Hi Tia, Silk is a natural fiber so it "breathes" - One would not wear silk (alone) to keep warm, however they do sell silk undergarments as insulators to wear under regular outer clothing. But for fashion purposes it would be more fitting to wear a sweater or vest over a silk blouse rather than a tee under it... since the luxury of wearing silk is how good it feels against your skin.

If it's warmth you are looking for in silk... I believe there is a website called winter silks (or something similar), where they specifically sell silk 'underwear' for the purpose of keeping warm without a lot of bulk.

I hope that helps answer your question.