A common question when installing an under-carpet heater is: should you lay felt under an under-carpet heater? The short answer is yes – a suitable thermal insulation felt or underlay is recommended beneath the film. Only a thin, suitable protective layer should go on top of the film. In this article we explain why, and the correct order of layers.
Why Is Underlay Insulation Needed?
Carbon film gives off heat in both directions. Without insulation beneath it, a significant share of the heat escapes into the cold concrete floor. The result:
- the room warms more slowly;
- the film runs for longer and the bill goes up;
- efficiency drops sharply, especially in ground-floor flats and flats above basements or open undercrofts.
Underlay insulation directs heat upwards into the room. We explain its effect on warm-up time in how long does an under-carpet heater take to warm up?
Which Underlay Materials Are Used?
| Material | Properties |
|---|---|
| Thermal insulation felt | Economical and flexible; widely used under carpet |
| Aluminium-faced reflective underlay | Reflects heat upwards; thin |
| XPS insulation board | High insulation on cold floors; suitable under laminate/parquet |
| Laminate underlay suitable for underfloor heating | Acoustic and thermal insulation in laminate systems |
Note: When using aluminium-faced underlay, place an insulating separator (e.g. PE foil) between the film and the aluminium as recommended by the manufacturer.
Correct Order of Layers
- Clean, level floor
- Thermal insulation felt or underlay
- Moisture barrier (if required)
- Carbon film heater
- Thin protective sheet (optional)
- Carpet, laminate or parquet
Can Felt Be Laid on Top of the Film?
A thin protective sheet can be laid over the film to protect it from mechanical damage. However, thick felt or foam must not be used, because it:
- slows the transfer of heat into the room;
- causes heat to build up between the film and the carpet;
- increases consumption.
The total thermal resistance of the carpet and upper layers should be kept low. Details: which carpets suit carbon film heaters.
Ready-Made Under-Carpet Heaters with Felt
Some under-carpet heaters are ready-made products with the film and felt stitched together, which is practical for portable use. See our under-carpet heater with felt and thermostat as an example. On cold floors, adding extra insulation underneath these products further improves efficiency.
Frequently Asked Questions
What happens without underlay insulation?
The system still works, but some heat escapes into the floor; warm-up is slower and bills are higher.
Is my existing carpet underlay sufficient?
Thin carpet underlays are not sufficient for insulation. Use felt or underlay designed specifically for thermal insulation.
Conclusion
Laying thermal insulation felt or underlay beneath an under-carpet heater is important for savings and fast warm-up, while only thin layers should be used on top of the film. See our installation guide for the steps, or contact us for materials.
