Screens near you

Approved Document B Volume 1 sets out what an emergency escape window has to achieve, and the wording is unhelpful to anyone planning to cover it. Windows providing emergency escape should have an unobstructed openable area complying with all of the following: a minimum area of 0.33m²; a minimum height of 450mm and a minimum width of 450mm, where the route through the window may be at an angle rather than straight through; and the bottom of the openable area a maximum of 1100mm above the floor.

It goes on to address what may be fitted to such a window. Locks, with or without removable keys, and opening stays with child-resistant release catches may be fitted to escape windows, and windows should be capable of remaining open without being held. Notice what that list contains and what it does not: locks and stays are expressly contemplated, and a fixed mesh panel across the opening is not.

The practical rule that follows is simple. On any window that serves as a means of escape — and in most British houses that includes the bedrooms — a screen must be removable or openable from the inside, quickly, in the dark, by whoever sleeps in that room, without tools. Hinged and magnetic screens that swing or pull clear satisfy that. A fixed-frame screen screwed into the reveal does not, and should not be fitted there. If you do not know which of your windows are escape windows, that is the question to answer before ordering anything.

Approved Document F Volume 1 requires a system for purge ventilation to be provided in each habitable room, capable of extracting at least four air changes per hour per room directly to the outside, delivered either through openings such as windows or doors, or through a mechanical extract system. Where it is delivered through openings, minimum opening areas apply: for hinged or pivot windows with an opening angle of 15 to 30 degrees, one tenth of the floor area of the room; for hinged or pivot windows with an opening angle of 30 degrees or more, and for opening sash windows and external doors, one twentieth of the floor area of the room.

Those figures are based on BS 5925 and assume single-sided ventilation, an urban environment, a wind speed of 2.1 metres per second and a temperature difference of 3°C between inside and outside air. Approved Document F also states that hinged or pivot windows with an opening angle of less than 15 degrees are not suitable for purge ventilation at all, and that if smaller openings than those in the table are specified, expert advice should be sought to ensure the design complies.

A mesh screen does not change how far a window opens, so it does not change the opening area in the arithmetic — but it does reduce the air that actually passes, and a fine mesh reduces it more than a coarse one. On a room that was designed close to the minimum, a heavy screen across the only opening is a real reduction in purge capability. Ask the supplier what the mesh's free area is, choose the most open mesh that keeps out what you are trying to keep out, and be cautious about screening every window in a room rather than some of them.

Screens are usually reversible and internal, which keeps them out of most planning arguments, but appearance is still the trigger where a building is protected. The Planning Portal's position is that in a listed building you need listed building consent for any significant works, internal or external, and that in a conservation area, national park or area of outstanding natural beauty there may be additional restrictions, with councils able to make an Article 4 Direction withdrawing permitted development rights. A screen visible from the street on a listed elevation is worth a call to the conservation officer.

The more common obstacle is physical damage rather than planning. Any screen that is screwed into a frame puts holes in it, and on a uPVC or aluminium frame those holes can breach the frame's own drainage and weather detailing, while on a historic timber sash they are simply damage. Magnetic and tension-fitted screens avoid the issue entirely and are the sensible default on anything old or anything not yours.

For renters, that last point is the practical one. Screwing a screen into a landlord's window frames is an alteration, and one that is visible at check-out. Magnetic, tension or freestanding screens leave nothing behind, need no permission and come with you when you move. If a fixed screen really is necessary, ask the landlord in writing first and keep the reply.