Cellar insulation - indispensable for energy-efficient renovation

When it comes to energy-efficient construction and renovation - ie thermal insulation of the building - the focus is usually on the insulation of the roof and façade. However, even an uninsulated cellar causes energy losses of about ten percent of the heat energy used. In addition, a basement insulation contributes to the protection of the building fabric and to a pleasant living environment on the ground floor of the house.

To what extent a basement insulation is made, depends on the basement use. If the basement is unheated and essentially not used except as a storage and storage room, the insulation of the basement ceiling is sufficient to optimize the energy efficiency of the building basement. For heated and used for example as an economic or hobby room basements a complete basement insulation is recommended.

Table 1: Cost of a cellar insulation

DämmungsartCost per m2 (EUR)
Basement ceiling insulation from above70 – 160
Basement ceiling insulation from below15 – 25
Basement walls insulate outside (perimeter insulation)40 - 60 (+ earthworks)
Basement walls insulate inside80
Damm the basement floor70 – 160

Why the cellar insulation is important

Residents living above an uninsulated basement regularly have cause for complaint at least in winter - cold feet and increased heating costs are likely to be noticeable consequences of the lack of thermal insulation. Already by the insulation of the basement ceiling, the floor temperature rises significantly on the ground floor, which results in an increase in living and quality of life.

Cellar insulation reduces the risk of moisture and mold damage

In addition, cellar insulation reduces the risk of rising damp and mold on the walls. In an uninsulated cellar, condensed humidity quickly settles on the walls and can lead to damp spots and mold growth. Since the insulation increases the temperature on and in the outer walls, this problem is solved permanently. For this already the inner insulation of the cellar rooms is sufficient.

Tips & Tricks

A basement insulation can be made as pure interior insulation (cellar ceiling, walls, floor) or from the outside as Perimeterdämmung. In new buildings, the base plate is also mitigated by perimeter insulation. Subsequent insulation of the floor slab is not possible, so it is necessary for old buildings even with a perimeter insulation floor insulation.

Perimeter insulation - comprehensive moisture protection and thermal insulation

A little more effort is needed to prevent the ingress of water from the soil into the basement walls. For a comprehensive moisture protection of the outer wall a Perimeterdämmung - ie the insulation of the basement from the outside - is necessary. Despite the greater expense and higher costs, it makes sense if a cellar is used intensively or even temporarily occupied. A Perimeterdämmung includes the cellar insulation and in new buildings and the bottom plate. Compared to pure interior insulation, it is the more efficient form of insulation.

Perimeter insulation must be robust and resilient

Perimeter insulation optimizes the moisture balance of the basement and the ground floor by positioning the dew point so that - unlike internal insulation of basements - moisture can not collect between the walls and the insulation layer. A perimeter insulation has to cope with cold, moisture and pressure - so it requires a robust insulation material. Typically, extruded polystyrene (XPS) is used for this type of basement insulation, but it is also possible to use EPS / Styrofoam or foam glass panels, for example.

The moisture exchange between basement and soil is prevented

A perimeter insulation prevents any moisture exchange between the basement wall and soil. In the basement itself resulting moisture can not diffuse through the wall to the outside. Prerequisite for the introduction of such insulation are therefore adequate ventilation options for the basement rooms. For reliable moisture and mildew protection, even minimal thermal bridges must be eliminated. Structurally conditioned thermal bridges can be well remedied by a professional external insulation.

Structural differences between perimeter insulation in new and old buildings

In a new building, the perimeter insulation is installed immediately after the basement installation, the floor slab is mitgedämmt. Since the entire weight rests on the insulating layer under the floor slab, a very strong material - usually XPS or PUR / PIR - must be used here. When renovating old buildings, the earth is excavated around the house; eventually, the causes of damp walls must be found and eliminated before the start of insulation work. Subsequent external insulation of the floor slab of the house is not possible.

Insulation of the basement ceiling

In an unheated and hardly actively used cellar, the insulation of the basement ceiling is sufficient for an energetic renovation. For a simple Kellerdeckendämmung insulation boards are glued or dowelled from the bottom of the ceiling - the choice of attachment method depends on the nature of the surface. Mineral wool (stone or glass wool), EPS / polystyrene or PUR / PIR are often used as insulation materials. An additional insulation of the upper 50 cm of the outer wall in the soil helps to avoid thermal bridges.

Apply the basement ceiling insulation on the ground floor

If the basement ceiling is too low, arched or heavily covered with cables and piping, the insulating layer can be applied from the ground floor, albeit at a higher cost. For this purpose, a watertight film is laid on the ground floor, on which stand firm insulation boards made of EPS / Styrofoam, XPS, cork or hemp and the floor covering. A thinner screed may make it unnecessary to adjust doors and landings.

Various methods for a cellar-side insulation

If the basement ceiling has a cavity, blown insulation may be an efficient alternative to panel insulation. Arched ceilings can also be insulated on the basement with the help of pre-assembled substructures. Another possibility is to apply an insulating layer in the so-called spray-on process in the form of a mixture of water and cellulose flakes. This method is also suitable, for example, for ceilings with numerous pipelines.

Table 2: Selected insulating materials for cellar insulation

insulationThermal conductivity (W / mK)Minimum insulation thickness (cm)Cost / m2 (EUR)
XPS0,035 – 0,0451418 – 30
PUR / PIR0,02 – 0,0251010 – 20
EPS / Styrofoam0,035 – 0,045145 – 20
glass wool0,032 – 0,0401410 – 20
cellulose0,04 – 0,0451610 – 20

Insulation of basement walls and cellar floor

For an intensively used cellar, the thermal insulation of the basement walls and the basement floor is also necessary. The wall insulation is done either from the outside as Perimeterdämmung or as internal insulation. The insulation of the basement floor is - with the exception of new buildings as part of perimeter insulation - basically from the inside. However, such a floor insulation reduces the height of the basement rooms, as well as an adaptation of doors and stair treads is necessary. As with the ceiling insulation, a film is pulled in under the insulation layer as a vapor barrier. Naturally, the use of hard-wearing and resilient insulating materials is also important here.

Public support for the basement insulation

A perimeter insulation or the internal insulation of the entire basement are associated with a larger investment. If the minimum requirement of a heat transfer coefficient (U-value) of 0.24 W / (m²K) stipulated by the Energy Saving Ordinance (EnEV) 2014 is exceeded, public funding can be obtained through a KfW construction subsidy or a KfW loan.

Tips & Tricks

All forms of cellar insulation should not be carried out alone, but always by a specialist. In order to achieve optimum moisture protection and high insulation performance, expert knowledge and professional thermal insulation are indispensable.

Video Board: 132. Insulation 101: A net-zero builder's formula for insulating a super energy efficient home