Understanding Performance

In this episode, Chris sits down with Andy to tackle a question that's become increasingly relevant after last week's 30°C+ temperatures across the UK — why do well-insulated homes still overheat? Andy explains that U-values, while important, are just one piece of the puzzle, and that a singular focus on thermal conductivity often leads to buildings that perform well in winter but become uncomfortable in summer. He walks through the key factors at play — airtightness, window orientation, and crucially, the thermal properties of the insulation itself — and makes the case for wood fibre as a material that genuinely addresses the overheating problem. With a decrement delay of over 16 hours compared to around two and a half for fibreglass, wood fibre slows heat movement through the fabric dramatically, meaning peak external temperatures never really make it inside. A timely watch for anyone who's been sweating in a supposedly energy-efficient home.

In this episode, Chris sits down with Marion for her first appearance on Can I Just Ask, talking about PAS2035, a subject that's well within her wheelhouse as a retrofit specialist. Marion explains that PAS2035 is a framework designed to make sure retrofit projects are delivered properly, from initial assessment all the way through to post-occupancy evaluation, with the right people involved at every stage. She walks through the key roles (retrofit coordinators, retrofit designers and installers working under the companion standard PAS2030) and clarifies that the scheme only applies to government-funded projects like ECO or Warm Homes Grant. It's a clear, jargon-light breakdown of a framework that affects a huge amount of retrofit activity in the UK but rarely gets explained in plain English.

Wood fibre insulation being made from natural material does raise the odd eyebrow when it comes to pests, but the concern is largely unfounded. The manufacturing process removes the sugars that wood-boring insects are actually after, and the material doesn't provide the dense structure they need. Rodents will nest in any insulation if they can get to it, but that's a detailing problem, not a material one. Good airtightness and careful sealing at junctions is what keeps pests out.

Wood fibre insulation earns its place in specifications for technical reasons as much as environmental ones. Its high thermal mass slows heat transfer and helps keep buildings cool in summer, which is increasingly important. It friction-fits into place without gaps, making real-world performance much closer to designed performance. It also handles moisture well, allowing vapour to move safely through the structure rather than trapping it. Practical, reliable, and genuinely low-carbon — it's one of the few materials that delivers on all fronts.

PIR has a lower thermal conductivity than wood fibre, but that single metric misses a lot. Wood fibre outperforms it on acoustic absorption, handles moisture on site without losing performance, and its thermal mass helps prevent summer overheating — something PIR does nothing to address. From a fire safety perspective, PIR can produce toxic fumes before flames take hold, while wood fibre burns slowly and predictably. Add in the carbon footprint difference and the easier installation, and wood fibre makes a compelling case across the board.

Wood fibre is combustible, but that's not the full picture. It chars rather than melts, and that char layer slows the spread of fire and protects the structure beneath. Crucially, it produces very little toxic smoke compared to synthetic insulants like PIR, which can off-gas cyanides under fire conditions. With the right build-up, wood fibre systems can achieve 30, 60, or 90 minute fire resistance ratings — no chemical flame retardants needed. In a fire, buying time and limiting toxicity matters as much as non-combustibility.

Natural ventilation sounds appealing but in a well-sealed, high-performance home it rarely delivers consistent results. Relying on open windows means CO2 can climb well above comfortable levels overnight, and moisture from cooking or drying clothes builds up without effective extraction. MVHR offers a more reliable alternative, running quietly in the background to supply fresh filtered air and extract stale air continuously. In an airtight home, natural ventilation is often just uncontrolled ventilation — and that's not the same thing at all.

Insulation is about more than cutting energy bills. By reducing heat loss, it raises the surface temperature of walls and floors, which makes a real difference to comfort — cold surfaces absorb body heat and make a room feel colder than the air temperature suggests. Warmer surfaces also mean less condensation, which is where mould and poor air quality start. Modern living generates a lot of indoor moisture, and without insulation, older buildings struggle to cope. Done right, insulation improves energy performance, comfort, and building health all at once.

Plasterboard gets a bad reputation in natural building circles, but much of it is undeserved. It's made from gypsum, a natural mineral, and is actually highly vapour permeable — in some cases more so than lime. Used correctly as part of a well-specified system, particularly alongside hygroscopic insulation like wood fibre, it performs well and helps buffer moisture. It also has low embodied energy compared to lime and carries good fire ratings. The issues people associate with it usually come down to misuse rather than the material itself.