Pitched Roof Insulation (Above or Between Rafters)

Insulating a converted loft either above the rafters (when re-roofing) or between them (from inside).

How does pitched roof insulation work?

With a converted and heated loft, the roof pitch forms the thermal envelope. There are two basic variants for insulation. Above-rafter insulation places a continuous insulation layer of 20 to 30 cm above the rafters, directly beneath the new roof covering. It is thermally the best solution because the rafters do not interrupt the insulation layer and no linear thermal bridges arise; the U-value drops from about 1.5 to 0.12 to 0.18 W/(m²·K). Between-rafter insulation fills the bays between the rafters from the inside with mineral wool, cellulose or wood fibre. If the rafter depth is insufficient, below-rafter insulation is added. In both cases, three details are decisive: a room-side vapour barrier or vapour retarder that keeps humid room air away from the structure, a windtight layer on the outside, and carefully executed junctions at eaves, ridge, verge and roof windows.

Which buildings benefit from pitched roof insulation?

Typical buildings are detached houses with loft conversions and top-floor flats in apartment buildings. Above-rafter insulation is economical when the roof is due to be re-covered anyway, because the covering costs are incurred in any case. Between-rafter insulation from the inside is the cheaper choice if the roof is watertight and the internal lining can be renewed. A build-up verified in terms of building physics is a prerequisite, especially for unventilated constructions. If the loft is unheated, however, top-floor ceiling insulation is the simpler and cheaper measure.

What does pitched roof insulation actually deliver?

Heating energy savings are 15 to 30 %. For a roof area of 140 m², the saving is around 15,000 kWh of natural gas per year, equivalent to about 3.0 t of CO2. Above-rafter insulation with re-covering costs 180 to 280 €/m², between-rafter insulation from the inside 80 to 150 €/m². Payback is 12 to 25 years over a 40-year service life. A further advantage is summer heat protection: high-density insulation materials such as wood fibre significantly delay the heating-up of roof spaces.

Energy carrier

Erdgas, Heizöl, Pellet, Fernwärme, Strom → Erdgas, Heizöl, Pellet, Fernwärme, Strom

Savings potential

15-30 %

Worked example

A detached house with a converted loft has 140 m² of uninsulated roof pitch with a U-value of 1.5 W/(m²·K). Since the roof covering has to be renewed anyway, above-rafter insulation with 24 cm of insulation is applied, reducing the U-value to 0.15 W/(m²·K). The house is heated by a gas boiler with 90 % efficiency, and the location has 3,500 heating degree days. The saving is calculated from area, U-value difference, heating degree days and efficiency, multiplied by the correction factor of 0.85: 140 × 1.35 × 3,500 × 0.024 / 0.90 × 0.85 = 14,994 kWh/a. The building thus saves around 15,000 kWh of natural gas per year and about 3.0 t of CO2. In addition, summer thermal protection in the loft improves noticeably.

Investment & payback

Aufsparren mit Neueindeckung: 180-280 €/m²; Zwischensparren von innen: 80-150 €/m² · Service life: 40 a · Payback: 12-25 a

Applicability

Requirements

Typical buildings

Frequently asked questions

How much does roof insulation cost per square metre?

Above-rafter insulation including re-covering costs 180 to 280 €/m² of roof area, between-rafter insulation from the inside 80 to 150 €/m² including vapour retarder and internal lining. For 140 m² of roof area, that comes to 25,000 to 39,000 € or 11,000 to 21,000 € respectively. Above-rafter insulation is only economical if the roof covering has to be renewed anyway.

What is the difference between above-rafter and between-rafter insulation?

Above-rafter insulation lies as a continuous board above the rafters beneath the roof covering and is practically free of thermal bridges; it requires re-covering and is more expensive. Between-rafter insulation is installed from the inside between the rafters; the timber rafters interrupt the insulation layer and form linear thermal bridges, which is why below-rafter insulation is often added. It is cheaper and can be implemented without opening the roof.

Does roof insulation need a vapour barrier?

Yes, a room-side vapour barrier or vapour retarder is indispensable with pitched roof insulation. It prevents warm, humid room air from penetrating the insulation and condensing on the cold outer side, which would lead to mould and rot on the rafters. All joints and junctions with walls, roof windows and penetrations must be airtightly taped. Humidity-variable vapour retarders allow drying back in summer.

How thick does roof insulation need to be?

20 to 30 cm of insulation is recommended to achieve a U-value of 0.12 to 0.18 W/(m²·K). With 24 cm of insulation, the result is typically 0.15 W/(m²·K). OIB Guideline 6 requires no more than 0.20 W/(m²·K) for refurbishments in Austria. With between-rafter insulation, the rafter depth is usually limited to 14 to 18 cm; the missing thickness is then added as below-rafter insulation.

Does roof insulation also help against summer heat?

Yes, pitched roof insulation also significantly reduces summer heat gain. Besides insulation thickness, the density of the insulation material is decisive: wood fibre or cellulose with high heat storage capacity delay heat passing through the build-up by several hours, so that the heat peak only arrives in the cooler evening. Mineral wool insulates equally well in winter but offers less phase shift in summer.

Related measures

Standards & sources

Metering points in the EDM Toolbox

Feuchtesensoren in der Konstruktion (z.B. zwischen Dampfbremse und Dämmung) bei kritischen Aufbauten zur Schadensfrüherkennung sinnvoll.

All measures