Design & Build with Wood Fibre Insulation - Learning Lab

Retrofit insulation rarely fails in the middle of a wall panel — it's the junctions, apertures, and service penetrations where things go wrong. In this episode, Chris and Andy dig into the details that most commonly cause failures in practice, from incomplete airtightness membranes to poorly handled window reveals, and why a 30% failure rate in IWI schemes is almost always a detailing problem rather than a system one. If you're specifying or installing internal wall insulation, this one's worth your time.

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.

Chimneys are one of the most persistent sources of heat loss and air leakage in traditional buildings. This episode covers how to insulate and seal a chimney properly — from lining and backfilling the flue, to airtightness at the opening and insulating the breast — without creating moisture problems or fire risks.

Mixing external and internal wall insulation on the same building is sometimes unavoidable, especially where planning or architectural constraints limit what can be done on certain elevations. This episode covers how to manage the junctions between EWI and IWI — keeping the thermal envelope continuous, minimising transitions, and avoiding the cold spots and moisture risks that come from getting the detailing wrong.

In timber frame construction, where you put the OSB matters more than most people realise. In timber frame construction, where you put the OSB matters more than most people realise. Placing it on the internal face of the frame gives you better moisture control by keeping the higher-resistance layer on the warm side, makes airtightness detailing far simpler, and helps absorb the natural movement of the timber before it reaches your plasterboard. If OSB has to go on the outside, you need a high-resistance internal membrane and solid airtightness to avoid condensation problems.

Not all moisture analysis methods are created equal. The Glaser method is widely used and fine for simpler constructions, but it was never designed for solid masonry walls or vapour-open materials, and it misses too many real-world variables to be reliable. For internal wall insulation projects, dynamic hygrothermal simulation using WUFI gives a far more accurate picture of how moisture moves through a wall over time, helping you avoid the kind of slow moisture build-up that causes serious long-term damage.

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.

Suspended timber floors are a common source of heat loss in older homes, and insulating them is usually straightforward — friction-fitting a flexible batt between joists, supported by a breather membrane below and a vapour control layer above. The critical part is managing moisture in the floor void. Cross-ventilation, hit-and-miss dwarf walls, and a ground membrane are all essential. Where reliable ventilation isn't possible, replacing the suspended floor with a solid construction using something like Foamglas aggregate is often the safer long-term solution.

More insulation isn't always better when it comes to internal wall insulation. The safe limits depend on wall type, exposure to weather, and what's embedded in the masonry. Solid brick walls typically allow 60 to 80mm before moisture risk becomes a real concern, while stone walls need an even more cautious approach. The bigger threat isn't always condensation — it's the slow build-up of humidity that can cause timber decay long before any visible damage appears.

U-values are a useful starting point for retrofit projects, but the right target depends heavily on how you're insulating and what you're working with. Building Regulations point to 0.3 W/m2K for thermal elements, though 0.7 W/m2K is the compliance threshold in certain circumstances. For external insulation, 0.3 is achievable and relatively low risk. For internal insulation, a more conservative 0.5 to 0.7 is often wiser — thinner insulation layers carry less moisture risk and still deliver real comfort improvements.

Insulation only works when it's continuous. Even small gaps allow air to move through the build-up, and that convective movement can strip heat away far faster than conduction alone. With flexible batts, it's better to slightly oversize so you get a snug friction fit with no voids. For internal wall insulation using capillary-active materials like wood fibre, gaps are doubly problematic — they break the contact with the masonry that the material relies on to manage moisture, turning a potential buffer into a condensation risk.

Brick creams are siloxane-based hydrophobic treatments that soak into masonry and reduce water ingestion without fully sealing the surface. They can be a useful tool, particularly in retrofit projects where adding internal insulation reduces the wall's ability to dry out. By cutting down how much rainwater gets in, they help restore some balance. That said, they're irreversible, they degrade over time, and they won't fix underlying problems like failing gutters or poor pointing. Useful in the right circumstances, but not a substitute for good detailing.

Insulation slows heat loss through materials, but it can't stop warm air escaping through gaps — that's a job for airtightness. Without it, air leakage bypasses the insulation entirely, and the warm moist air it carries can condense inside the wall build-up causing moisture damage over time. The two need to be designed together. Airtightness doesn't mean sealing a building shut — controlled ventilation still comes in, just on your terms rather than through accidental gaps in the fabric.

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.

When you add external wood fibre insulation to a solid wall, the outer surface becomes much colder, meaning a traditional lime render stays wet for longer after rain. Silicone render solves this by shedding rainwater effectively while still allowing vapour to escape from inside. That balance of water repellency and breathability keeps the insulation drier, extends its service life, and reduces the risk of freeze-thaw damage or biological growth. It works well on other substrates too, not just wood fibre systems.

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.

For most natural insulation work, a formal certification isn't essential. What matters far more is attention to detail — a carefully installed 20mm of insulation will outperform a poorly fitted 100mm every time. Natural materials use familiar trade skills, and resources like Fibres Academy mean good guidance is readily available for anyone who wants it. That said, some specific systems like NatureWall do have a recommended training course, not as a box-ticking exercise, but because consistent detailing is what makes the system perform.

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.

Partial insulation is possible but comes with real risks. Insulating only part of a building creates a mix of warm and cold surfaces, and the colder uninsulated areas end up carrying a greater moisture burden, increasing the risk of condensation and mould. Where insulation stops, thermal bridging becomes a problem too. If a staged approach is necessary, keep the gap between phases short and prioritise a thin continuous layer across the whole envelope over a thick layer in just one area — heat will always find the easiest route out.

Airtightness, breathability, and vapour control are often treated as conflicting priorities but they work together when detailed correctly. Airtightness controls bulk air movement through gaps and cracks, while breathability refers to vapour diffusing through materials molecule by molecule — the two are not the same thing. A lime plaster or intelligent membrane can be airtight and vapour-open at the same time. The key is choosing the right vapour control layer for the build-up, and making sure the wall can dry in the right direction when conditions change.

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.

External insulation wraps the building like a coat, keeping disruption out of the home and making it easier to achieve thicker insulation values with lower moisture risk. The downsides are planning constraints, tricky detailing around windows and junctions, and weather-dependent installation. Internal insulation is more flexible and preserves the external appearance, but it shifts the dew point inward, making moisture management critical. The right choice depends on the building, the site, and the budget — and in many cases a combination of both is the best answer.

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.