20mm Loft Insulation

20mm loft insulation sits at the very thin end of the rigid thermal spectrum. It is not intended to transform a freezing loft into a low-energy roof on its own; it’s a precision layer used where space is tight and detailing matters.

A 20mm thick board is not a bulk insulator. It is more of a supplementary control layer within a wider insulation build-up that adds measurable thermal resistance, reduces cold bridging over timbers, and helps tighten up irregular junctions that thicker boards simply cannot fit into.

What is 20mm loft insulation thickness?

20mm thick loft insulation is usually supplied as a rigid insulation panel – most commonly PIR (polyisocyanurate) foam – and is typically foil-faced, which improves vapour resistance when correctly detailed.

A typical 20mm PIR insulation board:

• Has a thermal conductivity (lambda value) of about 0.022 W/mK. Check individual manufacturer data sheets for confirmation.
• Delivers an R-value of approximately 0.90–0.91m²K/W depending on certified lambda value.
• It is lightweight but dimensionally stable.
• Comes in standard sheet sizes (often 2400mm × 1200mm).
• It can contribute to a vapour control layer when foil-faced and fully sealed, enhancing the overall roof insulation. The overall build-up must be assessed for condensation risk.

What is 20mm loft insulation used for?

Thin 20mm loft insulation is especially useful in loft conversions where every millimetre of head height matters. It tends to solve detailing problems without necessarily being the main thermal layer in the roof insulation. Here’s where it makes sense:

Floor applications

Used in loft conversions or upper-floor structures as a thin thermal layer within a restricted floor build-up. For example, 20mm PIR can be laid where height is restricted and overboarded with chipboard or plywood. It improves surface comfort but does not replace deeper floor insulation where Building Regulations Part L require excellent thermal performance.

Framing walls

Where timber or metal studwork forms part of the external envelope – such as dormer cheeks, gable walls, or insulated knee walls – 20mm PIR can be added continuously across the face of the studs before plasterboard.

This reduces thermal bridging through the frame and helps improve the overall U-value of the wall build-up. The full build-up should be assessed for condensation risk in line with BS 5250 and Building Regulations.

Internal masonry walls

Fixed to the inside face of blockwork using wall insulation adhesive applied in dabs or ribbons or mechanical fixings, then overboarded. This is common in retrofit where space is limited, and you cannot afford to lose too much interior space.

Pitched roof build-ups

A 20mm loft insulation thickness is often used as a secondary layer under the rafters. For example:

• 100mm–120mm between rafters
• 20mm across rafters
• Plasterboard below

This cross-layer reduces cold bridging through timber, which conducts heat more than insulation, and so covering it leads to overall high performance. Exact thickness depends on the target U-value for the thermal insulation.

Types of 20mm thick loft insulation

Insulation Wholesale mainly stocks 20mm thick PIR loft insulation. Our PIR loft boards offer exceptional thermal efficiency for the depth and are easy to cut with a fine saw or sharp knife.

Phenolic boards are less commonly specified at 20mm, but are available. They offer slightly better lambda values (around 0.020W/m · K) but are often more expensive.

EPS and XPS boards are also available in 20mm. These have lower thermal performance (around 0.030–0.036 W/m·K) and are therefore less commonly specified in pitched roof build-ups than PIR or phenolic insulation products.

Glass mineral wool roll for loft thermal applications is rarely supplied at 20mm, as performance at that depth would be minimal.