Does Celotex Cause Condensation?

Celotex is a widely used rigid foam insulation, typically made from polyisocyanurate (PIR). It is popular for internal walls, floors, lofts, and roofs because it delivers high thermal performance in relatively thin boards. But one question often arises in retrofit and new build projects: does Celotex cause condensation?

The short answer is: Celotex does not inherently cause condensation, but poor installation, missing vapour control layers, inadequate ventilation, and misunderstanding how moisture behaves in a building can create conditions where condensation becomes a problem. In other cases, Celotex can actually reduce condensation risk when used properly.

 For a deeper look at damp-related issues, see our guide on Does Celotex cause damp?.

Two Types of Condensation

To know whether Celotex can contribute to condensation, it helps to distinguish between two forms:

1. Surface Condensation

This is the visible moisture you see on cold surfaces such as walls, ceilings, or windows. It occurs when warm, humid air inside a room cools rapidly and reaches its dew point on a colder surface. Typical signs include droplets, patches of mould, or a damp feel on interior surfaces.

2. Interstitial Condensation

This occurs within the layers of a building—behind insulation, inside cavities, or within the structure. It is usually hidden and can cause structural decay, mould, smells, and a reduction in insulation performance. Interstitial condensation is often more damaging because it can persist unnoticed.

Why Celotex Can Reduce Condensation (When Installed Correctly)

• Warmer Internal Surfaces: Celotex raises the internal surface temperature of walls and ceilings. Warmer surfaces mean less chance for internal air to cool to its dew point, reducing surface condensation risk.
• Reduced Heat Loss: Celotex helps maintain stable room temperatures, lowering the likelihood of moisture condensing on surfaces.
• A Continuous Thermal Layer: When boards are tightly fitted, taped at joints, and paired with a vapour control layer (VCL), they create an airtight envelope that prevents cold spots.
• Supporting Ventilation Strategies: Insulation works alongside mechanical or passive ventilation systems to keep moisture under control.

Why Celotex Can Contribute to Condensation Problems (When Installed Poorly)

Although Celotex itself doesn't create moisture, improper use can lead to condensation—especially hidden, interstitial condensation. Common risk factors include:

1. Vapour Resistance and Moisture Trapping

Celotex is vapour-closed, which is an advantage when installed correctly. But if moisture gets behind the insulation—through gaps or external issues—it can become trapped and condense inside the structure.

2. Missing or Poor Vapour Control Layer (VCL)

A VCL prevents indoor moisture from reaching colder surfaces behind the insulation. Missing or poorly sealed VCLs are one of the leading causes of interstitial condensation.

3. Inadequate Ventilation

Older homes often lack modern ventilation. Cooking, showering, and even breathing release litres of moisture into the air. Without ventilation, moisture builds up and drives condensation problems.

4. Thermal Bridging and Cold Spots

Gaps between boards or uninsulated junctions create cold bridges—prime locations for moisture to settle and condense, whether visibly indoors or hidden within the building fabric.

Special Considerations for Older Buildings

Traditionally built homes with solid masonry walls are vapour-open. Adding a vapour-closed material like Celotex inside these walls disrupts how the building was designed to “breathe.” Risks include trapped moisture, hidden mould, and decay. In many cases, breathable insulation such as mineral wool or wood fibre is a better choice unless a moisture assessment is completed.

Common Real-World Problems People Encounter

Homeowners who encounter issues after installing PIR insulation almost always face system or installation problems, not product faults. Typical examples include:

• Loft conversions without a VCL
• Garage conversions lacking ventilation
• Internal solid wall insulation leading to trapped moisture
• Floors insulated without addressing rising damp or subfloor airflow

How to Use Celotex Without Creating Condensation

• Install a Continuous Vapour Control Layer — all joints taped, all penetrations sealed.
• Ensure Good Ventilation — extractor fans, trickle vents, or MVHR systems.
• Eliminate Thermal Bridges — neat cuts, sealed edges, properly insulated junctions.
• Assess the Building Fabric — especially in older, solid-walled properties.
• Follow Manufacturer Guidance — Celotex publish detailed installation best practices.

Celotex does not inherently cause condensation. When installed correctly, it reduces condensation risk. When installed poorly—especially without a VCL or adequate ventilation—it can contribute to both surface and interstitial condensation.

Ultimately, condensation risk depends on how the entire building system works together: insulation, ventilation, vapour control, and the construction type itself.