1. What Changes Dynamically in FVS

The FVS multilayer film system is a movable additional thermal protection layer. When closed, several thin films are arranged one behind another at defined distances, forming air cavities between them. When open, the films are rolled up or pushed together, largely withdrawing the additional insulation layer from the effective cross-section of the component.

What changes dynamically is not the thermal conductivity of an individual material, but the assembly: the number and geometry of the effective air cavities, long-wave radiative exchange and thermal coupling between indoors and outdoors. Scientific literature describes such approaches as dynamic or switchable thermal insulation. Their common aim is to vary heat transfer over time rather than maintain a permanently fixed thermal resistance.

The film is not effective because twelve micrometres of plastic provide good insulation. It subdivides the available space into several air cavities and has a low-emissivity surface.

2. Three Mechanisms of Heat Transfer

Heat passes through a multilayer assembly by three mechanisms: conduction, convection and long-wave thermal radiation. FVS does not act on just one of these pathways; it changes all three to different degrees.

For the one-sided aluminised PET film used in the early test specimen, Fraunhofer ISE determined a corrected long-wave emissivity of approximately ε = 0.03 on the aluminium side and approximately ε = 0.48 on the PET side. Because the film’s long-wave transmittance was measured to be negligible, ε = 0.03 corresponds approximately to 97% reflectance in the long-wave thermal radiation range. This does not mean that 97% of total heat transfer or solar radiation is reflected.

3. Why Several Air Cavities Are More Effective Than One Large Void

A single large air layer can develop circulation when height and temperature difference are sufficient. The moving air then carries additional heat. Several narrow cavities shorten the free flow path. At the same time, every additional low-E interface adds resistance to long-wave radiative exchange.

The Fraunhofer report analysed heat transfer through an individual cavity using convection and radiation models. For the subsequent multi-cavity analysis, a cavity thickness of approximately 10 mm was chosen to stay in a predominantly conduction-dominated regime. The report describes this as a design choice for the vertical arrangement studied. It does not establish a universal rule that 10 mm is optimal for all temperatures, orientations and gas fills.

In simplified terms, the thermal resistances of cavities, covers and surfaces add together. The U-value is approximately the inverse of the total resistance. In a real system, however, edge connections, frame, roll-up mechanism, airtightness and installation conditions also matter. Neither the number of films nor their spacing alone is sufficient to establish a reliable product U-value.

More films mean more cavities and more low-radiation-exchange interfaces. How much of the benefit is retained in the installed component depends on the assembly as a whole.

4. Open and Closed Are Distinct Thermal Operating States

Static insulation restricts heat flow regardless of whether heat is to be retained inside the building or released to the outside. FVS is intended to switch between open and closed states. Any benefit depends on choosing the appropriate state at the appropriate time.

This table outlines the underlying physical logic, not a finished control strategy. The optimum position cannot be determined from the calendar alone. It depends on outdoor and indoor temperatures, available solar energy, orientation, shading, thermal mass, overheating risk and actual demand for heat. Real controls also require hysteresis, user override and fail-safe operating states.

5. The Same Principle Applied to Windows and Façades

Windows

In a window retrofit, FVS is positioned on the room side of the existing window. When closed, it is intended to cover the glazing and, as far as possible, the frame. It supplements the thermal resistance of the existing window and also provides solar shading. When open, the view, daylight and solar transmittance of the existing window remain largely available. The overall effect depends especially on the side guides, upper and lower seals, and air exchange between the window recess and the room.

Façades

In a façade application, FVS may be installed behind a transparent or energy-active external skin and in front of a massive wall. When open, solar radiation may reach an absorber or the thermal mass. When closed, the film stack forms an additional thermal protection layer. Combining thermal protection, solar input and thermal mass in this way is a technical system concept. Its effect must be calculated and measured for the particular layer arrangement, orientation and moisture conditions.

Windows and façades thus share the same physical principle but not the same validation requirements. At the window, the focus is on additional insulation, solar shading, the view and connections to the window recess. At the façade, the external skin, rear ventilation, moisture, fire safety, fixings and thermal mass are additional factors.

6. What Has Been Measured So Far

External Investigation by Fraunhofer ISE

In 2016, Fraunhofer ISE investigated an early fixed test specimen of the film principle on behalf of I[n]solation UG. It consisted of ten vertical parallel films with air gaps of approximately 9 mm, two Plexiglas covers and a total of eleven air cavities. The films did not move in this test configuration. Five U-values between 0.35 and 0.45 W/(m²K) were measured, with stated uncertainties of ±0.03 or ±0.04 W/(m²K), respectively. The 12 µm PET film used at the time was aluminised on one side; the current material metallised on both sides must be distinguished from it. The 2016 report tested neither the current movable prototype nor a production-ready window or façade system.

