Solar control glass in passive houses – which values are crucial?

ISOLAR SOLARLUX ZAL TechCenter Hamburg Germany Copyright Arnold Glas 03

A passive house is more than an energy-efficient building – it's a finely tuned system of heat transfer, radiation and daylight. Every surface, every window, every glazing affects the balance between energy gain and comfort.

While walls and roofs minimize heat loss, glass is the "active" component of the building envelope: in winter it provides solar heat, in summer it must deflect it. This makes solar control glass a key element of the passive house concept. But which performance values are crucial – and how do you find the right balance?

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Performance data: U-value, g-value and LT-value

Three values determine how glass performs in a passive house:

U-value (thermal transmittance) indicates how much heat escapes to the outside. Passive house windows must not exceed 0.8 W/(m²K).
g-value (solar heat gain coefficient) shows how much solar energy enters the interior. It determines heating gains in winter and overheating in summer.
LT (light transmission) represents the transmittance of visible light – the higher it is, the brighter the room.

The ratio LT/g = selectivity describes efficiency: the larger the quotient, the more light with less heat. Selectivity values above 1.6 are considered particularly high-quality.

Solar control glass and passive house – the optimal interaction

Passive houses benefit from high g-values (around 0.5) in winter, but must avoid solar overheating in summer. The glazing must therefore do both: admit energy and block it.

In practice, optimal results are achieved when several values are coordinated: U-value, g-value, LT and Fc-value (shading coefficient). Depending on orientation, the following are recommended:

South-facing: g ≈ 0.45 – 0.55 (high winter gains, good daylight transmission)
East / West: g ≈ 0.25 – 0.35 (low heat entry with low-angle sun)
North: g ≈ 0.55 – 0.65 (no sun protection needed, focus on light)

This means: not every façade needs the same glass. A differentiated glazing strategy based on orientation is more efficient than a "one-glass-fits-all" approach.

The ISOLAR SOLARLUX variodirect glazing at the main gate of Kiel University showcases modern architecture, high energy efficiency, and transparent façade design in educational buildings.

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How solar control glass manages energy

Spectral selectivity instead of mirror effect

Modern solar control glass filters selectively by wavelength. Ultra-fine metallic Low-E and Low-g coatings reflect infrared radiation, but allow the visible portion of solar radiation to pass through almost unimpeded. This preserves daylight while heat radiation is reflected outside – resulting in high daylight transmission with a low g-value.

What is the g-value?

Current systems achieve Ug ≈ 0.6 – 1.0 W/(m²K) and g ≈ 0.25 – 0.46, with LT ≈ 40 – 70%. This means the glass fulfils both thermal insulation requirements and summer comfort needs.

Example systems

Example products such as the SOLARLUX® A and D series from ISOLAR® demonstrate how finely graduated modern solar control glass can be. Values range from g = 0.23 to 0.46 with LT = 40 – 70%, allowing flexible adaptation to orientation and use – all with neutral colouring and no mirror effect.

Shading – the second control element

Even selective glass needs support. External venetian blinds, screens or roof overhangs prevent radiation from reaching the glass at all.

The Fc-value (shading coefficient) shows effectiveness: an external venetian blind with 75° slat angle achieves Fc ≈ 0.07, while internal roller blinds only achieve ≈ 0.7. Even more elegant are integrated solutions such as SOLARLUX® variodirekt, where blinds sit in the cavity between panes – maintenance-free, wind-resistant and adjustable between g_eff ≈ 0.25 and 0.07.

This makes shading part of the glazing – and thus an integral component of the building envelope.

Standards and certification

Designers refer to the following regulations:

EN 410 – Measurement of light and energy transmission
EN 673 / ISO 10077 – Calculation of U-values
DIN 4108-2 – Summer thermal protection and verification via Fc-value

The Passive House Institute (PHI) classifies glass according to g/Ug and LT/g. For Central Europe (climate zone 3), the following apply: Ug ≤ 0.8 W/(m²K), g/Ug ≥ 0.65, LT/g ≥ 1.6. Such performance values ensure a harmonious balance between transparency and thermal protection.

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The ISOLAR SOLARLUX glazing at the MaRo Opera Quarter in Frankfurt am Main showcases premium architecture, high transparency, and energy-efficient façade technology in an urban landmark project.

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Integrated planning

During the design phase, glazing, orientation and shading should be considered together. Simulations with PHPP show early on whether overheating temperatures > 25 °C will occur – and how they can be avoided through g-value or Fc-value adjustments.

The structural connection is also important: edge seals with "warm edge" spacers, triple sealing levels and airtight installation (n₅₀ < 0.6 /h) prevent thermal bridges.

The goal is always the same: an envelope that doesn't store energy, but controls it.

Cost-effectiveness and funding

Solar control glass costs more than standard triple glazing – typically +20 to 40%. But the investment pays off: lower cooling loads, more stable room temperatures and higher daylight quality.

Various government and local authority energy efficiency grants may be available depending on your location and project type. In addition, well-considered glass selection increases property value and long-term comfort.

Sustainability and future technologies

Solar control glass has long been part of climate strategy.

Recyclable raw materials, EPD-certified products and regional manufacturing reduce embodied energy. ISOLAR® uses sustainable production processes for SOLARLUX® and offers optional functions such as acoustic insulation, alarm or bird-protection glass – an aspect that architects are increasingly demanding.

The next generation focuses on electrochromic and thermochromic glass, whose transmission rate adjusts automatically. In combination with BIPV glass (building-integrated photovoltaics), a new category emerges: solar control, daylight and energy generation in one function.

Balance instead of compromise

Solar control glass in passive houses is not an accessory, but a key energy component. It balances heat, light and comfort – and determines whether a building retains or loses its efficiency.

The formula for designers is:

Ug ≤ 0.8 W/(m²K) × g ≈ 0.45 × LT > 60% × Fc < 0.10

Those who integrate these parameters and combine them appropriately for the location achieve a building envelope that actively adapts to the climate – without losing the view outside.

FAQ

What is the difference between solar control glass and thermal insulation glass?

Thermal insulation glass reduces heat loss, while solar control glass additionally reduces heat gain.
Selective coatings combine both properties.

What g-value is optimal in a passive house?

South: 0.45–0.55 / East–West: 0.25–0.35 / North: 0.55–0.65.

How does shading affect performance?

External shading systems can reduce the effective g-value (g_eff) to below 0.10 — internal systems have almost no effect.

Is solar control glass eligible for funding?

In Germany, yes — for example through BEG WG individual measures, provided that an improvement in energy efficiency can be demonstrated.
In the UK, funding schemes differ: certain programmes may support window or glazing upgrades as part of energy-efficiency measures, but eligibility depends on the specific scheme and criteria (e.g. household income, building efficiency). Solar control glass is not automatically eligible.