Interlining (also known as interfacing) isn't just an optional extra which professionals use to charge you more for your draperies! It has a number of features which make it very useful when incorporated as part of your window treatments.
No. There are a few instances where it's not needed.
One is where you're using a very thick fabric, such as velvet. (Note: there's a difference between drapery velvet and upholstery velvet. With upholstery velvet the pile is locked in place, which makes it harder wearing.)
Or a very thick weave may be just too bulky if you include it.
Another might be if you have curtains at a small window. You don't want them to take up too much room, they're there to provide some color and softness. Including interlining would make them hang over too much of the window area.
But in the majority of cases it will certainly have its benefits. So what are they?
It's used initially for the improvement it gives to the looks of your drapery.
Because it adds 'body', interlining helps the drapes hang nicely. The is especially important when using thin to medium weight fabrics in larger windows. Using lining in these situations certainly helps, but interlining adds extra quality.
Another advantage is insulation. It's remarkable what a difference it makes when used in drapery at a large window. The heat loss, especially in cold weather, is greatly reduced. This can actually make a difference to your heating costs.
One reason I particularly like using it is the finish it allows on hems and sides. Because of the way curtains are constructed, you can achieve a finish without showing any stitch marks on the visible side of the fabric.
Using interlining makes your curtains look better, hang better, keeps you warm and makes your curtains last longer.
But it doesn't stop with your main curtains.
Including it is almost essential for swags, jabots and other treatments. When you come to make your cornice or pelmet (you really are going to start making your own drapery, aren't you?) using a thick version will enable you to get a great looking finish.
They come in many different thicknesses. The thinnest is sometimes called 'Domette'. The thickest is known in some countries as 'bump'.
Most curtains will benefit from a medium thickness interlining. Domette is useful where you don't really need to use an interlining for 'body', but you want some of the other advantages it offers. For example, if you use a thin version with a velvet which isn't too heavy, you'll avoid the stitch marks showing on the hems.
In most instances your drapes will benefit from using it in their construction.