Why It Gets So Hot Under a Skylight, and What Actually Fixes It

Most offices with a skylight have one desk nobody wants after lunch. By 1 p.m., the person sitting there has turned their chair, tilted the monitor, or moved to a meeting room. The window a few steps away doesn’t cause the same trouble, even with the same sun on the same building. The difference comes down to which way the glass faces.

That difference also changes how skylight window film should be chosen. Below, we cover why overhead glass runs hot, which skylights can take film, and how dark to go. We also walk through the roof work, using a recent commercial skylight job where we removed old film and installed 3M Prestige 20 Exterior.

Why Overhead Glass Takes More Sun Than Any Window

A skylight faces the sky, so the summer sun strikes it at a steep angle for most of the day. A wall window gets direct sun for only part of the day.

Pacific Northwest National Laboratory runs the Building America Solution Center, and its shading guide puts a number on this. On a midsummer day, a square foot of roof can take about three to four times the solar radiation of a north- or south-facing wall. The same guide says to shade skylights first, then east- and west-facing windows.

That guide is written mainly for homes, but the physics is the same on an office roof in Seattle or Bellevue. The heat lands on whatever sits under the glass: desks, carpet, shelving and people. Glare follows the same path, so screens under a skylight wash out while screens near the windows look fine. Before picking a film for skylight heat reduction, though, you need to know what the skylight is made of.

Glass, Acrylic or Polycarbonate: Check This First

Skylights are made of glass or plastic, and the material decides what can go on them. Standard solar film is made for flat glass, and 3M’s warranty bulletin for flat glass applications states that plastics such as polycarbonate are not covered. It also says film on curved glass voids coverage.

Plastic can react to film, too. In its Silver 15 Plastic product bulletin, 3M notes that some plastics outgas when heated, which can form bubbles under the film.

Most commercial skylights fall into one of four groups. Each one points to a different answer.

• Flat glass panels in a metal frame are the usual candidates for exterior solar film. Wired or laminated glass gets a closer review first, which our article on film-to-glass compatibility explains.
• Curved or arched glass is usually left out, since 3M designs these films for flat glass only.
• Plastic domes, the bubble-shaped units on many flat roofs, are often acrylic or polycarbonate, according to windowfilm.com. American Window Film says film won’t adhere to acrylic dome shapes.
• Flat plastic panels are where people ask whether you can tint a polycarbonate skylight. Standard film is the wrong product here. 3M makes Silver 15 Plastic for some polycarbonate and acrylic glazing, and it recommends testing the panel before a full install.

If you’re not sure which group yours is in, look for a small nameplate on the skylight frame. Windowfilm.com explains that it carries a manufacturer code you can look up to confirm the glazing. A clear photo from below and one from the roof also tells us a lot.

How Dark Should the Film Go?

The first question about skylight tint is usually how dark it should be. Heat rejection changes less across the range than most people expect. The table below uses 3M’s figures for three exterior films on 1/4-inch clear single-pane glass, taken from its Technical Data Sheets Summary.

From the lightest film to the darkest, visible light drops by 50 points, while heat rejection rises by only 14. The figures shift on double-pane and tinted glass, so we read the data for the glass you actually have.

The real choice is about light and skylight glare control. What the space below is used for decides the shade:

• Desks and screens right under the glass usually need the glare cut of a darker film like Prestige 20 Exterior.
• An atrium, lobby or corridor that relies on the skylight for daylight often does better with Prestige 40 or 70 Exterior. Both keep the room bright.

Prestige 40 Exterior sits in the middle of the range. 3M calls it an “excellent film for skylight applications” on its product data sheet. For atrium window film and mixed spaces, we look at where people sit for most of the day and choose for them. Our commercial solar film page covers the wider range.

Why Most Skylight Film Goes on the Outside

Exterior film for skylights stops much of the sun’s energy at the outer surface, before it passes into the glass. Many commercial skylights also sit high over a working floor, where the roof side is easier to reach than the room side. Sloped glass has its own warranty terms and edge sealing rules, which we cover in Inside or Outside the Glass?

Getting Old Film Off a Skylight

Plenty of skylights already have film on them, often put on years ago. From below, failed film shows up as bubbles near the edges or peeling at corners. It can also take on a purple or yellow cast, or a hazy look that dulls the light.

On a skylight, these signs are easy to miss because nobody looks straight up for long. Our guide to when commercial window film needs replacing walks through each one.

Old film has to come off before new film goes on. New film needs clean glass to bond evenly, and a fresh layer over a failing one carries the old defects forward. Skylight film removal means peeling the film and then clearing the leftover adhesive from every panel. If the old film is on the outside, all of that happens on the roof and needs the same dry weather as the install.

When we see old film on a site visit, removal goes into the scope from the start. That way the schedule reflects the full job and there are no surprises on install day.

What Skylight Work Looks Like on the Roof

Exterior skylight work happens on the roof, so roof access comes first. We confirm how the crew gets up, where they can tie off, and who on site needs to sign off on roof work. Installers work on the sloped glass itself using safety lines, which is how our crew handled the job described below.

Weather sets the schedule. Film needs dry, clean glass, and Puget Sound weather doesn’t always cooperate. We plan skylight work around a dry stretch and keep the date flexible until the forecast holds.

The space below usually stays open. Because the crew works from the roof, people under the skylight can keep working through the install. If there are hours when the area must stay quiet, tell us and we’ll plan around them.

What We Did on a Multi-Panel Skylight

One recent job shows how these steps fit together. The building had a large multi-panel skylight, and heat and glare were affecting the people working below. The skylight already had old, worn film, so our crew removed it first to get the new film onto clean glass.

We then installed 3M Prestige 20 Exterior, a metal-free solar film made for the outside of the glass. The crew worked from the roof, on the sloped panels, using safety lines. With people working right under the glass, glare control came first, and 3M lists 76% glare reduction for this film on 1/4-inch clear glass. From outside, the skylight now has a clean, even look, and you can see the photos in the full case study.

Get a first opinion on your skylight. Send us a photo of your skylight from below and tell us what the space is used for. We’ll check the material and tell you whether film is the right fix, and which shade suits the room. We work on commercial skylight film across Seattle, Bellevue, Tukwila and the greater Puget Sound. Contact us to get started.