Twin-Skin Built-Up Cladding | U-Value Reference

Q: What is a twin-skin or built-up cladding system?

A built-up cladding system uses separate liner sheet, spacer rails, insulation, and outer weathersheet assembled on site, in contrast to factory-bonded composite sandwich panels.

Q: Why does cold bridging matter in a built-up roof?

Steel spacer bars conduct heat more than the surrounding mineral wool insulation, creating thermal bridges. Actual U-values are typically 0.03-0.06 W/m²K higher than centre-of-insulation values. BR 443 provides the calculation method for correction factors.

Q: Where should the vapour control layer go in a built-up roof?

The VCL should be on the warm side of the insulation — immediately above the liner sheet, below the insulation quilt. This prevents warm moist air condensing on the cold outer sheet.

Q: Can a built-up system achieve Part L U-values for roofs?

Yes. To achieve a corrected U-value of 0.25 W/m²K, a built-up roof typically needs approximately 200mm of mineral wool quilt with a good spacer system, or mixed mineral wool and PIR. Composite panels achieve the same value at 80-100mm PIR core depth.

Twin-skin built-up cladding systems

Twin-skin or built-up cladding uses separate liner sheets, insulation, and outer weathersheets assembled on site. It is an alternative to composite (sandwich) panels and remains widely used, particularly for walls, retro-fits, and where thicker insulation is needed at lower material cost. This page covers standard roof and wall build-ups, typical U-values, and how built-up systems compare with composite panels.

Roof build-ups Wall build-ups Built-up vs composite

Standard twin-skin built-up roof

The most common built-up roof system for UK portal frame buildings. Liner sheet on the inside, mineral wool quilt over spacer rails, outer weathersheet on top.

Layers (outside to inside)

#	Element	Typical specification
1	Outer weathersheet	Trapezoidal/box profile sheet, 0.7mm coated steel (e.g. Steadmans AS35, Cladco 32/1000)
2	Spacer rail	Steel hat-section spacer bars at 600mm centres on top of liner sheet. Depth determines insulation thickness.
3	Insulation quilt	Mineral wool (e.g. ROCKWOOL RW3 or Isover SP15). Supported on liner, compressed at spacers.
4	Vapour control layer (VCL)	Polythene sheet or foil-faced quilt. Installed on warm side of insulation (immediately above liner in most UK practice).
5	Liner sheet	Shallow trapezoidal liner sheet (e.g. Steadmans AS20/1000, 0.4mm), spanning between purlins.

Indicative U-values

Centre-of-insulation values only — actual as-built values will be higher due to cold bridging at spacer bars and fixings. Apply the BR 443 correction factor method for compliance calculations.

Quilt/insulation thickness (mm)	U-value (W/m²K) — centre of insulation	Notes
100	0.4	Centre-of-insulation only
150	0.27	Centre-of-insulation only
200	0.21	Centre-of-insulation only
250	0.17	Centre-of-insulation only

Spacer rail systems

Manufacturer	Product	Depth range	Notes
Metsec	SpeedDek roof spacer system	80–200mm	Pre-engineered steel hat sections. Proprietary clip-fix to liner.
Steadmans	Built-up roof system guide	Variable	Coordinate with Steadmans AS20 liner and AS35 outer sheet.
Kingspan	Supabuild built-up system	80–200mm	Uses Kingspan insulation products with steel spacer rails.

Key design points

Liner sheet spans between purlins — typically 0.4mm gauge is structurally adequate for standard purlin centres
Outer sheet carries all wind and snow loads — 0.7mm gauge is standard for most UK conditions
VCL must be continuous and carefully lapped — breaks in VCL lead to interstitial condensation
Spacer depth must accommodate compressed quilt thickness — allow for compression at clips
Cold bridging at spacer bars reduces actual U-value vs centre-of-insulation calculation. Use correction factor per BR 443.

Standing seam over liner (warm roof)

Liner sheet fixed to purlins, continuous rigid insulation board or quilt over liner, standing seam outer skin on clips. Reduces cold bridging vs rail-and-quilt systems.

Layers (outside to inside)

#	Element	Typical specification
1	Standing seam outer sheet	0.6–0.7mm secret-fix standing seam (e.g. Euroclad Vieo, Lindab SRP25N)
2	Insulation	Rigid PIR board or dense mineral wool over liner surface, clipped back to liner via thermal break clips
3	VCL	Above liner if quilt used; PIR board acts as VCL if vapour closed
4	Liner sheet	Standard liner profile spanning purlins

Indicative U-values

Centre-of-insulation values only — actual as-built values will be higher due to cold bridging at spacer bars and fixings. Apply the BR 443 correction factor method for compliance calculations.

