Flat Roof Insulation (Warm Roof)

Adding an insulation layer to the existing flat roof — usually in the course of renewing the waterproofing.

How does flat roof insulation work?

With flat roof insulation, an additional insulation layer is applied to the existing roof, usually as part of renewing the roof waterproofing. The warm roof build-up is: existing roof, vapour barrier, 24 to 30 cm of PIR, PUR or mineral wool insulation, new waterproofing, then gravel ballast or green roof. The vapour barrier on the warm side prevents moisture from the building entering the insulation layer and condensing there. With at least 24 cm of insulation, the U-value drops from 0.5 to 1.5 W/(m²·K) in the existing state to 0.16 to 0.18 W/(m²·K). Because the insulation lies on the outside of the load-bearing structure, it remains free of thermal bridges; junctions with parapets, rooflights, roof outlets and penetrations must, however, be carefully carried up. The additional load of 30 to 80 kg/m² must be verified structurally.

Which buildings benefit from flat roof insulation?

Typical buildings are apartment buildings with flat roofs, office buildings and industrial halls. The right time is when the roof waterproofing is due for refurbishment anyway, usually after 25 to 35 years, because waterproofing costs are incurred in any case and only the insulation counts as additional cost. Requirements are a structurally sound roof construction and resolved junction details at parapets and rooftop structures. The measure can be ideally combined with a mounted photovoltaic system or extensive green roof, which additionally reduces roof heating in summer.

What does flat roof insulation actually deliver?

Heating energy savings are 10 to 20 %. For an apartment building with a 350 m² flat roof, the saving is around 20,600 kWh of natural gas per year, equivalent to about 4.1 t of CO2. Costs are 150 to 280 €/m² including new waterproofing, with a payback of 12 to 22 years over a 35-year service life. Because cooling demand on the top floor falls, the measure also works in summer. Together with a PV system, this creates a bundle that reduces heat losses and generates electricity at the same time.

Energy carrier

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

Savings potential

10-20 %

Worked example

An apartment building from the 1960s with a 350 m² flat roof has to be refurbished because of leaking waterproofing. The roof has thin existing insulation with a U-value of 0.9 W/(m²·K). As part of the waterproofing refurbishment, a vapour barrier and 24 cm of insulation are applied, reducing the U-value to 0.16 W/(m²·K). The building is heated by gas at 90 % efficiency, and the location has 3,500 heating degree days. With a correction factor of 0.85 for thermal bridges and occupant behaviour, the calculation is: 350 × (0.9 − 0.16) × 3,500 × 0.024 / 0.9 × 0.85 = 20,580 kWh/a. The building thus saves around 20,600 kWh of natural gas per year and about 4.1 t of CO2. Because the waterproofing had to be renewed anyway, the insulation pays back considerably faster than the nominal payback period.

Investment & payback

150-280 €/m² (inkl. Abdichtung) · Service life: 35 a · Payback: 12-22 a

Applicability

Requirements

Typical buildings

Frequently asked questions

How much does flat roof insulation cost per square metre?

Flat roof insulation as a warm roof costs 150 to 280 €/m² including vapour barrier, 24 cm of insulation and new waterproofing. For 350 m² of roof area, that is about 52,000 to 98,000 €. Since the waterproofing has to be renewed anyway after 25 to 35 years, the pure additional cost of the insulation, at about 40 to 80 €/m², is considerably lower. Gravel ballast, green roof or PV mounting come on top.

How thick does insulation on a flat roof need to be?

At least 24 cm of insulation is recommended to achieve a U-value of 0.18 W/(m²·K) or better; 24 to 30 cm is common. With PIR or PUR boards of lower thermal conductivity, the same insulating effect can be achieved with less thickness, which helps where parapet height is limited. OIB Guideline 6 requires no more than 0.20 W/(m²·K) for refurbishments in Austria; in Germany, the same value applies under GEG.

What is the difference between a warm roof and an inverted roof?

In a warm roof, the insulation lies beneath the waterproofing and is protected against moisture from inside by a vapour barrier; the waterproofing is the top layer. In an inverted roof, the XPS insulation lies above the waterproofing and is weighed down with gravel or planting. The inverted roof protects the waterproofing from temperature fluctuations and UV radiation, but requires a moisture-resistant insulation material and has somewhat higher heat losses due to water flowing beneath the insulation.

Can a flat roof bear the additional insulation?

A structural engineer must check this. Flat roof insulation with new waterproofing adds 30 to 80 kg/m² of load, gravel ballast or extensive planting 50 to 150 kg/m², and PV mounting a further 15 to 30 kg/m². With reinforced concrete slabs, the reserve is usually sufficient; with trapezoidal sheet roofs on industrial halls or old timber structures, it can be tight. Structural verification is a prerequisite for execution.

Can flat roof insulation be combined with photovoltaics or a green roof?

Yes, the combination is actually recommended. Refurbished waterproofing with high insulation is the ideal basis for a mounted PV system, because the roof then does not need to be opened again over the lifetime of the modules. Extensive planting protects the waterproofing, cools the roof in summer and improves the yield of the PV modules through lower ambient temperatures. The additional loads must be taken into account structurally.

Related measures

Standards & sources

Metering points in the EDM Toolbox

Beim Bündel mit PV: gemeinsamer Stromzähler PV, Vergleich Heizenergie vor/nach mit HGT-Korrektur. Optional Dachoberflächentemperatur als Indikator für sommerlichen Kühlbedarf.

All measures