Insulate traditional buildings without trapping moisture
A vapour-permeable insulation system for solid-wall, heritage and hard-to-treat properties. Declared values, build-up detail and specification resources.
This page exists to let you decide whether to shortlist BreatheTherm in about a minute, and to specify it without needing to call us. If something you need is missing, tell us and we will add it.
The problem with conventional insulation
Solid-wall buildings constructed before about 1919 were built to a different moisture model. Lime mortar, solid masonry and permeable finishes allow the wall to absorb moisture and release it again. There is no cavity, no membrane and no designed drainage plane; the drying capacity of the assembly is the moisture strategy.
Introducing a vapour-resistant insulation layer on the internal face changes two things at once. It lowers the temperature of the masonry behind the insulation, which increases the relative humidity within the wall, and it removes the drying path that previously ran inwards. The result is accumulation at the insulation–masonry interface.
The consequences are well documented and familiar to anyone who has surveyed a failed retrofit:
- Interstitial condensation at the cold face of the insulation
- Elevated moisture content in embedded timbers, particularly joist ends built into the wall
- Frost damage and spalling where masonry cannot dry before a freeze cycle
- Salt migration and efflorescence at the interface
- Mould growth at thermal bridges, typically reveals and floor junctions
- Degraded thermal performance, since wet masonry conducts heat substantially faster than dry
Traditional wall + impermeable insulation
Vapour drive meets a high-Sd layer and stops. Moisture accumulates at the interface, where it is coldest and dries slowest.
Moisture trapped
Traditional wall + BreatheTherm
Every layer is vapour-open and capillary-active. Moisture migrates through the assembly and is buffered by the hygroscopic lime-perlite matrix.
Moisture can migrate and dry
Sammy's brief calls for a simple two-panel graphic showing vapour drive meeting an impermeable layer versus migrating through a vapour-open assembly. The panel above is a text stand-in. A drawn figure will carry far more weight with a specifier than prose.
How it compares to the common alternatives
| System | Typical λ | Vapour behaviour | Capillary active | Junction / reveal detailing |
|---|---|---|---|---|
| PIR board | 0.022 | Vapour-tight; foil facings give very high Sd | No | Rigid board; difficult on irregular substrates, cold bridging at fixings |
| Mineral wool (rockwool) on battens | 0.035 | Vapour-open itself, but normally installed behind a vapour control layer | No | Void behind the insulation is a condensation risk |
| Wood fibre board | 0.038–0.045 | Vapour-open | Yes | Organic; moisture tolerance is finite, and build-up depth is greater |
| Wood wool slab | 0.065–0.085 | Vapour-open | Partial | Usually a carrier board rather than the primary insulant |
| BreatheTherm (perlite) | 0.064 | Vapour-open, Sd 0.2–0.4 m at 50 mm | Yes | Applied wet, monolithic, follows irregular substrates with no void or fixings |
| BreatheTherm (aerogel layer) | 0.017 | Vapour-open, Sd 0.04–0.08 m | Yes | Used selectively where build depth is critically constrained |
Comparator lambda values are typical published figures for the generic product category, not for any named manufacturer's product. BreatheTherm figures are our own declared values.
Why the system exists
BreatheTherm was developed by two builders rather than a materials company. Josh Tetley founded HF Limited in 2003; Greg Podolak joined in 2007, and together they launched HF Building in 2016 specialising in conservation and retrofit of period properties.
Across two decades of that work the same pattern recurred: products specified in good faith were damaging the buildings they were installed to improve. The system was developed to combine the vapour-open behaviour of traditional lime construction with thermal performance closer to that of modern insulation, rather than asking a specifier to choose between the two.
How the system works
The wall assembly
Applied directly to prepared masonry. No frame, no battens, no service void, no membrane. The absence of a cavity behind the insulation is deliberate — an air gap at the cold face of an insulation layer is where interstitial condensation collects.
- Existing substrate — solid brick, stone, cob, flint or timber-frame infill, prepared back to sound material
- PerliScratch — lime-perlite scratch coat providing a keyed, high-adhesion base
- PerliTherm — expanded perlite bound with hydraulic lime; the primary insulating and moisture-buffering layer
- AeroBond + mesh — fibre-reinforced lime-polymer adhesive and basecoat with reinforcement, replacing separate primer, adhesive and basecoat
- Aerogel layer — optional, used selectively where build depth is constrained
- PerliFinish — breathable lime finish coat
How moisture moves through the assembly
Three mechanisms operate together. Vapour diffusion moves moisture through the assembly along the vapour pressure gradient; because every layer has a low Sd value, there is no point at which that flow is arrested. Capillary transport moves liquid water back towards the drying face through the pore structure of the lime-perlite matrix, which is the mechanism conventional insulation lacks entirely. Hygroscopic buffering absorbs moisture at humidity peaks and releases it as conditions dry, damping the spikes that produce surface condensation.
