If you are building your own home you are in control of the specification of every aspect of the design, incl­uding how the walls are to be constructed. There are many different walling systems to choose from, ranging from concrete blockwork to various forms of timber frame, plus modern alternatives such as hollow clay blocks, insulated concrete formwork (ICF) and structural insulated panels (SIPs).

A major consideration when choosing a walling system will be its thermal performance, or energy efficiency, which is measured in U-values. This is an important factor in determining the running costs of your home so it will be in your interests to maximise thermal performance and to improve on the minimum standards required by the Building Regulations (Building Standards in Scotland).

The current maximum elemental U-value for external walls in England and Wales is 0.30W/m²K. In 2013 the maximum is expected to be reduced to 0.20W/m²K, (Code Level 3 of the Code for Sustainable Homes) and by 2016, when new homes will have to achieve ‘Zero Carbon’ standard it is expected to be 0.10W/m²K (Code Level 6). The PassivHaus ‘Gold Standard’ for sustainable homes requires a maximum U-value of 0.15W/m²K for walls.

There are many different ways to improve the thermal performance of external walling systems, usually by combining different types, layers and thicknesses of insulation. The choice of external cladding has a relatively limited impact on thermal performance, as does the use of wet plaster or dry-lining.

Here we look at the most popular options and their performance.

Timber Frame Systems

Timber frame and brick (316mm)

Insulation between studs, and insulated plasterboard

  • Consists of: Facing brick 102.5mm (1); 50mm clear vented cavity (2); breathable membrane (3); 9mm oriented strand board (OSB) or plywood sheathing (4); 89mm softwood studs (5) infilled with 70mm Kingspan Kooltherm® K12 (6), held in place with timber stop battens (7) forming external void; 62.5mm Kingspan Kooltherm® K18 insulated plasterboard with integral vapour control layer (8); 3mm skim coat
  • Finished Wall U-value: 0.17W/m²K

Timber frame and rendered blockwork (330mm)

High-performance option for render finish

  • Consists of: 10mm polymer render (1); 100mm blockwork (medium density) (2); 50mm clear vented cavity (3); 50mm Kingspan Kooltherm® K12 (4); breathable membrane (5); 9mm OSB or plywood sheathing (6); 89mm softwood studs (7) infilled with 50mm Kingspan Kooltherm® K12 (8), held in place with timber stop batten to form internal void (9); vapour control layer (10); 3mm skim coated 15 mm plasterboard (11)
  • Finished Wall U-value: 0.17W/m²K

Timber frame with hung tile, render or timber cladding (238.5mm)

Thinnest conventional timber system

  • Consists of: Plain clay tiles (1) on horizontal timber battens 25mm (2); or 10mm polymer rendered calcium silicate board (3); or timber cladding (4); breathable membrane (5); 9mm OSB or plywood sheathing (6); 89mm softwood studs (7) infilled with 70mm Kingspan Kooltherm® K12 (8), held in place with timber stop battens (9); 62.5mm Kingspan Kooltherm® K18 insulated plasterboard with integral vapour control layer (10); 3mm skim coat
  • Finished Wall U-value: 0.17W/m²K

Timber frame with hung tile, render or timber cladding (264mm)

Double layer of insulation achieves great results

  • Consists of: Plain clay tiles 13mm (1); on horizontal timber batten 25mm (2); or 10mm polymer rendered calcium silicate board (3); or timber cladding (4); vertical timber batten 50mm (5); 60mm Kingspan Kooltherm® K12 Framing Board (6); breathable membrane (7); OSB (oriented strand board) or plywood sheathing 9mm (8); 89mm softwood studs (9) infilled with 60mm Kingspan Kooltherm® K12 (10), held in place with timber stop batten (11); vapour control layer (12); 15mm plasterboard (13); 3mm skim coat
  • Finished Wall U-value: 0.17W/m²K

Timber frame and brick (358.5mm)

Insulation in cavity rather than between studs

  • Consists of: 102.5mm facing brick (1); 50mm clear vented cavity (2); 90mm Kingspan Kooltherm® K12 Framing Board (3); breathable membrane (4); 9mm OSB or plywood sheathing (5); 89mm softwood studs (containing only services etc.) (6); 15mm plasterboard (7); 3mm skim coated
  • Finished Wall U-value: 0.17W/m²K

Timber frame and brick (358.5mm)

Insulation between and over studs

  • Upgrade option from Solo Timber Frame
  • Consists of: 102.5mm facing brick (1); 50mm clear vented cavity (2); foil-faced breathable membrane (3); 9mm OSB or plywood sheathing (4); 140mm softwood studs (5) with 90mm rigid foil-faced insulation such as Kingspan (6), and 50mm flexible insulation (mineral wool) (7); 50mm rigid foil-faced insulation (8); 15mm plasterboard; 3mm skim coat
  • Finished Wall U-value: 0.13W/m²K

Timber frame and brick (358mm)

Insulated cavity with added multi-foil insulation

  • Consists of: 102.5mm facing brick (1); 50mm clear vented cavity (2); foil-faced reflective breathable membrane (3); 12mm OSB or plywood sheathing (4); 140mm softwood studs (5) with 120mm rigid foil-faced insulation (6) held in place with 20mm timber spacer battens to form an internal cavity (7); 38mm counter batten (8) with TLX Silver multi-layer foil insulation (9); 12.5mm foil-backed plasterboard (10); 3mm skim coat
  • Finished Wall U-value: 0.15W/m²K

(MORE: Timber Frame Guide)

Improving Thermal Performance

All of these build systems can be upgraded to result in a lower U-value. On our timber frame walls, increasing the depth of insulation between the studs (and the studs themselves) will make a big difference, as will switching to a foil-faced breather membrane. On the blockwork walls, the cavities may be widened to accommodate extra insulation, and the blockwork itself may be upgraded to thin joint lightweight (aerated) where not already used. Both systems can be topped off with insulated plasterboard. The choice of wet plaster, dry-lining or plasterboard and skim makes little difference to the performance of the wall, although plasterboard on dabs of glue adds marginally.

