In the field of thermal analysis, winter serves as the ultimate stress test for building performance. In cold climates, the effectiveness of an energy audit is critical for operational survival and carbon reduction. Understanding a building’s envelope needs more than a visual snapshot. It requires the “ground truth” of accurate heat flow measurement.
Infrared thermography reveals surface anomalies. However, heat flux measurement quantifies the actual rate of loss. The EKO HF-01S Heat Flux Sensor is specifically designed. It detects underperformance in highly insulated walls. Here, thermal bridges may be too faint for standard infrared thermography.
The U-value Gap: Design Intent vs. As-Built Truth
Most energy audits rely on theoretical U-value calculations based on architectural plans. However, as-built performance often deviates significantly due to factors such as the difference between theoretical vs actual thermal properties as well as non-uniform materials, settling insulation, inconsistent construction quality, and decay. Field studies routinely find large gaps between design U-values and measured in-situ performance, especially where the calculations are based on theoretical values and unrealistic simple assumptions. A study by researchers from Delft University of Technology showed that relying on tabulated insulation values (instead of measuring) in some cases can overestimate the implied U-value by up to nearly 400%.
How to Measure U-value and R-value on Site?
Quantifying Thermal Transmittance (U-Value): By measuring the actual heat flow rate (using HF-01S heat flux sensor) across a temperature gradient (using 2 temperature sensors) through an element, you can determine the on-site U-value and R-value, providing more accurate data than decades-old blueprints. Estimate the U-Value as follows:
U = q / ΔT and R = ΔT / q
Where:
q = heat flux (W/m²)
ΔT = temperature difference across the element (K or °C)
The EKO QRU-100 monitoring kit is an example of a complete, user-friendly set to capture the required data for this application.
Why the HF-01S is the Auditor’s Standard
High-accuracy, high-precision heat flux measurement requires a sensor that does not disturb the heat flow it is measuring. The EKO HF-01S is engineered with a small, thin (2 mm) body to keep its thermal resistance minimal.
Engineered for Compliance and Confidence
The HF-01S provides audit-ready data through several key technical features:
- ISO 9869 & ASTM C1046-C1155 Compliance: The sensor is designed to meet rigorous international standards for in-situ thermal resistance and transmittance measurements.
- Built-in Thermal Guard: Reduces lateral heat spreading so the sensor approximates 1D heat flow. The facings help ensure stable, even heat flux distribution across the sensing area of the sensor.
- High Sensitivity: With a nominal sensitivity of 55 µV/W/m², the sensor captures very low heat flows even in high-performance insulations.
- Size is the key: An ultra-light compact design with thin body and small area ensures minimal thermal resistance and disturbance on the natural heat flow as well as wider application range, and makes the sensor easy to install and fit in places where larger sensors don’t.
The ISO 9869 Methodology: The Auditor’s Roadmap
Following the ISO 9869 (equivalent to ASTM C1046 and C1155) Heat Flow Meter method allows auditors to deliver defensible, traceable and globally-acceptable results. What matters:
- Placement: Place sensors on a representative, thermally-homogeneous location of the wall, away from direct sunlight or HVAC vents & drafts (because convective and radiative effects can mess with surface temps)
- Thermal Contact: Using a conductive medium like thermal compound is critical; air gaps can create “mini insulation pockets” that skew data.
- Ensure a large temperature difference exists between the interior and exterior. This creates an ideal environment for accurate in-situ measurement.
- The 72-Hour Minimum: To account for high thermal storage and weather fluctuations, standards suggest a minimum of 3 days (72 hours) of continuous data to produce a valid, average R-/U-value.
Limitations & Best Practices:
To make the best out of your heat flux measurements:
- Avoid solar loading and radiant asymmetry
- Provide stable temperature control to get faster results
- Confirm sensor adhesion/contact quality
- Avoid placing sensor over studs/thermal bridges unless intentionally measuring them
Watch our Installation Video and read the Quick Start Guide for a quick understanding of how HF-01S can be applied on surfaces.
Conclusion: Data-Driven Retrofits
Effective management of the built environment requires precise measurement. In cold climates, where heating accounts for the majority of energy consumption, the HF-01S unmasks hidden thermal failures that simulations often miss. Integrating high-precision heat flux sensors into energy audits moves the process from guesswork to thermal certainty, ensuring retrofit investments are applied where they will have the greatest impact on energy efficiency.
