With 2026 the UK’s hottest summer on record, homeowners and specifiers are increasingly asking whether their insulation can keep homes cooler as well as warmer.
When choosing insulation, most people focus solely on keeping heat in during winter. While reducing heating bills is important, rising UK summer temperatures mean homeowners, architects, and builders must ask a new question: Will this insulation keep the home cool when the sun is beating down?
This is where natural-fibre insulation like Sisalwool makes a massive difference.
Why Standard Insulation Fails in Summer
Most traditional insulation is judged strictly on winter performance. However, roofs and buildings work incredibly hard in the summer.
On a sunny day, walls and roofs bear the brunt of solar radiation - this intense heat slowly penetrates the structure and the building can absorb a significant amount of solar energy. If the insulation material cannot manage that thermal energy, it creates a stuffy, uncomfortable living space that stays hot long into the night.
To prevent this, high-performance insulation must provide year-round comfort, not just winter resistance.
The Science of Decrement Delay…. What Is It?
"Decrement delay" is the technical term for how long it takes heat to travel through a building and enter a room. The longer the delay, the cooler the home stays during peak sun hours.
Sisalwool acts like a ‘thermal sponge’ due to its natural composition. Instead of allowing a sudden rush of heat into the living space, its natural fibres absorb the thermal energy and release it gradually.
By delaying the peak heat transfer until late evening when outdoor temperatures drop, homeowners can easily flush the warmth out through open windows or rooflights. This drastically reduces overheating in vulnerable spaces, including:
- Loft conversions and top-floor bedrooms
- Rooms with large, south-facing rooflights
- Low-energy and eco-build designs
How Does Sisalwool Compare to PIR?
Rigid PIR boards are popular because they offer high thermal resistance (low U-values) in restricted, thin rafter spaces. While foil-faced PIR reflects radiant heat, it cannot provide a heat-buffering effect.
Consequently, a PIR roof and a Sisalwool roof might achieve the exact same winter U-value, yet perform completely differently during a summer heatwave. Sisalwool prevents extreme indoor temperature swings by moderating both seasons: it traps heat indoors during January, and blocks solar heat in July.
Proven Heat Capacity Data
Sisalwool’s thermal buffering is backed by empirical data, not just marketing claims. In April 2026, specimens of Sisalwool were independently tested for specific heat capacity at the University of Bath.
The samples recorded values between 2,000 and 2,600 J/kgK. This high capacity validates Sisalwool’s ability to absorb large amounts of thermal energy before transferring it indoors.
Four Additional Benefits of Natural Fibres
Beyond temperature regulation, Sisalwool’s make-up solves several structural and environmental challenges:
- Breathability: Helps manage structural moisture naturally.
- Acoustic Insulation: The dense, fibrous material absorbs airborne noise, significantly quietening rain, traffic, and aircraft sounds.
- Lower Carbon Footprint: Offers a sustainable, renewable alternative to fossil-fuel-derived petrochemical insulation.
- Installer Friendly: The soft, flexible batts fit tightly around irregular rafters without causing the itching associated with synthetic alternatives.
Conclusion: Designing for Every Season
An optimised building is about more than chasing the thinnest possible profile. It is about creating spaces that remain comfortable on cold January mornings and humid August nights.
For architects, developers, and self-builders, choosing Sisalwool means delivering acoustic comfort, moisture safety, lower embodied carbon, and true year-round thermal stability.
the sisalwool collection
