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If you’re planning to insulate your home, you’ve likely come across Celotex insulation. It’s one of the UK’s most widely used insulation materials—lightweight, highly effective and easy to install. But some homeowners report damp problems after using Celotex, leading to one very common question:
Does Celotex cause damp?
No, Celotex itself does not cause damp.
However, incorrect installation or using the wrong insulation system can cause condensation issues that show up as damp. To understand why, we need to look at how Celotex behaves, how moisture moves in a building, and how insulation affects that balance.
Celotex is a rigid PIR (polyisocyanurate) foam insulation board with foil facings. It provides excellent thermal performance in a relatively thin board, making it ideal for:
PIR insulation is moisture-resistant, meaning it does not easily absorb water. Many people assume this means damp cannot occur around it—but that’s not entirely true.
The material isn’t the problem. Moisture movement is.
Damp in homes typically comes from:
Celotex cannot cause the first two, and it doesn’t attract moisture. The only damp issue associated with Celotex installations is condensation due to poor design or installation—not the product itself.
If damp appears after installation, one of these issues is usually the cause:
Warm air contains moisture. If this air escapes behind insulation and hits cold surfaces, it will condense. A vapour control layer prevents that by keeping warm, moist air inside the room.
For best results, a high-quality VCL such as the 500g Vapour Control Layer should be installed on the warm side and fully taped.
If the VCL is missing, damaged or poorly sealed:
Insulation reduces natural airflow. If ventilation pathways are blocked, moisture builds up and settles on cold surfaces. Roofs are especially vulnerable: cold roofs require a 50mm ventilation gap, while loft spaces need airflow through the eaves.
If Celotex boards are not tightly butted together or joints are not taped, cold spots (cold bridges) can form. Moisture gravitates to these colder areas, creating mould along the joints.
Older solid-walled homes (pre-1919) need breathable insulation. Using PIR boards against a solid wall traps moisture and causes damp.
These require perfect vapour control and airflow. Even small gaps can create condensation.
Cold external surfaces make moisture control critical.
PIR insulation is not usually suitable without a specialist breathable system.
High moisture levels increase the risk of condensation behind insulation.
The good news? Damp issues around Celotex are almost always preventable.
A proper VCL—such as a 500g membrane—must be:
Taping joints ensures airtightness and eliminates cold bridging.
Traditional walls need breathable materials. Using PIR on solid walls can trap moisture.
No, Celotex does not cause damp. Damp happens due to:
When installed correctly, Celotex performs extremely well and reduces condensation by keeping indoor surfaces warmer.
Insulation isn’t simply about fitting boards—it’s about controlling moisture movement. Celotex is a high-performance insulation product, but it must be paired with proper moisture management, especially a well-installed vapour control layer and good ventilation.
If you have damp issues after installing Celotex, or you're unsure whether your home is suitable, feel free to ask for help. I can walk you through what’s going on and what to do next.
