---

📋 Part of our Skylight Guide

Related: Roof Skylights | Skylight Leaks

A glass skylight can be fixed or operable, but that choice is only part of the specification.

For a U.S. project, the skylight also needs to be evaluated for glazing safety, structural performance, water management, thermal performance and operating requirements.

The current AAMA/WDMA/CSA 101/I.S.2/A440-26 (NAFS-26) is the latest North American fenestration standard and covers windows, doors, roof windows, tubular daylighting devices and unit skylights. It supersedes the 2022 edition.

The correct selection process is:

Fixed or venting → glazing configuration → structural rating → thermal performance → water-management detail → installation requirements.

---

Quick Summary

The key point: fixed vs venting determines whether the skylight is also intended to provide ventilation. It does not, by itself, determine the product’s thermal or weather performance.

---

What Is a Glass Skylight System?

A glass skylight system is a roof-mounted fenestration assembly that may include:

• aluminum or other framing
• insulated glass
• safety glazing
• gaskets and sealants
• drainage provisions
• flashing interfaces
• curb or curb-less installation
• operating hardware where applicable
• motor and controls for powered units

The visible glass is only one component.

For commercial projects, the skylight must be coordinated with the roof structure and building envelope rather than treated as a stand-alone glazing product.

---

Fixed vs Venting Skylights

The main functional difference is whether the unit opens.

Fixed Skylight

A fixed skylight does not intentionally open.

It is generally selected when the design objective is:

• daylight
• visual connection to the sky
• reduced mechanical complexity
• limited maintenance

Because there is no operating mechanism, a fixed unit has fewer moving components.

That can simplify maintenance, but it does not automatically mean the whole product is cheaper or more durable. Price still depends on size, glazing, framing, performance rating and installation scope.

---

Venting Skylight

A venting skylight is designed to open.

Possible functions include:

• natural ventilation
• removal of warm air
• purge ventilation
• additional daylight

Some systems can use:

• manual operators
• electric operators
• chain actuators
• remote controls
• rain sensors
• building automation interfaces

But these are product-specific options.

A rain sensor should therefore be described as an optional control feature, not as a standard requirement for every venting skylight.

---

Does a Venting Skylight Really Improve Ventilation?

It can, but performance depends on the building and opening configuration.

A venting skylight may support natural ventilation when there is an appropriate path for outdoor air to enter and leave the building.

The result depends on:

• opening size
• room volume
• inlet openings
• temperature difference
• wind conditions
• skylight position
• control strategy

The term stack effect is therefore better used as a mechanism rather than a guaranteed performance outcome.

A skylight that opens without an effective air path may provide much less ventilation than expected.

---

Skylight Safety Glazing

The original article was too absolute when it said:

“Both types use safety glass because they are overhead.”

The direction is correct, but the U.S. code framework is more specific.

Current model-code provisions for sloped glazing and skylights allow several glazing materials subject to the applicable limitations, including laminated glass, fully tempered glass and heat-strengthened glass, with additional requirements depending on the construction and application.

Therefore, the correct specification principle is:

Specify glazing that satisfies the applicable adopted code, product standard and project configuration.

Common options include:

Laminated Glass

Laminated glass uses an interlayer between glass plies.

When fractured, the interlayer helps retain the broken glass.

This can be valuable where post-breakage retention is an important safety objective.

Fully Tempered Glass

Tempered glass is heat-treated for increased strength.

When it breaks, it generally fractures into relatively small particles rather than large sharp shards.

Heat-Strengthened Glass

Heat-strengthened glass has increased strength compared with annealed glass but behaves differently from fully tempered glass after fracture.

Its use therefore depends on the applicable code, glass design and project requirements.

---

Why “Laminated vs Tempered” Is Not the Whole Glazing Specification

For a professional project, safety glazing is only one part of the glass selection.

Also define:

• glass thickness
• IGU configuration
• Low-E coating
• solar-control coating
• cavity gas
• spacer
• visible transmittance
• SHGC
• U-factor
• acoustic requirements where applicable
• impact requirements where applicable

A statement such as:

“Use tempered glass.”

is therefore incomplete.

