Under rafter insulation - well suited for retrofitting the attic

Under rafter insulation is a common way to insulate a roof. It is particularly suitable when subsequently the expansion of an attic takes place or the thermal insulation of an already inhabited attic is optimized. The insulating effect of an intermediate rafter insulation can optimize them to a great extent.

Private builders often consider under-rafter insulation as the least favorable way to insulate a roof. The problem: In the case of under rafter insulation, the insulation material is installed underneath the rafters, which of course means that the living space in the attic is lower. However, with a pitched roof, the available space and the shelves for furniture are also limited by the sloping ceilings themselves, so that this argument against a sub-rafter insulation is not really significant. Overall, it is an energy-efficient and very effective way to insulate a roof. Its strengths unfolds a Untersparrendämmung especially if it is combined with other forms of roof insulation.

Table 1: Minimum insulation thicknesses, thermal conductivity and m2 costs of selected insulation materials

insulationrock woolglass woolEPS / Styrofoamhemp
Minimum insulation thickness (cm)14141416
Thermal conductivity in W / (mK)0,035 – 0,0400,032 – 0,0400,035 – 0,0450,04 – 0,045
Cost per m2 (EUR)10 – 2010 – 205 – 2010 – 27

Under rafter insulation and rafter insulation - an optimal duo

For example, under rafter insulation and rafter insulation form a nearly perfect duo. Depending on the insulation material, the energy efficiency of the roof insulation can be increased by around a quarter thanks to this combination. A pure Untersparrendämmung is theoretically also possible, but in practice, with the exception of the subsequent thermal insulation of an already developed and inhabited roof rather rare.

Under-rafter insulation prevents thermal bridges

Another advantage of a sub-rafter insulation is that this thermal bridges in the rafter area are minimized or avoided altogether. The timber of the rafters has three to four times greater thermal conductivity than the insulation of an intermediate rafter insulation. Thermal bridges and thus energy losses are therefore often difficult to prevent in such constructions.

Which buildings are suitable for under-rafter insulation?

A Untersparrendämmung or the combination of Untersparren- and Zwischensparrendämmung is particularly suitable for a subsequent expansion of the attic. For new buildings, this form of roof insulation, however, makes little sense. In this case, the decision is usually always for a Aufsparendämmung fall, which is the best solution from a building physics perspective, as it ensures a continuous and seamless thermal insulation without loss of space in the attic. The same applies to old buildings, in which a complete roof renovation is due anyway.

A sub-rafter insulation optimizes thermal protection

Like all other insulation materials, a sub-rafter insulation not only optimizes the energy efficiency, but also the thermal insulation of the house. If necessary, this aspect also plays a role in the choice of the insulating material. As a rule of thumb, natural insulating materials - such as hemp insulation, wood fiber, cellulose or flax - have much better heat protection properties than conventional mineral wool or EPS / Styrofoam insulation with nearly identical insulation performance.

Tips & Tricks

A sub-rafter insulation is suitable for the subsequent thermal insulation of an already developed roof as well as for renovation projects in which not the entire roof is to be renewed. Particularly effective is a combination insulation of sub-rafter insulation and intermediate rafter insulation. Compared to a pure intermediate rafter insulation, it reduces energy losses through the roof of the house by a further 25%.

Insulation of the upper floor ceiling or under rafter insulation

For many older single-family homes, the roof insulation is rather poor. The Energy Saving Ordinance (EnEV) 2014 stipulates that for self-used houses with a maximum of two apartments, thermal insulation can be dispensed with, as long as there is a minimum thermal insulation through an approx. 5 cm thick insulating layer and the current owner already has the building before 1 February 2001 inhabited. For all other buildings, at least the thermal insulation of the top storey ceiling is mandatory - with regard to the energy efficiency of the house, this type of insulation has the same effectiveness as a completely insulated roof. Homeowners who subsequently opt for insulation measures in the roof area, are often well-advised with a complete roof insulation, as for a conversion of the previously unused attic in living room then only the interior is pending.

Sub-rafter insulation for roofs already removed

A sub-rafter insulation is also suitable for optimizing the thermal insulation of an already developed and inhabited roof - for example, because the owner must meet the requirements of the EnEV 2014 for thermal insulation. In this case, the insulating layer is mounted from the inside on an existing wall, then a new wall cladding must be made. The associated loss of space is faced with the fact that in this way no complete renovation of the roof is needed.

How is a sub-rafter insulation made?

