UK insulation thickness guide: meeting Part L without overspending

insulation-thickness-guide-uk-part-l-compressed

Meeting Part L is rarely about chasing the thickest insulation you can fit. It is about hitting the right thermal performance, keeping moisture risks under control, and spending money where it actually reduces heat loss.

If you are working on a new build, extension, conversion, or a refurbishment that triggers Building Control, the practical question tends to be the same: what thickness do I need for the roof, walls, and floors in the UK, and how do I avoid paying for insulation I do not need? This guide puts typical thicknesses into context and explains where the costs creep in.

Part L targets performance (U-values), not a fixed thickness.
After you're compliant, continuity and detailing usually beat extra millimetres.
Pick insulation to suit the location: depth available, moisture strategy, and buildability.

Jump to thickness table

Part L in plain English: it targets performance, not a particular thickness

Part L of the Building Regulations sets energy efficiency requirements for dwellings. In day to day terms, that usually means your design has to meet target U-values (how quickly heat passes through an element, measured in W/m²K). Lower is better.

Because Part L is performance-based, the same U-value can be achieved with different materials and different thicknesses. A thin layer of high-performance PIR can do the job of a much thicker layer of mineral wool. That trade-off is where most budgets are won or lost.

There is also a difference between:

New dwellings: tighter targets and whole-house modelling.
Existing dwellings: "reasonable provision" upgrades when renovating, plus limits that vary by element and scenario.

If you work across the UK, keep an eye on the local Approved Documents and national equivalents. Scotland, for example, often expects lower U-values than England and Wales, which changes the thickness you end up specifying.

Avoid a common mix-up You will see both element limits (maximum U-values per roof/wall/floor) and whole-dwelling targets (a notional/spec used for modelling). Many new-build designs need to beat the element limits in places to pass the overall calculation.

Thickness comes from two numbers: the target U-value and the material's lambda

Manufacturers publish a thermal conductivity value (lambda, λ) for insulation products. Typical ballpark figures:

Mineral wool: about 0.035 to 0.044 W/mK
PIR/phenolic: about 0.022 W/mK
EPS: often around 0.030 to 0.038 W/mK (product dependent)

Lower λ means more thermal resistance per millimetre, so you can use less thickness. That is why PIR is popular where space is tight (internal linings, warm roofs, floors), while mineral wool dominates lofts where depth is available and cost per m² is attractive.

The table below gives practical thickness ranges that are commonly used to get close to typical Part L-style performance. Treat them as starting points, not a substitute for a calculation.

Indicative thickness ranges (starting points)

Building element	Typical target (context)	Cost-effective insulation choices	Indicative thickness range (common UK practice)	Notes that affect the final spec
Loft (cold roof at ceiling level)	~0.16 W/m²K (common backstop; varies by nation/method)	Mineral wool rolls or slabs	270 to 300 mm	Easy wins, low cost, but do not compress the quilt and keep eaves ventilation clear
Pitched roof (warm roof between/under rafters)	~0.16 to 0.18 W/m²K (design-led)	PIR plus insulated plasterboard, or multi-layer build-up	120 to 170 mm PIR equivalent (often split layers)	Depth limits, rafter thermal bridging, and condensation control drive the design
Cavity wall (newer construction)	~0.26 W/m²K (common backstop; varies by nation/method)	Full-fill mineral wool batt, PIR board, beads	80 to 100 mm PIR, or 100 to 125 mm mineral wool	Cavity width, wall ties, and workmanship at openings matter as much as the product
Solid wall upgrade (IWI/EWI)	~0.30 W/m²K (typical refurb aim; where feasible)	PIR internal lining, mineral wool on studs, EPS external	70 to 100 mm PIR (IWI) or 100 to 150 mm EPS (EWI)	Moisture risk assessment is essential, plus detailing around floors and windows
Ground floor	~0.18 W/m²K (common backstop; varies by nation/method)	PIR/XPS under slab, mineral wool between joists	100 to 150 mm PIR/XPS, or 150 to 200 mm wool	Edge insulation, air leakage, and service penetrations can dominate real-world performance

These ranges are a starting point. Always confirm with a U-value calculation for the exact build-up and a condensation/moisture strategy (especially for roofs and internal wall insulation).

Where people overspend (and how to spot it early)

Insulation has diminishing returns. The first increments of thickness make a big difference, but later additions often buy small improvements at a high cost, especially once you are already near the target U-value.

After you have met the requirement, the biggest gains often come from continuity and detailing rather than another 25 mm of board. Before adding thickness, ask what is actually limiting performance: gaps, thermal bridges, loft hatches, unsealed penetrations, or poor junction details.

