Interfacings mostly aren’t given the attention as they deserve. But they play a tricky part in sewing and garment construction, and it gets bad if they aren’t chosen correctly and fused properly. 

In schools they usually don’t teach about fabrics and Interfacings and it’s left to students to experiment and learn from their mistakes. Expensive experience, at least it was for me.

There are two types of Interfacings:

• Fusible Interfacing
• Sew in Interfacing

Fusible Interfacings

Fusible interfacings can be fused by pressing with hot Iron.
This type of interfacing provide additional body after the fusing process. Decide on a fusible interfacing only after preparing a test sample and observing the results.

Advantages:  Quick use and you do not need to baste. Shape can be built into a garment as interfacing is fused in place; no need to do hand or machine pad stitching. Adds firmness to an area. Good for fabrics that travel easily

Disadvantages: if not used carefully it may damage some fabrics that cannot be steamed and pressed. If this happens in testing, purchase an interfacing that is recommended for dry iron application or a low temperature application. Fusing adhesive may come through lightweight fabrics, sheers, or open structures such as eyelets. The fusing process may flatten the surface of some fabrics, such as those with nap or crinkle finishes. Test a sample. If fusing distorts the fabric, try low temperature fusibles. 

Fusible interfacing are more commonly used in garment construction and I’ll demonstrate fusible interfacings in my video, that comes in 3 different types:
Woven
Non woven
Knit interfacing

Woven interfacings are very durable and similar to fabrics and have grainlines, So when you cut your fabrics these interfacing should follow the same grainlines of the fabrics. For ex. If you are cutting your pattern on bias your interfacing should be cut on the bias as well. Make sure your interfacing is preshrunk before you apply. This type of interfacings are mostly used on garments, and hope projects, it gives extra body to the fabric, and definitely changes the drape of the fabric. These are the examples of different weight of interfacings and how it affects the drape of the fabric.

Non woven interfacing has no grainlines and kind of fuzzy materials. It’s not as durable as woven and I personally don’t use on garments. I use them mostly for home projects like pillow cases etc, or when i need little extra cushiony feel and thickness to my top fabric.  

Knit interfacings like knit fabric this type of interfacings have stretch. You still can use them for woven fabrics that has little flexibility and little stretch or no stretch at all.  This type of interfacing stretches on cross grains not on straight grain so make sure when you cut it for your patterns, the actual fabric and the interfacing should have stretched in the same way.
They come in  different weights:  lightweight, mid weight and heavyweight.

When you are using lightweight fabric, such as lightweight satin and sheer fabrics such as Georgette and if you need a little support for example around the collar. I recommend choosing the same weight fabrics and interfacing or lighter on interfacing. Do not choose heavy weight interfacing on lightweight fabric, the result can be very bad. 

And always make a little 5X5 example of fused fabrics with different types of interfacing if you have doubts and examine the result before you apply it entirely. Especially if you are using sheer fabrics, silks and stretchy fabrics. 

So let’s say you have a very sheer fabric such as a chiffon! I recommend to double the fabric instead of using any type of interfacing, and if you need volume or stiffness, double it with organza fabric instead.  

Sew In Interfacings

Sew in Interfacings are not fusible but are sewn between top and lining fabrics. These type of interfacings are mostly used for decorative or fancy fabrics that cannot take high heat, like Velvet, lame and metallic fabrics and embellished fabrics with sequins, beads and laces.
When you want to select your  Sew in interfacing, try to take your garment fabric and  lining fabric with you. Layer your actual fabric, lining and your potential interfacing together. Observe the thickness and the way it drapes, fold together.
Make the final decision for a sew-in interfacing based on your desired final look.  Volume, heaviness, if you like it crisper or softer touch and look.

Advantages: Gives softer, more supple shaping. Can be used with woven and knit fabrics.

Disadvantages: May shrink; preshrink before cutting out. May need to be basted in place. A nonwoven may buckle in an area such as a collar where it is completely enclosed because the interfacing is too heavy for the fabric. To avoid this, use lighter interfacing.  May soften after washing. May need machine or hand pad stitching for firm shaping.