Blockwork Systems

Rendered dense concrete blockwork (377.5mm)

Regular blocks, extra-wide cavity

  • Consists of: 15mm render (1); 100mm dense concrete blockwork (2); 150mm cavity with 150mm full-fill mineral wool insulation (3); 100mm dense concrete blockwork (4); 12.5mm gypsum plaster (5)
  • Finished Wall U-value: 0.20W/m²K

Lightweight blockwork with brick (365mm)

Lightweight blockwork with conventional mineral wool insulation

  • Consists of: 102.5mm brick (1); 150mm cavity with 150mm full-fill mineral wool insulation (three layers) (2); 100mm lightweight concrete (aircrete) blockwork (3); 12.5mm gypsum plasterboard on dabs (4)
  • Finished Wall U-value: 0.18W/m²K

Thin-joint blockwork and brick with Partial-fill cavity (355mm)

For areas where full-fill is unacceptable

  • Consists of: 102.5mm facing brick (1); 50mm clear cavity (2); 90mm Kingspan TW50 insulation (3); 100mm standard-grade thin-joint aerated concrete block (4); 12.5mm dry-lining (5)
  • Finished Wall U-value: 0.18W/m²K

Concrete blockwork and brick (315mm)

How a conventional block and brick wall achieves low U-values

  • Consists of: 102.5mm brick (1); 100mm full-fill insulation interlocking (CavityTherm) (2); 100mm concrete blockwork (3); 12.5mm dry-lining (4)
  • Finished Wall U-value: 0.17W/m²K

Thin-joint blockwork and brick (350mm)

Extra-wide cavity aids performance

  • Based on Design & Materials Ltd. PointOne (0845 404 0400)
  • Consists of: 102.5mm brick (1); 150mm insulation interlocking (2); 100mm thin-joint Jumbo ‘Bloks’ from H+H Celcon (3); 12.5mm dry-lining or 15mm wet plaster (4)
  • Finished Wall U-value: 0.10W/m²K

Blockwork and render (383mm)

Using high-performance ‘solar-grade’ blocks on a render finished wall

  • Consists of: 15mm render or cladding finish (1); 140mm (EPS) expanded polystyrene insulation (2); 215mm solar-grade concrete blockwork (3); 12.5mm dry-lining (4)
  • Finished Wall U-value: 0.15W/m²K

(MORE: A Guide to Blockwork)

Alternative Systems

Insulated concrete formwork (383mm)

Classic polystyrene block system

  • Consists of: 20mm brick slips (which give the appearance of brickwork) (1); 350mm EPS expanded polystyrene insulation formwork (210mm of EPS insulation) (2) filled with steel-reinforced concrete (3); 12.5mm dry-lining (4)
  • Finished Wall U-value: 0.15W/m²K

(MORE: ICF Guide)

Cellular clay blockwork (474mm)

Increasingly popular, simple solid wall

  • Based on ThermoPlan 365mm Ziegel blocks (natural-building.co.uk)
  • Consists of: 15mm render or cladding finish (1); 60mm (EPS) expanded polystyrene insulation (2); 386mm hollow clay blocks (3); 12.5mm dry-lining or wet plaster (4)
  • Finished Wall U-value: 0.20W/m²K

Structural insulated panels (SIPs) (355mm)

High performance pre-insulated panels

  • Based on Build It Green’s 175mm SIPs (0845 9000 326)
  • Consists of: 102.5mm brick (1); 50mm clear cavity with low-conductivity wall ties (2); breathable membrane (3); 12mm oriented strand board (OSB) (4); 151mm rigid polyurethane insulation core (5); 12mm OSB (as 4); 15mm timber battens (6); 12.5mm dry-lining (7)
  • Finished Wall U-value: 0.14W/m²K

(MORE: SIPs Guide)

Oak Frame (195mm)

Updated version of traditional oak stud with principal studs visible inside and out

  • Based on Oakwrights’ Type 3i Panel (01432 353353)
  • Consists of: 195mm principal structural oak studs (1) infilled with: 15mm external render; 22mm plywood panel gasketed (2); 60mm rigid polyurethane ‘primary’ insulation (3); 20mm rigid polyurethane ‘solid break’ insulation (4); 40mm timber battens (5) infilled with 40mm polyurethane insulation (6); 12.5mm plasterboard (7) with 2.5mm skim coat finish
  • Finished Wall U-value: 0.19W/m²K

(MORE: Oak Frame Guide)

Engineered timber frame (480mm)

High-performance I-beam system

  • Based on the Masonite TRADIS system (01494 771242)
  • Consists of: 102.5mm brick (1); 50mm clear cavity (2) with low-conductivity wall ties; 300mm wall panel consisting of 9.2mm Panelvent sheathing board (3); 284mm Masonite I-beams (4) infilled with Warmcel 500 cellulose insulation (5); and 6.4mm Paneline sheathing board (6); 15mm timber battens (7); 12.5mm dry-lining (8)
  • Finished Wall U-value: 0.13W/m²K

U-values Explained

The parameter U, or the U-value, denotes the thermal transmittance of a material, or group of materials. The units for U are W/m²K (watts per square metre, per degree of temperature difference).

As the value of U increases, the amount of heat that will flow through a material under a constant temperature differential will increase. Therefore, a material with good insulating properties will have a low value of U and a poor insulator will have a high U.

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