Solar Control Film vs Replacing Office Glazing

A hot desk by the window has three possible fixes

The symptom is familiar in older office buildings. The perimeter desks with the best views sit empty after lunch, blinds stay down all day with the lights on behind them, and the air conditioning overcools the core to keep the window zone tolerable. The floor is paying rent for space that people avoid.

There are two common misconceptions about what to do. One is that window film is a cheap fix that works on any glass. The other is that the only real answer is new glass. Both skip the question that decides the answer: what exactly is the problem? Heat from the sun, glare, fading from UV, heat loss and noise, and condensation all have different causes, and the existing glass sets limits on what can safely be done to it.

For a CEO or CFO the decision has three parts: which option addresses the dominant problem, what it risks on the existing facade, and whether the landlord will allow it at all.

What each option fixes and what it does not

Two glass properties sit behind every row. Solar heat gain coefficient (SHGC) is the fraction of solar energy striking the glass that ends up inside the room. Visible light transmission (VLT) is the fraction of visible light that passes through. A dark film can lower both, cutting heat and glare but also daylight, which leaves the floor gloomy and the lights on. Spectrally selective films and coatings aim to reduce heat while keeping more of the light, which is why the two figures should always be compared together rather than chasing the lowest SHGC.

Glass performance figures are determined by methods such as EN 410 and ISO 9050. For film, ask for figures calculated for the film on your actual glass, not for the film alone or on a reference glass that is not in your building.

Check the glass before applying film

This is the step most often skipped, and the one with the most expensive failure.

A film that absorbs solar energy makes the glass in the sun run hotter. The edge of the pane, held in the frame and shaded by it, stays cooler. That temperature difference puts the edge of the glass in tension, and glass breaks from the edge. The risk is higher with:

• annealed glass, which has not been heat-treated
• tinted or already absorbing glass, which is hot before any film is added
• large panes, and panes partly shaded by a column, reveal or neighbouring building
• glass with chipped or damaged edges
• insulating units, where film on the inner pane changes the temperatures inside the unit and at its edge seal

So the first step is identifying what is installed: annealed, heat-strengthened or toughened; single glass or a double-glazed unit; clear, tinted or coated. Reputable film suppliers assess the specific film against the specific glass, size and elevation before applying it.

The second step is the warranty. Some glass and insulating unit manufacturers restrict or exclude cover where film has been applied, and film suppliers set their own conditions. Get both positions in writing. A cracked pane on a high floor is a facade repair, not an interior one.

The landlord owns the facade

In a multi-tenant building the glazing is part of the facade, and the facade belongs to the landlord. Even an internal film can change how the building looks from outside, in colour, reflectivity or a patchwork of treated and untreated floors. Fit-out guidelines commonly restrict films, blinds and anything visible from the street, and external films or shading are facade alterations needing approval. Film applied by a tenant may also be listed for removal at lease end.

Check the building's rules before any of this is priced; see reading the landlord's fit-out guidelines.

Internal blinds versus external shading

Internal blinds are the cheapest and most flexible glare control, and for many floors they are part of the answer. They are weak on heat, because solar energy that has passed through the glass is already inside the room. Light-coloured or reflective-backed blinds handle this better than dark ones, which absorb heat and release it into the room.

External shading is the most effective way to stop solar heat, because it acts before the sun reaches the glass. It is also a change to the building's appearance and structure, which puts it in the landlord's or owner's hands. For an owner-occupier planning facade works, it belongs in that conversation.

When replacement is the real answer

Film and blinds treat the symptoms of glass that is otherwise sound. Replacement becomes the realistic option when:

• The glass has failed. Misted or fogged insulating units have lost their edge seal, and cracked panes are a safety matter.
• The problem is not the sun. Heat loss, noise and condensation are not solved by solar film. They need a better unit, such as an insulating unit with a low-e coating.
• Film is ruled out. A thermal stress assessment or warranty position may leave no safe film option.
• The owner is planning facade works anyway. Then the glazing specification should be set as part of that project, not patched floor by floor.

Replacement is a facade project. The existing frames must be checked for whether they can take the new glass, its thickness, weight and the wind loads it will carry, and that check belongs to the facade engineer. The options are covered in facade retrofit and recladding, and how to set a glazing specification in the commercial glazing selection guide. ECBC 2017 sets envelope provisions for fenestration in buildings within its scope; whether and how they apply to a particular retrofit is for the project's consultants to establish.

Common mistakes

• Choosing the darkest film. Heat and glare drop, and so does daylight; the lights stay on all day.
• Applying film without identifying the glass. Thermal stress cracking is avoidable only if someone checks first.
• Comparing film data on reference glass instead of on the glass actually installed.
• Ignoring the warranty position on insulating units until a unit fails.
• Expecting film to fix heat loss, noise or condensation. It addresses sunlight.
• Treating internal blinds as heat control. They control glare; the heat is already in.
• Skipping the landlord. A film the building will not accept has to come off.

What to ask before choosing

• Is the main problem heat, glare, UV, heat loss, noise or condensation, and on which elevations?
• What glass is installed: annealed, heat-strengthened or toughened; single or insulating; clear, tinted or coated?
• Has the film supplier assessed thermal stress for this glass, pane size and orientation, in writing?
• What are the SHGC and VLT of the film on this glass, and how were they determined?
• What do the glass or insulating unit warranties say about applied film?
• What do the landlord's guidelines allow, and must it be removed at lease end?
• If replacement is considered, can the existing frames take the new glass?

Standards referenced

Energy performance provisions for building envelopes, including fenestration, in ECBC 2017; luminous and solar characteristics of glazing determined under EN 410 and ISO 9050; energy and light considerations in the use of glass in buildings in IS 16231 (Part 2). Glass performance, thermal stress suitability, frame capacity and any statutory requirement for a specific facade must be established by the project's facade engineer, consultants and the relevant manufacturers; nothing on this page replaces that assessment.