Reading a mesh openness number

Every screen mesh is a woven fabric with holes in it. The openness factor is simply how much of the surface is hole rather than yarn, expressed as a percentage. Almost everything you will notice about living with a screen follows from that one number.

More openness means more air through the weave, a clearer view out, and more daylight. It also means less sun blocked and less glare cut. An insect mesh sits at the open end of the range: its job is to stop bugs while changing as little else as possible.

If the complaint about your porch is mosquitoes rather than heat, open is what you want, and buying a dense solar mesh to solve an insect problem will leave you sitting in a dim room.

Less openness means more solar gain stopped, less glare, and more privacy — at the cost of view and airflow. Solar meshes are published across a range of openness factors, and the difference between the open end and the closed end of that range is dramatic in practice, not marginal.

This is the mesh for a west or south elevation that becomes unusable in the late afternoon. It is the wrong mesh for the side of the house you sit on to look at the garden.

At the closed end are privacy fabrics. Twitchell's guidance on their Textilene Dim Out product is worth quoting because it manages expectations better than most marketing: Dim Out is a privacy fabric, not a blackout fabric. At around half a percent openness you should still expect pinprick 'star lighting' through the weave, and visible weave striations. Both are normal characteristics of the material.

If what you need is genuine blackout, a retractable screen is the wrong product and no openness number will get you there.

Openness gets the attention, but the swatch data behind these fabrics carries more than that: roll widths, ASTM test data and shading coefficients differ across the families we specify from — Twitchell Textilene and Dim Out, OmegaTEX, Phifer SunTex and SheerWeave among them. Two meshes at the same openness are not automatically interchangeable.

One consequence is worth planning around: on screens, the fabric warranty term follows the fabric, because it is passed through from the mesh manufacturer. Your mesh choice is a warranty choice as well as an appearance choice.

Do not choose from a photograph on a screen — a mesh's whole job is to change how light behaves, which is exactly what a photograph cannot show you. Look at a physical swatch, outdoors, held up against the view you will actually be looking at.

And be prepared for the answer to differ by elevation. A house with a shaded east porch and an exposed west patio has two mesh problems, not one.