A sub-rafter insulation is applied to an existing intermediate rafter insulation or directly to the roof cladding. In the case of a combination of under- and intermediate-spar insulation, additional battens are installed across the rafters, the interstices of which are later filled completely by the insulating material. At the same time, this lathing serves as a basis for the wall structure, which can be done with gypsum fiber or OSB boards, for example.

Construction in a total of five layers

A combination of intermediate and sub-rafter insulation consists of a total of five layers:

  • Insulation between the rafters
  • Steam brake / vapor barrier, if required by construction
  • Battens and under rafter insulation
  • wall covering
  • Wallpaper or painting.

Vapor barrier or diffusion-open system?

A vapor barrier or vapor barrier is not mandatory for under rafter insulation. The general trend of thermal insulation, especially in renovation of old buildings, is very clearly towards diffusion-open systems that are able to permanently and actively regulate their moisture balance. Here as well - in addition to the general construction of the roof and the structural conditions - the nature of the insulation material is very important. Most natural insulating materials are highly permeable and capillary active. For a diffusion-open insulation structure but also special vapor barrier films come into question, which minimize the moisture input into the insulation, without stopping the moisture regulation through the insulation layer as a whole. The decision whether a sub-rafter insulation with or without vapor barrier / vapor barrier is made, should be made only by a person skilled in the art.

An additional installation level through insulation

Another advantage of the sub-rafter insulation is that an additional installation level can be created by this, for example, to lay electrical lines. If such a construction is not necessary, the insulation can also be made with insulation boards, which are circumferentially equipped with tongue and groove and are screwed from below to the rafters. In this way, a continuous layer of insulation is created quickly and with little work.

Which insulating materials are suitable for under-rafter insulation?

The choice of the optimal insulating material depends on the individual construction of the roof truss. The maximum possible insulation thickness depends largely on the height of the slats of the roof truss. If the minimum requirements of the EnEV 2014 for achieving a heat transfer coefficient (U-value) of 0.24 W (m2K) can not be achieved, it is necessary to create an additional battening even for a sub-rafter insulation that is not combined with an intermediate rafter insulation, For combinations of the two types of insulation, the sub-rafter insulation may be designed to be thinner.

Flexible insulation choice

For a Untersparrendämmung many insulating materials are suitable. Especially in the insulation of older roofs flexible insulation materials in the form of fleeces or mats are optimal, since they are able to completely fill the gaps in the roof battens and thereby compensate for misalignments of the slats. Many builders will decide - also due to the low price - for a sub-rafter insulation with mats made of stone or glass wool, which also optimize the fire protection properties. If necessary, insulation boards made of EPS / polystyrene come into question. In addition, many natural insulating materials are suitable for under-rafter insulation, but require a slightly larger insulation thickness to achieve the EnEV specifications.

Cost of a sub-rafter insulation

The m2 costs for a sub-rafter insulation are between 30 and 70 euros. For a detached house with 80 m2 of floor space, 140 m2 of living space and a sloping roof area of ​​120 m2 total costs between 3,600 and 8,400 euros. For comparison, a Aufsparrendämmung for the same house would cost between 18,000 and 24,000 euros. If the under-rafter insulation is combined with a new intermediate-rafter insulation, however, further investments in the amount of 6,000 to 9,600 euros will be required for them.

Table 2: Costs of under rafter insulation and intermediate rafter insulation

DämmungsartUnder raftersInsulation between the rafters
Cost per m2 (EUR)30 – 7050 – 80
Cost Family house (EUR)3.600 – 8.4006.000 – 9.600

Detached house: building with a roof pitch of 120 m2

A sub-rafter insulation can be publicly funded

For sub-rafter insulation as well as for all other insulation solutions public funding can be applied for. The prerequisite for a KfW grant or a KfW loan is the expert opinion of an energy expert. In addition, the planned insulation generally must exceed the EnEV minimum thermal insulation requirements. Funding options include the KfW subsidy 4230 (10% of the total investment, a maximum of 5,000 euros) or the KfW loan 152 (0.75% interest, 7.5% amortization subsidy, maximum 50,000 euros).

Tips & Tricks

A Untersparrendämmung is executed by skilled DIYers also as own contribution. For the planning of this insulation measure, however, expert advice is required in any case. Decisions on the choice of the optimal insulating material as well as the diffusion-permeable or diffusion-tight structure of the system should always be made by a competent consultant in order to avoid planning mistakes and later costly renovations.

Video Board: Insulating Between & Under Rafters | Loft Conversion Project 4.0