Building Regs U-Values: Limiting & Notional Values

Table of Building Regulations U-Values for New Dwellings in England

Within Approved Document L (Part L) there are two sets of u-values that apply to new build dwellings:

• Limiting U-Values
• Notional Building U-Values

A u-value measures how fast heat escapes through a building element. Lower is better. For construction elements, it’s calculated using all the layers and materials used to build the element. Understanding the difference between limiting and notional u-values will make complying with building regulations easier.

Notional & Limiting Explained

The limiting u-value is the maximum u-value an element can be, a hard cap. Designing to this value is not an automatic SAP fail but it probably won’t pass overall.

The notional values should be your target.

During a SAP Calculation, a notional dwelling is created that passes SAP. This dwelling uses the notional u values and your design is weighed against it to determine a pass or fail.

In short, the notional building is your benchmark. You’ll need to hit its fabric heat loss (TFEE), carbon emissions (TER) and primary energy (TPER) targets.

The reason the two sets exist is to give design freedom and to fall short of mandating a recipe for all building projects. You can ‘borrow’ from elements and make up in others as long as no element exceeds the back stop limiting values. Need a little leeway on the walls? No problem, just make sure your floor and roof are under.

Common Build Ups That Pass Building Regs

We’ve created a quick reference guide for the most common elements to show what you’d need to reach the notional u-values. We’ve used reference materials in Elmhurst’s u-value calculator. Specific products will have a different thermal resistance and may not achieve the figures below.

Cavity Walls – Notional Target of 0.18 W/m²K

A typical build up, starting from the outer leaf would be:

• Standard Brick
• Air Gap (50mm)
• 120mm Polyurethane Insulation
• Lightweight Block
• Dot & Dabbed Plasterboard
• Skim

Thermal bridging has been taken into account for the mortar and dabs. It assumes plastic wall ties, which have no thermal bridging effect on u-values. This build up achieves 0.17 W/m²K.

Floors – Notional Target of 0.13 W/m²K

Floor u-values differ with overall size and the length of the exposed perimeter. A semi will be different than a detached. Let’s assume a 7m x 5m semi detached and look at a floor build up that would meet the notional 0.13 W/m²K target:

• Concrete Slab (100mm)
• 150mm Polyurethane Insulation
• Screed (75mm)

This build up achieves 0.13 W/m²K.

Roofs – Notional Target of 0.11 W/m²K

For a standard pitched roof with a cold loft space and horizontal insulation at ceiling level:

• Plasterboard
• 150mm Timber Frame*
• 150mm Mineral Wool Insulation Between Joists
• 250mm Mineral Wool Cross Laid Above Joists

*Thermal bridging via the timber has been taken into consideration. We’ve assumed a 50mm width at 500mm centres and noggins every 1.2m.

This build up matches the 0.11 W/m²K notional value.

Airtightness

U-values aren’t the only fabric metric. Airtightness is important too.

All new dwellings must be pressure tested. Like u-values, the notional figure (5) is your real target. Hitting 8 and hoping for the best will mean compensating elsewhere.

Note: Below 5 is good for energy. Below 3 triggers mechanical ventilation requirements under Part F.

Conversions & Extensions

The values above apply to new build dwellings only.

Conversions (e.g. barn to dwelling, commercial to residential) have different u-value requirements based on the existing structure.

Extensions don’t use the notional dwelling comparison. They still need to meet specific u-values to avoid a full home SAP calculation as well as keeping the glazing under thresholds too. We’ve got a page dedicated to extension building regulations compliance.