Measuring the Invisible: HTC and the Future of Retrofit

As the winter heating season hits its peak, the gap between predicted building performance and lived reality is impossible to ignore. For decades, the industry relied on SAP ratings and EPC assumptions – essentially “paper performance”. But today, attention is shifting to the metric that reflects what actually happens in occupied homes: the Heat Transfer Coefficient (HTC).

This blog explores the key themes from the latest episode of the Stroma Insights podcast, where our experts discuss why HTC is becoming central to retrofit, heat pump design, and performance assurance. You can listen to the full episode below, or keep reading for a clear overview of the insights and practical takeaways.

Episode 3: Measuring the Invisible – HTC and the Future of Retrofit

Key Takeaways

What is the Heat Transfer Coefficient? Looking Beyond U-Values

Think of the Heat Transfer Coefficient (HTC) as the “whole-building” thermal signature. While U-values show you the heat loss of a specific, isolated component (e.g., a window or wall), HTC captures the combined effect of the entire building fabric. It accounts for all heat lost through walls, floors, and roofs, as well as losses from infiltration and ventilation.

• HTC is focused on the entire building / assembly
• It’s the sum of all U-values × area, + ventilation + thermal bridge losses
• This is measured in Watts per Kelvin (W/K)
• The lower the figure, the better the building is at retaining heat
• Results can be used to determine and optimise the building’s energy demand

In practical terms, HTC indicates the total rate of heat escaping from a building for every degree of temperature difference between inside and outside. These measurements help us understand a building’s heating demand, identify the gap between expected and actual building performance, and target and validate investments in energy-efficient retrofits.

Why Whole-Building Heat Loss Matters

Knowing how individual elements perform does not tell you how a building behaves as a whole. Heat does not escape through walls, roofs or windows in isolation; rather, it moves through the building as a connected system, influenced by construction, air movement, and occupant use.

A whole-house metric like the Heat Transfer Coefficient (HTC) brings these interactions together into a single, foundational metric. It reflects how all elements work collectively, rather than assuming each component performs exactly as specified.

For asset owners, designers, and retrofit assessors, this is crucial because heating demand, comfort, and operating costs are primarily driven by total heat loss. HTC provides a system-level view that component testing alone cannot deliver.

The Role of HTC in Closing the Performance Gap

Studies show that as-built fabric performance can be significantly worse than design expectations. This difference, known as the “performance gap”, means retrofit upgrades are often designed based on assumptions about materials, workmanship, and occupant behaviour, rather than on measured data.

In reality, a building may be losing more or less heat than expected at the design stage. For an energy-efficient retrofit assessor, that uncertainty can lead to fabric upgrades being applied where they offer limited benefit, or heating systems being incorrectly sized due to an unreliable view of actual heat loss.

As a result, the Heat Transfer Coefficient (HTC) is increasingly being treated as a core part of retrofit quality assurance. By measuring real whole-house heat loss, it provides an as-built performance check that helps ensure retrofit decisions are grounded in evidence rather than assumptions.

Applying the Heat Transfer Coefficient in 2026

Historically, measuring the Heat Transfer Coefficient (HTC) required co-heating tests that involved vacating a property and installing temporary heaters. While accurate, this approach was impractical for most occupied homes and large-scale retrofit programmes.

That has now changed. Smart HTC combines monitoring technology with advanced data analysis to deliver reliable results with minimal disruption:

• Small, discrete sensors are installed, or data is drawn from existing smart thermostats
• Monitoring runs for approximately three weeks during the heating season
• Algorithms analyse internal temperatures against local weather data and energy use
• The process accounts for “occupancy noise”, such as cooking or door openings

This process allows a retrofit assessor to understand true building performance without interrupting residents’ daily lives.

The Business Case: “Right First Time” and Retrofit ROI

Measuring the Heat Transfer Coefficient (HTC) turns energy performance from an assumption into a known value. This creates a more reliable basis for decisions about heating systems, fabric upgrades, and overall building performance. It also establishes a clear baseline that can be used to assess improvement later.

In practice, a measured HTC helps deliver:

• Correctly sized heat pumps based on peak heat demand in watts
• Lower risk of costly remedial works and long-term performance problems
• More targeted investment in energy-efficient retrofits
• Clear evidence that interventions have achieved the intended outcome

In summary, HTC measurements can play a key role in reducing uncertainty, improving cost control, and ensuring projects are delivered “right first time”.

Targeted Retrofit and Post-Works Validation

In retrofit projects, HTC data supports smarter and more defensible investment decisions. Evaluating whole-house heat loss helps identify which homes or building types are performing worst, allowing teams to prioritise retrofit house upgrades based on evidence rather than assumption.

Re-measuring HTC after works can also confirm that performance has genuinely improved. This helps retrofit assessors demonstrate that any interventions have delivered the expected benefits, and for lenders and insurers, it’s the ultimate proof that the work actually achieved the intended carbon savings.

The Future: Live HTC and Digital Twins

The role of the Heat Transfer Coefficient (HTC) is expanding beyond one-off assessments. With the rise of Digital Twins, measured HTC can now serve as a live performance input rather than a static result, enabling continuous monitoring of building performance over time.

By using real operating data, changes in heat loss can reveal emerging issues, such as moisture ingress, degraded insulation, or failed seals, often before they are visible to occupants. This allows for earlier intervention and helps protect long-term energy performance.

Digital Twins represents a wider shift for the industry. Energy performance is moving away from static certificates and toward live, data-driven oversight, where buildings are monitored, understood, and managed based on how they perform in practice.

How Stroma is Leading the Shift to Measured Retrofit Performance

At Stroma Group, we believe the future of retrofit lies in measurement. As the Heat Transfer Coefficient (HTC) becomes the key metric linking design to real-world outcomes, we focus on making that shift practical, scalable, and trusted. Find out more about our retrofit services here.

By combining non-invasive Smart HTC measurement, robust data analysis, and decades of building performance expertise, Stroma supports retrofit assessors, landlords, and delivery teams at every stage. From right-sizing heat pumps to validating post-retrofit works, measured performance enables better decisions, investments, and buildings.

This approach doesn’t just reduce risk – it builds trust with residents, funders, insurers, and the wider supply chain. If you’re planning energy-efficient retrofits, managing a retrofit house project, or looking to close the performance gap across your stock, stop guessing and start measuring.

FAQs About HTC and the Future of Retrofits

What is the difference between the Heat Transfer Coefficient (HTC) and an EPC?

An EPC (Energy Performance Certificate) is an estimated, standardised rating (A-G) of a building’s energy efficiency based on a model. HTC (Heat Transfer Coefficient), on the other hand, is a measured, precise technical metric of a building’s total fabric heat loss.

Can HTC be measured in an occupied retrofit house?

Yes. Smart HTC uses non-invasive monitoring, allowing accurate measurement without asking residents to vacate the property.

Is HTC only relevant for heat pumps?

No. While HTC is essential for heat pump sizing, it also supports fabric-first decisions, post-works validation, and long-term monitoring for energy-efficient retrofits.