Why This Comparison Matters
If you’re looking at external wall insulation, you might have noticed it’s not as simple as picking “the warmest option.” Two insulations might seem similar on paper, but once they’re installed, they behave very differently.
The real difference is how they manage moisture, comfort and long-term performance in your walls. This guide compares wood fibre boards and Rockwool insulation so you can make the right choice for your home, whether it’s a solid brick Victorian terrace or a newer cavity wall property.
What Is Wood Fibre Insulation?
Wood fibre insulation comes in rigid boards made from compressed timber. It’s breathable, so it lets moisture pass through the wall rather than trapping it. Its density also gives your walls a bit of “thermal mass,” which helps keep indoor temperatures stable and warmer in winter and cooler in summer.
It works really well on older homes with solid walls, stone houses or buildings with lime mortar. Basically, if your property was built before the 1920s, wood fibre often makes a lot of sense. It’s designed to work with the wall, not against it, which helps prevent damp and keeps your home comfortable.
What Is Rockwool Insulation?
Rockwool is a brand of mineral wool insulation. Think of it as stone spun into fluffy slabs that insulate your walls. While it’s a brand, it behaves very similarly to other mineral wool products.
It’s non-combustible, lightweight and easy to handle. It’s commonly used on modern homes with cavity walls or projects that need a high fire rating. Rockwool keeps heat in well and also helps with noise reduction, though it doesn’t manage moisture in the same way wood fibre does.
Moisture and Breathability
This is often the most important difference, especially in older homes. Solid walls absorb moisture and they need to be able to dry naturally. If insulation traps moisture, you can end up with damp patches, mould, cold walls or peeling paint, not fun.
Wood fibre absorbs and releases moisture slowly, keeping walls dry and reducing the risk of condensation. This makes it perfect for solid walls, lime mortar construction or any older property prone to damp.
Rockwool is vapour permeable, so it allows moisture to pass through but it doesn’t buffer or regulate it inside the wall. That’s usually fine for modern cavity walls, but for older solid walls, wood fibre is often safer in the long term.
Thermal Performance
Rockwool slightly outperforms wood fibre per millimetre in raw insulation, so a thinner layer can achieve the same U-value. But wood fibre’s density gives it thermal mass, which helps keep indoor temperatures steady. Walls cool more slowly overnight and the house feels more consistently comfortable.
In other words, Rockwool keeps heat out, while wood fibre helps your home feel cosy all day long.
Fire Safety
Rockwool is non-combustible and rated Euroclass A1, making it extremely fire safe. Wood fibre boards are treated for fire resistance and meet building regulations when used in a full EWI system, but they’re not rated A1. If fire safety is a top priority, Rockwool has the edge.
Acoustic Comfort
Both materials help make your home quieter. Wood fibre’s density absorbs external impact noise, while Rockwool disrupts airborne sound like voices or higher-frequency noise. In a typical home, the difference is small either choice will improve comfort noticeably compared to an uninsulated wall.
Environmental Impact
Wood fibre is made from timber by-products and stores carbon in the building fabric. It’s low-energy to produce, which makes it a good choice if you’re thinking about sustainability.
Rockwool is durable and recyclable, but it needs very high temperatures to make, so it has a higher carbon footprint. If you care about eco-friendly retrofits or breathable wall construction, wood fibre usually comes out ahead.
Installation
Rockwool slabs are lightweight, easy to cut and quick to install. They’re forgiving on uneven walls, which makes them ideal for modern homes or larger projects.
Wood fibre boards are heavier and need careful fixing. Installation can take longer, but once it’s done, your wall build-up is solid, long-lasting and very robust.
Which Insulation Should You Choose?
If your home is older, pre-1920, solid brick or stone, lime mortar construction or prone to damp wood fibre, it is often the safer choice. It works with your walls rather than against them.
If your home is modern, or you want faster installation, high fire rating, or cavity wall insulation, Rockwool usually makes more sense.
The key is matching the insulation to your property. The right choice can make your walls healthier, your home more comfortable and your heating bills lower.
Both materials are excellent insulations, but they do slightly different jobs. Rockwool offers strong fire protection, ease of installation and good heat retention. Wood fibre gives better moisture management, sustainable credentials and all-day comfort.
Choosing the right one depends on your home, not just the product. Get it right and your EWI project will keep your house warm, dry and comfortable for decades. For advice tailored to your property, the EWI Store team can help you pick the best insulation system.