A Homeowner’s Guide to Internal Wall Insulation (IWI)
Thinking about insulating your home? This guide explains everything you need to know about Internal Wall Insulation (IWI) — how it works, when to use it, and what to watch out for in older solid wall homes
Internal wall insulation (IWI) can make older homes warmer and more comfortable by reducing heat loss through external walls. It is most often used in solid-walled properties or where cavity walls cannot be insulated. This guide explains the main approaches, the risks to be aware of, and what we currently consider to be the most reliable solutions.
WHY CONSIDER INTERNAL WALL INSULATION?
In homes with uninsulated walls, up to a third of heat can be lost directly outside. Adding insulation to the inner face reduces this loss, raises internal surface temperatures, and cuts down on condensation and mould. Unlike external wall insulation, which changes the outside appearance, IWI is fitted room by room on the inside.
Sometimes the best results come from combining internal and external wall insulation, depending on the house and its exposure. We would be happy to advise you further on what will be the best solutions for your project.
TYPES OF INSULATION
BREATHABLE (VAPOUR-OPEN) SYSTEMS
These use natural materials such as wood fibre boards, cork, or calcium silicate. They allow moisture vapour to pass through, helping the wall to dry in both directions. This reduces the risk of damp and keeps embedded timbers safe.
VAPOUR-CLOSED SYSTEMS
Typically foil-faced PIR or phenolic boards, often sold as insulated plasterboard. They give high thermal performance in a thin layer, but they are vapour-tight. If moisture enters behind the boards, it has nowhere to go, creating a risk of mould, damp staining, and rot in timbers.
KEY RISKS TO AVOID
Moisture accumulation: Our greatest concern with foil-faced boards is condensation forming on the cold face of the insulation, or within the external wall, where it cannot dry. Over time this can damage masonry and accelerate rot in joists or rafters built into the wall.
Hidden mould: Any voids or gaps behind insulation can collect moist air, leading to unseen mould growth that can spread spores into the home.
Thermal bridging: Poor detailing around reveals and junctions can leave cold spots where condensation and mould form.
Signal loss: Foil facings can reduce mobile phone and Wi-Fi reception, effectively blocking radio signals.
Poor workmanship: Dot-and-dab fixing, cavities behind boards, or unsealed joints all increase the risk of failure.
BENEFITS OF BREATHABLE SYSTEMS
Breathable boards such as wood fibre are thicker for the same U-value, but they provide:
Moisture safety: Hygroscopic materials absorb and release vapour, keeping walls and timbers drier.
Improved comfort: They moderate humidity and temperature swings, making rooms feel more stable.
Sustainability: Made from renewable materials with lower embodied carbon.
Acoustics and fire: Dense fibre boards help with sound insulation and have good fire safety ratings.
Finding installers with relevant experience can sometimes be harder, as many do not like deviating from what they are used to. However, we often find that builders who have worked with wood fibre insulation sing its praises for ease of use and the quality of results.
HOW MUCH INSULATION IS ENOUGH?
Adding insulation quickly reduces heat loss, but benefits taper off as thickness increases. For example:
40 mm wood fibre can cut heat loss by around 65%
60 mm wood fibre improves this to about 75%
100–120 mm brings further gains, but the extra savings are relatively small
For most projects, 40–60 mm of wood fibre strikes the best balance of performance, cost, and practicality.
COMPARISON OF COMMON SYSTEMS
PIR board + plasterboard (vapour-closed):
Typical build-up: 50 mm PIR + 12 mm plasterboard
U-value range on 220 mm solid brick wall: around 0.30–0.35 W/m²K
Notes: Very thin solution, but carries a high risk of trapped moisture and loss of mobile phone signal.
Wood fibre + lime plaster (breathable):
Typical build-up: 60 mm wood fibre + 10 mm lime plaster
U-value range on 220 mm solid brick wall: around 0.45–0.50 W/m²K
Notes: Safe and robust option that allows the wall to breathe. Thicker and more expensive than PIR, but far lower risk of moisture problems.
Wood fibre + plasterboard + breathable paint (hybrid):
Typical build-up: 60 mm wood fibre + 12 mm plasterboard
U-value range on 220 mm solid brick wall: around 0.45–0.50 W/m²K
Notes: Our current preferred balance for most homes. Offers the moisture safety of wood fibre combined with the practicality and cost benefits of a plasterboard finish.
PRACTICAL POINTS
IWI reduces room size slightly; allow for radiators, sockets, and skirtings to be adjusted.
Ensure walls are sound, dry, and repaired before installing.
Plan for ventilation improvements to keep indoor humidity under control.
Seek installers who show a genuine understanding of your project rather than a one-size-fits-all mentality. The best results come from tradespeople who consider detailing and building health as much as insulation thickness.
CONCLUSION
Internal wall insulation can greatly improve comfort in older homes, but only if the right system is chosen and carefully installed. While foil-faced PIR looks attractive on paper, the risks of hidden damp and timber decay make it unsuitable in most solid wall situations. From our experience, a build-up of 40–60 mm wood fibre insulation finished with plasterboard and breathable paint provides the most reliable balance of performance, safety, and practicality for many domestic projects.
Disclaimer: This article is provided as general guidance only and does not constitute professional advice. Always seek tailored advice for your own project.
