Insulation - which one to choose?
- Louise Mcleod
- Oct 15
- 4 min read
Whether you're planning a new build, adding an extension, or retrofitting your existing house, insulation is key to keeping your home warm and reducing your energy bills. But with so many products out there, how do you choose the right one?
Standard insulation - 'the plastic option'
The most standard option, PIR (Polyisocyanurate), is the most widely used. It's the cheapest, most widely available, and incorrectly thought to be longer lasting and more effective than the 'greener' alternatives. Unfortunately, at the moment, alternatives are still seen as much more expensive and with an incorrect view that you need more of them to do the same job.
But in fact, that's a huge myth. Many of them are just as—if not more—effective, and the result is a more breathable and sustainable material that creates a healthier home environment.
The 'alternatives'
Wood Fibre
Wood fibre is definitely an option worth considering as an alternative to PIR board. Steico, one of the main manufacturers, has been bought by Kingspan (main manufacturer of PIR), so we're hoping to see a drop in price and an uptake in availability. It comes in boards like PIR and is very easy to cut and fix.
Wood fibre boards offer excellent thermal performance and have superior acoustic properties compared to PIR. They're also highly breathable, which helps regulate moisture in your walls and reduces the risk of condensation and mould. Available in various thicknesses, they can be used for walls, roofs, and floors in both new builds and retrofit projects.
Hemp
Hemp insulation is a quickly growing alternative. This is not hempcrete, FYI—it's a soft material (shown in foreground of left pic) used in the same way as you would any soft insulation, with incredibly good insulation value. Hemp grows rapidly without pesticides, absorbs CO2 as it grows, and creates a carbon-negative product. It's naturally resistant to pests and mould, and won't irritate your skin during installation.
Hemp batts typically achieve high thermal conductivity, matching or exceeding mineral wool performance. They're particularly good for stud walls, between rafters, and under floors.
Sheep's Wool
Sheep's wool is brilliant (yes, this one you need a bit more thickness for) but is very low cost. It only comes loose at the moment but is great for your lofts where depth isnt an issue and stops you being itchy when you go and get your Christmas decorations and suitcases from the loft every year.
Wool has natural properties that manage moisture brilliantly—it can absorb up to 33% of its weight in moisture without losing insulating properties. It's also naturally fire-resistant and biodegradable. While the thermal conductivity is slightly lower than some alternatives, it's offset by its superior moisture management.
Cork
Cork is an excellent natural insulator that's often overlooked. Harvested from the bark of cork oak trees without harming them, it's completely renewable and sustainable. Cork boards are naturally resistant to moisture, mould, and pests. and can be used as finished surface on exterior walls.
Cork is particularly good for external wall insulation, under-floor insulation, and flat roofs. It's also incredibly durable and won't degrade over time.
Cellulose (Recycled Paper)
Made from recycled newspaper treated with non-toxic fire retardants, cellulose is one of the most eco-friendly options available. It can be blown into cavities, making it perfect for retrofit projects where access is limited. The blown-in application means it fills every gap and crevice, eliminating cold spots and air leakage that can occur with batt insulation.
Woodwool
Woodwool (also called wood strand) consists of long wood fibres bound together with a binder. These boards are rigid, strong, and provide both insulation and structural support. They're particularly popular for walls and ceilings where you need something robust.
They offer excellent acoustic properties and are often used in music rooms and home cinemas.
Foam Glass (Recycled Glass) Insulation
Foam glass is an excellent choice for floor insulation, particularly under or instead of concrete slabs and ground floors. Made from 100% recycled glass that's been crushed, heated, and expanded into a lightweight cellular structure, it's one of the most durable and eco-friendly options available. It offers good insulation performance while being completely waterproof, non-combustible, and resistant to rodents and insects.
Unlike many other insulation materials, foam glass won't degrade over time, compress under load, or absorb moisture—making it ideal for damp-prone areas like basements and ground floors. It's also highly breathable and creates a capillary break that prevents ground moisture from rising into your building. The material is particularly popular in Passivhaus projects and sustainable builds because it's carbon-negative when you account for the recycled content.
Emerging Alternatives
At CAT (Centre for Alternative Technology), they're looking at lots of other sustainable and renewable fibres to use to make insulation. Sunflower stalks are proving to be very good—an easy and quick to grow option that could become more widely available in coming years.
Other innovations we could see on its way too are Mushroom insulation (mycelium-based) and Seaweed insulation (particularly interesting for coastal areas)
Making the right choice for your project
The perception that natural insulation materials are more expensive or less effective is outdated. While the upfront cost might be slightly higher in some cases, the long-term benefits—improved air quality, better moisture management, lower embodied carbon, and comparable thermal performance—make them a wise investment for any home improvement project.
For new builds, you have the freedom to design with any insulation type. Consider using wood fibre boards for walls and roofs, with cellulose or hemp for stud cavities.
For extensions, match or exceed the performance of your existing home. Natural materials often work better with older building fabric, especially in period properties where breathability is crucial.
For retrofit projects, blown cellulose is often the most practical for cavity walls, while sheep's wool or hemp batts work well for accessible lofts and internal walls.
As availability increases and more mainstream manufacturers (like Kingspan acquiring Steico) enter the natural insulation market, prices will continue to fall, making sustainable choices accessible to everyone.
Your home isn't just about keeping warm—it's about creating a healthy, comfortable space that treads lightly on the planet. Choosing the right insulation is one of the most impactful decisions you'll make.
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