25mm loft insulation is a thin thermal layer specifically designed for tight spaces where overall depth is restricted.
This insulation depth, on its own, is not installed to reduce heat loss and improve overall energy efficiency. It is more about fine-tuning a detail, lining a junction, or adding controlled thermal resistance without raising levels too much.
A 25mm loft insulation thickness will not transform a cold roof by itself, but in the right place, it addresses common site detailing constraints.
What is 25mm thick loft insulation?
25mm loft insulation thickness means a board or slab manufactured to a nominal thickness of 25mm deep across its full face. The thickness directly affects the thermal resistance it provides.
In rigid PIR form, 25mm thick loft insulation is a closed-cell foam board faced with foil. The foil can contribute to a vapour control layer when foil faces, joints, and penetrations are fully sealed and correctly positioned within the build-up. Typical thermal conductivity (lambda) for PIR is 0.022–0.023 W/m.K, depending on product.
In mineral wool form, such as rigid and semi-rigid stone wool products, the fibres trap air. Lambda is typically around 0.035–0.040 W/m·K, depending on density.
Thinner board = lower total thermal resistance of roof insulation.
Better lambda = more performance per millimetre of high-performance insulation.
So a thin 2400mm x 1200mm x 25mm PIR loft insulation board gives noticeably more resistance than 25mm mineral wool, simply because the high-performance foam conducts less heat.
But 25mm loft thin insulation is still a shallow layer. It is usually used as part of a build-up, not as the only insulation solution in a loft, to meet the Part L requirements of the UK Building Regulations.
What is 25mm loft insulation thickness used for?
25mm loft insulation comes in handy in a variety of applications:
Floor applications
In loft conversions or raised deck details, 25mm PIR can sit above joists before chipboard goes down. Why use it?
You need a thin thermal layer within a restricted floor build-up.
To correct thermal bridging at joist lines.
You cannot afford to lose ceiling height below.
It improves comfort underfoot.
It does not replace the deeper floor insulation required between or below joists to achieve regulatory U-values.
Framing walls
In timber or metal stud partitions, a 25mm layer can be added internally when:
You’ve already insulated between studs.
You want a continuous thermal layer across the face.
You’re improving SAP calculation results.
Boards are fixed over the frame, joints taped, and then plasterboard is applied for excellent thermal insulation.
Internal masonry walls
On solid brick or block, 25mm PIR can be fixed using insulation adhesive (dot-and-dab) on specific systems or mechanical fixings before plasterboard. It is a common wall insulation solution where:
Room size is limited.
You are upgrading a single cold wall.
Full internal wall systems are too thick.
Again, it improves thermal comfort in your living space, but it will not achieve deep retrofit targets when used alone.
Pitched roof
In a rafter roof, 25mm PIR is often used:
As a thermal layer beneath rafters.
To maintain ventilation space above thicker insulation.
As a correcting layer of roof insulation to reduce cold bridging.
For example, 100mm between rafters + 25mm across the underside reduces overall U-value by reducing thermal bridging through the rafters.
Types of 25mm loft insulation
We stock two main categories of loft insulation materials at 25mm thick. What you choose depends on the problem you hope to solve: fire performance, thermal efficiency, or acoustic insulation.
PIR (rigid foam board)
High-performance rigid boards for insulating the loft have the following features:
Lambda ≈ 0.022 W/mK
High compressive strength
Foil-faced
Precise, easy to cut using a fine-toothed saw or sharp insulation knife with a straight edge
Best for tight spaces where exceptional thermal output per millimetre is desired.
Rock mineral wool (rigid or semi-rigid slab)
Lambda ≈ 0.035–0.040 W/mK
Non-combustible (Euroclass A1)
Excellent acoustic absorption
Vapour permeable
Best where fire resistance and sound control are priorities over maximum thermal performance per depth.
Benefits of thin 25mm loft insulation thickness
Installing 25mm loft insulation is useful in these ways:
Minimal build-up height
Useful for thermal detailing and junctions
Reduces thermal bridging
Lightweight and easy to install
Works well in layered systems
Adaptable for targeted retrofit upgrades
At Insulation Wholesale, you can purchase 25mm Loft Insulation at low wholesale prices with fast delivery for most of items within 2-5 days. Competitive rates guaranteed.
FAQ
Where to buy 25mm loft insulation?
You can buy 25mm loft insulation online from Insulation Wholesale. We stock high-quality PIR and mineral wool options, all held as UK stock and ready for fast nationwide delivery. We offer competitive pricing, pallet deals, and trade discounts on all our thermal insulation products.
What is the thermal resistance of 25mm loft insulation?
Thermal resistance (R-value) depends on lambda. It is calculated by dividing thickness (in metres) by lambda.
PIR (λ ≈ 0.022W/mK) has an R-value of ≈ 1.13m²K/W.
Mineral wool (λ ≈ 0.037 W/mK average) has an R-value of ≈ 0.68 m²K/W.
Is 25mm insulation enough for a garage?
25mm thick loft insulation may help reduce surface condensation when combined with adequate ventilation. It is not enough on its own to meet modern standards in a heated garage or workshop. Combine it with thicker layers.
How to install a 25mm layer of insulation in a loft?
For effective installation of PIR insulation board:
Measure accurately between rafters or across joists.
Cut boards accurately and secure them using appropriate mechanical fixings or adhesives.
Tape foil joints for air tightness.
For mineral wool slabs:
Cut with an insulation knife.
Push it gently so it rests snugly between structural members.
Avoid compressing the material. Compression removes the trapped air that provides the insulation.
Is 25mm insulation enough for a shed?
25mm loft thin insulation will reduce temperature swings and internal condensation in a garden shed used occasionally. You will need thicker insulation in the walls and roof for year-round use or a garden office.
How effective is 25mm Ecotherm?
25mm Ecotherm PIR board’s effectiveness comes from its low lambda (around 0.022 W/mK). It performs effectively for its depth, but remember that depth influences performance. A thin board cannot match the resistance of 100mm thermal insulation, even with good foam chemistry.
We supply 25mm loft insulation at low wholesale prices and offer fast nationwide delivery. Contact our team for trade discounts, bulk pricing, or a fast quote today.
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