Performance stars

U-value performance

Thermal performance, calculated the same way every time, so that one manufacturer’s figure means the same as the next one’s.

The scheme applies a standardised analysis and a defined U-value threshold. A product must meet that threshold before it can be listed in the public directory.

Why standardising the method matters

A U-value needs its context. Rooflights and roof windows may be assessed in different planes and to different standards. The assessment basis must be understood before figures are compared.

The scheme records the assessment basis alongside the U-value, so a number can be traced back to the product and the method used.

Condensation Risk performance

Meeting a U-value does not mean condensation is not an issue. This is a gap that can cause trouble once a product is installed.

Roof glazing sits at the top of the room where warm, moist air collects. It faces the night sky, and the frame perimeter can be a cold point where moisture becomes visible.

What we look at

Assessment covers surface condensation risk at the coldest point of the assembly under defined internal conditions, and the unit’s ability to manage and drain the water that does form. A minimum threshold has to be met before the star is awarded.

Acoustic performance

Noise is an issue whether it is a busy road or a flight path. If this star is to be attained, a threshold level must be met.

Sound insulation is often designed into the walls and windows, then roof glazing is chosen on looks and daylight. Its contribution to the acoustic performance of the room needs the same attention.

Rain noise matters too. A large glazed or plastics panel at a shallow pitch can be disruptive in heavy rain. It is worth discussing this with a client before it becomes a conversation after installation.

The whole unit matters

Assessment covers the sound reduction performance of the whole unit as installed, including the perimeter, rather than the glazing specification alone. A threshold level must be met before the star is awarded.

Solar performance

Because of the orientation of the product, solar control is a vital part of the performance equation.

Roof glazing faces the sky, so its exposure to solar radiation needs careful consideration. A high g-value allows more solar heat through the glass and can contribute to overheating.

Balance solar control and daylight

A minimum low solar factor threshold is needed, and that is what this star provides. Publishing g-value and light transmittance together allows the trade-off between solar control and useful daylight to be considered properly.

The product is one part of the design. Glazed area, orientation, shading and ventilation also influence comfort in the finished room.

Weather performance

The scheme uses a purpose-built methodology that assesses the whole unit in the orientation it is designed to be installed in.

A rooflight is not a vertical window. At shallow pitches, water can remain on the surface or collect around the perimeter. Wind-driven rain and snow melt also need to be considered in the product’s intended roof application.

Assessing the unit as a whole, at the intended orientation, gives a clearer picture of its weather performance where weathertightness is fundamental.

Testing at your site

The scheme has portable equipment and site test engineers who can carry out this testing at your own site. That avoids shipping large units to a laboratory and can reduce the time and cost of the exercise.

Security performance

Because of the orientation and product type, a specialised security test method is used.

A security assessment needs to reflect the product being tested and the way it is installed. A rooflight at a shallow pitch presents a different arrangement from a vertical window.

The star records that the product has met the scheme’s security threshold. The specification for a particular building must still consider the required security performance and the accessibility of the opening.

Tested in one visit

Where a manufacturer wants both, security assessment can be carried out at the same site visit as weather testing.