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
Flat roofs often feature knee walls that are separated from the roof pitch. The partition wall separating the living space from the roof area, which is usually around 80 cm high, consists either of a lightweight concrete slab, a lightweight wood wool board or wood-based panels. Brick-built partition walls made of solid bricks or river stones are also found. There are often small openings through which one can access the cavity. In older buildings, neither the floor – whether concrete or timber-beamed – nor the sloping roof surfaces nor the partition wall separating the living space from the cavity are insulated. Heat loss through the concrete ceiling (from the floor below upwards) or the partition wall is very high. As the partition cavity is often no more than 80 cm high and the depth of the room is also 80 cm, it is often difficult to install conventional retrofit insulation; consequently, blowing insulation into the full width of the partition cavity is an obvious option for improving a building’s thermal performance.
A good solution is blow-in insulation, which can be installed without having to enter the eaves space. The cavity of the eaves can be completely filled with insulation material. This can be done regardless of whether a roof underlay is present. Blown-in insulation is installed from the inside through the eaves wall and the hatches or openings often found there, or, if this is not possible, from the outside through the roof cladding or the underlay membrane that has been cut open. Here’s how it’s done.
Which insulation materials are suitable? As the existing underlay membrane is often vapour-tight, it prevents moisture from the interior from entering the structure. On cold, frosty nights, however, a small amount of condensation may form directly beneath the membrane. Particularly suitable are loose-fill insulation materials made from cellulose, wood fibre, glass wool or rock wool, which form an even layer where the fibres interlock and are therefore not displaced by occasional draughts. There must be a small air gap between the underlay and the roof structure to allow moisture to dry out in the wind. If the membrane is not fitted, the space between the rafters must be kept clear of any insulation material to allow a drying air flow, as condensation can sometimes form even beneath the roof tiles on cold nights.
The cavity is completely blown out. An average detached house requires 8–10 m³ of insulation material.
All the benefits at a glance
Improved thermal insulation: If the so-called heat transfer coefficient (U-value) of the ceiling is 2 W/m²K and that of the partition wall is between 3 and 5 W/m²K, these values can be reduced to 0.05 W/m²K – or 0.2 W/m²K at the narrowest points – thanks to the enormous insulation thickness of up to 80 cm.
Lower heating costs: This measure can reduce the building’s heating costs by 3 per cent.
Further benefits:
- Draughts are minimised
- Comfort is improved due to higher temperatures on the inside of the external walls and beneath the ceiling
- Mould in the corners of the room beneath the lintel disappears
- CO₂ emissions are reduced
- The process is minimally invasive
Costs/cost-effectiveness: The cost of the blow-in insulation is approximately €50 to €60 per metre.
Financial support: This measure is subsidised by the government at a rate of 20 per cent, which you can claim as a tax deduction on your income tax return.
Can I install the insulation myself? No, this insulation method requires a blow-in machine, which only specialist contractors have. Your assistance (e.g. helping out, feeding the insulation material into the machine) can help reduce the costs. Ask your specialist contractor about this!
You can find qualified tradespeople here
