Roof
roof pitch
Drempel
Flat roof
Upper floor slab
Solid ceiling
timber-beamed ceiling
Nail binder
Attic conversion
External wall
Façade
Partition joint in a house
cellar wall
Ground floor
cellar ceiling
Stairway to the cellar
Solid ceiling
Cap ceiling
Timber joist floor/cavity
Special components
Many older buildings have converted attics where the sloping roof sections (approximately 2–4 metres in length from the eaves to the upper floor ceiling) are either not insulated at all or are inadequately insulated. From the loft, one can look into the empty spaces between the rafters. It is often evident that there are no insulation layers, or only very thin ones, on the internal cladding – for example, lightweight wood wool boards mounted on roof battens, which are plastered on the room-facing side. Furthermore, there is no underlay membrane (water-bearing sub-roof) beneath the roof cladding made of roof tiles. The retrofitting of insulation boards or mats cannot be carried out properly without dismantling the roof covering or the interior cladding.
An ‘insulation bag’ manufactured specifically for this method (consisting of two layers: a vapour barrier on the inside and a diffusion-open, waterproof underlay membrane on the outside, sealed at the bottom) is inserted into the cavities from the loft. A spacer batten is placed on top of this bag; the insulation bags are then filled from above with fibrous blow-in insulation and, if desired, sealed at the top. Whilst this method is cost-effective to implement, it does have its drawbacks: an air barrier cannot be installed. Furthermore, cavities in which, for example, an exhaust air pipe is laid in the centre, or very narrow cavities at the edge of the roof or beneath roof windows, cannot be insulated.
An informative YouTube video by the ‘Energy-Saving Commissioner’, Carsten Herbert.
Due to the usually very narrow rafter thicknesses, a maximum insulation thickness of 15 cm can be achieved.
All the benefits at a glance
Improved thermal insulation: If the roof’s so-called heat transfer coefficient (U-value) is 2 W/m²K before insulation work, this can be improved to as low as 0.3 W/m²K. This means that thermal insulation is up to seven times as effective as before.
Lower heating costs: Depending on the specific situation, this measure can reduce the building’s heating costs by 10 per cent.
Further benefits:
- Draughts are minimised
- Comfort is enhanced due to higher temperatures on the inside of the external walls
- Mould problems are eliminated
- Summer heat protection is significantly improved
- CO₂ emissions are reduced
- The work is minimally invasive
Costs/cost-effectiveness:
The cost of the insulation bag method is around €50 per m². Other ways of insulating a roof (such as a new roof, etc.) can cost up to €250 per m² in comparison. It is also worth bearing in mind that when installing a new roof, the entire roof must be replaced. A partial replacement (up to the loft) is not possible. In a typical detached house, this area is usually 150 m².
The insulation bag method is only used in the roof area of the living space, which is usually 60 m². This method is not used in the loft area. Here, the floor should be insulated generously, i.e. to a thickness of at least 30 cm. You can find more useful information in the article ‘Roof insulation vs ceiling insulation’.
Financial support:
The insulation bag method is not eligible for financial support. However, it pays for itself quickly, can be implemented on a small budget and delivers immediate energy and cost savings.
The more complex method of on-roof insulation, on the other hand, is eligible for a 20 per cent tax relief or, under the BEG (Federal Funding Scheme for Efficient Buildings), for a grant of up to 20 per cent of the total costs (as part of an individual renovation roadmap, iSFP) in the form of an investment grant.
The BEG funding scheme is administered by the Federal Office for Economic Affairs and Export Control (BAFA). The eligible investment amount is capped at 60,000 euros per calendar year and residential unit, and at 600,000 euros per building. To receive the funding, you must engage a qualified energy adviser before work begins.
Can I carry out the insulation work myself?
No, you need a blow-in machine for this insulation method. Your assistance (helping out, feeding the insulation material into the machine, inserting the insulation bag into the roof spaces, etc.) can help reduce costs. Ask the specialist contractor about this!
You can find specialist companies here