A section drawing showing vapour diffusion, capillary return and hygroscopic buffering across the assembly, with an indicative temperature and dewpoint profile overlaid. This is the single most persuasive graphic the page could carry.
Why perlite, and why lime
Perlite is a naturally occurring volcanic mineral. Heated, it expands and traps air, producing a lightweight aggregate that is insulating, non-combustible and open-pored. Bound in hydraulic lime it produces a render that is softer than the masonry it is applied to, vapour-open, and capillary active — three properties a cement- or gypsum-based product cannot offer simultaneously.
Lime matters for compatibility as much as for breathability. A binder harder than the substrate concentrates stress at the interface and causes the render to fail before the wall does. On historic masonry the sacrificial layer should always be the finish, not the fabric.
Why aerogel
Silica aerogel has the lowest thermal conductivity of any usable solid insulant at 0.017 W/m·K, and it remains vapour-open. Its role in the system is specific rather than general: it is used where build depth is critically constrained and perlite alone cannot reach the target U-value — window reveals, recessed frames, tight junctions and rooms where losing 50 mm from the wall face would alter the proportions unacceptably. It costs roughly five times as much per millimetre of build-up saved, so specifying it across whole elevations is rarely justified.
Technical performance
Thermal
| Layer | Function | Thickness | λ (W/m·K) | Sd (m) | Guide price /m² |
|---|---|---|---|---|---|
| PerliScratch | Lime-perlite scratch / key coat | 3–5 mm | 0.080 | 0.02–0.04 | £4.60 |
| PerliTherm | Expanded perlite insulating render, hydraulic lime bound | 50 mm | 0.064 | 0.2–0.4 | £45.00 |
| AeroBond | Fibre-reinforced lime-polymer adhesive and basecoat | 3–6 mm | 0.12 | 0.02–0.04 | £20.00 |
| Aerogel layer | Silica aerogel blanket for constrained build-ups | 10 mm | 0.017 | 0.04–0.08 | £23.70 |
| PerliFinish | Breathable lime finish coat | 20 mm | — | 0.10–0.20 | £6.60 |
Guide prices are supply only, excluding VAT, and are indicative. Every build-up is specified per project. See the full cost guide.
| Layer | Function | Thickness | λ (W/m·K) | Sd (m) | Guide price /m² |
|---|---|---|---|---|---|
| PerliMat | Vapour-permeable geotextile separator | 3.5 mm | n/a | Vapour open | £3.00 |
| PerliBase | Mineral insulating floor underbase | 75–200 mm | 0.045 | 0.3–0.6 | £59.25 |
| PerliStruct | Lime-perlite structural screed / underbase | 30–150 mm | 0.080–0.090 | 0.20–0.40 | from £46.00 |
| AeroBond | Fibre-reinforced lime-polymer adhesive and basecoat | 5 mm | 0.12 | 0.02–0.04 | £20.00 |
| Aerogel layer | Silica aerogel blanket | 5 mm | 0.017 | 0.04–0.08 | £23.25 |
| PerliScreed | Vapour-open, membrane-free floor screed | 75–200 mm | 0.045 | 0.3–0.6 | £59.25 |
Guide prices are supply only, excluding VAT, and are indicative. Every build-up is specified per project. See the full cost guide.
| Substrate | Build-up | U-value (W/m²K) |
|---|---|---|
| 215 mm solid brick | 50 mm PerliTherm system | ≈ 0.35 |
| Constrained reveal / deep retrofit | PerliTherm + 10 mm aerogel layer | 0.17 |
| Range across all specifications | Varies by substrate and available depth | 0.17 – 0.45 |
A site-specific calculation is produced for every project. Generic figures are indicative only — the same build-up on a sheltered elevation and an exposed gable will not perform identically.
Moisture
Sd values for every layer are given in the tables above. A complete 50 mm wall build-up including scratch coat, insulation, basecoat and finish has a total equivalent air layer thickness in the region of 0.35–0.7 m, which places it firmly in the vapour-open range used in WTA guidance for internal insulation of historic buildings.
Condensation risk analysis is produced per project. See hygrothermal evidence below.
Fire
Required: reaction-to-fire classification to BS EN 13501-1 for each product and for the system as installed; the test standard and notified body; any restrictions on use in buildings over 18 m or in relevant buildings under the Building Safety Act. Perlite and silica aerogel are both mineral and non-combustible in raw form, but the bound system's classification must come from actual test data. Do not publish an inferred figure.
Durability and maintenance
Required: expected service life of the system, maintenance requirements for the lime finish, redecoration guidance including which paint systems are compatible (mineral and silicate paints only, presumably, but this needs confirming), and repairability of damaged areas.
