Passive house — termPIR over-rafter system for EUco 15 kWh/m²/year

What “passive house” means

In short: a house that consumes a maximum of 15 kWh/m²/year of energy for heating. The standard was set by the Passive House Institute (PHI) in Darmstadt in 1990. For comparison:

• Typical house per WT 2021: 60–80 kWh/m²/year
• NF40 (energy-efficient): 40 kWh/m²/year
• NF15 / passive: ≤15 kWh/m²/year
• Plus-energy: ≤0 kWh/m²/year (produces more than it consumes)

15 kWh is roughly the energy of 1.5 litres of heating oil per m² per year. For a 150 m² house = 225 l of oil/year. In reality, a passive house barely needs heating — the sun heats it + heat recovery from the ventilation unit + body heat.

To achieve this standard, every building envelope element (roof, wall, floor, window) must meet rigorous U-value requirements. Here we show how to do it in the roof using termPIR®.

Roof requirements for the passive standard

PHI Darmstadt recommends:

For comparison, WT 2021 for the roof = 0.15 W/(m²·K) — the passive standard is 33% stricter.

What does that mean in terms of insulation thickness?

U = 1 / (R + R_other_layers) ≈ 1 / R (the insulation layer dominates). Hence for U = 0.10 → R ≥ 10 m²·K/W.

Insulation thickness for R = 10:

• Mineral wool (λD 0.036): 360 mm = 36 cm
• EPS (λD 0.038): 380 mm = 38 cm
• PIR with Al foil (λD 0.022): 220 mm
• PIR MAX 19 (λD 0.019): 190 mm

So PIR saves ~15 cm of thickness for the same R standard. That’s a lot when you’re designing a rafter structure — the difference between 24 cm and 36 cm rafters means significant money and structural weight.

Why over-rafter = king for a passive house

In a typical pitched roof you have 3 insulation systems:

1. Between-rafter — insulation inside the structure. Plus: doesn’t increase roof height. Minus: thermal bridges across the rafters (wood has λD 0.16 — 7× worse than PIR). The roof U drops by 10–15% due to the rafters.
2. Under-rafter — insulation under the rafters on the interior side. Plus: an additional layer that eliminates bridges. Minus: reduces room height, vapour permeability requires attention.
3. Over-rafter — insulation above the rafters (outside the structure), under the roof covering. Plus: zero thermal bridges (entire envelope homogeneous), anchor counter-battens can be fixed through the insulation. Minus: roof covering sits higher (+ insulation thickness), requires long screws.

For the passive standard, over-rafter is almost essential — without it, it’s hard to achieve U = 0.10 without internal additions that reduce room height.

A concrete system for EUco 15 kWh

Variant A — full over-rafter (simplest)

[roof tile / metal sheet]
[batten]
[counter-batten 80×40 mm]
[wind-tight membrane]
[termPIR® AL 220 mm  — over-rafter]        ← R 10.0
[wooden rafter 60×120 mm — not loaded with insulation]
[vapour-permeable foil]
[steel grid CD60 + 1× plasterboard 12.5 mm]

• Roof U-value: 0.10 W/(m²·K) ✓ passive
• Rafter height: 120 mm (small, as it doesn’t carry insulation)
• Total envelope height: ~400 mm (220 insul + 120 rafter + ~60 rest)
• Thermal bridges: zero (rafters below insulation)

Variant B — MAX 19 for a thinner envelope

[roof tile]
[batten + counter-batten 80×40]
[wind-tight membrane]
[termPIR® MAX 19 AL 190 mm  — over-rafter]    ← R 10.0
[rafter 60×120 mm]
[vapour-permeable foil]
[CD60 + plasterboard 12.5 mm]

• Roof U-value: 0.10 W/(m²·K) ✓ passive
• Thickness saving: 30 mm vs Variant A
• Cost: +25–35% for insulation (MAX 19 is more expensive than AL)
• When it makes sense: when the designer has limited ridge height or wants to preserve aesthetic roof proportions

Variant C — hybrid (between-rafter + over-rafter)

Sometimes the designer wants to keep rafters visible inside (highlander or loft aesthetic):

