Moisture management is the primary purpose of a rainscreen facade. That is the whole point of the design: an outer cladding layer, a ventilated cavity open at top and bottom, and an inner through-wall behind it. Water that gets past the outer panel drains down the cavity and evaporates, and there is no direct path for wind-driven rain to reach the inner leaf.
So why do buildings with rainscreen facades still suffer damp sheathing, corroded fixings, degraded insulation and mould?
Almost always because of decisions made behind the cladding rather than in front of it. The through-wall does the real work, and moisture problems come from the products specified within it, the interfaces between them, and how long they sit exposed before the facade goes on.
Know what you are protecting
There is no strict definition of "through-wall", but it is widely taken to mean the part of the wall that provides most of the fire, thermal and acoustic performance. In practice, that is everything from the internal plasterboard lining to the external sheathing board, fixed to light gauge steel framing, and sometimes including the insulation on the face of the sheathing board.
A typical build-up runs: internal lining, air and vapour control layer, SFS studwork with infill insulation, sheathing board, breather membrane, rainscreen insulation with fixings, helping hand brackets and cladding rails, ventilated cavity, external cladding.
Every one of those layers has a role in moisture control. Get one wrong, and the others cannot compensate.
The four places moisture actually gets you
- Penetrating rain through the cladding. Open-jointed facades let water into the cavity by design. Anything behind that joint is in an area of severe exposure whether or not the site itself is.
- Interstitial condensation. Water vapour generated inside the building diffuses outwards through the wall. If it meets a layer it cannot pass through, it condenses inside the construction where nobody can see it.
- Construction-phase exposure. Steel goes up, boards go on, and the facade may not follow for months. Getting the building watertight so internal trades can start is often the critical path, and the sheathing board and membrane are what deliver that.
- Interfaces and penetrations. Windows, doors, lintels, insulation fixings and membrane laps. The NHBC and other warranty providers concentrate on water resistance for a reason: most warranty claims come from water damage and ingress.
Specify the breather membrane on evidence, not habit
KlasseWRAP KW015 FR, a breather membrane, or vapour permeable membrane, is designed to let vapour permeate through it rather than control it. Vapour from inside diffuses through the wall, passes through the membrane into the ventilated cavity, and drains or dissipates to the outside. Any water coming the other way is deflected because the membrane is water resistant.
This is not the same product as an air and vapour control layer, which sits on the warm side of the insulation and controls how much air and vapour leaves the building in the first place. Whether you need an AVCL should be settled by a condensation risk analysis rather than assumed.
When you specify the external membrane, check it against the standards rather than the product name:
- Water resistance of at least Class W2 to BS EN 13859-2 with no leakage during testing. In areas of very severe exposure, or where liquid water penetration of the cladding is anticipated, such as open-jointed cladding, use Class W1.
- Vapour openness. BS 5250, the code of practice for the management of moisture in buildings, defines a breather membrane as having an Sd value no greater than 0.12, equivalent to 0.6 MNs/g. If it does not breathe, it is not doing the job.
- UV and ageing performance. If the cladding is open-jointed, or the membrane will be exposed longer than normal during construction, performance should be based on tensile and water resistance after artificial ageing. A membrane that meets its figures on day one and fails after several months of UV is a liability, not a specification.
- Fire classification. Membranes forming part of an external wall above ground level should achieve at least Class B-s3,d0 to BS EN 13501-1.
- Installation. Joints protected so moisture drains outwards, laps of at least 100mm at horizontal joints and 150mm at vertical joints. Black membranes suit open-jointed systems visually.
Perforations and poor sealing let vapour and water straight into the cavity, so the detail matters as much as the product.
The sheathing board can be part of the answer
Sheathing board choice is usually discussed in terms of structure and fire, but it is also a weather barrier during and after construction. Predicted exposure, site location and likely weather during the build should all feed the decision.
Different board types sit under different standards. Fibre cement and calcium silicate boards like Klasse C-board, are covered by BS EN 12467, where Category B covers heat, moisture and occasional frost and Category A adds high moisture and severe frost. As a guide from experience, Category B boards can be left exposed up to around six months and Category A up to twelve, though the harmonised standard sets no period itself. Gypsum-based boards like Klasse G-board, fall under BS EN 15283, with GM-H1 the optimum classification, and are dimensionally stable enough to maintain weather protection for up to twelve months during construction.
Klasse G-board go further than weather protection alone. A board that is water resistant and also open to vapour delivers airtightness while still letting damaging moisture escape outwards from the through-wall, and on the right build-up that removes the need for a separate breather membrane. That is a programme saving as well as a specification decision, because it takes a trade and a sequence step out of the critical path to watertight.
Always read the certification rather than the headline. It will tell you how long the board can be left exposed and under what conditions, and some products will require a membrane or sealed joints regardless. Where joints and penetrations do need sealing, follow the board manufacturer's own detail.
