- Understanding the Problem: Why Does Damp Occur?
- How EWI Should Work to Prevent Damp
- The Importance of Correct Installation
- What Are Thermal Bridges?
- When Poor Installation Leads to Damp
- Ventilation and Moisture Management
- The Role of Breathable Systems
- Common Causes of Damp After EWI
- Maintenance After Installation
- Red Flags: How to Spot Damp Risks Early
- Final Thoughts
External wall insulation (EWI) is designed to improve your home’s energy efficiency by wrapping it in a protective, heat-retaining layer. In most cases, it also helps reduce condensation and internal damp.
However, if it’s installed poorly or without proper consideration for ventilation, EWI can actually contribute to moisture build-up and mould. Let’s explore why this happens, how to avoid it and the red flags to watch for.
Understanding the Problem: Why Does Damp Occur?
Traditionally built homes, especially those constructed before the 1920s, used breathable materials that allowed moisture to escape naturally. Adding insulation can disrupt this process if ventilation isn’t factored in.
Blocking draughts is one of the benefits of insulation, but it also reduces airflow, potentially trapping moisture inside. The result? Condensation, damp patches and even mould growth.
How EWI Should Work to Prevent Damp
When installed correctly, EWI actually helps stop damp. Cold interior walls are often where condensation forms as warm, humid air hits a chilly surface, particularly in bathrooms and kitchens.
By raising the internal wall temperature, EWI minimises this effect. In homes with proper ventilation, EWI significantly reduces condensation and related damp issues.
The Importance of Correct Installation
An EWI system is made up of rigid insulation boards fixed with adhesives and mechanical anchors. For the system to perform correctly, boards must fit snugly together with no gaps. Otherwise, water can creep in, creating damp and cold spots.
A correctly installed system:
- Prevents water ingress
- Eliminates cold spots
- Reduces the risk of mould
Starter tracks at the base of the wall are especially important as they stop rising damp and create a level line for the first row of boards.
What Are Thermal Bridges?
A thermal bridge is an area where heat escapes faster due to gaps or weaknesses in the insulation layer. These cold zones attract condensation, which can then soak into the wall fabric and encourage mould growth.
Filling all gaps between insulation boards (often with spray foam adhesive or expanding foam tape) ensures a continuous protective layer and prevents thermal bridges from forming.
When Poor Installation Leads to Damp
Poor workmanship doesn’t just reduce energy efficiency - it can actively create damp problems. Common issues include:
- Water getting trapped behind insulation boards
- Adhesives breaking down
- Internal mould patches appearing
If you notice damp forming on walls after EWI installation, it may be a sign of water ingress. Always have the system checked by a professional before damage worsens.
Ventilation and Moisture Management
Insulation reduces heat loss, but it also reduces airflow. Without proper ventilation, humidity has nowhere to go and condensation can quickly develop.
Solutions include:
- Extractor fans in bathrooms and kitchens
- Trickle vents on windows to encourage background airflow
- Mechanical Ventilation with Heat Recovery (MVHR) systems in airtight homes
Balancing airtightness with ventilation is essential to keeping your home both warm and dry.
The Role of Breathable Systems
Choosing the right insulation material makes a big difference:
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EPS (Expanded Polystyrene): Cost-effective, widely used, but less vapour-permeable.
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Mineral Wool: Fire-resistant and breathable, making it a good choice for homes where vapour movement is important.
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Wood Fibre: A natural, highly breathable material suited for older, heritage buildings.
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Kingspan Kooltherm K5: A premium phenolic insulation board with excellent thermal performance. It has lower vapour permeability than Mineral Wool or Wood Fibre, so it’s typically recommended for modern properties where maximum insulation value is needed in a slim profile, but ventilation must be carefully managed.
Breathable systems allow moisture to move through the wall structure rather than trapping it inside, ideal for traditional homes.
Common Causes of Damp After EWI
Even with good materials, certain detailing mistakes can cause damp to appear. These include:
- Insulation bridging the damp proof course (DPC)
- Poor sealing around windows and doors
- Inadequate roof overhangs
- Gaps around pipes, cables or vents
Attention to detail during installation is just as important as the insulation boards themselves.
Check out our blog on Treating the Rooflien and DPC on a Retrofit
Maintenance After Installation
A well-installed EWI system is designed to last decades, but some light maintenance helps prevent future issues:
- Inspect sealants around windows and doors every few years
- Clean rendered surfaces to remove dirt or algae growth
- Check for hairline cracks after storms or freezing conditions
- Ensure gutters and downpipes are directing water away from walls
Proactive checks keep the system performing as intended and prevent small issues from becoming costly repairs.
Check out our blog on Maintenance for External Wall Insulation or How to Clean and Maintain Your Render
Red Flags: How to Spot Damp Risks Early
You don’t need to wait until black mould appears to take action. Watch out for these early warning signs after EWI installation:
- Peeling paint or blistering on interior walls
- Musty odours around skirting boards
- Discoloured patches behind furniture or curtains
- Condensation build-up on internal windows despite insulation
These symptoms suggest either moisture entering the system or vapour being trapped inside your walls. If caught early, remediation is often straightforward.
(Mould forming on an internal wall after EWI)
Final Thoughts
External wall insulation, when installed properly, should prevent damp, not cause it. But if shortcuts are taken or details are overlooked, risks such as thermal bridges, water ingress and internal mould increase dramatically.
If you’re planning EWI or suspect damp after installation, always consult an experienced professional. The right installation will protect your home, improve efficiency and keep walls warm and dry for years to come.