I'm having a problem wtih mat board warping, not exactly warping, but acting like it has water damage.

I live in a dry environment, Northern AZ. When the mat board is delivered it is fine... then when I take it out of the bag it slowly over a period of about 20 minutes curls and buckles in different directions.

We have a cheap humidity gauge that say sit is about 40%.

It doesn't affect all boards the same, but the board it does buckle is 100% rag, it is buffered...

Does anyone have any ideas as what is going on? I don't think 40% is terribly high humidity.

Thank You In Advance,