Paying for premium boards where depth is not restricted
Adding thickness without fixing air leakage paths
Ignoring thermal bridges at joist ends, lintels, and eaves
Doubling insulation in one area while leaving weak spots elsewhere

A practical element-by-element guide to thickness choices

Loft insulation (cold roof): the cheapest route to a strong U-value

If you have a conventional loft with joists, mineral wool is usually the most cost-effective option. Part L style targets often translate to around 270 to 300 mm total depth in many standard roofs.

Most installers achieve this with two layers: one between joists, then a second layer laid across to reduce gaps. Keep the insulation lofted, not squashed. Compressed quilt performs worse, and it is a common reason people "meet the thickness" but miss the performance.

Before you start stacking rolls, make sure you are not blocking ventilation at the eaves. Cold lofts rely on ventilation to manage moisture.

Pitched roofs and loft conversions: thickness is constrained, so specs get smarter

Conversions and rooms in the roof bring two realities: rafter depth is limited and the build-up must manage condensation risk.

A typical approach is a combination of PIR between rafters with an insulated plasterboard layer under, or a continuous layer under the rafters to reduce thermal bridging. The thickness is driven by the U-value calculation and the rafter size, not what "feels like enough".

Ventilation strategy: cold roof with ventilation, or warm roof with controlled vapour layer, not a halfway house
Continuity at eaves: insulation should connect roof to wall without gaps
Service voids: keep penetrations to a minimum or seal them properly
Loft hatches and downlights: Loft hatches and rated covers can save more heat than extra board thickness

Cavity walls: use the cavity you have, and be realistic about what fits

For new cavity walls, the most cost-effective route is often a full-fill batt or beads that use the full cavity width, rather than chasing thinner high-performance boards to save a few millimetres you do not need.

If space is tight (narrow cavities, design constraints at the façade), PIR can meet the same U-value at less thickness. That makes it useful in certain details, but it is not automatically the cheapest route across the whole job.

What matters most in cavity walls is workmanship: tight joints, clean edges around openings, and correct placement of cavity closers and insulation at reveals.

Solid walls: cost is in the detailing, not just the insulation

Solid wall upgrades are where overspending happens quickly, because the work often expands beyond insulation. Moving sockets, radiators, skirting, and making good can dwarf the insulation material cost.

Internal wall insulation (IWI) with PIR lining boards can deliver strong performance with 70 to 100 mm in many common refurb aims, but it needs careful moisture management. External wall insulation (EWI) often needs 100 to 150 mm EPS (or alternatives) to reach similar targets, but it brings scaffold and façade detailing costs.

Solid walls also magnify thermal bridge problems at intermediate floors and party wall junctions. A slightly thinner insulation layer installed continuously can outperform a thicker layer that is full of interruptions.

Floors: thickness is only half the story

For slab floors, insulation thickness is relatively easy to specify, but edges and penetrations can wreck performance. Perimeter insulation and good continuity to wall insulation are key.

For suspended timber floors, mineral wool between joists is common, but air movement can short-circuit the insulation if the floor is draughty. In those cases, the spend that pays back is often on sealing, membranes, and careful fitting, not simply thicker quilt.

Spending less while staying compliant: a procurement and buildability mindset

If your goal is compliance without waste, treat insulation as a system with a target, not a product with a thickness.

Pick the material to suit the location: mineral wool in deep lofts; rigid boards where space is limited; cavity solutions that match the cavity width
Avoid "hero thickness": meeting the U-value in one element does not compensate for weak junctions and thermal bridges elsewhere
Design for install: simple repeatable details reduce labour time and cut the risk of gaps

This is also where a builders' merchant setup can help. Being able to order standard board sizes, mix thicknesses for a multi-layer roof build-up, and schedule deliveries around the programme can reduce both waste and delays. Many projects run smoother when the same account covers web ordering and depot support, with the option of collection for last-minute items and regional delivery for bulk materials.

Paperwork and checks that save you from costly rework

Building Control and warranty providers tend to care about evidence. Even if you are confident on site, it helps to have a clear trail:

U-value calculations that match the exact build-up
Product datasheets showing λ-values and thickness
Details for junctions (eaves, reveals, floor edges)
Condensation risk strategy for roofs and IWI (and how ventilation is maintained)

A common cost trap is changing one layer mid-job because of lead times, then discovering the U-value no longer stacks up. If substitutions are likely, agree acceptable alternatives early and keep the calculations up to date.

A quick way to sanity-check thickness before you order

If you want a fast reasonableness check before running full calculations, use these rules of thumb:

Cold lofts usually land around 270 to 300 mm mineral wool for strong Part L-style performance.
Cavity walls aiming around 0.26 often work with roughly 80 to 100 mm PIR, or a full-fill mineral wool batt sized to the cavity.
Floors that need around 0.18 often sit around 100 to 150 mm PIR/XPS, with careful perimeter detailing.