All about solar control films
Hardly a day goes by without someone asking me for a window film that stops people looking in, doesn't alter the look looking out, blocks heat and glare, but without reducing the light coming in. Unfortunately, it can't be done and in this section I'll try to explain why.
As you can imagine, there are dozens of different manufacturers producing dozens of different films but I'm going to concentrate on the main film types here. These are my own findings and assume that the film is fitted internally.
A perfect film, in a lot of people's eyes will be the one I mentioned above, that is, it gives one way privacy, blocks heat, but without reducing the light. As there's no such thing as a perfect film, everything is going to be a trade off, one quality against another.
The are two main types of solar control film, reflective and absorptive:
All film types reflect and absorb solar energy to some extent, but reflective films are more reflective than absorptive. Single reflective films have the same reflective surface on both sides of the film and so have similar qualities on both sides.
A reflective film is the one needed for one way privacy. The amount of privacy, or reflection is dictated by the amount of tint, and the difference in light on each side of the glass. If you're standing in your garden, on a bright day and it's dark inside the house, you'll see a reflection when looking in (referred to as daytime privacy) and a tint looking out. The greater level of tint, the greater level of reflection from outside and the darker it will be when looking out.
The photo below shows a daytime view looking out through a 35% single reflective film on the left and a 35% dual layer film on the right. The middle windows are clear glass
The photo below shows a daytime view looking in through a 35% single reflective film on the right and a 35% dual layer film on the left. The middle windows are clear glass.
As it gets darker outside and the light levels between inside and outside start to equalise, the level of reflection, or privacy, from the outside will start to diminish and the view looking out will start to become reflective. When the light levels are the same on both sides, the appearance will be the same on both sides. A sort of reflective, tinted effect.
When it's fully dark outside and you have the lights on inside, you'll be able to see in from outside and when trying to look out, you'll see a reflection of yourself. You will already see a reflection in plain glass in these circumstances as plain glass has 8% reflectance. A 20% single reflective film (lets in 20% light) has around 50% reflectance.
The photo below shows a night time view looking out through a 35% single reflective film on the left and a 35% dual layer film on the right. The middle windows are clear glass.
The photo below shows a night time view looking in through a 35% single reflective film on the right and a 35% dual layer film on the left. The middle windows are clear glass.
The greater the level of tint, the greater the level of reflection, from films with 60% VLT (visible light transmission) all the way up to 10% VLT films that are sold as one way privacy films and the ones you see on TV in the police interview rooms. They only work when the bad guy is in the well lit room and the police sit in a dark room.
Another example is a household mirror which would be the equivalent of 0% VLT. It's a piece of glass (think of this as your window). The glass is painted silver on the back (the reflective film) and is then painted in dark paint (a very dark room).
Don’t practice this at your neighbour's house but what do you do if you want to look through a window at night? You cup your hands round your face and get close to the window - but why? It blocks out the light in that area making it darker. If it’s darker than inside the room, you can now see in!
Single reflective films are usually some of the most cost effective solar control films to install and have the best qualities in terms of heat and glare rejection, daytime privacy and heat gain coefficient (the reflective qualities reduce heat loss back through the glass) and, as the energy is being reflected away from the film, the surface doesn't get too hot.
Single reflective films are a great choice for conservatory roofs, on any vertical glass where blinds or curtains can be used to hide the internal reflection at night or indeed, if you don't mind the internal reflection. Just be aware that at night, people can see in and you can't see out.
Absorptive (or Absorbative) films
As mentioned above, all film types reflect and absorb solar energy to some extent, but absorptive films are more absorptive than reflective.
As the name suggests, an absorptive film works by absorbing solar energy. It’s a film more commonly seen on cars where the airflow over a moving vehicle helps to keep the glass cool. Appearance-wise, as it has low reflection, it generally offers little in the way of privacy other than being a dark tint. On the plus side, you don’t have the internal reflection at night and it has a nice grey look. Think about limousine film, it’s an absorptive film with 5% VLT. As the film is blocking out 95% of light from the back windows, it’s going to be darker inside the car. The person in the back of the car can see out because it's lighter outside and you can’t see because of the reflective qualities of the glass and the film and it’s darker inside the car. If you want to see in, you cup your hands round your eyes and press your nose against the window again! You may then get to see the one way mirror in the police interview room!!
Depending on the level of tint of the absorptive film, it will absorb a certain amount of heat. As the glass isn’t cooled by air flow like a car, the glass gets hot when the sun is on it and, when the sun goes in, the glass starts to cool down again. I’ve recorded glass temperatures of 67oc in full sun and, in cases where there is a lot of glass and a small room, the heat radiated from the glass can make the room hotter rather than cooler.
Absorptive films are a good choice where the heat build up isn’t going to be an issue and where a low reflection is important. Compare the ratio of solar energy absorbed to reflected, as the greater the reflective qualities, the more it will help with daytime privacy and heat rejection.
That covers the two main categories of solar film, but there are a couple of noteworthy variations:
Dual layer or hybrid films
In my view, these films offer the best of both worlds. They are made from two layers of film, a reflective layer on the side facing outwards and an absorptive layer facing inwards. The reflective layer offers daytime privacy and reflects heat and glare and the absorptive layer cuts down the internal reflection and has a slightly reflective grey look.
Available in 25%, 35% and 50% VLT these films are ideal for all applications in residential, hospitality and commercial applications.
Spectrally selective films
These films work by rejecting the maximum amount of heat while also allowing in the maximum amount of visible light. Typically, a spectrally selective film will have about 70% VLT but reject 50% heat and 25% glare.
They are the ideal film if you want heat rejection and to keep a natural look to the windows as it's barely visible once fitted. They offer nothing in the way of privacy and have a low internal reflection. As you can imagine, these films cost more and are more labour intensive to install as the edges need to be sealed once fitted.
Internal or externally installed?
Some films are available in an external version and, once again, there are pros and cons. With an internally fitted film on a double glazed unit, the sun's rays pass through two layers of glass and an air gap before hitting the film. They are then either reflected back or absorbed in to the film depending on the film type. With an external film, it’s the film that is doing all the work so the glass and the air gap are staying cooler. The disadvantage is that they are more labour intensive to install as the edges need to be sealed once fitted and they normally only have half the life expectancy (or warranty) of an internally fitted film. An external, single reflective film in 35% VLT is the only safe option where laminated glass is fitted.
I hope this goes some way to explaining the differences between the main film types before deciding which solar control film you need. I will of course be happy to help you in your decision.
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