Summer Thermal Performance: What DIN 4108-2 Requires

31.07.2026 4 min read

Summer thermal protection is usually treated as a comfort issue. In fact it is a compliance issue: DIN 4108-2 requires it for new buildings and for certain alterations to existing buildings — regardless of whether cooling is installed or not.

The standard works with an index, not a temperature

The simplified method under DIN 4108-2 Section 8 compares an actual solar gain index S_vorh against a permissible S_zul. Compliance is demonstrated when:

S_vorh ≤ S_zul

S_vorh results essentially from the ratio of window area to the room’s net floor area, weighted by the total solar energy transmittance of the glazing and the reduction factor of the solar shading device.

S_zul is made up of components that depend, among other things, on:

the summer climate region — Germany is divided into regions A, B and C, from cool to hot summers
the construction type, i.e. the effective thermal storage capacity: light, medium or heavy
the night-time ventilation — increased night-time ventilation may be credited
the window inclination and orientation
whether passive cooling is present

If the simplified method is insufficient or exceeded, compliance must be demonstrated by a thermal building simulation under DIN 4108-2 in conjunction with DIN EN ISO 52016-1. Assessment is then based on excess temperature degree hours.

The order of effectiveness is clear

In practice, discussion often starts with the glazing. This is rarely the biggest lever. The ranking:

External, movable solar shading. Venetian blinds, roller shutters, awnings. It intercepts radiation before it passes through the pane. The reduction factor F_C here is typically around 0.25 depending on the design, while internal devices are considerably less effective.
Window size and orientation. West-facing windows are the most critical in summer: the sun sits low in the afternoon, strikes the façade at a shallow angle, and the building fabric is already heated up. South-facing windows, with the high summer sun, can be effectively shaded by an overhang; west-facing windows can barely be shaded this way.
Effective thermal storage capacity. Heavy construction buffers the daily peak. It is precisely this mass that internal insulation decouples from the room — a point worth bearing in mind when choosing between internal and external insulation.
Night-time ventilation. Free, but only effective if windows can be safely opened at night and the outdoor temperature actually drops. In inner-city heat islands, both conditions are often absent.
The total solar energy transmittance of the glazing. Important, but secondary — and with a trade-off: a low g-value reduces solar gain in summer, but also reduces solar gains in winter.

The emissivity of the outer surface belongs on the short-wave side

For the summer heating-up of a façade or roof, the decisive parameter is not the long-wave emissivity but the short-wave absorptance for solar radiation — and this depends on colour.

A dark surface absorbs a considerably greater share of incident radiation than a light one. On a sun-exposed roof surface, this makes a difference of several tens of kelvin in surface temperature at the height of summer.

Confusing these two quantities is a common mistake: colour determines the absorption of short-wave solar radiation, while long-wave emissivity determines the emission of thermal radiation at ambient temperature. These are different wavelength ranges, different figures and different effects. For more detail: What emissivity really describes.

Two cases where compliance also applies to existing buildings

Summer thermal protection is not purely a new-build issue. It also becomes relevant for alterations to existing buildings, in particular:

when converting a previously unheated loft — the most critical scenario of all: large glazed areas in an inclined position, little storage mass, direct solar radiation
in the case of a substantial enlargement of window areas

In both cases, the question should be raised before the work is carried out, not in the first summer afterwards.

What this means for a client conversation

Three points that hold up in conversation:

Compliance is room-specific. It is demonstrated for the least favourable room, not for the building as a whole. “The house is well insulated” is not an answer to the question of summer thermal protection.
External beats internal. Anyone fitting solar shading internally has already let the energy into the room. The difference between external and internal shading is greater than any difference between two types of glazing.
No figure without boundary conditions. Climate region, construction type, night-time ventilation and orientation belong with every result — otherwise it cannot be verified.

Sources

DIN 4108-2, Section 8 — Minimum requirements for summer thermal protection, solar gain index method. Note: as of May 2026, the 2026-05 edition applies, replacing 2013-02.
DIN EN ISO 52016-1 — Energy performance of buildings, thermal simulation
DIN EN ISO 52022-1/-3 — Solar shading devices, reduction factors
Gebäudeenergiegesetz (GEG), requirements for summer thermal protection

The figures given are calculated values under the standard’s specified boundary conditions. They do not replace a project-specific assessment. The version of the standard in force at the time is decisive in each individual case.