Insulation thickness (mm)	Insulation type	U-value (W/m²K) — centre of insulation	Notes
80	PIR board	0.25	Approximate — depends on PIR lambda
100	PIR board	0.2	Approximate
120	PIR board	0.17	Approximate

Key design points

Eliminates rail cold bridging — thermally better than quilt-over-rails
PIR board must be vapour closed or VCL added
Clip fixing through insulation must be designed to avoid point loads on PIR

Standard twin-skin built-up wall

The built-up wall equivalent of the twin-skin roof. Liner sheet on the inside spanning between rails, mineral wool batts or quilt between Z-section or hat-section spacers, outer wall sheet.

Layers (outside to inside)

#	Element	Typical specification
1	Outer wall sheet	Trapezoidal wall profile (e.g. Steadmans AS35, 0.7mm coated steel)
2	Z or hat-section spacers	Steel Z-sections or hat rails at 600mm centres fixing outer sheet back to structure
3	Mineral wool batts	ROCKWOOL RW3 or similar. Friction-fit between spacers and liner.
4	VCL	Polythene or foil-backed quilt on warm side
5	Liner sheet	Shallow liner profile (e.g. AS20/1000, 0.4mm) spanning between wall rails

Indicative U-values (wall)

Centre-of-insulation values only — cold bridging through Z-section spacers typically adds 0.02–0.04 W/m²K to the actual value.

Insulation thickness (mm)	U-value (W/m²K) — centre of insulation	Notes
75	0.5	Centre-of-insulation only
100	0.38	Centre-of-insulation only
150	0.26	Centre-of-insulation only
200	0.2	Centre-of-insulation only

Key design points

Wall liner spans horizontally between wall rails (secondary steelwork fixed to portal frame columns)
Batt insulation must fill the full depth of the spacer without compression to achieve stated U-value
Wall sheets are typically fixed with self-drilling screws into the Z-section, not into primary structure
Below-DPC detailing is critical — wall liner and outer sheet must both terminate correctly
Cold bridging through Z-sections is significant — correction factor of 0.02–0.04 W/m²K typically applied

Built-up vs composite panels

	Built-up / twin-skin	Composite sandwich panel
Pros	Lower material cost for thick insulation depths Can be adjusted/upgraded later Wide range of liner and outer sheet combinations	Factory-controlled quality — U-value guaranteed Faster installation — single element Better airtightness achievable Lower cold bridge risk
Cons	More components — slower installation More detailing for VCL continuity Higher cold bridge correction needed Quality depends heavily on installation workmanship	Higher material cost at same insulation depth Liner face colour/texture fixed at manufacture Panel replacement if damaged Larger delivery units

Common questions

What is a twin-skin or built-up cladding system?

A twin-skin or built-up cladding system uses separate components — a liner sheet, spacer rails, insulation quilt or batts, and an outer weathersheet — assembled on site. This is in contrast to a composite sandwich panel, which has all three elements factory-bonded. Built-up systems allow greater flexibility in insulation depth and material selection, and are often used for retrofit overcladding or wall applications.

Why does cold bridging matter in a built-up roof?

Steel spacer bars conduct heat significantly better than the mineral wool insulation around them — a thermal bridge. At each spacer bar, the effective insulation is much lower than in the field of the insulation between bars. The centre-of-insulation U-value calculation ignores this effect. Actual U-values for built-up systems are typically 0.03–0.06 W/m²K higher than the centre-of-insulation value. BR 443 provides the calculation method for applying the correction factor. For Part L compliance, always use the corrected value.

Where should the vapour control layer (VCL) go in a built-up roof?

In a standard UK portal frame built-up roof, the VCL should be on the warm side of the insulation — in practice, immediately above the liner sheet, below the insulation quilt. This prevents warm moist air from inside the building migrating into the insulation layer and condensing on the cold outer sheet. A common mistake is omitting or discontinuing the VCL at purlins — this is where the most significant condensation risk exists. Some systems use foil-faced insulation quilt which acts as both insulation and VCL when lapped correctly.

Can a built-up system achieve Part L U-values for roofs?

Yes, with sufficient insulation depth. To achieve a corrected U-value of 0.25 W/m²K (the Part L roof target for many non-domestic new builds), a built-up roof typically needs approximately 200mm of mineral wool quilt with a well-performing spacer system, or a mix of mineral wool and PIR to reduce depth. Composite panels achieve the same value more reliably at 80–100mm PIR core depth, with lower cold bridge correction. The choice between systems is often driven by cost, programme, and whether the building is new-build or refurbishment.