here is that there are no gaps between the boards. In turn, this will prevent water ingress, which will be the main source of damp. Starter tracks are the ideal solution to preventing damp spreading from the DPC. Starter tracks also serve as structurally integral, straight guidelines.

Thermal Bridges

In the context of external wall insulation and damp, avoiding gaps is crucial. Gaps in the super insulation envelope are called thermal bridges. During installation, any gaps should be filled using a spray foam adhesive which will act as an insulator.

Thermal bridges are essentially places where there is a higher rate of heat transfer than the surrounding materials. They act as ‘heat highways’, resulting in an overall reduction in thermal insulation of construction. Thermal bridges are responsible for up to 35% of thermal losses and increased condensation and mould growth.

Take for example an insulation system where gaps have been left between the boards. The temperature within the gap is going to be a lot colder than the surrounding insulation boards. The condensation will gather in that area and damp will occur.

You can read more about thermal bridges here

Poor Quality Insulation

Damp insulation means ineffective insulation. Not only this, but moisture gathering behind the insulation boards can lead to erosion of the adhesive, which can cause serious damage to the system. 

If you start to notice damp on your internal walls, then this could be a sign that water is getting behind your EWI system. Signs of damp caused by external wall insulation should be an immediate cause for concern, and a professional should be called out to assess the situation promptly. 


16 thoughts on “Why Has External Wall Insulation Made My Home Damp?

  1. Hi,

    Since EWI was installed, damp patches have appeared on a closed off chimney breast on the ground floor. Is there a possible link?

    Rest of the house is fine.

  2. I am going to use EWI on an old mixed material building. I’ve stepped back to the stone in one part to like render but was going to leave the bricks. However, they are looking really damp. Should I put up a barrier or will the external insulation get rid of the issues? I’m ALSO putting in MVHR.

    1. If you’ve stripped the old render down, the wall should dry out fairly quickly, then you can proceed with insulation. Going forward, we’d definitely recommend any of our Silicone Renders for the finish, as they’re hydrophobic. If you’re looking for a bit more advice on what to do and which insulation and render to choose, give our sales team a call on 02045717492

  3. Bedroom is smelling damp after external insulation a few weeks ago. Could this be damp in the walls coming out and will pass in future?

  4. I am having EWI Fitted. They have almost completed the work. The top and sides have not been capped or sealed properly. There are obvious unfilled gaps in the boards and this can be seen at roof level. My roof has no felt and is not water tight and the guttering is failing.
    Roof extenders were priced for and suppose to be installed to protect the insulation. This has not happened.
    Push vents and air box vents have been installed to a very poor standard, ill fitting and not fit for purpose. I am very concerned and worried about this.

    1. Hi Carol,

      Can you give our sales team a call for some advice on this? (02045719589) All of this needs to be addressed correctly to prevent water ingress, especially the extension of the roof soffit. Thanks

  5. Hello,
    I am intending to clad external walls of my house with cement board cladding. Part of my house was already cladded with same material as this was an extension which was constructed in accordance to new regs and fully insulated, however the rest of the house is fairly cold.
    My plan was to fix vertical 25mm battens to the building, then fix a foil vapour barrier to the battens (so leaving a 25mm ventilation gap between the wall and the barrier, then fix double battens (25mm) and insert 25mm foam insulation boards between battens, then cover with another vapour foil barrier on top, and ultimately fix the cladding. However I have since discovered that the cladding also requires to breath and therefore needs to have a gap left too so I’m considering triple battens, however this pushes the total out by another 25mm, totalling to 75mm worth of battens plus cement board overlap.
    In regards to exterior walls, I have read mixed feedbacks on whether I could apply the insulation board directly to the wall in between the first set of battens and then cover the lot with foil vapour barrier, before fixing double batten and then cladding onto those, or whether I should indeed leave a gap between the wall and insulation board, as well as between board and cladding, so effectively having 2 ventilation gaps?
    Any thoughts on this please?

    1. Hi Ed,

      We would suggest applying render carrier boards to the first battens, and then applying the insulation on top of these boards (we stick the insulation on and then use supplementary fixings).

      If you need some more help, give our Technical Team a call on 08001337072 and they can advise regarding this issue


      1. Thanks Adam, my problem is that I have already got the materials, so the Celotex, foil vapour barrier and cement board cladding for the whole project.

        However if my understanding of your recommendation is correct, aside from using the render boards as an iterim layer between the first battens and the insulation, you would still then need to apply extra battens to the insulation as otherwise how would you fix the cement board cladding as a final finish?

