Flat Roof Insulation: Materials, Thickness and Prices 2026

Flat roof insulation is one of the most important factors in a building’s energy efficiency – up to 25% of total heat can be lost through a poorly insulated roof. In this article we cover which insulation materials are suitable for flat roofs, what thickness is needed depending on building class, how to choose between mineral wool, XPS and EPS, what it costs in Lithuania in 2026 and the most common mistakes builders make.

Why flat roof insulation is critical

Hot air rises – so a poorly insulated flat roof becomes the main heat-loss path in winter and a heat source in summer. Properly installed insulation:

• Reduces heating bills by up to 30%;
• Extends waterproofing life (smaller temperature swings);
• Ensures summer comfort – rooms don’t overheat;
• Protects the structure from condensation and mould;
• Increases property value and energy class.

Insulation materials for flat roofs

Mineral wool (rock wool)

The traditional and most reliable insulation material for flat roofs. Key advantages – non-combustibility (A1 class), good acoustic insulation and breathability. Drawbacks – heavier than polystyrene, more expensive and requires dry working conditions.

Thermal conductivity (λ): 0.034–0.040 W/(m·K). Typical thickness: 25–30 cm.

XPS – extruded polystyrene

Denser and mechanically stronger than EPS, water-resistant. Ideal for inverted (upside-down) roofs where insulation is laid above waterproofing. Also a good choice for trafficable flat roofs and terraces.

Thermal conductivity (λ): 0.032–0.036 W/(m·K). Typical thickness: 22–25 cm. Compressive strength: 300–700 kPa.

EPS – expanded polystyrene (Neopor, EPS 100/150)

The lightest and cheapest insulation material. Suitable for classic compact flat roofs. Modern EPS variants (Neopor with graphite additives) have better insulation values than traditional white polystyrene.

Thermal conductivity (λ): 0.031–0.038 W/(m·K). Typical thickness: 25–28 cm. Cheapest option.

PIR / PUR boards

Top-tier insulation with the lowest thermal conductivity. Used when structural depth is limited (e.g. renovation with low parapets).

Thermal conductivity (λ): 0.022–0.025 W/(m·K). Typical thickness: 14–18 cm – achieves the same U-value as 25 cm of wool.

How thick should the insulation be? Table by building class

Lithuanian building regulations set minimum U-values for flat roofs. The post-2021 requirement is U ≤ 0.16 W/m²K. Practical thickness recommendation by material and building class:

• B-class building (renovation): 18–22 cm wool / 16–20 cm XPS;
• A-class new building: 25–28 cm wool / 22–24 cm XPS;
• A+ and A++ class: 30–35 cm wool / 25–30 cm XPS / 18–22 cm PIR;
• Passive house: 35–40 cm wool or equivalent (U ≤ 0.10 W/m²K).

Note: insulation that is too thin not only loses heat but can cause condensation and mould. It’s better to choose a slightly thicker layer than the minimum.

Insulating from outside vs inside

External insulation (above the slab) is the technically correct option – it eliminates thermal bridges, protects the structure and ensures longevity. Internal insulation of a flat roof is rarely suitable in practice – it causes condensation, mould and structural damage. The only case for internal insulation is heritage protection requirements, but it requires extremely careful vapour barrier design.

Insulation installation steps

1. Substrate preparation and drying;
2. Vapour barrier laid with sealed seams;
3. First insulation layer with overlapping joints (avoiding thermal bridges);
4. Second layer rotated 90°;
5. Mechanical fixing or bonding;
6. Waterproofing applied immediately, before insulation gets wet.

Flat roof insulation prices 2026

Insulation cost depends on material, thickness and roof accessibility. Indicative prices (materials + labour):

• Mineral wool 25 cm: 22–32 EUR / m²;
• EPS 100, 25 cm: 15–22 EUR / m²;
• EPS Neopor, 25 cm: 18–25 EUR / m²;
• XPS, 22 cm: 25–35 EUR / m²;
• PIR, 16 cm: 32–45 EUR / m²;
• Tapered slope formation: +5–12 EUR / m².

Price typically includes: material, vapour barrier, mechanical fixing and labour. Waterproofing is priced separately.

Most common insulation mistakes

• Layer too thin. Saving now means heating losses for decades.
• Skipping the vapour barrier. Insulation gets wet, losing up to 50% of efficiency.
• Single-layer installation. Two layers rotated 90° eliminate thermal bridges.
• Wrong material for trafficable roofs. Requires XPS or EPS 200, not standard EPS 70.
• Leaky parapet detailing. Cold spreads from outside inwards.