This question is a bit more complicated than it seems. The answer is both and neither. Let us explain.
Your hair shaft consists of a surrounding layer of overlapping cells (also called the cuticle). The cuticle “opens” or “closes” when exposed to hot and cold temperatures.
Washing your hair “with hot or warm water separates the overlapping cells of the cuticle, allowing the shampoo to penetrate the hair’s shaft deeper.
But the same thing that makes warm water good for washing makes it bad for conditioning.” – Alexis Bennett, writer at Self
This is because hot water can dissolve the natural oils (called lipids) as well as remove any conditioners you added to your hair. This is making you waste time and money, and is not contributing at all to taking good care of your hair. On the other hand, cold water “closes” the hair cuticle cells, which can make it great for conditioning and sealing in moisture.
So What Should You Do?
Which is it? Hot or cold? As we mentioned above, both.
“As soon as you step into the shower, rinsing and washing your hair with hot water is a great first step, followed by a cold water rinse.” – Advaitaa Ravi, writer at Skinkraft
Shampoo your hair with warm (not scalding hot!) water to provide a deep, thorough cleanse. After, apply your conditioner, hair mask, or any other moisturizing product you normally use and rinse away the excess with cold water. This will help lock in moisture and close the cuticle, leaving the hair shiny and with less frizz.