        Also your suggestion seems to go in line with my assumption that I need to have two ventilation gaps, one between wall and insulation, and one between insulation and cladding, so in essence still create a sort of sandwich wit 3 layers, but instead of 3 battens overlap as below:
        1. wall
        2. vertical batten
        3. foil vapour barrier
        4. horizontal batten-celotex filled gaps
        5. foil vapour barrier
        6. vertical batten
        7. cement board

        Have arender board added in?
        1. wall
        2. vertical batten
        3. render board
        4. celotex/insulation fixed to above
        5. vertical batten
        6. cement board

        1. Hi Ed, just to clarify our first answer. What we are saying is you have your sheathing board (treated or wrapped OSB) that closes off your insulated stud work. Then you would install additional battens to create a drainage cavity. Another cement board can sit on these battens which can take the insulated render. The insulated render comprises of a non-combustive Rockwool Dual Density slab, which is then directly rendered. Hopefully this makes sense!  

  6. Hi, I’ve viewed a bungalow today which has damp problems and it was said a new damp proof course is required from outside only on all external walls.
    The outside has external brick effect installation – will this be the cause and is an outside damp proof course all that’s required??
    Any help you can advise would be much appreciated thanks…

    1. Hi Kath, external brick effect insulation can sometimes contribute to damp issues, especially if it has been installed without proper consideration for moisture management. It can trap moisture against the wall or create a bridge for moisture to bypass existing damp proofing measures. However, this doesn’t automatically mean it’s the root cause of your damp problems. The first step would be to organise a comprehensive damp survey, and then assess the options. It’s likely that the DPC will need to be replaced.

  7. >
    Screwfix Community Forum
    Builders’ Talk
    Improving living conditions for my mum in damp and humid small bungalow.
    air gap
    polystyrene insulation
    Thread ToolsUnwatch Thread
    New Member
    My mum wants me to add a layer of insulation around the outside walls of her small bungalow and clad over it to improve her damp and unhealthy living conditions inside. However I want to understand the problem and make sure her idea is the solution.
    The existing external cladding is wood with a breathable paint. Inside that is a 20mm layer of fibreglass wool packed tightly against a 30mm layer of polystyrene which is lined with a plastic non breathable sheet. The Internal cladding is wood and there is no air gap between any of these layers. These layers appear dry when I exposed a small area.
    The roof is the same with thick bitumous felt on the outside which seems to be sound.
    The floor is elevated off the ground with good ventilation underneath but there is no insulation – just a plastic sheet and a layer of underlay under the lino floor.
    Inside she has double glazed windows with no vents in the frames. She has one air vent in the wall which is open and an extractor fan in the bathroom not the kitchen.
    Her humidity inside is regularly in the 90s, heating is provided by plug in electrical heaters- it’s a small space. Basically a cabin. She lives with her dog.
    The walls are mouldy and even the curtains and her clothes but there are no signs of leaks.
    Even though she keeps a window open most of the time and her dehumidifier running; the high humidity presumably comes from the dog, cooking, etc etc and the plastic sheet inside the walls and lack of ventilation in the window frames prevents it escaping so condenses on the colder walls and floor where the insulation is insufficient- have I got this right?
    Is it possible the felt roof let’s in moisture even though it’s not actually leaking?
    If so then an extractor fan in the kitchen and one in the living space is needed plus floor insulation and a bit more roof and wall insulation.
    Am I right so far?
    If so then my next question is about installing the insulation and whether to start again by ripping down the existing external cladding and replacing the polystyrene OR rip out the internal cladding OR my mum’s idea which leave everything as it is and at another layer on top.

    Thank you.

    1. Hi Minty, this seems to be a multi-layered issue with damp. We’d recommend a chat with the Technical Team (0800 1337072, info@ewipro.com) due to the complexity of the issues. However, your course of action is likely to be one of the following:
      – Ripping Down External Cladding: This approach allows you to inspect the structure for any hidden issues, such as moisture damage or mould growth. It also provides an opportunity to install a more effective insulation solution and incorporate a breathable membrane that could help manage moisture more effectively. However, it’s likely to be the most disruptive and potentially the most expensive option.

      Ripping Out the Internal Cladding: This could be less disruptive to the building’s exterior appearance and might provide a chance to add insulation from the inside. However, it could limit your insulation options and would not address any potential issues with the external cladding.

      Adding Another Layer on Top: Adding another layer of insulation on top of the existing structure might be the least disruptive option and could improve thermal performance. However, without addressing the underlying issues of ventilation and moisture management, it might not fully resolve the problem. It’s also important to ensure that any additional layers don’t trap moisture within the walls.

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