20 SPF is not enough for face sunscreen. Dermatologists say that the absolute minimum SPF you should be using, whether on your face or body, is SPF 30.
Using SPF products such as foundation or moisturizer that includes SPF is enough, but make sure that these products are SPF 30 or higher. Layering a foundation that is SPF 15 and a moisturizer that is SPF 15 does not equal SPF 30!