15mm Loft Insulation
15mm loft insulation is designed to enhance thermal control in tight spaces. It is a precision layer that plays an essential supporting role in floors, roof details, and wall build-ups connected to loft spaces.
What is 15mm loft insulation thickness?
15mm thick loft insulation refers to a rigid insulation board with a fixed depth of 15mm. In the UK, this thickness is most commonly manufactured from PIR (polyisocyanurate) foam.
- PIR has a low thermal conductivity (approximately 0.022W/mK).
- At 15mm thickness, PIR still provides measurable thermal resistance due to its low thermal conductivity.
- Most 15mm PIR are foil-faced, which improves vapour resistance when correctly detailed.
15mm PIR has an R-value of roughly 0.68m²K/W, which is nowhere near enough to meet typical Part L elemental U-values requirements on its own, but it works well as:
- A thermal break
- A lining layer
- A correction layer over timbers to insulate and improve thermal performance.
What is 15mm loft insulation used for?
15mm PIR boards are used in wall, roof, and floor insulation, especially where headroom or detailing limits thickness.
However, 15mm on its own will not achieve the current Part L U-value targets for these structures. Its R-value (roughly 0.68m²K/W) is far below the resistance required to achieve typical new-build elemental targets.
Instead, 15mm insulation is used as a supplementary layer on:
Floors (loft conversions & refurbs)
On loft conversion jobs, space is gold. A 15mm board can:
- Sit over existing timber joists before chipboard is installed
- Reduce cold bridging through joists
- Improve surface warmth without raising floor levels much
Roof details
In pitched roofs, 15mm thick loft insulation may be used:
- Under rafters as a continuous layer
- As a thermal break below structural timber
- Around dormer cheeks where depth is restricted
Being rigid and foil-faced, it also helps with airtightness when foil joints are taped properly, edges sealed, and service penetrations properly detailed.
Walls around loft spaces
In knee walls or dormer structures, 15mm PIR:
- Lines the internal face
- Adds extra resistance to timber framing
- Improves comfort in tight wall cavities
Types of 15mm thick loft insulation
At Insulation Wholesale, we stock PIR loft insulation 15mm thick. Its main features include:
- Thermal conductivity of ~0.022W/mK
- It is lightweight
- It has a high strength-to-thickness ratio, which is particularly advantageous for rigid PIR insulation applications.
- It is foil-faced
This is the standard specification for this thickness.
FAQ
Where to buy thin 15mm loft insulation?
You can buy 15mm loft insulation online from Insulation Wholesale. We stock UK-manufactured PIR loft insulation boards in 15mm. We offer pallet deals, low wholesale pricing, and delivery to your site. Fast quotation turnaround is available.
Can you cut 15mm-thick loft insulation?
Yes. PIR boards at 15mm are easy to cut.
Use:
- A fine-tooth handsaw, or
- A sharp insulation knife and a straight edge
Score deeply on both sides and cut through the core for a clean edge. Seal joints with foil tape to ensure airtightness. Always measure carefully so there are no gaps.
How to fix 15mm loft insulation thickness?
The 15mm rigid PIR insulation board is easy to install. Fixing methods vary by installation location:
Under rafters or ceilings
- Mechanical fixings with insulation washers, followed by plasterboard.
On floors
- Laid loose over timber, and then the decking is mechanically fixed through the insulation into joists.
On walls
- Adhesive-backed systems or mechanical fixings
What is 15mm insulation typically made of for lofts?
Most 15mm loft insulation in the UK is PIR (polyisocyanurate). It’s a rigid foam formed through a chemical reaction between polyols and isocyanates.
It contains:
- Closed-cell foam structure
- Blowing agents that help achieve low thermal conductivity
- Foil facings for moisture resistance
Mineral wool is not produced at 15mm because it would offer very little resistance at that depth.
What is the thinnest loft insulation available?
For rigid loft boards, 15mm is typically the thinnest commonly stocked PIR thickness in the UK, with options available up to 200mm. Below that, you may find multifoil reflective layers and specialist thermal laminates.
But in standard board form, 15mm thick loft insulation is usually the starting point for rigid PIR insulation.
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15mm Unilin Thin-R PIR Insulation Board 2400mm x 1200mm (8′ x 4′) Pallet of 160 (460.80m2)
15mm 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.
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