Solid ceiling

After the external walls, roofs and upper floor slabs are the largest surfaces through which valuable heating energy is lost. If the loft space under the roof is unoccupied and therefore unheated, insulating the roof is pointless, as it is far too expensive and the amount of heating energy that can be saved is always less than if, as described below, the ceiling is insulated. If the ceiling above the unheated loft consists of a concrete slab, its thermal performance can be significantly improved using simple methods and blow-in insulation. In principle, the ceiling can be insulated from above using any insulation material – the most cost-effective, technically sound and environmentally friendly options are listed below.

How is it insulated?

The concrete ceiling is insulated from above. An insulation thickness of at least 30 cm is recommended (eligible for subsidies, sustainable). It is possible and advisable to install a sort of ‘walkway’ leading to the chimney for the chimney sweep. If cellulose blow-in insulation is used, the chimney must be clad with non-combustible insulation to a thickness of at least 10 cm. Tip: you should consider whether the loft is to be used as a storage space or similar, or just part of it. Installing ‘accessible’ insulation involves considerably more work and is therefore at least twice as expensive. Tip: The loft access area should also be insulated.

Cost-effectiveness of ceiling insulation

Which insulation materials are suitable? The upper floor slab can be insulated with any insulation material, including board or mat insulation of any kind. However, we recommend using blow-in insulation, as these are 1. the best value for money, 2. can be applied completely seamlessly and without any offcuts, and 3. the insulation can be fitted in just a few hours.

You can find qualified tradespeople here.

Insulation thickness

Insulation of the upper floor slab has been a legal requirement for many years. A U-value of at least 0.24 W/m²K must be achieved. (GEG – Building Energy Act). This corresponds to an insulation thickness of 14 cm when using standard insulation materials (WLS 035 to 040). If the federal subsidy (BAFA, tax incentive) is claimed, a U-value of 0.14 W/m² must be achieved. This corresponds to an insulation thickness of 25 cm. Insulation thicknesses of up to 40 cm, however, are economical, sustainable and environmentally friendly.

This measure pays for itself in 2.7 years and is therefore the most cost-effective insulation method of all.

Temporary
insulation: If you are considering converting the loft into living space ‘at some point’, insulating the ceiling is not a hindrance. However, the ceiling insulation would then be superfluous and would ‘take up’ valuable living space. In this case, it is advisable to remove the insulation from the ceiling at a later date and reuse it in the roof slopes. See https://www.daemmatlas.de/sites/default/files/2024-03/Fach-Info%204_D%C3%A4mmung%20zwischenparken.pdf

All the benefits at a glance

Thermal insulation: If the U-value of the upper floor slab was previously around 2 W/(m²K), it can be reduced to around 0.1 W/(m²K) through insulation measures. This represents a thermal improvement of approximately 95 per cent for the upper floor slab.

Summer thermal insulation: the temperature in rooms beneath uninsulated ceilings can reach up to 35°C in summer. Insulating the ceiling can reduce the temperature by up to 10°C (provided solar gain through windows etc. is prevented).

Heating costs for a single-storey building can be reduced by up to 30 per cent. In multi-storey buildings, the percentage saving is lower, but the absolute energy saving for the entire building remains the same.

Further benefits:

Comfort is enhanced due to higher temperatures in the upper-storey ceiling
Summer heat protection is significantly improved
CO₂ emissions are reduced.
Heating energy consumption and heating costs are reduced

Costs/cost-effectiveness: Depending on the level of difficulty and the size of the property, the measure can cost up to €40 per m² with an insulation thickness of 30–40 cm. Thermal insulation can be improved by up to 95 per cent (depending on the initial situation). The measure pays for itself in less than 4 years.

What insulation thickness offers the best value for money?

(Assumptions: Insulation material costs ‘turnkey’ €90 per m³. Heat price €0.13/kWh, energy price increase 3% per annum)

Financial support: This measure is subsidised by the government at 15 per cent of the total costs via the BAFA (an energy adviser is required in each case; an additional 5 per cent is available if an iSFP is drawn up) or via tax relief (Section 35c of the Income Tax Act (EstG)) – in this case, a 20 per cent tax relief for owner-occupied homes.

Can I install the insulation myself? Not the blown-in insulation; you need specialised blowing equipment. However, costs can be reduced by lending a hand (setting up scaffolding on site, feeding the insulation material into the blow-in machine, cleaning up and dismantling the scaffolding, and, if necessary, constructing a walkway for the chimney sweep). Ask a specialist contractor! It is also possible to insulate the roof structure yourself using mat insulation materials.

Please note: there are building materials suppliers who sell blow-in insulation materials to DIY enthusiasts and hire out the blow-in machine. This is not permitted! The products lose their building regulations approvals, there is no guarantee in the event of damage, and you will not be eligible for any financial assistance either.

Historical note: As early as 1939, the article ‘More Care in Construction’ appeared in *Bauwelt*: