Insulation and vapour barrier

Stone wool, glass wool, EPS and XPS · U-value requirements · Vapour barrier on the correct side

Proper insulation is the single most cost-effective investment you can make in a building — whether it is new construction or a retrofit. The choice of insulation type, thickness and vapour barrier position determines energy consumption, indoor climate and the long-term durability of the structure. This guide gives you the foundation to make the right decisions.

Insulation types

Stone wool (Rockwool, Paroc, Knauf)

• Made from molten volcanic rock — non-combustible (Euroclass A1 / ASTM E136)
• Excellent acoustic and thermal performance; used in walls, roofs, floors and building services
• Handles normal moisture exposure well (hydrophobic surface treatment) — prolonged submersion should still be avoided
• Thermal conductivity (λ) typically 0.033–0.037 W/mK
• Available as flexible rolls (walls/roofs) and rigid boards (floors, flat roofs)

Glass wool (Isover, Ursa, Owens Corning)

• Made from recycled glass — lightweight and flexible, easier to cut than stone wool
• Slightly better λ-value than stone wool (typically 0.030–0.034 W/mK)
• Less suited to moisture-exposed environments than stone wool — prefer stone wool in crawl spaces and other damp locations
• Available as rolls — the lowest-cost option for interior stud walls and loft floors

EPS (expanded polystyrene)

• Rigid foam board — water-repellent; used under concrete slabs, on foundation walls and flat roofs
• λ-value typically 0.032–0.038 W/mK
• Compressive strength 100–300 kPa (select by load: Type II for light loads, Type IX below heavy slabs on ground)
• Combustible — always require a non-combustible covering (drywall or concrete) for interior applications

XPS (extruded polystyrene)

• Denser and more water-resistant than EPS — ideal for foundation walls, basement floors and decks
• λ-value typically 0.029–0.034 W/mK
• Higher cost than EPS, but withstands prolonged moisture exposure better. For below-grade or load-bearing use, select any board by its declared water absorption, compressive strength and the manufacturer's approved application — some high-grade EPS products are also approved below ground, so follow the data sheet rather than a blanket "EPS vs XPS" rule
• Combustible — always require a non-combustible covering

Wall build-up and vapour barrier

In most cold-climate constructions, the vapour barrier (or vapour-control layer) goes on the warm side of the insulation (the room side) — normally not in the cold zone. Incorrect placement can cause interstitial condensation, rot and mould growth inside the wall. This is not a universal rule: the correct strategy, and how vapour-tight the layer should be (a low-permeability barrier versus a variable / “intelligent” vapour-open membrane), depends on your climate, the assembly and local building-physics guidance — verify it for your project rather than copying a single figure. See the exceptions below.

• External wall: insulation in stud cavity → vapour barrier on the room side → plasterboard
• Pitched roof insulated from inside: insulation between rafters → vapour barrier → lining board. Typically 23–50 mm ventilated air gap towards the roof covering
• Flat roof (conventional): vapour barrier on top of the structural deck, below the insulation. Inverted roof: insulation sits above the waterproof membrane — the membrane itself acts as the combined air barrier and vapour barrier
• Floor over basement or crawl space: insulation below the floor joists, vapour barrier on the warm side (above the insulation, facing the room). The crawl space itself needs its own moisture strategy — a sealed ground membrane plus either deliberate ventilation or a sealed/conditioned design — and any cold-side wind barrier must be vapour-open. Get this wrong and the floor structure rots from below

Building code U-value requirements

Building regulations set maximum U-values (thermal transmittance) for each building element. The U-value depends on insulation thickness, the λ-value of the material and the effect of any thermal bridges.

The U-values shown are typical requirements for new buildings in cold climates (based on Nordic/EU building code levels). Check your local building regulations for the exact figures. In practice, thermal bridge correction means you will typically need 20–30 mm additional insulation to reliably meet the target U-value.

Insulation selection guide

Understanding thermal bridges

A thermal bridge is any point in the building envelope where heat flows significantly faster than through the surrounding insulated area. Common examples include:

• Structural studs: timber conducts heat roughly 5× faster than mineral wool — at 600 mm (600 mm c/c) stud centres, the studs alone can account for 15–20% of total wall heat loss
• Concrete columns and beams: concrete is a poor insulator; exposed structural elements can dramatically reduce the effective U-value of a wall
• Window and door reveals: the junction between a frame and the surrounding wall is a common cold spot and interstitial condensation risk
• Floor-to-wall junctions: a concrete floor slab extending through to the outside face of the wall bypasses the wall insulation entirely

To compensate for thermal bridges in a standard timber-stud wall, add a continuous inner layer of insulation (e.g. 50 mm rigid board) running perpendicular to the studs before fixing the plasterboard. This breaks the direct conduction path through each stud and can improve the effective U-value by 0.03–0.05 W/m²K. Check two things first: that the added layer keeps the assembly's vapour control correct for your climate (a low-permeability inner board can shift the dew point), and that any plastic-foam board is covered by the required fire-protective lining — combustible insulation normally must not be left exposed.

Calculating quantities — rolls and packs

Consumption: Rolls = ⌈Area ÷ Roll coverage⌉

• Stone wool 100 mm, standard roll: typically approx. 3.6 m² — varies by manufacturer and format
• Stone wool 150 mm: typically approx. 2.4 m² per roll
• Glass wool 200 mm: typically approx. 4.8 m² per roll
• EPS/XPS boards: sold in packs covering 4.0 m²–8.0 m² — always check the packaging for exact coverage
• Always add 10% for cutting waste around studs, pipes and trim

Vapour barrier installation checklist

• Use 0.15 mm polyethylene film or an approved intelligent vapour-control membrane
• Overlap all joints by at least 200 mm
• Seal every joint and overlap with approved vapour-barrier tape — do not rely on staples alone
• Seal around all service penetrations (pipes, cables, ventilation ducts) with preformed collars or flexible mastic
• Seal the membrane to the structure at all edges — floor, ceiling and abutting walls
• Inspect the completed membrane with a torch held at a low angle before closing the wall with plasterboard
• Document the installation with photographs before covering — useful evidence if problems arise later

Common mistakes

• ✗Vapour barrier on the wrong side — condensation collects inside the wall and causes rot and mould over time
• ✗Air gaps and loose fitting between studs — sound reduction can fall by 3–6 dB and a thermal bridge forms at every gap
• ✗Unsealed service penetrations through the vapour barrier (pipes, cables) — destroys the entire airtightness function
• ✗Insufficient insulation below ground-floor slabs — floors and basements are the most common sources of heat loss in older buildings
• ✗EPS used where XPS is needed (basement walls, foundations) — EPS absorbs moisture over time and loses insulation performance
• ✗Insulation left hanging loose or falling out in roof spaces — creates thermal bridges and uneven indoor climate
• ✗Vapour-barrier joints not taped with approved tape — laps open up after a few years if only stapled or friction-held

Calculate your materials

Use the insulation calculator to plan quantities and cost: