Thermal insulation: how to insulate your house properly

We want our home to be nice and warm when outside temperatures are low but stay cool in the summer. It’s easy to control the temperature thanks to heating and possibly air conditioning. But there is another factor that plays a key role: thermal insulation.

A house with modern insulation in the summer sun.
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In this article

Why thermal insulation makes sense

The type and quality of insulation determines how much heat escapes on cold days and how much heat can enter the house from outside on warm days. This is exactly what is measured by the U-value (formerly known as K-value). It describes the heat transfer in watts per square meter and kelvin (W/m² K). An exterior wall made of solid wood without additional thermal insulation can have a U-value of around 0.5 W/m² K, for example. The lower this heat transfer coefficient, the better the insulation. This allows the desired temperature to be maintained indoors, which means not only more comfort for you, but also lower costs for heating and air conditioning.

Where thermal insulation is an option

A house can lose or gain heat in many different places. The roof and facade are particularly affected. But energy can also be lost elsewhere, such as through windows, doors and even the basement.

A young woman sitting with her cat in front of a radiator.

The effort will not be wasted: good insulation saves money because the house or apartment owner won’t have to heat the property as much.

Roof insulation for pitched and flat roofs

In older houses, the U-value of the roof can quickly reach or even exceed 1.0 W/m² K. Yet an insulated roof can reduce this heat transfer coefficient many times over.

If you have a pitched roof, the first thing to consider is whether you want to use the attic as living space or for storage. Using an attic for additional space requires insulation of the inclined surface and perhaps even of the ceiling. In this case, the insulation is most complex and can be done in two ways:

The first option is to install the insulation material between the rafters, for instance. And this offers one crucial advantage: the roof does not need to be covered first. Experienced DIY enthusiasts can do it themselves if they have a little time to spare, but a company specializing in insulation will usually get the job done more quickly. In addition to the insulation between the rafters, it is worth fitting insulation underneath them as well. This will allow you to achieve even better protection against cold and heat and reduce the U-value even further.

A craftsman applies a layer of insulation to a roof.

For above-rafter insulation, the roof needs to be removed and re-covered.

The other alternative is to opt for above-rafter insulation – which is the best choice if the roof has to be re-covered anyway (or the additional work is not a problem). This type of insulation is mounted on the rafters, which means that there are no gaps. A vapor barrier is absolutely vital. It ensures that no moisture can collect between the insulation material and the wood. What’s more, re-covering the roof is the perfect time to install a photovoltaic or solar unit at the same time.
If you only want to use the attic for storage, ceiling insulation is often sufficient. It is cheap and quick to install. The insulation material is fitted between the living area and the attic.

On a flat roof, however, the thermal insulation is usually above the concrete layer and separated from it by a vapor barrier. The rest of the structure depends on the use that is to be made of the roof. For example, an accessible roof will have more layers than a non-accessible roof.

Facade insulation

As the largest surface area of a house, the exterior wall is also responsible for the greatest energy loss. You can minimize this with facade insulation. If the house has a double-shell outer wall (two walls positioned next to each other and connected with wire ties), insulation can be fitted using the blow-in technique. Professionals blow the insulation material into the area between the walls by drilling several holes. It then spreads itself out evenly in the available space. In new buildings, mats are often installed between the inner and outer wall. This is also known as core insulation.

There are two possibilities available for external insulation: a thermal insulation composite system (ETICS) and a rear-ventilated curtain wall. The ETICS consists of insulation panels attached to the old facade, and a layer that acts as a buffer to the new exterior plaster. A thermal insulation composite system is compact and efficient – and therefore usually the best choice for facade insulation.

A rear-ventilated curtain wall allows greater design freedom as far as the outer appearance is concerned. Here, too, the insulation panels are located directly on the old facade, but a curtain is placed at a certain distance in front. Air is therefore allowed to circulate between the insulation and the new facade, preventing condensation from forming on the masonry.

Craftsmen on scaffolding during facade work on several buildings.

Since the facade is usually responsible for the greatest energy loss in the house, this complex facade insulation is worthwhile in most cases.

Internal insulation

If external insulation is not possible, for example on a protected historical building or a construction that fails to meet the structural requirements, the only remaining option is usually internal insulation. A supporting structure on the walls is the basis for this type of insulation. A vapor barrier is absolutely essential. Otherwise, moisture will quickly accumulate, causing mold to form in the living areas. The insulation cladding can then be wallpapered or plastered.

One problem with interior insulation is that it reduces the living space somewhat. The thicker walls take up several square meters, spread over the house.

