Internal Insulation in Existing Buildings: What WTA Requires

31.07.2026 5 min read

Insulating from the outside is the simpler approach in building-physics terms. Only it is often blocked — by listed-building status, a clinker-brick façade, the neighbouring plot, or simply the owners’ association. That leaves internal insulation. It works, but it forgives far less, and there is an identifiable reason for that.

What reverses when insulating from the inside

With external insulation, the load-bearing wall sits on the warm side of the insulation layer. It stays tempered, stores heat and dries towards the inside.

With internal insulation, it sits on the cold side. Three things follow inevitably:

The wall becomes colder than before. This is not a side effect but the purpose: the heat flow from the room is slowed, so less of it reaches the wall.
The drying reserve towards the inside is largely lost. Moisture that could previously dry into the room now has to find another path.
The thermal mass is decoupled from the room. The room heats up faster and cools down faster.

The critical point is the interface between the insulation and the existing wall. It is cold there, water vapour from the room arrives there, and it can condense there — invisibly, behind the lining.

The WTA has its own set of rules for this

The Wissenschaftlich-Technische Arbeitsgemeinschaft für Bauwerkserhaltung und Denkmalpflege (Scientific-Technical Working Group for Building Conservation and Heritage Protection) maintains two interlinked guidance documents:

WTA-Merkblatt 6-4, “Innendämmung nach WTA I: Planungsleitfaden”, issue 10.2016
WTA-Merkblatt 6-5, “Innendämmung nach WTA II: Nachweis von Innendämmsystemen mittels numerischer Berechnungsverfahren”, issue 2014

The structure is two-tiered and geared towards practice. Merkblatt 6-4 describes a simplified verification procedure with clearly named boundary conditions that must be met. Anyone who stays within these boundary conditions does not need a simulation.

Anyone who steps outside them — greater insulation thicknesses, vapour-open systems, unusual constructions — needs the route via Merkblatt 6-5: a simulation of coupled heat and moisture transport in accordance with WTA-Merkblätter 6-1 and 6-2, with the boundary conditions and evaluation criteria for the result set out in 6-5.

The point to take away from this structure: a moisture-technical verification is not optional with internal insulation. Without it, there is no way to judge whether the construction will cause consequential damage.

What must be clarified before planning

The survey of existing conditions determines success or failure, and it is more extensive for internal insulation than for an ordinary coating.

To clarify	Why
Driving-rain exposure of the façade	A wall that gets soaked from the outside dries out less well after internal insulation is applied
Condition of joints, render and connections	Any leak becomes more critical because of the colder wall
Rising damp at the plinth	Must be stopped beforehand, otherwise it migrates into the insulation layer
Connecting elements: floor slabs, internal walls, balcony slabs	These are where the thermal bridges arise that internal insulation does not address
Timber joist ends in the external wall	The classic failure case — the joist end sits colder after the works
Use of the room and moisture load	Bathrooms, kitchens and bedrooms have different boundary conditions than an office

The timber joist end deserves its own line, because it is the most common serious defect. In many older buildings, the floor joists sit in pockets in the external wall. After internal insulation, this area becomes colder and damper than before — with a component made of organic material sitting right in the middle of it. Detailed planning of this point belongs in every internal insulation project.

Thermal bridges do not shrink, they become more visible

Internal insulation insulates the plane surface, not the connections. Wherever an internal wall or a floor slab ties into the external wall, an uninsulated link to the cold side remains.

The result: the relative difference between the plane surface and the connection grows. The surface becomes warmer, the connection stays where it was. Where moisture previously occurred evenly distributed across the whole wall, it now collects at the few cold spots.

This is exactly where the criterion from DIN 4108-2 applies: at the most unfavourable point of the room-side surface, a temperature factor f_Rsi ≥ 0,70 must be maintained, which under the boundary conditions of the verification corresponds to a minimum surface temperature of 12,6 °C. Anyone who improves the plane surface and forgets the connection shifts the problem instead of solving it. Discussed in detail here: Why mould is not a question of heating.

The substrate must be right beforehand

The same principle applies to execution as for any coating work — only with higher stakes, because internal insulation is not accessible afterwards. The substrate must be dry, sound and load-bearing; the site-standard checks for this are described in BFS-Merkblatt Nr. 20, and the duty to raise concerns in VOB/C ATV DIN 18363 Section 3.1.1 and VOB/B § 4 Abs. 3.

Anyone who cuts corners here seals the defect behind a lining. Details on the scope of testing: Checking the substrate: what the contractor owes — and what not.

Where internal insulation has its strength

For all the caution — there are situations where it is clearly the right choice:

Listed façades or façades bound by design constraints, where nothing may be changed on the outside.
Rarely used rooms, which need to warm up quickly and do not depend on thermal mass.
Individual rooms, when a full-scale measure is not achievable economically or under property law.
Tight boundary development, where there simply is no space on the outside.

In all four cases, the same applies: the measure stands or falls with the planning of the connections and with the moisture-technical verification. Both are effort spent before the first step is taken — and both are cheaper than the damage they prevent.

Sources

WTA-Merkblatt 6-4, “Innendämmung nach WTA I: Planungsleitfaden”, issue 10.2016
WTA-Merkblatt 6-5, “Innendämmung nach WTA II”, issue 2014, together with the referenced Merkblätter 6-1 and 6-2 on coupled heat and moisture transport
DIN 4108-2 — Minimum requirements for thermal insulation, temperature factor f_Rsi
DIN 4108-3 — Climate-related moisture protection
VOB/C ATV DIN 18363 Section 3.1.1 and VOB/B § 4 Abs. 3
BFS-Merkblatt Nr. 20, “Baustellenübliche Prüfungen zur Beurteilung des Untergrundes”

This article reflects the current state of the referenced standards and does not replace individual planning or legal advice. The applicable version in each case is decisive.