For an order-of-magnitude comparison, the report considered a 95 mm thick insulation board with an assumed thermal conductivity of λ = 0.035 W/(mK). Under the boundary conditions used, this also gives approximately U = 0.35 W/(m²K). At the same time, Fraunhofer suspected air leakage in the test specimen’s cavities because the measured thermal resistance varied more strongly than expected with temperature and temperature difference.

This investigation establishes the material properties and approximate heat-transmission performance of an early, statically tested multi-cavity assembly. It is not a test certificate for the current movable window prototype, nor does it establish a universal Uw-value for an installed window or façade solution.

Own Exploratory Measurements

In an in-house winter measurement, FVS installed at an existing triple-glazed window was compared with a 50 mm EPS element fitted airtight. The measured temperatures in the intermediate space were of a similar order of magnitude. This is a plausibility comparison under real conditions. The temperature relationships alone cannot establish a normative U- or Uw-value, however, because heat flows, surface resistances, leakage and changing boundary conditions were not fully captured.

In in-house summer comparisons, temperatures of approximately 26.2 to 26.3 °C were measured on the room-facing side of the FVS, compared with approximately 33.1 to 33.3 °C on a commercially available thermal roller blind. In one test, the outdoor temperature was 37.8 °C and the room temperature 25.9 °C. Identical black measurement surfaces were used for the pyrometer. The results show a difference in room-facing surface temperature in the specific assembly, but they do not directly measure heat flow or establish an energy saving. Temperature measurements on reflective surfaces require correct allowance for emissivity and reflected ambient radiation; the comparative values must therefore not be treated as standardised product performance figures.

7. Film Thickness and Effective Assembly Depth

The PET film studied in the Fraunhofer report was 12 µm thick. Ten such films have a combined nominal material thickness of 0.12 mm, and twenty films 0.24 mm. This figure describes only the sum of the film thicknesses. It is neither the installation depth of the closed insulation layer nor evidence of its U-value.

The thermal effect requires spacing between the films. With cavities of approximately 10 mm, the effective installation depth increases to several centimetres as the number of cavities rises. Covers, frames, guides, roller shaft, drive and edge connections must also be accommodated. Low film-material consumption is therefore a separate advantage, but it must not be confused with an equally small system depth.

8. What FVS Already Demonstrates—and What Remains Open

The underlying physical principle is established: narrow air cavities, low long-wave emissivity and multiple thermally series-connected layers can significantly reduce heat transfer. External measurements covered the film properties and an early fixed test specimen. Existing movable prototypes demonstrate that the film assembly can be deployed and retracted; in-house temperature measurements provide initial plausibility observations for winter and summer operation.

A generally valid performance figure for the current movable system as a whole has not yet been established. A reliable product rating requires a defined assembly and standardised testing, including frames, edge connections and airtightness. Depending on the application, further open questions include durability, soiling, moisture and condensation, fire safety, acoustics, operating cycles, maintenance and life-cycle impacts.

The development objective is therefore not merely to add more films. What matters is transferring the measured multi-cavity effect to a movable, airtight and durable construction. Only then can performance characteristics be established for the complete window or façade system.

Related articles in the Knowledge section: “Windows: Energy Balance Rather Than U-Value”, “Window Retrofit – More Thermal Protection Without Window Replacement” and “From the Component to the Building Energy Balance”.

9. Conclusion

FVS provides dynamic thermal protection by forming and withdrawing a multilayer assembly as required. In the closed state, multiple narrow air cavities are present. The cavities limit convection, while the metallised surfaces reduce long-wave radiative exchange. In the open state, this additional thermal resistance is largely removed from the effective component assembly.

Thermal protection thereby becomes a time-dependent function. In winter, the open state can admit solar gains and the closed state can limit losses. In summer, the same closed assembly can reduce incoming solar heat; under suitable conditions, the open state can support heat release. Whether and when these effects are beneficial must be assessed separately for windows and façades.

FVS is not a thin insulation material with a fixed “miracle” rating. It is a switchable multi-cavity assembly whose performance results from the combined effects of air layers, surface properties, airtightness, geometry and operating state.