[numpad0]

The Sun Protection Factor(SPF) is defined "a measure of the fraction of sunburn-producing UV rays that reach the skin" when applied as instructed and only over a defined period, usually 2hr[1]. That means there is twofold difference even between a pane of glass(<1% whole body) and correctly applied sunscreen(~2% for 2hr @ SPF50), before taking walls into accounts.

1: https://en.wikipedia.org/wiki/Sunscreen#Sun_protection_facto...

[LorenPechtel]

This. Note that even single-layer garments generally don't have a protection factor of 100. Simply being outside in warm weather will give you more UV even if you use sunscreen perfectly--and I don't believe perfect use of sunscreen happens in the real world. I don't care how well you apply it, the protection will be degraded by movement. It will be degraded even more by touching things.