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.
At Insulation Wholesale, you can purchase 20mm Loft Insulation at low wholesale prices with fast delivery for most of items within 2-5 days. Competitive rates guaranteed.
FAQ
Where to buy 20mm loft insulation?
You can buy high-performance insulation 20mm thick online from Insulation Wholesale. We sell various 2400mm x 1200mm x 20mm PIR insulating panels suitable for the loft. Technical support is available to help you select the appropriate insulation solution and organise fast delivery to your site or home.
Can you cut 20mm-thick loft insulation?
Yes, 20mm rigid foam gives excellent insulation for lofts when cut properly.
Use:
A fine-toothed handsaw or an insulation saw for straight cuts
A sharp utility knife for scoring and cutting through the core
A straight edge to keep lines true
Avoid gaps as they reduce performance, can cause air leakage, and increase energy bills.
How to fix 20mm loft insulation thickness?
Your fixing method depends on where you install the PIR board:
To fix on masonry: Use insulation adhesive plus mechanical fixings.
Across studs and rafters: Secure it with insulation fixings or drywall screws with large washers.
To floors: Lay the boards tightly together beneath decking, which is mechanically fixed through into the structure.
Seal joints with foil tape to maintain the vapour layer where required. In roof build-ups, follow manufacturer guidance and Building Regulations to reduce the risk of condensation and optimise roof insulation.
What is the thinnest loft insulation available?
For rigid boards, 15–20mm is generally the thinnest insulation option commonly stocked. Below that, products move into reflective multi-foil membranes or specialist acoustic insulation layers rather than true thermal boards.
Remember, thinner means lower R-value. Depth still wins when it comes to serious heat retention. At 2026 recommended guidance for cold lofts typically is 270mm mineral wool at ceiling level to meet energy efficiency targets.
Need a few 20mm loft insulation boards or full pallet quantities? We can sort it. Get a fast quote, unlock trade discounts, and secure competitive pricing on bulk orders.
20mm Mannok PIR Insulation Board is an excellent choice for anyone looking for a high-performance insulation solution that provides superior thermal efficiency. It is ideal for use in any construction project, such as commercial and residential, in wall applications as well as roofing and flooring applications.
20mm Unilin Thin-R PIR insulation is a foil faced, high-performance insulation solution that offers exceptional thermal efficiency and durability. This polyisocyanurate insulation board, formerly known as Xtratherm, is ideal for a range of applications, such as floors, pitched roofs, and walls.
All three models 20mm Kingspan Therma PIR Insulation Board:
20mm Thermapitch TP10
20mm Thermafloor TF70
20mm Thermawall TW55
share the same high-performance PIR core and low-emissivity foil facings, offering excellent thermal efficiency and easy installation. Although labeled differently, these boards share identical technical specifications.
