How to Select a Polycarbonate System for a Daylighting Brief

Most architects who specify polycarbonate do it backwards. They look at the products available, pick one that seems appropriate for the budget and the application, and then justify it against the brief. The result is often a product that technically satisfies the specification but doesn't actually deliver the daylighting performance the building needs.

The correct approach starts with the brief and works towards the product. This post is a framework for doing that — covering the key variables in the right sequence.

Step 1: Define the Space Type and Lux Target

Every polycarbonate specification should begin with a lux target. Not a panel, not a thickness — a performance outcome for the space.

Refer to the IES Lighting Handbook, CIBSE Code for Lighting, or the reference table in our companion post to establish the maintained average illuminance for the space type you're designing. For a standard office, that's 300–500 lux at the working plane. For a school classroom, 300–500 lux. For a warehouse aisle, 100–200 lux. For a hospital ward, 100–300 lux general, 500–1,000 lux at examination areas.

Write this number into the brief before you open a product catalogue.

Step 2: Establish the Orientation and Its Implications

As covered in our orientation guide, the orientation of the skylight or translucent roof determines:

Whether direct sun enters the space and when
The heat gain associated with the aperture
The glare risk and the diffusion requirement

From this analysis, you can establish three things:

Whether you need a diffusing panel (south, east, west-facing: yes in most task environments; north-facing: sometimes, depending on the application)
Whether you need infrared control (south-facing in India: almost always yes; north-facing: lower priority)
Whether an antiglare finish is required (yes for any task environment where occupants may be in direct view of the aperture during direct sun hours)

Step 3: Determine the Required Light Transmission Range

This is a calculation, not a guess. The required LT depends on:

The lux target from Step 1
The available daylight outside (governed by climate and location)
The geometry of the aperture and the space below it (height, floor area, room depth)
The internal surface reflectances

A simplified approach for early-stage design in India's climate:

For horizontal or near-horizontal skylights, a rough target lux at working plane is approximately:

Lux ≈ LT% × external illuminance × (skylight area / floor area)

In a central Indian climate on an overcast day, external horizontal illuminance might be approximately 7,000–10,000 lux. For a space with a skylight-to-floor area ratio of 15% targeting 400 lux:

LT required ≈ 400 / (8,500 × 0.15) ≈ 31%

This is a rough starting estimate — proper calculation requires daylight simulation software. But the calculation indicates that for a 15% roof glazing ratio in this climate, a 40–50% LT panel is sufficient — not 75% or 80%.

Oversizing the LT to "ensure adequate light" without doing this calculation is how buildings end up with glare problems.

Step 4: Select the U-Value Requirement

The thermal performance of the translucent element should be specified based on:

The building's thermal envelope requirements (check the energy code for the project location)
The HVAC load budget for the space (a higher-U glazing element means a larger HVAC capacity needed)
Whether the space is air-conditioned and if so, to what temperature set-point

As a reference:

U-value ≤ 2.5 W/m²K: adequate for most conditioned commercial buildings in Indian climatic zones 1–3
U-value ≤ 1.6 W/m²K: good thermal performance; appropriate for premium commercial, institutional, and healthcare buildings
U-value ≤ 1.0 W/m²K: high performance; appropriate for energy-sensitive specifications or buildings targeting GRIHA/LEED energy credits

Higher thermal performance is achieved with greater panel thickness (more cell layers) in multiwall and multicell systems. The trade-off is cost and panel weight.

Step 5: Match to a System

With the lux target, orientation implications (diffusion/antiglare/IR needs), LT range, and U-value target established, a system can be selected:

For industrial skylights (factory roofs, warehouse skylights, logistics facilities):

Lux requirement: 150–300 lux at floor level
Orientation: often south or mixed
System: Multicell or X-Fix, 16–22mm thickness, moderate LT (40–60%), antiglare or standard depending on task type, U-value 1.6–2.4 W/m²K

For commercial and institutional skylights (offices, schools, hospitals, courts):

Lux requirement: 300–500 lux at working plane
Orientation: considered carefully; north where possible
System: Multicell or X-Fix (16–25mm), with antiglare finish for south/east-facing; Prism for applications requiring diffusion quality; LT 40–60%; U-value ≤ 1.6 W/m²K

For facades (vertical translucent cladding, privacy screens, atria):

Orientation: variable — vertical facades receive direct sun at low angles
System: Snapwall (40mm, 0.96 W/m²K) for dry-fix facade; Vivid for multi-colour design expression; LT dependent on facade function (privacy vs transparency)

For premium daylighting quality (galleries, studios, premium commercial):

System: Prism — its prismatic structure redirects and diffuses direct sun, eliminating hotspots while maintaining high apparent brightness; suitable for south-facing applications where direct sun must be managed without reducing the daylit feel of the space

Step 6: Verify with Simulation

Simulation should not be treated as an optional extra. For any occupied building where daylighting is a design intent, a daylight simulation model using the selected system's tested LT value should be run to verify:

That the target lux is achieved at the working plane under representative sky conditions
That the uniformity ratio meets CIBSE/IES recommendations
That the annual sunlight exposure (ASE) does not exceed acceptable limits for the space type
That the Spatial Daylight Autonomy (sDA) — the proportion of the year when daylight alone meets the lux target — meets the project's ambition

Software for this: VELUX Daylight Visualiser (free, good for basic checks), Dialux Evo (free, detailed photometric analysis), Honeybee + Radiance (open source, full simulation), or commercial packages such as IES VE.

The simulation should use tested LT values from the manufacturer's datasheet — not estimated values or catalogue descriptions. Ask the polycarbonate manufacturer for tested light transmission data per thickness and colour before running the simulation.

The Brief as the Anchor

The selection process described above is only possible if the brief has been established before the product is chosen. An architect who can say "I need 400 lux at the working plane, north-facing, with a U-value below 1.6, and a diffusing panel because there are VDU workstations below" is in a position to select a product with confidence and defend that selection to the client and consultant team.

An architect who says "specify polycarbonate, 16mm, medium colour" is not specifying — they're shopping.

Coxwell provides technical product selection support for architectural projects. Supply your brief criteria — lux target, orientation, space type, U-value, and finish requirements — and we will recommend the appropriate system with tested performance data.