Basement insulation

Moisture and cold air can enter the house through the basement. When building a new house, perimeter insulation protects against this. It sits on the foundation and on the outer wall of the basement. This form of insulation can also be fitted in an existing house or an old building, but it involves extensive earthwork.

An alternative to this is internal insulation. This can be retrofitted at any time and without major structural work. However, ground frost, dew and groundwater are extremely problematic in basements and cellars. Internal insulation generally does not withstand these as well as perimeter insulation. The latter is therefore the better choice. If you don’t want to use the space as a living room or dry storage room, it may be enough to insulate it from the floors above. This involves attaching the insulation material to the basement ceiling.

Insulation of windows and doors

Well-insulated masonry is not sufficient, because a great deal of energy can be lost through windows and doors – up to 25 percent in fact. As a builder or house owner, you should therefore not attempt to make savings on insulation here. Double- instead of single-glazed windows make a clear difference. A frame made of plastic also seals better than one made of wood.
A draft under the doors also causes large quantities of energy to be lost. A brush sealing can help here. It reduces the exchange of air and thus ensures that rooms better maintain the desired temperature.

The following insulation materials are available:

Synthetic insulation materials

Expanded polystyrene (EPS): this insulating material is also known as “Styrofoam”. This is actually a brand name for products from the BASF Group. EPS is the standard insulation material for indoor and outdoor use. It is also available enriched with graphite under the brand name “Neopor”. This should further improve thermal insulation.

Extruded polystyrene (XPS): XPS is somewhat more pressure resistant and offers better protection against moisture. This insulation material, which can also be found under the brand name “Styrodur”, is therefore preferred for use in perimeter insulation or on flat roofs.

Polyurethane (PUR): the material used here is rigid foam. Due to its compactness and good insulating properties, PUR is particularly suitable for roof and facade insulation. Polyurethane can also be used in other areas, however.

Mineral insulation materials

Mineral wool: this material is primarily used for roof insulation in the form of glass or rock wool. It is easy to handle and can be inserted precisely between the rafters. However, good protection against moisture is important, because otherwise mineral wool can lose its insulating properties.

Foam glass: if the insulation material is exposed to moisture, foam glass can be a good choice. That’s why it is chosen for perimeter insulation, for example. It consists mainly of waste glass.

Perlite: perlite provides insulation around the house, usually in the form of a granulate. The installation is carried out by backfilling. Alternatively, perlite is also available in the form of panels. These are particularly versatile, for example for insulating ceilings.

Natural insulating materials

Wood fiber: wood fiber is used in fiber form for blow-in insulation or as panels for roof, facade or interior insulation. This allows wood waste to be reused and creates a good insulating effect.

Flax: matted flax fibers glued together with potato starch serve as the basis for insulation panels. They can then be attached to the roof, for example. Alternatively, the fibers can also be processed as stuffing wool, to insulate doors and windows for instance.

Cellulose: cellulose provides insulation within the building material either via the blow-in process or as bulk insulation. It is another versatile material that can be used in many places in the house.

Hemp: this insulation material looks like mineral wool. It can also be cut and handled just as easily. However, it is less suitable for exterior walls. It tends to be used in roof and interior insulation.

Cork: this material can be used not only for floors, but also as panels for insulating roofs and ceilings. If the cork is crushed, it can even be used to provide bulk insulation.

Sheep’s wool: this natural fiber is a genuine all-rounder and is used in textile production, for instance. One of the reasons for this is that it has good insulating properties. After special processing, sheep’s wool can also be used for roof and interior insulation.

A woman wrapped in a blanket reads a book in front of a closed window.

Windows and doors should also be well insulated, otherwise a great deal of heat is lost.

Subsidies available for thermal insulation

Good insulation improves the carbon footprint of a house. The Confederation and the cantons are therefore prepared to promote energy-efficient rehabilitation. Subsidies vary from canton to canton. As part of the buildings program, however, the cantons have agreed on a minimum amount of 40 francs per square meter of thermally insulated building components. However, the U-value must improve by at least 0.07 W/m² K as a result, and the building permit for the house must have been issued before 2000. Anyone interested in a subsidy can visit to obtain information directly about the regulations in their canton.

The cost of thermal insulation

The question of costs is difficult to answer because they can vary greatly. Depending on the type of insulation, the materials used and the structural conditions, the prices per square meter fluctuate considerably. That’s why it’s worth discussing the possible costs with a company specializing in thermal insulation right from the start.

Conclusion: thermal insulation is always worthwhile

Lower energy costs, indirect environmental protection and an increase in the value of the house: energy-efficient renovation has many advantages. The costs for the builder or homeowner can be significantly reduced thanks to subsidies from the respective canton. However, the type of insulation and the choice of material vary in each case. We hope that you have found the information in our guide useful.


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