Renovating

Roof insulation: types, subsidies and costs

Thomas Bott

Insulation is particularly important when carrying out energy-efficient renovation work or construction of a new building. The roof should not be underestimated. There are many different ways to insulate a roof, all of which have their advantages and disadvantages. Here you can find out what these are, how insulation works and what costs you should expect.

An attic during insulation work. Unclad insulation can be seen between the rafters.
© Getty Images/iStockphoto

In this article

Roof insulation: what makes it so important

It is a well-known fact that heat rises. If the insulation is insufficient, the warm air often escapes through the roof first – prompting a rise in heating costs. With the help of roof insulation, you can achieve significantly better energy efficiency – provided the other parts of the house are well insulated as well. In general, an uninsulated roof is responsible for about 15 to 25 percent of the heat loss in a house. If the area directly under the roof is to be inhabited and heated, the Swiss Energy Efficiency Ordinance (EnEV) requires energy-efficient renovation to be carried out anyway.

A thick layer of snow lies on the sloping roof of a wooden house. Some snow has melted around the chimney.

Good insulation not only helps to prevent heat from escaping, but also stops cold air from entering the house.

Types of insulation: four different options

When it comes to insulation, there are various approaches to choose from. The type of roof insulation that’s right for you depends entirely on the circumstances.

Rafter-mounted insulation

The roof structure consists of rafters, which in turn support the actual roof. With rafter-mounted insulation, the insulation material is placed on top of these rafters. These rafters form the actual roof construction. The formwork, consisting of wooden boards, is placed above the rafters. This is what determines the shape of the roof. A vapor barrier foil is placed on top to prevent moisture from penetrating the roof. With this method, the actual insulation material follows after this. Since the insulation is on the outside, there is of course also space for the tiles. That’s why counter battens are needed above the insulation. The support battens for the tiles are mounted on top.

As you may already have guessed, rafter-mounted insulation involves a great deal of work. For existing roofs, it is necessary to completely remove and re-cover the roof. This type of insulation is therefore particularly worthwhile if a new roof is needed anyway – or, of course, when constructing a new building. Rafter-mounted insulation has the best insulating effect. It does not create unwanted thermal bridges as it completely covers the roof. This means there are fewer areas that conduct heat better than others. Even the rafters themselves can form thermal bridges. Furthermore, the space in the attic is not affected.

A roofer lays panels above the insulation layer. The tiles will later be rested on top.

Work on the roof is necessary for rafter-mounted insulation. This should definitely be carried out by a professional.

Rafter-to-rafter insulation

When installing rafter-to-rafter insulation, the roof can remain untouched from the outside. The insulation material is fitted between the rafters. There is then a vapor barrier above the living area to prevent moisture from accumulating. Otherwise the wood could be damaged and the insulation could even become moldy. Since vapor barrier foil does not look very attractive, cladding is placed over it. This can be painted or wallpapered.

The effort required for this method is fairly low. Rafter-to-rafter insulation can be installed relatively quickly and no major preparatory work is required. This means that this type of insulation can be fitted at any time. However, in comparison to rafter-mounted insulation, thermal bridges (see above) can occur. This makes rafter-to-rafter insulation less efficient.

Picture of ready mounted rafter-to-rafter insulation. But the cladding has not yet been added.

You can also carry out rafter-to-rafter insulation yourself with a little manual skill.

Under-rafter insulation

Here, too, the name gives it away: under-rafter insulation is installed below the rafters, i.e. inside the attic. There is a vapor barrier in between to guard against moisture. Cross battens are screwed to the rafters to keep the insulation in place. Cladding is added here later. It can be individually decorated with paint or wallpaper.

Under-rafter insulation alone is only suitable in very few cases. This is due to the limited insulation properties resulting from the thin thickness of the insulation layer. However, under-rafter insulation is a very good complement to rafter-to-rafter insulation. It compensates a little for thermal bridges, making for a very efficient combination.

Floor-to-ceiling insulation

Floor-to-ceiling insulation is not direct roof insulation. With this method, the insulation material is located between the floors, i.e. it is attached above the ceiling of the floor below the attic. Less work is involved overall, as the area to be insulated is smaller and no extensive construction work is required on the roof. However, this type of insulation is only suitable if you don’t want to make use of the attic.

Insulation panels are laid out on the attic floor. Underneath is a vapor barrier, above it are wooden plates.

If you don’t want to use the attic as living space, floor-to-ceiling insulation may prove sufficient.

