# Solar Control Film vs Replacing Office Glazing

> Solar control film, secondary glazing and replacing the glass solve different problems. Film can cut solar heat gain, glare and UV on existing glass, but it lowers daylight and can raise thermal stress on some glass types. Secondary glazing mainly adds insulation and sound reduction. Only replacing the glass or insulating unit resets the glazing's performance, and the frame must be checked first.

## 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

| Option | Solar heat gain | Glare | UV | Daylight | Main caution |
| --- | --- | --- | --- | --- | --- |
| Solar control film on existing glass | Reduces, by an amount that depends on film and glass | Reduces | Reduces substantially | Reduced, more so with darker films | Thermal stress on some glass; warranty terms; appearance from outside |
| Secondary glazing (an inner pane or frame) | Little effect on its own | Little effect unless tinted or coated | Some | Slightly reduced | Mainly an insulation and acoustic measure; space and access for cleaning |
| Replacing the glass or insulating unit | Reset to the new specification | Set by the new glass | Set by the new glass | Set by the new glass | Frame capacity; landlord and facade works; longer programme |
| Internal blinds | Limited, as heat is already inside | Good | Partial | Adjustable | Heat still enters; dark blinds absorb and re-radiate it |
| External shading | Most effective, stops sun before the glass | Good | Good | Can be designed to preserve views | A facade alteration, rarely within a tenant's control |

Two glass properties sit behind every row. [Solar heat gain coefficient (SHGC)](/glossary/shgc)
is the fraction of solar energy striking the glass that ends up inside the room.
[Visible light transmission (VLT)](/glossary/visible-light-transmission) 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](/glossary/double-glazed-unit-dgu);
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](/blog/reading-landlord-fit-out-guidelines-before-design).

## 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](/glossary/low-e-glass).
- **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](/services/facade-glazing/facade-retrofit-and-recladding),
and how to set a glazing specification in the
[commercial glazing selection guide](/guides/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.

## Frequently asked questions

### Does solar control film actually reduce heat in an office?

It can reduce the solar heat that passes through the glass, which is the main source of heat at a sunny perimeter. How much depends on the film, the existing glass and the orientation, so ask for performance data calculated for the film on your actual glass. Film does little for heat conducted through the glass on a hot, overcast day, and it does nothing for heat from people and equipment.

### Can window film crack the glass?

It can increase the risk. A film that absorbs solar energy makes the glass in the sun run hotter while the edge held in the frame stays cooler, and that temperature difference creates stress. Annealed glass, tinted glass, large panes, partly shaded panes and glass with damaged edges are more vulnerable. The film maker should assess the specific glass and elevation before anything is applied.

### Will film void the glass or double-glazed unit warranty?

It may. Some glass and insulating unit manufacturers exclude or restrict warranty cover where film has been applied, particularly on insulating units, because the film changes the temperatures the unit and its edge seal experience. Check the warranty terms for the installed glass, and get the position in writing from the relevant manufacturer before applying film.

### Do internal blinds do the same job as film?

Not for heat. Internal blinds are good at controlling glare and privacy, but solar heat that has already passed through the glass is inside the room, and the blind mainly redistributes it. External shading stops heat before the glass but is a facade alteration. Film sits between the two: it acts at the glass, but on the inside face it still lets the glass itself warm up.

### When should the glass be replaced instead of filmed?

When the glass has already failed, such as a fogged or misted insulating unit or cracked panes; when the real problem is heat loss, noise or condensation rather than sun; when a thermal stress assessment rules film out; or when the building owner is planning facade works anyway. Replacement needs a check that the existing frames can take the new glass.

### Do I need the landlord's permission to apply film?

Usually, in a multi-tenant building. The glazing is part of the facade the landlord owns, and even an internal film can change how the building looks from outside, in colour or reflectivity. Fit-out guidelines often restrict films, blinds and anything visible from the street. Check the guidelines and ask in writing before ordering.

## Sources

- [ECBC 2017](https://beeindia.gov.in/) — Building envelope - fenestration
- [EN 410](https://standards.cencenelec.eu/ords/f?p=CEN:105) — Glass in building - determination of luminous and solar characteristics of glazing
- [ISO 9050](https://www.iso.org/standards.html) — Glass in building - determination of light transmittance, solar direct transmittance, total solar energy transmittance, ultraviolet transmittance and related glazing factors
- [IS 16231 (Part 2)](https://standards.bis.gov.in/website/published-standards/department-wise) — Use of glass in buildings - code of practice, energy and light

---

Source: https://hagerstone.com/blog/solar-control-film-vs-replacing-office-glazing