Insulation that stays dry and stays put
Rainscreen insulation is there for thermal performance, but it also has to survive being wet occasionally. KlasseROCK Rainscreen Insulation is a BBA-approved stone mineral wool insulation with a declared thermal conductivity of 0.035 W/mK and a Euroclass A1 reaction-to-fire classification. Stone mineral wool with water-repellent additives resists water absorption, and its dimensional stability matters given the heat that builds up in high-rise cladding cavities. A product that expands and contracts opens gaps, and gaps in insulation are where condensation risk and cold bridging start.
Robustness and compressive strength also protect against mechanical damage, and higher compressive strength improves the wall's resistance in service. Specify non-combustible Euroclass A1 where you can. Both stone and glass mineral wool are A1, but glass mineral wool has a low melting point and cannot deliver effective fire resistance.
On U-values, calculate the whole build-up. Metal rails and brackets have a significant effect on heat transmittance through the wall, and U-value calculations need to reflect the design as actually built.
Windows and openings are where it goes wrong
Openings are the highest-risk detail in the whole facade, and the market has already moved on the products used to seal them. Class E EPDM is no longer the default position. Current practice is a sealing membrane achieving a minimum of Class B-s3,d0 to BS EN 13501-1, applied either as a self-adhesive system or bedded with a wet adhesive.
That raises the fire baseline, but it does not settle the moisture question, and this is where openings still fail. A sealing membrane, KlasseSEAL KS007 Interface Sealing Membrane, with a very high Sd value is effectively impermeable, so vapour reaching the joint has nowhere to go. It condenses at the interface, and what follows is corrosion, a downgrade in the thermal performance of the insulation layer, and mould. The logic you apply to the wall applies here: the seal has to be airtight and watertight while still letting vapour escape outwards.
The attachment method then decides the fire classification you actually get on site, because the membrane and the adhesive are tested as a system. That works both ways. An adhesive tested with its membrane can lift a product from B-s3,d0 to B-s1,d0, which is a real gain rather than a marketing one. Equally, a membrane that passes on its own but is installed with an untested adhesive, or with a substitution made on site, has an unknown classification. Unknown is not a position anyone wants to defend at Gateway 2.
Do not trade fire against moisture
Regulation 7(2) sets the floor. Materials forming part of the external wall of a relevant building must achieve Class A1 or A2-s1,d0 to BS EN 13501-1, and Regulation 7(3) exempts a short list that includes membranes, seals, gaskets, fixings, sealants and backer rods.
Those exemptions are worth reading for what they are. They describe what the regulations tolerate, not what a good specification should contain. A breather membrane at the minimum B-s3,d0 is a combustible material sitting in a ventilated cavity with airflow designed into it. It is permitted. It is still fuel.
Compliance is the easy part. The harder question is how much combustible material you are willing to leave behind the cladding at all. Fibre cement, GRP-faced gypsum and calcium silicate boards are all A1. Stone mineral wool is A1. The framing, brackets and rails are steel or aluminium. On a well specified through-wall, the membrane, the opening seals and their adhesives are usually the only combustible elements left in the build-up.
That is why Klasse G-board, an A1 sheathing board which is water resistant and vapour open, and therefore needs no breather membrane, is the gold standard for this kind of wall. It takes the largest remaining combustible layer out of the cavity rather than certifying it and hoping the cavity barriers do the rest. And it pays for itself twice, because removing the membrane removes a product, a trade and a sequence step between steel erection and a watertight building.
Fire performance and moisture performance improve together, and the programme gets shorter. There is no trade to make.
What this looks like on site
- Has a condensation risk analysis been done, and does it tell you whether an AVCL is needed?
- Is the facade open-jointed? If so, W1 membrane and aged performance data.
- How long will the boards and membrane be exposed before the cladding goes on, and does the certification cover that period?
- Is the membrane Sd value 0.12 or below?
- Are the seals around openings vapour permeable, and were they fire tested with the adhesive you are actually using?
- Does the U-value calculation include the brackets and rails?
- Is there test evidence for the through-wall as a system, not just for individual products?
The performance gap between designed and in-use performance is largely a problem of interfaces, both between products and between trades. Under the Building Safety Act, and with the golden thread requiring evidence of compliance at every stage, tried and tested systems with proper test data are considerably easier to defend than assembled assumptions.
How Klasse can support rainscreen wall design
Klasse supplies products for several critical parts of the external wall, including:
Klasse G-board gypsum external sheathing board
Klasse C-board calcium silicate cement board
KlasseROCK rainscreen insulation
KlasseWRAP fire-rated breather membranes
KlasseSEAL interface sealing membranes
KlasseBOND membrane adhesives
Sheathing-board tapes, fixings and accessories
Our team can provide technical product information and help project teams review product compatibility within the proposed wall build-up.
Early coordination is essential. Drainage, ventilation, condensation control, fire performance and interface detailing should be resolved as parts of one external-wall strategy.
Contact the Klasse team to discuss your rainscreen specification or request technical information.
Further guidance
Approved Document C: Resistance to moisture
Klasse technical downloads