Accreditation and third-party evidence
The system is developed to WTA Merkblatt 6-4, the German technical standard for internal insulation of historic buildings, which is the reference document conservation officers and building physicists across Europe use for this application.
Required: current status of BBA certification, ETA, or any other third-party product approval, including in-progress applications with expected dates. Also: which independent laboratory produced the declared lambda and Sd values, and whether Declarations of Performance are available for download. Specifiers routinely filter on this, and an honest 'ETA application submitted, expected Q2 2027' is far better than silence.
Can I specify it for my project?
| Construction type | Suitable? | Notes |
|---|---|---|
| Solid brick walls | Yes | The core use case. Typically 215 mm or 330 mm solid brick with lime mortar. |
| Solid stone walls | Yes | Includes rubble-filled and Cotswold limestone construction. |
| Cob | Yes | Vapour-open throughout is essential; cob must be able to dry inwards and outwards. |
| Timber frame with infill | Yes | Wattle-and-daub and brick-nogged panels. Junction detailing is specified case by case. |
| Flint | Yes | Irregular substrate; a thicker levelling coat is usually required. |
| Listed buildings (all grades) | Yes | Subject to Listed Building Consent. We supply supporting documentation. |
| Conservation areas | Yes | Internal work does not alter external appearance, so area restrictions rarely apply. |
| Walls with active damp | Diagnose first | Rising damp, failed rainwater goods or defective pointing must be resolved before insulating. |
| Modern cavity walls | Possible | Not the intended application. Conventional measures are usually more cost-effective. |
| Basements below ground | Case by case | Depends on water table, existing tanking and drainage. Speak to the technical team. |
Detail drawings & specification resources
Available on request to specifiers:
NBS specification clauses
Ready-to-use clauses for direct incorporation into your specification.
RequestCAD detail library
Standard junction details including reveals, joist ends, floor-to-wall and party walls.
RequestInstallation manual
Application sequence, curing times, tolerances and substrate preparation.
RequestProject-specific build-up
Send drawings and we will return a proposed specification with calculated U-values.
Send drawingsEvery resource above is currently a mailto request, which introduces friction at exactly the point where a specifier is ready to act. Sammy's brief is explicit that this is where most manufacturer sites fail. Host the NBS clauses, CAD/DWG details, BIM objects, data sheets and installation manual as direct downloads behind an optional email capture, and replace these links. A U-value calculator would go further still.
Hygrothermal evidence
Designed around building physics rather than around a product range. Every project receives a site-specific hygrothermal assessment covering the proposed build-up, the existing wall construction, exposure and orientation, and internal moisture load.
Required: two or three worked modelling examples suitable for publication — ideally a 215 mm solid brick wall, a rubble stone wall and a timber-frame infill panel. For each: total water content over a five-year run, relative humidity at the insulation–masonry interface, moisture content at an embedded joist end, and the before/after comparison against an impermeable build-up. Sammy's brief identifies this as the strongest available differentiator, and it is the section a building physicist will look at first.
Where a project carries unusual risk — a highly exposed elevation, an embedded timber floor structure, or a wall with a history of moisture ingress — modelling output can be issued directly to support a Listed Building Consent application or to satisfy a client's own technical reviewer.
Technical case studies
17th century Cotswold farmhouse
Construction: solid limestone walls, cob partitions, original lime
plaster.
Constraint: Historic England requirements; no loss of internal
detail.
Result: 0.22 W/m²K, energy bills reduced 52%.
Mansion block, 12 units
Construction: solid brick, multi-occupancy.
Constraint: phased works across occupied flats.
Result: 0.19 W/m²K, bills reduced 71%, hygrothermal monitoring installed
throughout, zero moisture failures recorded at 36-month review.
Bedroom renovation, Upper Park Street
Construction: bare brick walls, retained lath ceiling.
Constraint: 25 mm maximum build-up.
Result: wall heat loss down 46%, ceiling down 38%, using PerliScratch,
AeroBond, mesh and PerliTherm.
Sammy's brief asks for technical case studies rather than marketing ones. For each project: building age, wall construction and thickness, existing defects identified, the specified build-up layer by layer, before and after U-values, the moisture considerations addressed, the conservation constraints, and photographs including during-works shots. Each completed study should become its own indexable page linked from here.
Technical FAQs
How does BreatheTherm compare to wood fibre systems?
How does it compare to PIR?
What happens if moisture enters the wall?
Internal or external application?
What thickness is typically required?
Can it be used with lime render or lime plaster?
Is hygrothermal analysis available?
Can it be specified on a listed building?
CPD & technical support
A RIBA-accredited CPD session on moisture in traditional buildings and vapour-open retrofit is available for practices, running approximately 1.5 hours and delivered in your office or remotely. Project-specific technical advice and drawing review are available at no charge during design stage.
For architects & specifiers
Send us drawings for review, request a project-specific build-up, or book a RIBA-accredited CPD session for your practice.