Insulation above the rafters

Insulating a pitched roof from the outside is common in Western Europe. Its advantage over insulating from the inside (between the rafters) is that air cannot pass through the ventilated layer (between the tiles and the roof structure) and appear between the rafters, so heat losses in winter are avoided and the insulation is more effective. The rafters are boarded from the outside, then the waterproofing is laid (e.g. a vapour-permeable bitumen membrane) and then the insulation, for which we recommend Austrotherm EPS A150, our hardest EPS board. It is needed because above the rafters the boards carry high loads (including compressive loads) from direct exposure to the weather (wind, rain, snow etc.). We recommend the following thicknesses of Austrotherm EPS A150, which give the U-values shown in the table:

1. 1roof tiles
2. 2battens (cross battens)
3. 3battens (counter battens)
4. 4Austrotherm EPS A150
5. 5waterproofing layer
6. 6full board sheathing
7. 7rafters

After laying the Austrotherm EPS A150 boards over the rafters, first fix the counter battens (in the direction of the rafters) and then the cross battens that carry the tiles. The two layers of battens create an air space that serves as additional insulation (air circulation, roof ventilation). The first counter batten is nailed or screwed through the EPS into the rafter; at the eaves a starter batten is fixed first, on which the first row of EPS boards rests — it also stops the boards from sliding off.

This way of insulating the roof has another advantage: the load-bearing roof structure remains visible after insulation (provided the work is done professionally, also with regard to fire protection). Make sure the waterproofing (usually bitumen sheets) is laid so that it seals well against wind. If you lay, for example, a 40 mm visible timber boarding under the waterproofing, you also get excellent sound insulation: a sound insulation value of 49 dB is quoted for this construction.

If the rafters (visible from inside with this type of roof insulation) are closed from the inside with plasterboard or similar materials, and the fields between the rafters as well, at least fire resistance class F30 must be ensured, i.e. at least 30 minutes of fire resistance.

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