Blown-in insulation in a timber-beamed 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 saved is less than if, as described below, the ceiling is insulated. Timber-beam ceilings – also known as purlin ceilings – often have a hollow core or are fitted with only narrow strips of insulation. Insulating the top of these hollow-core ceilings is almost thermally ineffective due to rear-ventilation effects.

How is it insulated?

First of all, a check is carried out to determine whether there is a hollow layer. To do this, either a floorboard is lifted up or a hole is drilled. A folding rule or a stiff piece of wire is inserted into the hole to determine the depth of the hollow layer.

If the cavity is thicker than 3 cm, specialist blow-in insulation contractors will either create small openings in the floorboards or – even more simply – remove a floorboard. A blow-in hose is fed through these openings, and the cavity is filled with blow-in insulation via this hose. The process is carried out in each cavity; within a few hours, the ceiling is completely filled with insulation and insulated.

Important: Insulation from above using any type of insulation material is thermally ineffective if the cavity is not completely filled with blow-in insulation. This is due to rear ventilation effects ‘from one side of the roof to the other’ beneath the floorboards. See

As there is a risk of slight condensation forming directly beneath the floorboards, only capillary-active insulation materials such as cellulose are suitable

You can find qualified tradespeople here.

Insulation thickness

The cavity in the ceiling structure is filled completely. This usually results in an insulation thickness of 12 to 16 cm.

All the benefits at a glance

Thermal insulation: If the U-value of the timber-beamed ceiling was previously around 1.3 W/(m²K), it can be reduced to around 0.23 W/(m²K) through insulation measures. This represents a thermal improvement of around 80 per cent for the timber-beamed ceiling.

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 that solar gain through windows etc. is prevented).

Heating costs: The heating costs of a single-storey building can be reduced by up to 20 per cent. (depending on the ceiling area)

Further benefits:

Comfort is improved 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
Tip: whilst the blow-in insulation contractor is on site, insulation material should also be installed on the ceiling (see ). This approach would then also be eligible for a grant and thus more cost-effective.

Costs/cost-effectiveness: Depending on the level of difficulty, the work can cost up to €30 per m². Thermal insulation can be improved by up to 72 per cent. The work pays for itself within 3–4 years.

Mice/martens:

... like to use the cavities to move about within the structure. Filling them with polystyrene beads or SLS20 permanently eliminates this problem: the resulting environment is extremely unattractive to these animals, so they will avoid it.

Financial support: Unfortunately, this measure alone is not eligible for financial support.

Tip 1: Have up to 25 cm of additional insulation installed on top whilst the work is being carried out. Then the grant will apply. (15–20 per cent)

Tip 2: Wooden beam ceilings often contain ash, slag or clay. These materials should not be removed, as their mass contributes significantly to thermal insulation in summer. The remaining cavity should be insulated and additional insulation blown in above the wooden floorboards.

Tip 3: Old PVC, linoleum or carpet is often found on top of the wooden floor. Most of these materials are fairly vapour-tight. (These materials were laid on the floor to protect it from any potential damp. As the underlying wooden floor is relatively permeable to water vapour, there is a risk of condensation forming directly beneath the wooden floorboards. If this is also hermetically sealed from above, the wooden floorboards can become permanently damp, which can lead to wood rot. Woodworm also thrives in such an environment. It is strongly recommended that these materials be removed as soon as possible!

Can I install the insulation myself? No, you need blow-in insulation equipment. However, the costs can be reduced with a bit of help (setting up the site, feeding the insulation material into the blow-in machine, and clearing up and dismantling the site). Ask a specialist contractor!

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