There are numerous window treatments available on the market today that are labelled and sold as block out products. By definition, the term “block out” means allowing no light to pass through.

Note that the definition specifies “no light.” If indeed even a small amount of light passes through a product – five per cent or more – that product is then categorised as “dim out” or maybe even “translucent.”

So what should one expect when purchasing a block out window covering? Obviously you would expect 100 per cent block from light. However, one needs to consider the actual construction and installation features to truly determine the effectiveness of a block out product.

Let’s look at roller blinds, which are simple, cost effective and minimal in style. Vast patterns and colours are available. If manufactured as a chain control or motorised blind without side channels, you must take into consideration a gap of about 11 to 13 millimetres in either side at the top that accommodates the pin and chain mechanisms.

If roller blinds are to be installed side by side, this gap can increase. The gap goes all the way down as the blind drops. Of course, these gaps will allow light to pass through uninhibited, thus affecting the true block out effectiveness, regardless of the block out capabilities of the fabric itself. You can minimise the gaps by adding side channels, but this can be expensive and ugly.

Plantation shutters are another popular product that is commonly misrepresented as block out by unscrupulous sales people in the window covering industry. If shutters were made using one plank of wood, rather like a door, there would be no issue. However, they are constructed of moving parts that allow slivers of light between the horizontal blades, as they are not necessarily designed to shut tight.

Also, where the blades meet the sides of the panel (stiles), small gaps are present. Gaps appear around the frame as well. Most shutters should never be sold as a 100 per cent block out solution.

Fabric products such as curtains and roman blinds can be block out if the fabric is coated or lined in a 3-pass or equivalent lining. Romans need to be situated as close to the window as possible, and will preferably overlap so that no gaps appear at the sides. If they are installed inside the window frame, expect five-millimetre gaps at the sides.

Curtains are the best block out solution, especially if teamed with a pelmet that will stop light entering from the top, and returning the curtains back to the wall to stop light gaps at the sides. Keep in mind that some coated fabrics will display stitch holes that can allow light to penetrate annoyingly, effectively negating the true block out effect.

So if you need absolute darkness – perhaps in a theatre room or bedroom – and ask for a block out window treatment, make sure you are aware of the above mentioned anomalies that will have an effect. You might even prefer to consider using two products in tandem to get the desired outcome. It pays to be well informed.