Hi Babypig. I have very fair skin always like to apply sunscreen even if my BB Cream of foundation also contains it. That way I don't have to worry if I've applied enough, as I tend to apply my makeup as lightly as I can then layer in areas I need more.
Most importantly, does your BB cream get rubbed off? the thing is moisturizers are suppose to sink into your skin, so they don't disappear or move around or get wiped off during the day, which means the area you covered with SPF are protected with SPF. For make up however, if they come off, their SPF comes off of course, then you are unprotected.
If your BB cream doesn't bulge, it should be ok. The general rule is you MUST wear spf 15 or higher, but no need to go above spf30 unless you are super pale or burn easily.You MUST look for something that protect against UVA and UVB, also termed "broad spectrum". Lastly, spf loose its effect after 2-3 hours. I work in an office so I figured that since it take me 10min to walk to the car and 2min from car to apt, 12min of unprotected sun should be good for vitamin D without burning. But if I plan to spend time outdoors after work, I apply a light layer of sunscreen over the T-zone and cheeks, basically areas that are easy to burn.
BTmod1 said it perfectly! Doubling up your sunscreen will not prolong the protection your receive. If your worried but the summer heat and saving your skin, try a mineral SPF that can be reapplied as often as you need it over your finished makeup.
Check out Peter Thomas Roth Instant Mineral Powder SPF 45.
I usually don't double up on my sunscreen if I plan on being in the office all day. Most days, even if I plan on being outdoors, my BB Cream is usually all I'll stick to. Keep in mind that even if you put another SPF on underneath, you will only get the highest amount of either product. If you plan on being outdoors, definitely opt for a product with a higher SPF! The Dr Jart+ Premium Beauty Balm with SPF 45 might be a better choice
Hope that helps!