Walls that are allowed to dry
Why vapour-open build-ups matter in solid masonry, and what “breathable” actually means on a pre-1919 wall.
Common questions
Questions?
Everything you need to know before your free assessment.
Still unsure? Call us on 01285 407088. We’re always delighted to talk through the specifics of your project.
Book an assessmentWhat is BreatheTherm?
BreatheTherm is a breathable, fabric-first retrofit system developed specifically for solid-wall and heritage buildings. Rather than offering a single product, we look at how the building behaves as a whole (moisture, condensation, comfort and energy performance) and then design an appropriate wall, floor or junction build-up around that.
Is this suitable for listed and heritage buildings?
Yes. All materials are mineral, vapour-open and designed to work with traditional construction. The approach supports drying and long-term fabric health, which generally aligns well with conservation principles.
Why can’t standard modern insulation be used in pre-1919 homes?
Pre-1919 buildings were built using breathable lime mortar and solid masonry designed to absorb and release moisture naturally. Modern PIR foam boards and mineral wool are vapour-resistant: when installed in old walls, they trap moisture inside the structure. This causes interstitial condensation, accelerated decay of historic fabric, and serious structural damp. BreatheTherm uses vapour-permeable mineral materials that work with the natural moisture cycle rather than against it.
What U-value can I achieve with BreatheTherm?
Depending on the thickness available and existing wall construction, BreatheTherm systems can achieve U-values between 0.17 and 0.45 W/m²K. A typical 50mm PerliTherm build-up on a 215mm solid brick wall achieves approximately 0.35 W/m²K. Combining PerliTherm with our 10mm Aerogel Booster in constrained reveals can reach 0.17 W/m²K. A site-specific hygrothermal calculation is produced for every project.
Does installing internal wall insulation require planning permission?
Internal insulation does not typically require planning permission as it doesn’t alter the building’s external appearance. However, if your property is Listed, you will need Listed Building Consent before carrying out works that affect its character. BreatheTherm can provide supporting hygrothermal modelling reports and documentation to assist consent applications.
How long does BreatheTherm installation take?
For a typical Victorian or Georgian home, internal wall insulation takes 2–5 days per room, or 1–3 weeks for a whole house depending on scope and preparation required. The system uses dry-application materials (lime plaster requires 3–7 days curing before decoration).
What problems does it address first?
Cold internal surfaces, condensation and comfort issues, while avoiding the moisture-trapping risks that can arise with more conventional retrofit approaches.
What is perlite?
Perlite is a natural volcanic mineral. When heated it expands and traps air, which makes our lime-based renders lightweight, insulating and vapour-open. Within the system it helps walls manage moisture safely by absorbing excess humidity and releasing it again as conditions dry.
What is aerogel made from?
Aerogel is a mineral material based on silica (the same basic constituent as sand and glass). During manufacture the liquid component is replaced with air, creating an extremely lightweight, non-combustible and highly insulating solid.
How do perlite and aerogel work together?
They are the component parts. Perlite forms the breathable, moisture-regulating backbone of the system. Aerogel is introduced selectively, typically in thin layers of around 5–30 mm, to enhance thermal performance where space, detailing or heritage constraints limit build-up thickness. They can be used together or independently, depending on the building and the priorities of the project.
Is BreatheTherm just insulation?
Not really. It’s a joined-up approach to the building fabric, considering floors, walls, junctions and finishes so that improvements work together safely rather than in isolation.
What is vapour-permeable (breathable) insulation?
Vapour-permeable insulation allows moisture vapour to pass through the wall assembly, preventing it accumulating within the structure. Materials like expanded perlite and silica aerogel are hygroscopic: they can absorb small amounts of moisture and release it as conditions dry. This capillary activity keeps walls dry without impermeable barriers, making them ideal for solid masonry and timber-framed heritage buildings.
What is WTA certification and why does it matter?
The WTA (Wissenschaftlich-Technische Arbeitsgemeinschaft) is the German scientific association for building maintenance and monument preservation. WTA Merkblatt 6-4 defines the internationally recognised technical standard for internal insulation of historic buildings. A WTA-compliant system has been tested to manage moisture correctly in heritage wall constructions, giving architects, conservation officers and homeowners confidence that the insulation will protect, not harm, the historic fabric.
Can BreatheTherm be used on timber-framed or cob buildings?
Yes. The vapour-permeable nature of BreatheTherm materials makes them particularly suitable for timber-frame, wattle-and-daub, cob, and flint construction, all of which need to breathe freely. We adapt the specification for each substrate. Our technical team will advise on the most appropriate system for your specific construction type on the free assessment visit.