A better procurement instruction is:

Provide a tested/rated skylight configuration using glazing compliant with the applicable code and project performance requirements.

---

Skylight Structural Performance

The skylight and glazing must resist the loads applicable to the project.

Depending on location and building type, these can include:

• wind pressure
• wind suction
• snow
• dead load
• roof live load
• thermal movement
• maintenance loads where applicable

The current North American NAFS-26 framework covers unit skylights and provides performance, testing and rating requirements.

For larger or custom skylights, the design may require project-specific engineering rather than relying only on a catalogue product.

This is particularly important when the skylight is:

• oversized
• unusually shaped
• installed at a high elevation
• exposed to high wind
• located in a heavy-snow region
• integrated into a custom roof system

---

Skylight Thermal Performance

A glass skylight should be evaluated using actual performance ratings rather than simply specifying “Low-E glass.”

For U.S. projects, the 2024 IECC states that U-factors for fenestration products such as skylights are determined using NFRC 100, while SHGC and VT are determined using NFRC 200.

This gives the procurement team a much more useful specification:

U-factor + SHGC + VT

rather than:

Low-E + argon

alone.

---

U-Factor, SHGC and VT

U-Factor

Measures heat transfer through the fenestration product.

Lower generally means lower heat transfer under the applicable test/calculation conditions.

SHGC

Solar Heat Gain Coefficient indicates how much incident solar radiation enters the building through the fenestration.

Lower SHGC can be beneficial where cooling loads and solar gain are important, but the correct value depends on climate and building design.

VT

Visible Transmittance indicates the fraction of visible light transmitted through the glazing.

This matters when daylighting is one of the main reasons for installing the skylight.

These three ratings should be considered together.

---

NFRC Certification for Skylights

NFRC’s residential certification program provides third-party verification of skylight performance, including:

• U-factor
• SHGC
• VT
• air leakage
• condensation index

NFRC also maintains a certified product directory that can be used to verify product ratings.

This is useful for procurement because it allows the project team to verify that the manufacturer’s quoted performance corresponds to a certified product configuration.

A real NFRC rating is more useful than a generic statement such as:

“Energy-efficient skylight.”

---

Skylight Condensation

Condensation occurs when an interior surface falls below the surrounding air’s dew point.

For skylights, potential contributors include:

• indoor relative humidity
• outdoor temperature
• glass U-factor
• frame thermal performance
• spacer
• curb detail
• roof insulation
• air leakage around the opening

An NFRC-certified product can include a condensation index, but condensation risk still depends on actual building conditions. NFRC’s certification program includes condensation index as one of its rated performance characteristics.

Therefore:

Low-E glass reduces heat transfer; it does not guarantee zero condensation.

---

Water Management: Fixed vs Venting

Both fixed and venting skylights need a properly designed roof interface.

A venting skylight has an additional consideration:

the moving frame and operating joints must maintain weather resistance when closed.

The complete detail may include:

• curb
• head flashing
• side flashing
• sill/apron flashing
• underlayment
• membrane
• drainage paths
• perimeter seals

For a fixed unit, there are fewer moving interfaces.

For an operable unit, weather performance also depends on:

• gasket compression
• operating hardware
• sash alignment
• seals
• closure pressure

Therefore, “venting = more leak-prone” is too broad. The more useful statement is that an operable unit has additional moving and sealing components that must be maintained.

---

Current U.S. Skylight Product Standard

The previous article should not refer generically to “AAMA/WDMA/CSA 101” without an edition.

The current edition is:

AAMA/WDMA/CSA 101/I.S.2/A440-26 — NAFS-26

WDMA identifies NAFS-26 as the latest edition and states that it supersedes the 2022 edition. It covers windows, doors, skylights, roof windows, tubular daylighting devices and secondary storm products.

This should be the current reference point for a 2026 U.S. skylight article.

