Applied film in Window 7.8 LBBL

[Julio Fernández Amodia]

Hi,

I am working on a project for an existing office building. This building is using an applied exterior vinyl film that acts like a mirror. There is no information about the whole assembly optical characteristics (applied film + existing windows) but i do have some information about the film and window, which i could use to find an equivalent in the library.

I wonder if is possible in Window 7.8 or optics to create a new window assembly combining these two elements and estimate the G-value of the whole assembly to carry out energy and thermal comfort simulations.

Please find attached the window and film properties.

Thanks

[Jacob C. Jonsson]

Hi Julio,

Not really, three major hurdles. You should really use spectral
data for this problem as both the window and the film are
spectrally selective.

For the window construction you would need to replace
the outer layer for the one with a film on it. And without
knowing what the inner layer or outer layer (pane) in the
window that seems very hard.

For applying the film, you would have to know the outer
layer properties (and if there is a coating on surface 2
it might be even harder), the properties of the substrate the
film is on, and the properties of the film as advertised
(assuming it is applied on glass, if data is for free-standing
film then there is nothing to be done).

Even though the film is fairly absorbing (~20%) it is
quite likely that the transmittance and g-value should be lower
than that reported for the film (assuming film data is for film
on clear glass).

Best,
Jacob

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[Julio Fernández Amodia]

Hi Jacob,

Thank you for your response. I understand your point, but I find myself going down a bit of a rabbit hole. I initially tried a rough estimation by multiplying the g-values of the film (0.2) and the glazing assembly (0.45), based on the technical datasheets previously shown. However, this results in a combined g-value of 0.2x0.45= 0.09, which seems unrealistically low. Interestingly, using this value in my simulations eliminates any overheating issues. Yet, the occupants, based on real-world experience, report significant overheating during summer evenings, particularly on the west façade, which receives intense direct solar radiation (near-normal to the glazing plane). This contradicts the simulation results.

Given that the building is around 20 years old and lacks spectral data for the glazing and film, I’ve been trying to find a more realistic way to model their combined behavior. My current workaround involves creating an equivalent layer in Window 7 and simulating an equivalent film in Optics, but I recognize this is just a temporary hack.

From my understanding, two key factors might be contributing to the discrepancy:

1. The film appears to be mirror-like and likely contains aluminum, which typically has a low emissivity (around 0.1–0.2). This means that once solar radiation is absorbed (about 0.26 in this case), very little longwave radiation is emitted outward. If the assembly cannot effectively radiate this heat back outside, it may lead to internal heat buildup, raising the mean radiant temperature, consistent with the occupants observations.

2. Outdoor air temperatures in Zaragoza during summer evenings often exceed 30°C, which reduces the glazing’s ability to dissipate heat via conduction and convection.

In essence, it seems the glazing assembly is acting as a heat trap for the absorbed solar energy component.

Does this interpretation make sense to you? And do you have any suggestions for a more accurate way to simulate this behavior given the limited data?

Thanks

[Jacob C. Jonsson]

Hi Julio,

On 10/1/2025 1:17 AM, Julio Fernández Amodia wrote:
> Hi Jacob,
>
> Thank you for your response. I understand your point, but I find myself
> going down a bit of a rabbit hole. I initially tried a rough estimation
> by multiplying the g-values of the film (0.2) and the glazing assembly
> (0.45), based on the technical datasheets previously shown. However,
> this results in a combined g-value of 0.2x0.45= 0.09, which seems
> unrealistically low.

Correct, this approach will always be wrong when you are dealing
with spectrally selective layers.

> to model their combined behavior. My current workaround involves
> creating an equivalent layer in Window 7 and simulating an equivalent
> film in Optics, but I recognize this is just a temporary hack.

It is a sensible approach, trying to build a system that
matches your setup and using optics to create what you are
building.
> *The film appears to be mirror-like and likely contains aluminum*,

> which typically has a low emissivity (around 0.1–0.2).

While there are some examples of interior films with exposed
metal it is very rare for external films. Maybe they have
managed it but it is much more common that there is a
protective film on top that has a typical 0.8-0.9 polymer
emissivity.

> This means
> that once solar radiation is absorbed (about 0.26 in this case),
> very little longwave radiation is emitted outward. If the assembly
> cannot effectively radiate this heat back outside, it may lead to
> internal heat buildup,

> *Outdoor air temperatures in Zaragoza during summer evenings often
> exceed 30°C*, which reduces the glazing’s ability to dissipate heat
> via conduction and convection.

I don't have the intuition to say which of these effects is
the most impactful. My hunch is that even with high emissivity
and 60-70C temperature of the pane there probably more
more incident energy than what you can emit, but that is mostly
a guess (based on incident sunlight can be of the order of 1000
W/m2 but you do not emit that much heat when you are at 60 C).
And if the pane is at 60C you can still have significant deltaT
for convection if there is enough air movement. So not trivial
to dismiss.

> Does this interpretation make sense to you? And do you have any
> suggestions for a more accurate way to simulate this behavior given the
> limited data?

Trying to build a system that matches the data you have. I like
"bracketing", i.e. building one system where both
window and film have lower SHGC than you know and one where
they have higher SHGC, that way you should be able to say
that your product is between the two.

There is another layer of issue with aging windows which is
the question if argon has leaked out? And if moisture has
infiltrated the cavity and oxidized the low-e coating. Both
of these would impact window performance as well. Simulating
with air instead of argon is easy but knowing the actual
properties of the glass is hard without taking apart and
measuring (which I don't recommend).

Best,
Jacob

[Julio Fernández Amodia]

Thank you Jacob.

 I took two similar films—one immediately below (Ts, Rs) and one immediately above (Ts, Rs)—and averaged them. I believe this is the closest approximation possible without measuring the entire system.
I also forgot to mention another factor that could be impacting occupant comfort: the film prevents reflected and re-emitted longwave radiation from escaping unlike an exterior shading protection (detached). As a result, the room accumulates heat throughout the day, especially if occupants close the blinds. In that case, the blinds reflect radiation back toward the glazing, trapping it between the blind and the glass and creating a very hot cavity that radiates heat toward the occupants. I think all these factors contribute to the uncertainty and explain the occupants discomfort.  

Regards,

Julio

[Collin Robinson]

Julio, it's not just your film that "prevents reflected and re-emitted longwave radiation for escaping," it is any glass material. Plain glass is transparent to some UV, most visible and near IR radiation, but opaque to longwave radiation, the heat we can feel. Jacob is right to ask for spectral data on the film. Certain metals such as the silver used in low-e coatings have a very effective visible light filtering effect, in addition to their low emissivity. See diagram below, and note the wavelength of an object whose heat we can feel (10µm) is not transmitted by plain glass. Other metals such as aluminum have different effects. I don't know what your film contains. In my work I have found it hard to provide the  effect of a silver low-e coating with other measures, with the exception of exterior shading as you noted. Although I have never tried it, I hear interior storm windows with double or silver low-e coatings on them will provide this effect. Putting these piece together helped me, I hope it helps you.