Outdoor mirrors can be used to reflect skylight and often sunlight into a room to lighten the room and reduce electric lighting use.
A small 6" deep mirror is unobtrusive, but adds noticeably to the interior light level. A 15" deep mirror can double interior light level. Although twice as much light is not entering the room, the upward reflected light is used more effectively, meeting a white ceiling rather than the darker floor.
Large mirrors are not popular due to poor external appearance, but can be a practical option for off-grid homes, sheds & workshops. For outbuilding use, the mirror can be hinged, thus acting as a fold-up shutter when not in use, and the horizontal mirror is only visible when the building is used.
The mirrors are usually mounted almost horizontally outside windows at cill level. The mirrors slope down at around 5 degrees to avoid glare, and shed water and dirt.
These mirrors also harvest a small amount of heat in winter, and are often enjoyed as cat sunbathing balconies.
Skylight & Sunlight
The main aim of these is to refect in skylight, not sunlight. Skylight is present all day long, on all sides of the house, and gives a diffuse white light on the ceiling.
I found aluminium foil an unsatisfactory reflector. It does not lie flat properly, and causes quite a lot of glare. Silvered mylar on greased sheet material would behave much better, and conventional glass mirrors, which I used, give the best result visually, as being perfectly flat they don't add glare.
Ali foil on cardboard can be held in place as a test to see how much extra light you get, but it does create a lot of glare which doesn't happen with a good quality reflector.
Glass mirrors should be painted on the rear to prevent eventual deterioration.
Sloping at 5 degrees gives no significant glare with very good light yield.
Tilting the mirror up further gives more light, but also glare. Tilting the mirror down gives less light.
A small 6" deep mirror is unobtrusive, but adds noticeably to the interior light level. The light is seen most on the ceiling above the window.
A 15" deep mirror can roughly double interior light level.
Larger mirrors can return more, but each additional inch of depth returns less extra light, as its further away from the window.
The mirrors give a small amount of winter heating payback, reducing primary fuel use slightly. ROI depends primarily on the cost of the mirror: the lower the cost, the better the payback. Use of second hand mirrors can ensure payback in under a year.
These mirrors are more useful on the north side of the house, where interior light levels are lower. If used on the south side, they will reflect heat in in summer. Much of this heat can be blocked by placing a plant on the shelf in summer, so that it casts a lot of shade on the mirror.
Details to Watch
The mirror should be secured well enough to survive storms. This is relatively easy with hinged mirrors, which can be locked shut when a storm approaches.
Mirrors should not be secured by their frames where fitted, such frames are neither strong enough nor durable enough. 2mm galvanised iron wire overlapping the mirror corners is a simple way to make a fixing framework tough enough to survive storms.
An alternative is to design the mirror light and flexible enough that it can become detached in severe weather, eg thin rigid plastic sheet with metallised mylar.
These mirrors should not be used on high rise buildings, as winds are more severe at higher floors, and a normally trivial detachment could have grave consequences.