A U-value calculation ends with one number. Getting to a number that properly represents the proposed wall is the harder part.
The insulation thickness matters, but it is only one part of the calculation. The sheathing, internal lining, SFS or timber frame, cavities, brackets, rails and fixings can all change the thermal performance of the complete wall.
So why can two walls containing the same thickness of insulation produce different U-values?
Because heat does not only travel through the insulation. It follows the easiest route through every part of the construction, particularly conductive materials such as steel and aluminium.
When you ask Klasse to complete a U-value calculation, we need the wall as it will actually be built, not just the insulation product you intend to specify.
Start with the complete through-wall
A typical rainscreen wall might include:
Internal plasterboard lining, service void, air and vapour control layer, SFS studwork with infill insulation, external sheathing board, breather membrane, KlasseROCK rainscreen insulation, facade brackets and rails, ventilated cavity and external cladding.
Every layer has an effect on the construction, even if some have a much greater thermal impact than others.
Klasse supplies several critical parts of this build-up, including external sheathing boards, fire-rated membranes, interface sealing products and KlasseROCK rainscreen insulation. Our technical team can use the proposed system information to assess the wall as a complete construction and determine how it performs against the project’s target U-value.
The calculation can only reflect the information supplied. If the drawing leaves out the brackets, assumes an unknown stud arrangement or shows insulation without a confirmed thickness, the result will contain assumptions rather than the final design.
Tell us about the project
Start with the basic project information:
- Project name and location
- Building type and intended use
- New build, refurbishment or recladding
- Current design stage
- Relevant UK nation
- Drawing number and revision
- Required return date
Location and project type matter because thermal requirements differ between England, Wales, Scotland and Northern Ireland, as well as between new and existing buildings.
We also need to know what the calculation is intended to demonstrate. Is it an early design exercise, a check against the project specification or a calculation for the final wall build-up?
That distinction matters. An early calculation can help establish a suitable insulation thickness, but it should not be treated as the final result if the frame, brackets or other components are still changing.
Give us the target U-value
Tell us the U-value the wall needs to achieve.
This might come from the project specification, the energy assessor, the applicable Building Regulations or a project-specific sustainability target.
If the target is known but the insulation thickness has not been decided, Klasse can assess the wall and help identify a suitable thickness of KlasseROCK.
If the insulation thickness has already been selected, we can calculate the resulting U-value and compare it with the target.
Do not send us an insulation thickness and assume that it automatically delivers a particular wall U-value. A 150mm insulation slab does not perform in isolation. The result depends on where it is installed and what passes through or around it.
Send a dimensioned wall section
The most useful document is a clear section through the complete external wall.
It should show every layer in the correct order, from inside to outside, with the thickness and material clearly identified.
For a typical SFS rainscreen wall, we need to see:
- Internal lining type and thickness
- Service void depth, where present
- Air and vapour control layer
- SFS zone and infill insulation
- External sheathing board
- Breather membrane, where required
- External rainscreen insulation
- Brackets, helping-hand system and rails
- Ventilated cavity
- External cladding
Where Klasse products are used, identify the exact product and thickness. For example, specify whether the sheathing is Klasse G-board or Klasse C-board, rather than describing it simply as “sheathing board”.
The same applies to the insulation. State the thickness of KlasseROCK Rainscreen Insulation and identify whether any additional insulation is installed between the studs.
KlasseROCK is a BBA-approved, Euroclass A1 stone mineral wool rainscreen insulation with a declared thermal conductivity of 0.035 W/mK. It is available in thicknesses from 40mm to 200mm, giving project teams flexibility when developing a wall against a specific thermal target.
For products outside the Klasse range, send the manufacturer’s current technical datasheet. We need a confirmed thermal conductivity or thermal resistance rather than a generic description.
The frame detail is not optional
The frame can have a significant effect on the result, particularly in SFS construction.
Steel studs pass through the infill insulation and create repeating thermal bridges. To model that part of the wall properly, we need:
- SFS manufacturer
- System or section reference
- Stud depth
- Stud width and flange dimensions
- Steel gauge or thickness
- Stud centres
- Infill insulation type and thickness
- Any internal or external secondary framing
A note saying “150mm SFS” is not enough. Two 150mm systems can contain different steel sections, gauges and centres, producing different thermal results.
If the project uses timber frame construction, provide the stud dimensions, centres and timber fraction where available, together with the position of the insulation and any additional battens or noggins.
The calculation needs the frame that has been specified, not a typical frame chosen for convenience.
Include the brackets, rails and fixings
This is the information most often missed.
Aluminium and steel facade supports pass through the external insulation and create highly conductive routes through the wall. Leaving them out can make a construction appear to perform better than it will in practice.
Klasse needs:
- Bracket manufacturer and reference
- Bracket material
- Bracket dimensions
- Number of brackets per square metre
- Rail type, dimensions and orientation
- Rail centres
- Thermal isolator or pad details
- Fixing material and diameter
- Number of insulation fixings per square metre
- Depth of penetration through the insulation
- Wall-tie details where relevant
Where the bracket manufacturer has calculated thermal data for the support system, send that information with the request.
Increasing the insulation thickness without addressing conductive brackets does not always improve the wall as much as expected. Once metalwork starts to dominate the heat flow, system design becomes as important as insulation thickness.
