The hidden subtleties of U-Value calculations
Calculating prospective U-Values is a routine part of the building design process, so routine that you could be forgiven for not giving all that much thought to the intricacies of each and every calculation.
However, the slightest of changes to certain variables can have a significant impact on the final result and mistakes made in the design stages can come back to haunt us.
Contents |
[edit] The impact of ground types
Let us consider the case of ground-bearing floors – the thermal properties of the underlying ground can have a notable impact on the overall thermal performance of a buildup. It is therefore important to ensure that the U-Value calculations for competing insulation products have been compiled using the same ground type to allow you to make an informed decision. To this end, you should always provide the ground type (if known) when requesting a calculation to ensure that the resulting U-Value is most accurate. BS EN ISO 13370:2017 states that there are three categories of ground type:
The standard is clear that when the ground type is unknown it should be assumed to be category 2. While there is a swathe of fantastic tools on the market to help produce U-Value calculations, some of them actually default to using category 1, so those performing the calculation need to make sure that this is changed when a prospective customer has not specified a ground type. To demonstrate the importance of using a consistent ground type across calculations, lets look at an example build-up involving 100mm of XPS insulation on a ground bearing slab with a P/A of 0.3. In this scenario, using an XPS board with thermal conductivity of 0.031 W/mK results in a U-Value of 0.20 W/m2K when calculating for ground type 2, meanwhile a board with thermal conductivity of 0.034 W/mK can achieve a U-Value of 0.19 W/m2K if we instead use ground type 1; unless you were checking for the difference in ground type you would think that the board with higher thermal conductivity was the superior solution! (see diagram above)
[edit] What elements should be included in the calculation?
BS EN ISO 13370:2017 is also somewhat ambiguous on how to account for the thermal properties of some elements of a ground bearing floor. While it is explicit that the hardcore below a concrete slab should not be included, it states the “thermal resistance of a dense concrete slab may be neglected”, leaving it to the individual performing the calculation to decide. This decision is quite a significant one, as typically the inclusion or exclusion of the concrete slab’s thermal resistance will cause a variance of 0.01 to 0.02 W/m2K in the final U-Value. The 2019 edition of Conventions for U-Value Calculations (BRE BR443) provides best practice advice to complement the standard. At first it is quite definitive, stating that “It is recommended for most calculations that dense floor slabs (ρ ≥ 1800 kg/m³) and floor coverings such as vinyl or carpets are not included in the calculation”, but frustratingly, it then immediately reintroduces the ambiguity of the standard by saying “but it is permissible to include them if their properties are adequately defined.” It is therefore important to check exactly what elements have been included in individual U-Value calculations before trying to compare them.
[edit] The need for a common standard
These are just a couple of examples demonstrating how difficult it can be to compare U-Value calculations when trying to specify insulation. We need to start calculating U-Values with a consistent and transparent methodology that will allow specifiers to compare the performance of different insulation products quickly and easily.
This article appears in the AT Journal no 148, winter 2023 as "The hidden subtleties of U-Value calculations" and was written by Joan Ferrer, Commercial Director UK & I RE, Ravago Building Solutions.
--CIAT
[edit] Related articles on Designing Buildings
- Accredited construction details ACDs.
- Air tightness in buildings.
- Assessing the effects of thermal bridging at junctions and around openings.
- Building fabric.
- Building performance.
- Cavity wall.
- Cavity wall insulation.
- Co-heating test.
- Computational fluid dynamics.
- Conduction
- Conventions for calculating linear thermal transmittance and temperature factors.
- Conventions for calculating linear thermal transmittance and temperature factors.
- Conventions for U-value calculations (BR443 2e 2019).
- Double glazing v triple glazing.
- Emissivity.
- Emission rates in the building regulations.
- Floor insulation.
- Free U-value calculators.
- g-value.
- Heat loss.
- Heat transfer.
- Insulation.
- Insulation for ground floors.
- Insulation specification.
- Interstitial condensation.
- k-value.
- Limiting fabric parameters.
- Mould growth.
- Reducing thermal bridging at junctions when designing and installing solid wall insulation FB 61.
- Roof insulation.
- R-value.
- Solid wall insulation.
- Standard Assessment Procedure SAP.
- Thermal admittance.
- Thermal bridge.
- Thermal bridging and the Future Homes Standard.
- Thermal comfort.
- Thermal imaging to improve energy efficiency in building design.
- Thermographic survey.
- Thermal mass.
- Thermal resistance.
- U-values
- U-value conventions in practice: Worked examples using BR 443.
- U-What?
- What do design professionals need to know about U-value calculation conventions?
Featured articles and news
Design and construction industry podcasts
Professional development, practice, the pandemic, platforms and podcasts. Have we missed anything?
C20 Society; Buildings at Risk List 2025
10 more buildings published with updates on the past decade of buildings featured.
Boiler Upgrade Scheme and certifications consultation
Summary of government consultation, closing 11 June 2025.
Deputy editor of AT, Tim Fraser, discusses the newly formed society with its current chair, Chris Halligan MCIAT.
Barratt Lo-E passivhaus standard homes planned enmasse
With an initial 728 Lo-E homes across two sites and many more planned for the future.
Government urged to uphold Warm Homes commitment
ECA and industry bodies write to Government concerning its 13.2 billion Warm Homes manifesto commitment.
From project managers to rising stars, sustainability pioneers and more.
Places of Worship in Britain and Ireland, 1929-1990. Book review.
The emancipation of women in art.
Call for independent National Grenfell oversight mechanism
MHCLG share findings of Building Safety Inquiry in letter to Secretary of State and Minister for Building Safety.
The Architectural Technology Awards
AT Awards now open for this the sixth decade of CIAT.
50th Golden anniversary ECA Edmundson awards
Deadline for submissions Friday 30 May 2025.
The benefits of precast, off-site foundation systems
Top ten benefits of this notable innovation.
Encouraging individuals to take action saving water at home, work, and in their communities.
Takes a community to support mental health and wellbeing
The why of becoming a Mental Health Instructor explained.
Mental health awareness week 13-18 May
The theme is communities, they can provide a sense of belonging, safety, support in hard times, and a sense purpose.
Mental health support on the rise but workers still struggling
CIOB Understanding Mental Health in the Built Environment 2025 shows.
Design and construction material libraries
Material, sample, product or detail libraries a key component of any architectural design practice.