Cork insulation vs Celotex - Choosing the right insulation for your project

Choosing the right kind of insulation can make a lot of difference in the day to day life of your home: it affects not only energy efficiency but also comfort, air quality and sustainability. In short - choose wisely!

People often compare cork and Celotex; both offer strong thermal performance, though they differ in composition, environmental impact and in how suitable they are for different types of buildings.

Here, we’ll explore their qualities: pros, cons and how to decide which one will best serve your project.

Summary

Understanding your insulation options

Insulation acts as a barrier to heat, to make the inside of your home less affected by changes of temperature outside and make it more comfortable. The right material for the job depends on several factors: like how conducive it is of heat, how it reacts when it gets moist, cost and ease of installation. Cork and Celotex perform well in most settings, but their qualities appeal to different people - natural performance and breathability versus high efficiency and affordability.

What is cork insulation?

The bark of the cork oak - also known simply as cork - is harvested without harming the tree, making it one of the most sustainable insulation materials available. It’s typically sold in the form of expanded cork boards or loose granules and is 100% biodegradable.

Key properties:

• Thermal performance: λ-value around 0.037-0.040 W/mK, offering solid energy efficiency.
• Acoustic insulation: cork’s natural cellular structure provides excellent sound absorption.
• Sustainability: carbon-negative production process, making it an eco-conscious choice.
• Moisture resistance: breathable and mould-resistant, ideal for older or solid wall buildings.
• Durability: long lifespan with minimal degradation over time.

Cork insulation works equally well in walls, floors and roofs. It's a common choice in heritage buildings and environmentally conscious projects. It especially suits situations that benefit from vapor permeability and natural moisture control.

What is Celotex?

The rigid insulation boards made from polyisocyanurate (PIR) foam - known by their brand name Celotex - are often faced with foil for added resistance to heat. They're one of the most common synthetic insulation products in modern construction.

Key properties:

• Thermal performance: λ-value around 0.022-0.026 W/mK, meaning high insulation efficiency in thin layers.
• Ease of installation: lightweight and easy to cut, making it ideal for large-scale projects.
• Moisture resistance: acts as a vapor barrier, suitable for cavity walls, roofs and floors.
• Affordability: generally cheaper than natural insulation options.
• Fire resistance: meets building regulations when properly installed with protective layers.

Celotex is popular for modern buildings, and especially for buildings requiring slim insulation layers due to its excellent thermal performance. It’s ideal for cavity walls and projects with tight space or budget constraints.

Pros and cons of cork insulation

Pros:

• 100% natural and renewable
• Breathable, preventing condensation and damp
• Excellent acoustic insulation
• Long lifespan
• Low maintenance
• Non-toxic and carbon negative

Cons:

• More expensive upfront than synthetic boards
• Heavier and harder to cut

For more information on this topic read our article Cork Insulation: All Pros and Cons Explained.

Pros and cons of Celotex

 Pros:

• High thermal efficiency with minimal thickness
• Lightweight and easy to install
• Cost-effective for large projects
• Widely available and compliant with building codes

Cons:

• Non-breathable and can trap moisture
• Made from petrochemical materials
• May emit small amounts of VOCs when new
• Less eco-friendly at end-of-life disposal

Which insulation is right for your project?

The best insulation depends on your priorities and the type of building you’re working on.

Choose cork insulation if:
You value natural materials, sustainability, and breathable walls. It’s perfect for older buildings, timber frames, or eco-homes where air quality and environmental impact matter.

Choose Celotex if:
You need maximum thermal performance in a limited space or are working on a new build where cost and efficiency take priority.
In some projects, they are even used together in a hybrid insulation system, combining their qualities.

Cost comparison and long-term value

Although Celotex is cheaper upfront, cork insulation may represent better long-term value. Aside from lasting a long time, it helps to regulate humidity and improves comfort.

Celotex, on the other hand, offers great efficiency for less money and is widely available, having become a go-to choice for homeowners looking for practicality.

Installation tips and considerations

• Cork insulation: handle with care and use fine-toothed saws for cutting. Because it’s breathable, avoid adding vapor barriers that block airflow.
• Celotex: ensure joints are sealed with foil tape to prevent thermal bridging and condensation. Always follow manufacturer safety guidelines for cutting and fitting.
• Hybrid tip: some renovators install a layer of cork on the inside to add breathability while keeping Celotex as the primary thermal barrier.

Proper installation is key to maximising insulation performance, regardless of the material.

Final thoughts - cork insulation vs Celotex

Both cork and Celotex will do great things for your home’s energy efficiency, but they serve different purposes. The right choice ultimately depends on your goals, budget and values: cork is the sustainable choice and shines when it comes to breathability and comfort, while Celotex is unbeatably efficient, cost-effective and practical.

Portuguese Treasures offers a wide range of cork insulation perfect for use in exterior, interior and cavity walls, slabs, floors, roofs, and ceilings.