That is why the calculation has to represent the whole facade support arrangement.
Confirm the cavity and cladding arrangement
Tell us the depth of the cavity and whether it is unventilated, slightly ventilated or well ventilated.
For a typical rainscreen facade, we also need to know:
- Cavity depth
- Open- or closed-jointed cladding
- Cladding material
- Rail direction
- Whether the cavity is continuously ventilated
- Any cavity closer or fire-barrier arrangement that affects the typical wall area
A well-ventilated cavity is treated differently from a sealed airspace. Calling both simply “a cavity” leaves an important part of the wall undefined.
What happens if the design is not complete?
Send us what you have.
Klasse can review an early wall build-up and identify the information still needed. Where reasonable assumptions are required, they should be recorded clearly so the calculation can be reviewed when the specification develops.
Typical provisional items include:
- Final SFS section
- Bracket frequency
- Infill insulation
- Cladding rail arrangement
- Fixing density
- Final insulation thickness
An assumption is useful during design development, but it should not become part of the final specification by accident.
Whenever the wall changes, send the revised drawing back for review. Changing the stud system, insulation, brackets, rails, fixings or cavity arrangement can all affect the result.
The drawing reference on the calculation should match the drawing being used by the project team.
What a U-value calculation does, and does not cover
A U-value describes heat flow through a building element, normally expressed in W/m²K. The lower the figure, the better the thermal performance.
Building-element U-values are commonly calculated using BS EN ISO 6946 and the conventions set out in BR 443.
The calculation can account for the typical wall build-up and repeating thermal bridges within it. It does not automatically assess every junction around the building.
Slab edges, parapets, corners, window openings and wall-to-floor or wall-to-roof junctions create non-repeating thermal bridges. These are normally assessed separately as linear thermal transmittance, or ψ-values.
Point thermal bridges may also need separate consideration where individual components, such as facade brackets, penetrate the insulation.
If you need a junction calculation rather than a typical wall U-value, make that clear at the start and provide a fully dimensioned junction detail.
A U-value calculation is also not a condensation risk analysis, whole-building energy assessment or substitute for approval by the project designer, energy assessor or Building Control body.
The information Klasse needs at a glance
Before sending your request, check that you have included:
- Project name, location and building type
- Applicable target U-value
- Latest dimensioned wall section
- Every material listed from inside to outside
- Thickness of every layer
- Technical data for non-Klasse products
- SFS or timber-frame dimensions and centres
- Infill insulation details
- KlasseROCK thickness
- Bracket, rail and thermal-isolator details
- Fixing type and frequency
- Cavity and cladding information
- Drawing number and revision
- Any provisional information clearly identified
If that information is present, we can spend our time calculating the wall rather than trying to reconstruct the design from incomplete notes.
Find and specify Klasse products in BuildDesk
Klasse products are now included in the extensive material library within BuildDesk U, professional software designed for calculating U-values across a wide range of building constructions.
Its graphical, drag-and-drop interface allows designers and specifiers to assemble a building element, review the individual construction layers and see how changes affect thermal performance in real time.
With Klasse product data available within BuildDesk, users can:
- Add Klasse products directly to a building element
- Calculate and compare U-values quickly
- Test different insulation products and thicknesses
- Review surface temperatures
- Carry out a separate condensation risk analysis
- Assess the thermal mass of the construction
- Reduce manual product-data entry
- Produce clear calculation reports for clients and regulators
This makes BuildDesk particularly useful during early design development. A project team can compare KlasseROCK thicknesses, test alternative wall arrangements and understand how individual specification decisions change the overall U-value.
The same rule still applies: the result is only as accurate as the construction being modelled. The frame, brackets, rails, fixings and cavity arrangement must reflect the proposed wall rather than an idealised version of it.
BuildDesk gives experienced users a fast route to explore and document options using Klasse products. Where the wall contains complex framing, facade supports, point thermal bridges or non-standard junctions, the Klasse technical team can help review the information required.
Explore BuildDesk U and start specifying Klasse products within your calculations.
How Klasse supports the complete wall
Klasse does more than supply an insulation slab.
Our external-wall range includes:
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Klasse G-board gypsum external sheathing board
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Klasse C-board cement external sheathing board
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KlasseROCK rainscreen insulation
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KlasseWRAP fire-rated breather membranes
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KlasseSEAL interface sealing membranes
- KlasseBOND membrane adhesives
- Sheathing-board tapes, fixings and accessories
Because Klasse products cover several critical parts of the external wall, our team can help project teams consider product compatibility, thermal performance, fire performance and installation as parts of one through-wall strategy.
We can review the proposed specification, support the selection of an appropriate KlasseROCK thickness and provide the technical product information needed for the project record.
A correct U-value starts with a correct wall build-up. Send us the actual construction, with its frame, brackets and fixings, and we can help turn it into a clear, defensible calculation.
Contact the Klasse team to discuss your wall build-up or request a U-value calculation.
Further guidance
- Klasse technical downloads
- Klasse project support
- BS EN ISO 6946: Thermal resistance and thermal transmittance
- BR 443: Conventions for U-value calculations
- Explore BuildDesk U
