Top-Floor Ceiling — termPIR® AL

When to use this system

A ceiling beneath an unheated attic is a classic configuration in single-family construction — it applies to:

• Single-family houses with an open attic — a non-usable loft used as storage or left completely empty.
• Multi-family buildings with an attic — the topmost storey is heated, and the attic above it serves as a thermal buffer.
• Buildings with an uninsulated roof — where the roof is not insulated (e.g. energy-efficient houses with an insulated ceiling but an uninsulated roof — the attic acts as a thermal buffer).
• Renovations of older houses — when an old attic has a timber floor and no insulation was provided between the loft and the residential storey below.

WT 2021 requirement: U-value of the ceiling above unheated space ≤ 0.25 W/m²·K (ti ≥ 16°C). Already met by 80 mm of termPIR® AL.

Top-floor ceiling vs interstorey ceiling

Aspect	Top-floor ceiling (this system)	Interstorey ceiling
Temperature above the ceiling	atmospheric (attic)	internal room temperature (≥ 16°C)
WT 2021 requirement	U ≤ 0.25	U ≤ 1.00 (Δti ≥ 8°C)
Typical termPIR® thickness	100–150 mm	20–50 mm
Waterproofing	not required	not required
Walkway over insulation	OSB walkway boards	concrete screed

Installation requirements

• termPIR® AL boards are to be laid in a staggered pattern on the reinforced concrete slab, with no need for bonding (their own weight keeps them in place).
• Board joints must be sealed with aluminium tape on the upper side (vapour tightness) — essential when the rooms below have elevated humidity.
• Cut boards should be sealed with PIR foam (chemical compatibility with the core).
• Adhesive fixing (optional) — polyurethane adhesive in spots, where there is a risk of board displacement (e.g. strong winds through attic ventilation openings).
• OSB walkway boards — laid on top of the insulation along the attic’s communication axis (for services and maintenance), at least 60 cm wide. Screwed together at joints.

Knee-wall zone

CRITICAL: at the junction of the ceiling with the knee wall (at the roof eaves), additional insulation is required — without it, the thermal bridge reduces the system’s effectiveness by 15–20%:

• termPIR® ETX (glass veil) — installed vertically on the knee wall, compatible with foamed-concrete blocks
• termPIR® AL on the knee wall + additional insulation of the wall plate (the roof’s load-bearing beam)

Technical documentation

termPIR® Catalogue — Residential Buildings (Gór-Stal, 2022-04-26, pp. 61, 63–64). The system is described as a simple variant (without an additional vapour barrier) for typical single-family construction with a non-usable attic.

Layer composition

U-value by insulation thickness

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