U-Value Calculations in London and Nationwide

What is a U-value?

A U-value is the rate of heat loss through one square metre of a wall, roof or floor for each degree of temperature difference, in W/m²K. The lower the number, the better the insulation. It counts every layer in the build-up and the repeating thermal bridges in it, such as studs, rafters and wall ties.

When you need a U-value calculation

• Building control. For extensions, conversions and loft conversions, building control asks for calculations showing each new element meets the Part L limits.
• SAP and SBEM. Every element in the SAP or SBEM needs a U-value that matches the specified build-up. Using a manufacturer's figure for a different build-up is a common reason for a query.
• Spec changes. A different insulation product or thickness on site means a new U-value, and the as-built SAP has to match.
• Material change of use and renovation. Conversions and upgrades to existing walls, floors and roofs have their own Part L targets.
• Existing buildings. Upgrades for an EPC B plan or a retrofit, where the existing build-up has to be worked out.

Part L limits (England, 2021 edition)

• External walls: 0.26 W/m²K for new dwellings; 0.18 W/m²K for extensions and new walls in existing homes.
• Roofs: 0.16 W/m²K for new dwellings; 0.15 W/m²K for extensions and new roofs in existing homes.
• Floors: 0.18 W/m²K for both.

New dwellings must also pass the SAP targets, which usually needs walls, roofs and floors well below the limits. Commercial buildings have their own limits in Approved Document L Volume 2. The new Part L starts on 24 March 2027, with new tables; we work to whichever edition your building is under.

How our U-value calculations work

1. You send the build-up for each element: every layer, product and thickness.
2. We calculate the U-value to BS EN ISO 6946 (BS EN ISO 13370 for ground floors), following BRE's BR 443 conventions, including the corrections for fixings, air gaps and repeating timber or steel.
3. If an element fails, we tell you the cheapest fix: a thicker board, a different product, or a change to the build-up.
4. The values go straight into our SAP or SBEM, so you see the effect on the result.

The BR 443 conventions we work to

• The manufacturer's declared thermal conductivity first; where there isn't one, the BR 443 value or BS EN ISO 10456. Every value is listed with its source.
• Timber studs, joists and battens are counted as a bridged layer, using BR 443's timber fractions (15% for a standard timber frame wall).
• Mortar is counted in lightweight blockwork and left out for brick and dense block, as BR 443 allows.
• Corrections for air gaps, wall ties and fixings are added where they change the result.
• Each cavity gets the right resistance for its size and venting. A well-ventilated cavity, such as behind tile hanging, is left out with everything outside it.
• Ground floors are calculated to BS EN ISO 13370 from the floor's area and exposed perimeter, counting walls to unheated spaces such as garages in the perimeter.
• Walls, floors and roofs next to an unheated space, such as a garage or stairwell, get an extra resistance for that space.
• The result is rounded to two significant figures, as BR 443 asks.

Junctions as well? See thermal bridging

A U-value covers the flat areas. Heat loss at the junctions is counted separately through PSI values. See our PSI value and thermal bridging calculations.

Condensation risk analysis

We also check your build-ups for condensation risk. Building control and warranty providers often ask for it on warm flat roofs, timber frame walls, internal wall insulation on solid walls and floors over unheated spaces.

We run the calculation to BS EN ISO 13788, following BS 5250:2021. It shows whether moisture will condense inside the build-up (interstitial condensation) over the year, and whether it dries out again. If it fails, we tell you the change that fixes it, usually a vapour control layer or moving the insulation.

Surface condensation and mould at junctions are checked with the fRsi value, which is part of our PSI value calculations.

The full service is on our condensation risk analysis page. Condensation risk analysis is priced with the U-values. Send us the build-ups for a free quote; turnaround is 5 working days.

What you get

• A calculation sheet for each element: layers, thicknesses, conductivities, corrections and the final U-value.
• A pass or fail against the Part L limit that applies.
• A summary for building control and the SAP or SBEM evidence pack.
• A condensation risk result for each build-up, where you ask for it.

What we need from you

• Wall, roof and floor build-ups, with product names and thicknesses.
• For ground floors: floor area and exposed perimeter, or plans so we can measure them.
• Floor plans and sections if we are doing the SAP or SBEM as well.

Price and turnaround

Priced by the number of different build-ups. Send the details for a free quote. Turnaround: 5 working days once we have everything we need. Booking the SAP or SBEM with us at the same time? We price both together.

Our experience

More than 17 years in energy assessment and more than 4,000 SAP calculations. We know which build-ups pass first time and which ones building control queries.

Questions we get asked

What U-value does an extension need?
Under the 2021 Part L in England: 0.18 W/m²K for walls and floors and 0.15 W/m²K for roofs. Windows and doors have their own limits.

Can I use the insulation manufacturer's U-value?
Only if their calculation matches your build-up exactly: same layers, thicknesses and fixings. If anything differs, it needs recalculating.

What is a good U-value for a wall?
Lower is better. For an extension in England the limit is 0.18 W/m²K. New dwellings usually need lower than that to pass the SAP.

Do I need a condensation risk analysis?
If building control or your warranty provider asks for one, yes. It is most common on warm flat roofs, timber frame walls and internal wall insulation, where moisture can get trapped. Send us the build-up and we'll tell you if it needs one.

How long does a U-value calculation take?
5 working days once we have the build-ups. Send them by email or WhatsApp for a free quote.

Related: PSI value and thermal bridging calculations, condensation risk analysis, SAP calculations and EPCs, SBEM and BRUKL, new dwellings, energy statements and EPC B upgrade plans.

Email your build-ups or WhatsApp us for a free quote.