Interlayer condensation is a boundary problem rather than a whole-wall one, and it rewards being treated that way. This guide stays on the plane where masonry meets board — what decides whether that plane stays dry, which figures are actually declared for it, and how a specification for external wall insulation keeps the drying path open across it. The calculation routes and the standards that frame them belong to the companion guide linked below; the interface itself is this page's subject.
Where Interlayer Condensation Forms in a Solid Wall Build-Up
Interlayer condensation forms at a single plane, the masonry-to-board joint, and BBA Certificate 13/5018 assumes a vapour resistance factor of 10 for 215 mm brickwork against 100 for dense blockwork — two very different drying paths. That contrast is the practical starting point, because it says the substrate side of the interface is not one material with one behaviour.
Interstitial condensation describes moisture deposited anywhere within the thickness of an element. Interlayer condensation is the narrower case at a boundary between two adjoining materials, and it earns separate treatment because boundaries are where transport slows: adhesive covers part of the plane, fixings penetrate it, and the original substrate is rarely flat. A build-up can be sound through every layer and still concentrate moisture at that one joint.
Why the Substrate Side Is Not One Material
The sample U-value calculations in BBA 13/5018 set out the assumptions they run on, and reading them across is more instructive than any generic table of material properties.
| Layer in the certificate's sample wall | Vapour resistance factor μ assumed |
|---|---|
| 13 mm internal plaster | 10 |
| 215 mm brickwork, protected, 17.1% mortar | 10 |
| 200 mm dense blockwork, 6.7% mortar | 100 |
| 14 mm external render, wet | 6 |
Those figures are the assumptions behind one certificate's sample calculations for the Atlas/Aval EPS system, not declared properties of every brick or block in the UK. Used at that scope they still carry the decision: a dense blockwork substrate is assumed ten times more resistant to vapour than protected brickwork in the same calculation set, so the two walls arrive at the interface with entirely different drying capacity behind them. Survey which one you have before selecting anything outboard of it.
What Changes at the Interface Once the Board Is Bonded
Bonding insulation to the outside face moves the temperature gradient off the masonry and into the board, which is the whole point of the exercise and the reason a correctly specified system leaves the structure warmer and drier than it found it. The graphite EPS insulation boards stocked here are declared at λD 0.032 W/mK under EN 13163:2012+A1:2015, and a 100 mm graphite board declares a thermal resistance of 3.10 m²K/W from its own declared row rather than from thickness divided by lambda.
That resistance is what carries the masonry across to the warm side. How much of it a given elevation needs, and how the temperature profile is produced, belongs to the U-value and insulation thickness guide; where each layer sits in sequence is set out in the EWI system build-up guide. What matters here is the consequence at one plane: warming the masonry only helps if vapour reaching the interface can continue outward, and BBA 13/5018 assesses that as a system property, recording that the system can contribute to limiting the risk of interstitial and surface condensation against the moisture requirements of the Building Regulations in England and Wales, Scotland and Northern Ireland.
Two figures from the same certificate frame the durability side. The system is assessed as remaining effective for at least 30 years under normal service conditions when installed and maintained to its terms, and the insulation components in isolation are classified E to BS EN 13501-1:2007 — a board property, not a classification of the finished wall.
How to Read Vapour Resistance Through the Render Finish
The finish is the last gate on the outward path, and it is the one layer with a declared, comparable number. Under EN 15824 a render's water vapour permeability is stated as a class with an equivalent air layer thickness, Sd, in metres — the lower the Sd, the less the coating slows vapour leaving the build-up. Two silicone render options on this shelf publish that line, and they do not land in the same class.
| Render | EN 15824 vapour class | Declared Sd | Water absorption |
|---|---|---|---|
| Ceresit CT 74, 1.5 mm grain | V1 | 0.14 m | W3, w < 0.1 kg/m²h⁰·⁵ |
| Ceresit CT 174 Machine, 1.0 mm grain | V2 | 0.14 to 1.4 m | W3, w < 0.1 kg/m²h⁰·⁵ |
Read that table before assuming a silicate-silicone product is automatically the more open choice. The silicone CT 74 declares the tighter figure at V1, Sd 0.14 m, while the machine-applied silicate-silicone declares V2, spanning 0.14 to 1.4 m — a band whose upper end is ten times the lower. Where outward drying at the interface is the governing concern, specify on the declared class of the actual product rather than on binder family, and ask for the sheet.
One scope trap sits inside the CT 74 sheet itself, and it is worth naming. The same document states a second permeability line under ETAG for the render working inside a Ceresit Ceretherm system, permitting Sd up to 1.0 m. That is the less permissive of the two and it is a system figure; the V1 at Sd 0.14 m is the product's own EN 15824 declaration. Quote each at its own scope, because reading the system row as the product's class inverts the comparison entirely. The sheet also describes the base as a water dispersion of silicone and acrylic resins, so binder names are a weaker guide here than declared classes.
Key Takeaway: interlayer condensation is decided at one plane, and three sourced numbers govern it — the substrate's assumed vapour resistance behind the interface, the declared thermal resistance of the board that warms it, and the declared Sd of the finish that lets it dry. A minimum 40% bonded area is what keeps that plane continuous, and it is the same 40% the wind-load case depends on.
