Ventilated flat roof – concrete structure

In many older blocks of flats, the top concrete floor slab is situated beneath a slightly sloping timber structure, forming a flat roof or a gently sloping roof. Alternatively, they are constructed as ‘slightly ventilated, double-shell roofs’ with lightweight concrete shells resting on pumice or sand-lime brick supports. Between the roof waterproofing (bitumen + gravel fill on timber sheathing or lightweight concrete slabs) and the living space ceiling (plastered concrete) there are cavities 20–40 cm thick, which are sometimes filled with a 6–8 cm layer of insulation. These constructions result in high heat loss in winter, whilst in summer, by contrast, the living spaces below heat up rapidly.

How is it insulated?

Hollow-layered flat roofs can be retrofitted with insulation with minimal effort. To do this, blow-in insulation of sufficient thickness is blown into the flat roof without leaving any gaps. The insulation is efficiently fed into the structure via blow-in hoses through existing or specially created openings. The openings created by the insulation contractors must then be professionally sealed by a roofing contractor and welded shut using bitumen membranes.

Insulation thickness

Approximately 20–30 cm thick.

All the benefits at a glance

Improved thermal insulation: If the U-value of the roof structure was previously approx. 2.3 – 1.0 W/(m²K), the insulation work can reduce this to approx. 0.14 – 0.1 W/(m²K), representing a thermal improvement of approx. 90 per cent.

Lower heating costs: Heating costs are significantly reduced. In relation to the roof area, heating costs are reduced by more than 90 per cent. It is not possible to specify a percentage saving for the building as a whole, as this depends on many other factors (building height, volume, other building elements such as windows and external walls, and the supply of heating energy).

Further benefits:

• There is no need for a complex and more expensive roof renovation involving on-roof insulation elements, parapet conversion and roof covering replacement. – On the contrary: due to the ventilation openings often located in the parapet and the associated cross-ventilation, above-roof insulation would in fact be almost ineffective.

• This insulation method is very cost-effective and pays for itself within a few years.

• No scaffolding is required for the building.

• Summer heat protection for the top floor is significantly improved.

• The rooms become more comfortable.

• Heating costs are significantly reduced.

• CO₂ emissions are reduced.

• Depending on the size of the roof, the insulation work can usually be carried out in a single day.

• Residents are not inconvenienced by the insulation work.

• The insulation material can be removed and reused at a later date.

Costs/cost-effectiveness: Approx. €20–25 per m², depending on the insulation thickness and quantity, plus any additional work where applicable. For a medium-sized block of flats, this results in total costs of approx. €7,000.00.

Subsidies: The GEG and BEG subsidy schemes require a U-value of 0.20 or 0.14 W/(m²K) respectively for the insulation of flat roofs; the latter requires insulation thicknesses of up to 30 cm. This individual measure usually meets the U-value requirement.

Can I install the insulation myself? No, this insulation method can only be carried out by experienced specialist contractors in cooperation with a roofing firm.

Download document:

Fördermittel - Bundesförderung effiziente Gebäude .pdf