Flat roof insulation: a special type of insulation

Even on flat roofs, good roof insulation offers a major energy advantage. A distinction should be made between three basic procedures:

  • Cold roof: this refers to summer thermal insulation. This is achieved during construction by trapping a layer of air between the insulation and the roofing. A vapor barrier is installed on the reinforced concrete ceiling, while the actual insulation is placed between the supporting beams above. This is followed by a separation layer made up of battens. This gives the air the space it needs. Wooden formwork encloses the air from above. After another separating layer comes the roof sealing.
  • Warm roof: with a warm roof there is no layer of air. This means that the structure doesn’t have a separating layer, supporting beams, battens or a wooden formwork. This makes the insulation much more compact, but also less effective than thermal insulation.
  • Inverted roof: here the order of the components is slightly different. Basically, however, an inverted roof corresponds to a warm roof, and its characteristics are the same. With an inverted roof, the sealing is at the very bottom of the reinforced concrete ceiling. The insulation panels are mounted above it. It is important to use moisture-repellent insulation, such as expanded polystyrene. A layer of fleece is laid on top of this to protect against damage. Finally comes backfill gravel.

Insulation materials: glass wool, polystyrene and more

There are many different insulation materials available on the market. They have different properties in terms of thermal conductivity, combustibility, building material class and sensitivity to moisture. The following materials are particularly suitable for roof insulation:

Inorganic insulation materials: rock wool and glass wool

These are soft insulation materials made of artificially produced fibers. The minerals of the same name serve as a basis for their manufacture. They are sold as sheets or rolls. Rock wool and glass wool are easy to cut to size, so they can be adjusted precisely and are easy to install. Mineral wool is not flammable but differs in other respects. Rock wool offers better sound insulation, but glass wool is slightly more flexible. Inorganic insulation materials are often used for rafter-to-rafter insulation.

A workman fits an inorganic insulation material between rafters and presses it firmly into place.

Inorganic insulating materials such as rock wool and glass wool are particularly popular for rafter-to-rafter insulation due to their great flexibility.

Foam insulation materials: polystyrene and polyurethane

Polystyrene and polyurethane are plastics. Polystyrene (EPS) is usually known by the brand name Styropor and is used in sectors such as the packaging industry. However, it is also suitable as insulation in the form of panels. Extruded polystyrene (XPS), also marketed under the brand name Styrodur, is a special form of polystyrene. Its denser structure makes it somewhat less sensitive to moisture and allows it to withstand higher loads. Polyurethane is often found in upholstered furniture, but it also has good insulating properties. The material tends to be used for rafter-mounted and floor-to-ceiling insulation.

Natural insulating materials: sheep’s wool, hemp, coconut wool and more

It is also possible to opt for insulating materials made of natural materials. These include fibers from sheep’s wool, hemp, coconut plants and wood. However, the insulation properties vary greatly. An expert can help you decide which to choose. Natural insulating materials are primarily suitable for rafter-to-rafter insulation, often in combination with under-rafter insulation made of foam insulation materials.

Costs: how expensive it is to insulate a roof

The cost of roof insulation depends a great deal on the type of insulation and on the materials used. Rafter-mounted insulation is particularly expensive, as the roof has to be re-covered in full. A combination of rafter-to-rafter and under-rafter insulation could be a good alternative. An overview of the costs to expect is shown in the following table:

  • Type of insulation
  • Cost per square meter
  • Rafter-mounted insulation with roofing
  • about 400 francs
  • Rafter-to-rafter insulation
  • about 100 francs
  • Under-rafter insulation
  • about 60 francs
  • Floor-to-ceiling insulation
  • about 50 francs
  • Flat roof insulation
  • about 80 francs

State funding: subsidies in Switzerland

You will not be left completely alone when it comes to paying for the energy-efficient renovation of your roof. This type of insulation is covered by the cantons’ harmonized subsidy model. It comes under the heading “Thermal insulation” (M-01) in the section on building renovation with individual measures. The minimum subsidy is 40 francs per square meter. Higher subsidies or additional support measures may be available depending on the canton and municipality. You can obtain information from them directly.

Conclusion: more efficiency thanks to roof insulation

Energy-efficient renovation of a roof is worthwhile in many cases. It is even possible to install the roof insulation yourself with a little bit of manual skill. However, you should always hire a professional for rafter-mounted insulation – simply because of the risk of falling from the roof.

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