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48hr SAP Fast-Track 07866153540 · enquiry@eacsouthwest.co.uk
EAC South West Energy Assessment Consultancy

Fabric performance

PSI and U-Value Calculations

Thermal bridging

What is a PSI value?

A PSI value is a measure of heat loss through junctions and thermal bridges in buildings (e.g., where walls meet floors, roofs, or openings). PSI-value calculations are a crucial part of building design, improving energy efficiency and supporting Building Regulations compliance.

Quantify junction heat loss

Replace generic/default values with project-specific junction performance.

Improve SAP outcomes

Better junction data reduces thermal bridging losses, often improving DER/DFEE results.

Reduce condensation risk

Sound detailing at junctions helps avoid cold spots and moisture issues.

Scope

Typical junctions we calculate

We can assess site-specific details or apply accredited/enhanced details where appropriate.

What are U-value calculations?

U-values measure how well a construction element (wall, roof, floor, opening) resists heat loss. The lower the U-value, the better the insulation performance — and the less heat lost. U-values feed into SAP energy calculations to size insulation and demonstrate Part L compliance.

We turn drawings and specs into SAP-ready numbers and straightforward guidance.

Lower is better

Better/thicker insulation tends to reduce U-values, improving fabric efficiency.

Used in SAP

U-values for each element inform SAP to determine required insulation and energy performance.

Part L targets

Minimum U-values exist, but overall compliance also weighs TER & TFEE and system efficiencies.

Method

How are U-values calculated?

We’ll translate the calculation into practical, cost-aware options so you can specify with confidence.

Design guidance

Practical considerations for better U-values

Openings

U-values for windows & doors

U-values for windows are usually supplied by the manufacturer and cover the whole window (frame + glazing). Where data isn’t available, SAP allows a suitable default, but confirmed product data generally improves accuracy and outcomes.

In SAP, we also enter dimensions of each element to assess heat-loss areas accurately. Openings are included with their specific U-values to reflect the real design.

Checklist

What we need to start

Send drawings/specs electronically or by post — we’ll take it from there.

Need PSI & U-Value Calculations?

Send your drawings and spec — we’ll deliver SAP-ready PSI and U-value calculations accompanied by clear, compliant advice fast.

  • SAP-ready
  • No hidden extras
  • Fast turnaround
Request a quote
or call 07866153540

PSI & U-Values – FAQs

Do you provide PSI and U-value calculations in Cornwall?

Yes — we cover Cornwall and the wider South West. We provide SAP-ready outputs and practical, buildable specs.

Will PSI values improve my SAP result?

Often yes. Replacing defaults with modelled ψ-values can reduce thermal bridging losses and strengthen compliance.

What do you need to start?

Drawings (plans/elevations/sections), fabric build-ups, window/door schedule and any available product data.