What Actually Traps Moisture at the Masonry-to-Board Plane
Interface performance is largely a workmanship outcome, and the certified system is explicit about the discipline it assumes. BBA 13/5018 describes EPS boards fixed with a minimum 40% coverage of adhesive, with supplementary fixings applied through the boards while the adhesive is setting.
- Bonded area is the load path as well as the moisture path. Of the four interfaces the certificate tests, the adhesive-to-insulation bond gives the lowest design resistance at 30 ÷ 9 = 3.3 kN/m², against 250 ÷ 9 = 27.8 kN/m² at the substrate-to-adhesive interface. The plane you are trying to keep dry is also the weakest one structurally, which is a good reason to work it properly.
- Voids behind the board are lateral pathways. Perimeter-and-dab work that falls short of the 40% minimum leaves continuous channels along which air can move sideways to colder spots. Filling the perimeter properly closes the route and raises the bonded area at the same time.
- Fixings are a thermal item at this plane. The certificate's sample U-values assume 4 fixings per m² at a point thermal transmittance of 0.003 W/K per steel pin, recalculated to BS EN ISO 6946:2017 for other types. Layout and density are covered in the fixing pattern and spacing guide, and a recessed plastic-pin plug costs the calculation less than that steel-pin assumption.
- Sequence protects the bond. The GENDERKA EPS 032 technical card asks for board faces to be sanded immediately before bonding, for the grey boards to be shielded from direct sunlight in storage and installation, and for mechanical fixing no earlier than 24 hours after bonding — an interval that lets the adhesive develop before the fixings load it.
How to Specify the Interface Out of Risk
The certificate sets one gate before anything else in the sequence, and it is the single most useful line in the document for retrofit work. On existing buildings the system should only be installed where there are no signs of dampness on the inner surface of the wall other than those caused solely by condensation — so a wall with rising or penetrating damp is diagnosed and resolved first, and the retrofit proceeds on a sound substrate.
- Establish the substrate and its current moisture state, and clear the dampness gate above before specifying anything outboard of it.
- Select the construction against the local wind-driven rain index, which the certificate places with the designer alongside detailing, workmanship and materials.
- Take the board's declared thermal resistance for the specified thickness from its own technical card, then check what that does to the temperature at the interface.
- Choose the finish on its declared EN 15824 class, keeping the most open path the exposure allows, and record the Sd figure in the specification.
- Confirm the bonded area target and the 24-hour fixing interval on the method statement rather than leaving both to the scaffold.
- Detail the head and the openings so water is kept out of the insulation: the certificate asks for the system to be protected by an overhang and for sills to direct water away from the building.
Material choice between graphite EPS and stone wool turns on the same interface question and is argued at board and slab level in the graphite EPS and mineral wool comparison. Where the plinth meets the wall, the transition and its boards are covered in the plinth and façade board comparison. The full calculation routes for condensation risk, and the standards behind them, sit in the dew point condensation risk guide, which owns that method.
Written by Mariusz Saja. Technically reviewed by Rafał Wyrzykowski. Last reviewed Aug 2026.
FAQ — Interface Moisture on Solid Wall Retrofit
What separates interlayer from interstitial condensation in practice?
Interstitial covers moisture anywhere in the thickness of the element; interlayer is the case at a boundary between two materials. The distinction earns its place because a boundary carries discontinuities a bulk layer does not — partial adhesive coverage, fixing penetrations and substrate irregularity — so the plane can concentrate moisture even where each layer either side performs as declared.
Does a silicone render always slow drying more than a silicate-silicone one?
Not on the declared figures read here. The silicone CT 74 declares V1 at Sd 0.14 m under EN 15824, while the machine-applied silicate-silicone declares V2 at 0.14 to 1.4 m. Binder family is a weaker predictor than the declared class, particularly since the CT 74 sheet describes its base as a dispersion of silicone and acrylic resins together. Specify on the class, from the sheet for the exact product and grain.
How much adhesive coverage does the interface actually need?
A minimum of 40% bonded area, in the system assessed by BBA 13/5018, and the certificate applies that same minimum to the two bond resistances it publishes. Where the calculated design resistance falls below the design wind load, the certificate's route is to increase the bonded area rather than to add fixings, since the fixings' contribution is not counted in the wind-load calculation at all.
Can a wall with existing damp be insulated externally?
Not until the cause is identified and resolved. The certificate limits installation on existing buildings to walls showing no signs of dampness on the inner surface other than those caused solely by condensation. Treating that as a survey gate keeps the retrofit straightforward, because a substrate that is already wet has no spare drying capacity at the interface to lend the new build-up.
Is mineral wool the safer board at this interface?
It is the more vapour-open route in principle, and on porous pre-1919 brick it is the usual specification. No declaration of performance for the stocked slabs was retrievable this session, so no permeability, density or fire figure is quoted for them here; the material argument is set out in the comparison guide linked above, and the slab documents can be supplied with a quotation.
Specifying the Build-Up Around the Interface
Every decision on this page resolves to one plane and to documents that can be produced on request: the substrate behind it, the declared resistance of the board that warms it, the declared Sd of the finish that drains it, and the bonded area that keeps it continuous. Renders World supplies the boards, adhesives, meshes and finishes grouped by their position in the build-up, so a specification can be assembled layer by layer with the technical card for each one to hand. Start from the EWI systems category and take the sheets for the layers either side of the interface before the first board is bonded. The two documents behind most of the figures above are the BBA Agrément Certificate 13/5018 and the Ceresit CT 74 technical data sheet.