Eco Priority Guide: Internal Window Glare Control

 

Overview

While daylighting improves the internal environment and saves on artificial lighting energy, it is also responsible for significant potential nuisance glare. There are a number of common factors influencing the levels of discomfort and/or visual impairment associated with glare in homes and offices, including:

• The contrast between ambient lighting levels and the brightness of the source;
• The contrast between ambient or background brightness and the lighting levels at the task;
• The size, number and location of glare sources with the potential to cause rogue reflections, and
• The length of time that the glare is present.

Enabling occupants to control glare by the use of blinds, screens or other active devices minimises glare by controlling direct sunlight on internal surfaces – including reflections of windows on computer screens. This also minimises eyestrain and discomfort resulting from glare. The key factor in reducing glare is to reduce the contrast levels. Blinds and screens that have a visual light transmission (VLT) of less than 10% are considered to be effective in this regard (GBCA 2006).

 

Eco Priorities

The following issues relate to both potential positive and negative issues associated with each product class:

Priority Order	Fabric: PVC coated  fibreglass	Fabric: PVC coated polyester	Fabric: Polyester solution dyed	Fabric: Polyester metalised backing	Anodised aluminium mini-louvres(interstitial)
1	GHG+	GHG+	GHG+	GHG+	Durability+
2	Resource	Resource	Resource+	Resource+	GHG+
3	Toxics	Toxics	Toxics	Toxics	Resource+
4					Toxics
Issues of concern / Red Lights? *	Potentially: VOCs	Potentially: VOCs

* Issues that are high-concern and are a potential eco-design basis for not using the product.

+ an overall positive characteristic of the product

GHG: production of greenhouse gases, ozone-depleting chemicals

Toxics: toxic and/or persistent and/or bioaccumulative emissions to the environment

Resources: the use of raw resources e.g. oil, metal ores.

 

Making a Decision

Commentary

Thermal factors relating to the fabric alone include: Solar Transmittance (Ts), Solar Reflectance (Rs), Solar Absorbance (As) and Solar Radiation. Optical factors relating to the fabric include: Visual Light Transmittance (VLT) and Openness Factor (OF) and Shading Coefficient (Sc). See explanations below.

• Ts: This factor measures the proportion of solar energy transmitted through the fabric. A low percentage means the fabric performs well at reducing solar energy.
• Rs: This factor measures the proportion of solar radiation reflected by the fabric. A high percentage means the fabric performs well at reflecting solar energy.
• As: This factor measures the proportion of solar radiation absorbed by the fabric. A low percentage means the fabric absorbs little solar energy.
• Solar radiation is always partially transmitted through, absorbed or reflected by the fabric. The sum of all 3 equals 100. Ts + Rs + As = 100 % of solar energy.
• VLT: This factor measures the percentage of visible light coming through the fabric that can be seen by the naked eye. It is related to the amount of light (brightness) a person receives through a glazing system. A low figure shows a very efficient fabric.
• OF: This factor measures the proportion of holes in a woven fabric. This parameter, together with other technical properties of the fabric, should be considered when determining the degree of visibility, and heat and glare control, that the fabric offers. The openness factor can vary slightly from colour to colour in the same fabric, and is often expressed as an Average OF. A low OF indicates that the fabric has a very close weave.
• Sc: This factor shows how effective the fabric is at filtering the heat from solar radiation. It is expressed as a factor between 0 and 1. A low figure means high protection from heat flow.

Other general issues that relate to internal window glare control include:

• Level of incoming natural light: For the same type of fabric, light colours let through more light than dark colours.
• Glare control: For the same type of fabric, dark colours provide better glare control than light.
• Protection against heat gain from sunlight: Light colours installed inside give better protection from the heat than dark colours. For external installations, the reverse applies.
• Quality of outward visibility: The quality of visibility does not only depend on the openness or light transmission, it also depends on colour. Darker colours will provide better outward visibility.

High performance glass vs internal shading:

High performance glass refers to glass that has thermal and visual properties above and beyond that of clear float, laminated or toughened glass. The ‘smart’ properties can be designed to reflect more heat (reflective), emit less heat (low emission or ‘Low E’), or lower overall heat or light transmission (heat/light absorbing). Generally purpose designed blinds and internal glare reduction systems also perform some of these functions and in some cases can do so more effectively than even some high performance glass provided the hardware setup is appropriately configured to provide a well sealed air space against the glass and all edges are well sealed (albeit neither is as efficient as properly designed external sunscreens). This is particularly the case in relation to low levels of VLT.

Decision Making Checklist

1. Does a thing have to be made or used? If so, does it create a net benefit?
2. Fate: Start with the end in mind. If the product is not reusable, fully biodegradable or highly recyclable at the end of life, or facilitating these activities, its not sustainable.
3. Energy: What will the product’s likely net energy balance be over its life? Will it save more energy than it uses?
4. Durability: Does the product embody an appropriate level of durability for its accessibility, criticality and maintenance profile?
5. Biodiversity: Is there a chance that the product has had a negative impact on biodiversity? Erosion of biodiversity is a one-way street.
6. Toxicity: Is the product toxic and or persistent in the environment at any stage in its life cycle? If so, don’t use it.
7. Resources: Does the product use rare resources/ create a net negative flow of resources (e.g. poor maintainability/ high maintenance requirements)
8. Is the product socially sustainable?
9. Does the product, or its use, contribute to delivering synergy benefits in other building systems?

Source: Adapted from Andrew Walker Morison

 

Quick Guide

Aluminium mini-louvres – Interstitial, 0.15mm Powdercoated
For Highly durable Combined effect of double glazing and blind is significantly better thermally than single glazing and internal blind Not able to be damaged Hermetically sealed so do not require cleaning Minimal maintenance required Can be angled upward to allow some daylight while excluding glare	Against Cost of double glazing Additional embodied energy or double glazing and aluminium in wider frame More difficult to access if maintenance required
Fabric – PVC coated fibreglass
For High durability Moderate embodied energy Large color range	Against Not recyclable or biodegradable Pthalate emissions in use (varies with manufacturer some very low) Depending on individual products can contain heavy metal stabilisers Toxic and carcinogenic components required for manufacture Releases organochlorines including dioxin during combustion
Fabric – PVC coated polyester
For High durability Moderate embodied energy Large color range	Against Not recyclable or biodegradable Pthalate emissions in use (although as external elements not a significant issue) Depending on individual products can contain heavy metal stabilisers Toxic and carcinogenic components required for manufacture Releases organochlorines including dioxin during combustion Petrochemical fossil fuel base
Fabric – ‘100%’ polyester
For High durability Moderate embodied energy Fully recyclable Very low VOC Reduced toxic and carcinogenic components required for manufacture	Against Petrochemical fossil fuel base Potentially less easy to clean than PVC under some circumstances
Fabric –  ‘100%’ polyester, metalised backing
For High durability Moderate embodied energy Fully recyclable Very low VOC Higher energy efficiency	Against Petrochemical fossil fuel base Potentially less easy to clean than PVC under some circumstances

 

Further Information

For more detailed information on this topic contact subscribers@ecospecifier.org.

 

References

Green Building Council of Australia (2006), Green Star Office Design v.2 –Technical Manual, Green Building Council of Australia, Sydney.

Lawson, W.L., (1995), Building Materials Energy and the Environment, RAIA, Canberra.

Mermet Australia Pty Ltd (2007), T-Screen 9603 Brochure, accessed at www.mermet.com.au on 30.07.07.