That Energy Blog is published by Kates Energy Modelling Inc.
A phone flashlight can reveal a useful clue about low-e glass. It cannot establish a window’s energy rating, and an inconclusive reflection check should stay inconclusive. If the result will affect an evaluation, a replacement decision or a dispute about what was supplied, confirm it with product records or a detector suited to that glazing.
What the original field check showed
In my original flashlight demonstration, one reflection appeared magenta while the others appeared white. That visible difference was useful. The second photograph, however, was described as proving that no coatings were present. That conclusion went beyond what the photograph established.
The practical correction is simple: record what you observed separately from what you inferred. “One reflection has a different colour” is an observation. “The unit has this particular coating, on this surface, with this U-factor” needs more evidence.
Use the light as a screening tool
Hold a small white LED light near the glass without scraping it, and look from an angle that separates the reflections. Change the viewing angle if reflections overlap. Record the viewing side, apparent pane count and any repeatable colour difference. A phone photograph is useful documentation, but it is not a calibrated coating or thermal-performance measurement.
Do not turn a lack of obvious colour difference into a universal negative test. Cardinal’s technical glass guide describes some low-e coatings as having little or no effect on reflected and transmitted colour. A colour-based check therefore cannot identify every product reliably.
Surface numbers describe location, not performance
For a conventional double-pane unit, the glass surfaces are numbered 1 to 4 from outdoors to indoors; triple-pane units have surfaces 1 to 6. Do not assume every low-e coating is inside the sealed cavity. Cardinal’s LoĒ-i89 technical bulletin identifies a room-side coating used on surface 4 of double glazing or surface 6 of triple glazing. The actual coating arrangement is product-specific.
Likewise, an applied film and a factory low-e coating should not be treated as interchangeable evidence. Ask what product is present and how it was identified. A reflection’s colour alone does not provide its specification.
Match the evidence to the question
| Question | Useful evidence | What remains unknown |
|---|---|---|
| Do the reflections look different? | Direct observation and a clear photograph | Exact coating, emissivity and rated performance |
| Is a coating detected? | A suitable detector used within its stated limits | Whole-window U-factor and solar heat gain |
| What glazing was supplied? | Manufacturer identification, order records and matching specifications | Whether a later replacement changed that unit |
| How does the whole window perform? | Matching certified product ratings and configuration | Actual installed air leakage and annual household savings |
Choose the instrument carefully. For example, EDTM’s CM1030 instructions require direct contact with the conductive coating and explicitly distinguish that test from detecting a coating inside an assembled sealed unit. “Electronic low-e detector” is not one interchangeable capability. Check the model’s intended pane count, coating types and operating instructions before relying on its result.
If records and observations disagree, keep the discrepancy open. Confirm the exact window or replacement glass with the supplier rather than assigning a convenient specification.
Why the rating still matters
Low-e is one part of a window assembly. NRCan’s window guidance explains U-factor as a measure of heat loss through the window assembly. Its ratings guide distinguishes U-factor from solar heat gain and other performance measures. Use the matching whole-window rating, not an inferred value from a coloured reflection or a centre-of-glass figure.
The example below isolates that distinction. It holds window area and temperature difference constant and changes only the assumed U-factor. The values are deliberately hypothetical: none comes from the photographed windows or a client evaluation.
| Assumed whole-window U-factor (W/m²·K) | Calculated heat-transfer rate (W) |
|---|---|
| 1.0 | 60 |
| 1.5 | 90 |
| 2.0 | 120 |
For the geeks
Heat-transfer rate: Q̇ = U × A × ΔT
Q̇ in watts (W); whole-window U-factor in W/(m²·K); whole-window area A in m²; indoor-to-outdoor temperature difference ΔT in kelvins (K). A temperature difference of 30 °C is also 30 K.
Example: 1.5 × 2.0 × 30 = 90 W. This is a steady-condition illustration using an assumed constant rated U-factor. It excludes solar gains, air leakage and installation effects. It does not calculate annual energy, fuel consumption or savings. No U-factor can be recovered from reflection colour using this equation.
The useful field conclusion
Use the flashlight to decide what needs verifying. Use appropriate testing and product documentation to support the specification. If the evidence is incomplete, record “coating unconfirmed” rather than converting uncertainty into “no low-e.” That keeps a quick, inexpensive check useful without asking it to do a job it cannot do.