[roof tile]
[batten + counter-batten 80×40]
[wind-tight membrane]
[termPIR® AL 140 mm  — over-rafter]        ← R 6.4
[rafter 60×220 mm visible inside]
[termPIR® AL 200 mm  — between rafters]   ← R 9.1 (with addition)
[vapour-permeable foil]
[visible pine boarding]

• Roof U-value: ~0.09 W/(m²·K) ✓ with margin
• Aesthetics: rafters visible inside (loft, highlander)
• Thermal bridges: minimised (most insulation above the rafters)
• Cost: higher (more PIR + thicker rafters), but visual effect

Cost example — house 150 m² floor area, pitched roof 220 m²

Variant A (classic passive, AL 220 mm over-rafter):

The investor has a passive requirement in the design. A one-off investment of ~PLN 60k means:

• Annual heating energy use: for 150 m² × 15 kWh = 2,250 kWh/year
• Annual heating cost (electricity for a heat pump COP 4, price ~PLN 0.80/kWh): ~PLN 450/year

Compared with a WT 2021 house (60 kWh/m²/year) = annual use of 9,000 kWh, cost ~PLN 1,800/year. Savings of ~PLN 1,350/year.

Plus the thermal modernisation tax relief: up to PLN 53,000 × 2 people = PLN 106,000 of deductions. Real tax savings of ~PLN 12–34k. The investment pays back in 7–10 years, the roof lasts 30+ years.

Pitfalls when designing a passive roof

Pitfall 1 — Thermal bridges at the eaves

If the over-rafter insulation doesn’t extend beyond the rafters to the eaves, you get a thermal bridge (rafters sticking out through the insulation). Solution: a thermally insulating eaves beam or extending the PIR beyond the lower roof outline by min. 200 mm.

Pitfall 2 — Counter-batten fixings too short

The counter-batten must be fixed with a screw through the insulation into the rafter. For 220 mm PIR you need a screw of min. 240 mm (e.g. telescopic 12×260). Shorter = the roof covering will come off in the first storm.

Pitfall 3 — No joint sealing

Every joint between PIR boards = potential 5% loss of U-value. All joints must be sealed with BOKKA® aluminium tape. Also in corners, around roof windows, chimneys.

Pitfall 4 — Wrong membrane

For an over-rafter system you need a wind-tight + waterproof membrane (SD ≤ 0.3 m), not an ordinary roofing foil. A bad membrane = moisture in the insulation, performance drop.

Pitfall 5 — Sub-covering ventilation

Between the insulation (PIR) and the roof covering (tile) there must be a ventilation gap of 30–60 mm (maintained by counter-battens). Without it — condensation, wet insulation, durability problems.

What BOKKA provides for passive roofs

We have all the key termPIR® thicknesses for the passive standard in stock: 140, 160, 180, 200, 220, 250 mm. Plus all the accessories:

• Telescopic screws SX 12×220–340 mm
• Aluminium and butyl tapes for sealing joints
• Wind-tight and vapour-permeable membranes
• Technical advice: we calculate R for your passive envelope + select thickness for a specific U-value

🤝 Free BOKKA technical consultation — we’ll help select the product and complete documentation for your project.

Summary

A passive house in Poland is realistic and cost-effective — it requires a good design, a competent contractor and the right materials. The roof is the key envelope element (60–70% of heat loss in a typical house).

For the passive standard U = 0.10 W/(m²·K) you need:

• 220 mm termPIR® AL or 190 mm termPIR® MAX 19 over-rafter
• Rafters 120–140 mm (not loaded with insulation)
• Wind-tight membrane + sub-covering ventilation 30–60 mm
• Telescopic screws min. 240 mm
• Sealing of all joints with aluminium tape

An investment of ~PLN 60k for 220 m² of roof, payback through thermal modernisation relief + heating savings in 7–10 years. After that, pure savings for 30–50 years.

🤝 Free BOKKA technical consultation — we’ll help select the product and complete documentation for your project.

Sources:

• Passive House Institute (PHI Darmstadt) — technical requirements for the passive house
• Polish Institute of Passive Building and Renewable Energy (PIBP) — national certifications
• Regulation of the Minister of Infrastructure — Technical Conditions 2021 (standard comparison)

👉 Full range and technical specs: termPIR® PIR insulation boards.