My dermatologist told me that even when I am at home and not directly hit by sunlight, I must still use sun block because I am still getting hit by the sun's rays. She even told me even when it's raining, which might still have a significant effect on my skin. My skin by the way is just average and not that sensitive. So, I am assuming that if I have to use sunblock when I am at home, then much more those who have a high sensitivity to sun's rays.
I apply sunblock to my face, hands and arms when I am at home. My apartment has a lot of windows and I spend more than 8 hours at my desk that is right by 4 large windows. I figure it is easier for me to use sunblock every day to prevent problems as opposed to having to deal with a problem down the road.
Unless you have very extremely pale skin, I think your dermatologist is giving you bad advice. If you are outside, sure wear sunscreen, but if you are not in direct sunlight you should be fine. Most windows could be said to have an spf factor as they absorb and reflect some UV light but if you are in line with the sun, better safe than sorry. It is true that some UV will penetrate a thin layer of cloud so on a bright cloudy day, you should wear screen. If the clouds are thick and dense enough to rain, they will stop UV rays. If it is sunny and you are outdoors gardening, for example, you may think you are working in the shade but get exposed to the sun periodically - this is usually where the danger lies as people don't feel hot when in mostly shade but get enough sun filtering through to cause problems. If you can look up and see the sun from your position, you are risking exposure but otherwise you are safe.
