Celotex Isover

Insulation for Balconies and Terraces

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Providing useable space on a flat roof is an attractive proposition for many building types. For people living in flats and apartments, a balcony is a way of accessing an outside space without being at ground floor level. Terraces, meanwhile, can serve larger dwellings and commercial spaces, providing a hard-landscaped gathering place for a variety of uses.

Where balconies and terraces sit over a habitable space, they form part of the building’s thermal envelope. They must be suitably insulated to meet targets for energy efficiency, while bearing the additional load associated with being used for more than occasional maintenance access.

A warm roof design, with insulation (Celotex GA4000 or XR4000 can be considered) laid over the structural timbers, remains the preferred option to a cold roof design. A warm roof does not need cross-ventilation, and lends itself to better detailing at junctions with other elements to address linear thermal bridging.

Celotex rigid polyisocyanurate (PIR) foam boards are lightweight, easy to cut and handle, and can be used to insulate areas of flat roof quickly and efficiently. While they already offer a high compressive strength, to resist the dynamic foot traffic load associated with balconies and terraces, it is recommended to install the boards with an additional layer of plywood over the top. The top layer of ply, like the structural timber deck below the insulation, should be a minimum of 18mm thick. The additional layer protects the insulation from damage, spreading the weight of the decorative roof finish and the extra foot traffic load more evenly throughout the insulation layer.

An insulated plasterboard (PL4000 can be considered) may be installed underneath the joists to improve the thermal efficiency of the roof. An alternative option can be a PIR insulation board (Celotex TB4000 or GA4000 can be considered) alongside a separate layer of plasterboard.

Key Considerations

When using Celotex or ISOVER products, you need to satisfy yourself that use of the product meets all relevant national Building Regulations and guidance as well as local, national and other applicable standards relevant for your construction or application, including requirements in relation to fire and applicable height restrictions. In addition to the product datasheet, please refer to the following product documents:

The building detail is for illustrative purposes only. It does not constitute advice and should not be relied upon.


Insulation products
Product Name Thickness Range Sizes Lambda
Celotex XR4000 110-200mm Width 1200mm, Length 2400mm 0.022 W/m.K
Celotex GA4000 50-100mm Width 1200mm, Length 2400mm 0.022 W/m.K
Celotex TB4000 20-40mm Width 1200mm, Length 2400mm 0.022 W/m.K
Celotex PL4000 25-65mm (+12.5mm Plasterboard) Width 1200mm, Length 2400mm 0.022 W/m.K