Sheen refers to the reflective quality or luster of paint after it dries. High-gloss paint has the greatest sheen, with semi-gloss paint being a step down in luster.
Flat paint has the least amount of sheen while semi-gloss paint reflects light more than flat paint. If you want an elegant look with subdued lighting, use a flat paint.
On the other hand, if you want to make the room seem less formal or create dramatic light effects, use a semi-gloss paint.
The texture of a glossy paint on a wall is smooth. While flat paint dries evenly, it does not have the slippery feel of semi-gloss paint.
Both types of painted walls are washable, but a damp cloth or sponge will move across a semi-gloss surface easier than one painted with flat paint, which may be significant in a room with high traffic and frequent fingerprints on the wall, such as an entry area or a children's bedroom. The walls in a living room, however, generally require less cleaning and a flat paint is suitable for this area.
Because glossy paints reflect more light than flat paints, they also reveal imperfections in the wall more readily. If your living room wall is in perfect condition, either semi-gloss or flat paint are acceptable.
However, if your living room wall has any cracks, pocks or damage, flat paint is a better choice since it will not draw attention to those defects. You may still want to repair damage to a wall before painting it, though, since even flat paint won't completely hide problems.
Semi-gloss and high-gloss paints are generally more durable than flat paints. Because of this, glossy paints are frequently used on areas of a house that are subject to high traffic, such as trim around doorways.
Living room walls, however, are not as likely to be touched or damaged. For a nice contrast in paints, use a flat paint on the living room walls and a semi-gloss paint on the trim.