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
Particularly in large cities, where living space is scarce and valuable, many unused attic spaces (lofts) are converted. The sloping roof surfaces must be insulated in order to create comfortable and heatable living spaces.
A roof underlay (water-bearing sub-roof) is usually not present. However, one must be installed, as otherwise any condensation (beneath the roof cladding), drifting snow or rainwater ingress (e.g. through broken or wind-displaced roof tiles or joints) could penetrate the insulation. As a result, the dampened insulation would no longer be effective and the rafters could be damaged by the moisture. In short: too much moisture penetrating into the interior would cause mould damage.
A water-diverting sub-roof is therefore essential!
A waterproof, vapour-permeable underlay (available from roofing suppliers) is cut to size and fixed on one side to the rafter using a thin structural batten (2 x 3 cm), directly beneath the roof battens. It then hangs into the loft space like a curtain; a further timber batten (4 x 5 cm) is screwed vertically beneath the roof battens, centred between the rafters. The underlay is then secured on the other side to the opposite rafter – again using a timber batten – and pulled taut. This creates a very gently sloping ‘V’ shape. To prevent any water that may collect from causing damage to the underlay later on, the underlay should be routed outwards at the base, over the top of the wall.
In older buildings, the rafters are usually 14–16 cm thick and are often not straight. Insulation of this rafter layer, at a thickness of 14–16 cm, would be insufficient and would not qualify for government funding (see below). The rafter must therefore be doubled up. This can be achieved by attaching a wooden plank at least 3 cm thick to the side of the rafter, the installation of which requires some skill. One solution is the ‘rafter expander’. This is a T-shaped wooden element consisting of a strip of plywood up to 24 cm high, glued together with a horizontal wooden batten measuring 4 x 6 cm.
This rafter expander can be easily fitted to the side of the rafters; a total insulation thickness of at least 30 cm should be achieved. Using the expander,
- the rafters to double the required insulation thickness, as well as
- compensate for uneven or crooked rafters.
In the next step, an airtight vapour barrier must be installed beneath the rafter extenders. Ensure that there are airtight seals at the wall, skylights, chimneys, etc.! The plaster battens can then be laid horizontally beneath the vapour barrier (please follow the relevant manufacturer’s instructions).
The resulting cavity is extremely irregular in shape, making it difficult to insulate with mats.
Blown-in insulation using fibrous materials provides a solution here. These products also offer the best value for money; they can be installed quickly, leave no gaps, there is no waste, and even in tricky areas such as dormer connections or the ridge purlin area, a joint-free installation is easily achievable. Alternatively, soft mat insulation can be cut to size and inserted between the rafters.
Once the insulation has been blown in, under-rafter insulation can also be installed in the space between the plaster battens to further improve thermal insulation. Finally, finish by fitting the finishing boards (e.g. plasterboard) or wooden panelling as the underside cladding for the sloping roof – and you’re done!
The total thickness of the insulation should be at least 28 cm.
All the benefits at a glance
Improved thermal insulation: The thermal insulation achieves a minimum U-value of 0.14 W/m²K (watts per square metre per kelvin) or better.
Lower heating costs: The newly built flat in the sloping-roof extension consumes little heating energy and offers excellent thermal insulation in summer. Very little heat enters the living space through the roof, so it remains very comfortable in both summer and winter.
Further advantages:
- Draughts are eliminated
- 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
Costs/cost-effectiveness: Most of the work can be carried out by the homeowner. If this is not possible, enquire with a company specialising in drywall construction about current prices. The insulation blow-in process is carried out by specially trained contractors, and the costs can be reduced with your assistance.
Funding: This measure is subsidised at a rate of 15 per cent under the Federal Scheme for Efficient Buildings (BEG) by the Federal Office for Economic Affairs and Export Control (BAFA). An energy adviser is required for this. If the adviser also draws up a so-called individual renovation roadmap (iSFP), the subsidy increases to 20 per cent.
Alternatively, a 20 per cent subsidy is available through a reduction in income tax. No application is required for this, but the subsidy is spread over a period of three years.
Can I carry out the insulation myself? A large part of the work can be carried out by yourself; only the blow-in process is carried out by specialist firms.