Short answer: Two numbers drive most fabric choices. The openness factor is the percentage of open area in the fabric—it controls visibility, airflow, and daytime privacy. The total solar factor (TSF) indicates how much solar heat passes through the fabric-glass system—it determines heat gain and is the key metric for comfort.
Openness factor: what it controls
Openness is expressed as a percentage — 1%, 3%, 5%, 10%, etc. — indicating the amount of open space in the weave. Lower values indicate a tighter weave.
| Openness | View out | Daytime privacy | Glare control | Common use |
|---|---|---|---|---|
| <1% (Privacy) | Minimal | Highest | Highest | Bedrooms, high-privacy facades |
| 1–3% | Restricted | High | High | West-facing glazing causes intense glare |
| 3–5% | Good | Moderate | Good | General-purpose range |
| 5–10% | Excellent | Lower | Moderate | Priority View Elevations |
Color: The Counterintuitive Part
Fabric color changes affect performance in two opposing ways:
- Light colorsWhite, beige, and gray reflect more solar radiation. They offer better thermal performance but scatter light, which can increase perceived glare and reduce view clarity.
- Dark colorsCharcoal, black, and bronze finishes absorb rather than reflect radiation, allowing more heat to reach the interior. However, they offer significantly improved view clarity and reduced glare.
Professional Compromise: A fabric with a dark interior and reflective exterior. The outer side reflects heat, while the inner side maintains visibility and controls glare.
GTOT and EN 14501: The standard that ties it all together
EN 14501 — Blinds and Shutters: Thermal and Visual Comfort, Performance Characteristics, and Classification — establishes the European framework that quantifies comfort with measurable metrics rather than relying on subjective descriptions.
g-value (total solar factor)— The fraction of solar energy that passes into the room through both the fabric and glazing. Lower values are better for heat control. Key point: gtot is a property of the combination, not the fabric alone. The same fabric yields different gtot values when installed in front of single glazing versus solar-control double glazing.
Practical reading rule: If a supplier quotes a total value without specifying the glazing it was calculated against, the number is unusable. Request the glazing assumption.
Mesh vs. Opaque: These are different products.
| Mesh / solar screen | Opaque PVC-coated | |
|---|---|---|
| Rain closure | No | Yes |
| View out | Yes | |
| Airflow | Yes | |
| Wind behavior | Passes some gas through | Withstands full wind load |
The wind impact must be stated clearly: opaque fabric presents a solid surface to the wind, so for the same opening it loads the tracks, tubes, and brackets more than mesh does. Fabric selection and wind class are interdependent decisions.
Questions to Ask Any Supplier
- What is the openness factor, and what is the total solar energy transmittance (g-value) for my actual glazing?
- Is the fabric two-tone (reflective on the outside, dark on the inside)?
- Mesh or opaque — and is the wind rating applicable to the fabric I'm selecting?
- What does the fabric warranty cover? Does it include color fastness and tensile strength separately?
- What is the cleaning regimen, and does it include a surface treatment?
- For zip systems: Is the retention bead welded or extruded? Can it be repaired in the field?
Written by the engineering team at GRAND HOMEDECOR (Guangdong Grand Homedecor Technology Co., Ltd.). EN 14501 defines GTOT as the combined fabric-plus-glazing solar factor and establishes visual comfort classes.