Paper/cover curl is the result of an increase or decrease in the relative humidity in the environment the books are located. The ideal humidity level to keep paper flat is around 45%RH. Print shops control their climate by adding or reducing the amount of moisture in the air to maintain the 45%RH target. Paper/cover curl occurs when the relative humidity number shifts in either direction by 10%. Should the RH# fall more than 15%, you will begin to see what are called “ocean” curls because the paper will begin to look like waves in the ocean.
As you can imagine, curled paper reeks havoc in the print production process and that is why print shops control the humidity level in their printing, binding, and paper storage areas. Paper/cover curl has nothing to do with the weight or quality of the paper. All paper and board stocks will curl should the relative humidity rate fall out of the safe zone. Even the heaviest board stocks are susceptible to curling.
The relative humidity rate fluctuates with the outdoor temperature, moisture content in the air, and geographical location. If you were to leave a book flat on the table over the course of the year, you would see the book cover curl, and return to a flat position, and curl again as the season, and weather, and thus the relative humidity, in the area changes over time. Book collectors control the climates in their storage facilities for this very reason. It’s been the bane of the printing industry since paper was invented.
Usually books will acclimate over time to their new location and the covers will once again lay flat. To help it’s best to leave the books in their boxes or tightly lined up on bookshelves. Some harsh climates like a desert or tropical climate are just not ideal for storing paper.