---

Fixed vs Venting: Decision Matrix

This is a better decision framework than simply saying “fixed is cheaper.”

---

Skylight Specification Checklist

---

What to Request From a Glass Skylight Manufacturer

For a B2B procurement package, request:

Product data

• model / series
• dimensions
• fixed or operable configuration
• frame material
• glass build-up

Performance

• U-factor
• SHGC
• VT
• air leakage
• condensation index where available
• structural performance

Glazing

• glass type
• thickness
• interlayer where applicable
• Low-E coating
• spacer
• gas fill

Installation

• curb requirements
• rough opening
• flashing details
• drainage
• sealants
• fastening
• roof interface

Compliance

• NAFS performance rating
• NFRC certification where required
• test reports
• product labels
• installation instructions

---

Fixed vs Venting Skylight: Practical Trade-Off

The important difference is not simply price.

It is:

static enclosure vs operable enclosure.

That distinction affects maintenance, controls, weather sealing and system coordination.

---

Frequently Asked Questions

What is the difference between fixed and venting skylights?

A fixed skylight is non-operable and is primarily used for daylighting. A venting skylight can open to provide outdoor air. Venting products may include manual or powered operators and optional automatic controls.

Which skylight is cheaper, fixed or venting?

A fixed unit will often have a simpler cost structure because it has fewer operating components, but price depends on size, glazing, frame, performance rating and installation. It is better to compare equivalent product specifications than assume a fixed skylight always costs less.

What glass is used in glass skylights?

Depending on the adopted code and configuration, skylights may use laminated, fully tempered or heat-strengthened glazing, subject to applicable limitations. The exact glass construction must be selected for the specific system and project.

Is tempered glass required for every skylight?

No. The applicable U.S. code path can permit several glazing constructions. The exact requirement depends on skylight configuration, size, height, location and adopted code provisions.

Is laminated glass better than tempered glass for skylights?

They solve different breakage-behaviour requirements. Laminated glass is designed to retain glass fragments after fracture, while fully tempered glass increases strength and typically breaks into relatively small particles. The appropriate choice depends on the code and project requirements.

Are venting skylights better for ventilation?

They can provide natural ventilation when the building has an effective air path and the skylight opening is properly sized and controlled. Simply adding an operable skylight does not guarantee effective whole-room ventilation.

Can a venting skylight have a rain sensor?

Yes. Some powered skylight systems offer rain sensors that automatically close the unit when rain is detected. This is a product option rather than a universal requirement.

What U-factor should a glass skylight have?

There is no single U-factor appropriate to every U.S. project. Under the 2024 IECC, skylight U-factors are determined according to NFRC 100 when rated values are used. The project requirement depends on climate, building type and applicable code.

Should skylights be NFRC certified?

Where rated fenestration performance is required, NFRC certification provides third-party verification of performance characteristics such as U-factor, SHGC and VT. NFRC maintains a searchable certified-product directory.

Does Low-E glass prevent skylight overheating?

Not by itself. Low-E and solar-control coatings can change solar and thermal performance, but the correct SHGC, VT and overall skylight configuration depend on the climate and building design.

What is NAFS-26?

NAFS-26 is the 2026 edition of the North American Fenestration Standard, AAMA/WDMA/CSA 101/I.S.2/A440-26. It covers windows, doors, roof windows, tubular daylighting devices and unit skylights and supersedes the 2022 edition.

---

Final Takeaway

A glass skylight specification should not stop at:

fixed vs venting + tempered glass.

A professional U.S. specification should coordinate:

operation → glazing safety → structural rating → U-factor → SHGC → VT → water management → installation → certification.

For 2026 projects, the current North American reference point is NAFS-26, while the 2024 IECC uses NFRC 100 for U-factor and NFRC 200 for SHGC and VT for fenestration products such as skylights.

The practical procurement rule is:

Do not choose a skylight only by whether it opens. Choose the complete rated system.

That is the distinction between buying glass for a roof and specifying a building-envelope product.