Blowing out the timber joist floor

Many older buildings, particularly those from the 1930s, have a wooden floor on the ground floor (often painted brown or red, depending on the region). Due to the way they are constructed, the floorboards are nailed from above into a framework of beams. These beams are at least 8 cm thick/high. Beneath the floorboards or between the supporting beams, there is sometimes – depending on the region – ash, slag or clay, though sometimes there is nothing at all.

Heat loss through the ground-floor floor is high, and living comfort is low (= ‘cold feet’)

How is it insulated?

Preparation and gaining access: Check the condition of the timber joists and ensure that you have safe access to the cavity beneath them.
The blow-in insulation contractor creates a blow-in opening (20 mm in diameter) in each compartment from above, through which the cavity can be quickly filled with polystyrene beads or SLS20 and thus insulated.
If you wish to use a fibre-based insulation material (cellulose, wood fibre, glass wool, rock wool), at least two floorboards must be removed from the edge of each room. The blow-in hoses are inserted as far as possible into the cavities through the opening created in this way, and the cavities are then filled using the blow-in method. The disadvantage of this method is that at least one wooden floorboard is damaged – and possibly a second one as well – and must be replaced. This is often not feasible with historic floors.
If opening the cavities from above (from the ground-floor rooms) is not desired or possible (for example, because the floor is a high-quality wooden floor or has been recently renovated), it is also possible to blow in free-flowing granules from the cellar ceiling. However, as this is more labour-intensive, it incurs higher costs. Fibrous insulation materials are difficult to install in this way.
Finishing work: The blow-in openings are neatly sealed from above. Drilled holes can be sealed with suitable wooden plugs (available from specialist timber suppliers).
Tip: Old plank floors (often varnished brown or red) can be sanded down and oiled or waxed after the insulation work has been completed. This will give you a beautiful wooden floor ‘with history’
You can find qualified tradespeople here

Insulation thickness

As thick as the joists are high.

All the benefits at a glance

Lower heating costs: Heating costs can be reduced by around 10% by insulating the timber joist floor with blow-in insulation.

Further benefits:

Improved comfort: Warmer floors and a more pleasant room temperature.
Longer lifespan: Protects timber from temperature fluctuations.
Increased property value: Attractive energy efficiency.
Environmentally friendly: Lower CO₂ emissions.
Quick installation: Efficient blow-in method.
No loss of space: Invisible insulation without taking up any room.

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

Insulation materials: Cellulose, mineral fibres, wood fibres (blown in through openings in the floorboards), expanded polystyrene (EPS), SLS20 (blown in through 20 mm diameter drill holes made from above or below)

Download document:

Fördermittel - Bundesförderung effiziente Gebäude .pdf