Putting a hatch without a vent will cause pressure build-up with temperature changes, and can cause delamination and even cracking. What you need is at minimum a vent to vent pressure. This vent has to be left open when storing and transporting, and you have to remember to close it when you take the boat on the water, OR the better alternative is a gore-tex vent. I use them in wood surfboards, and the one I use is actually a vented leash plug (used for tying the board leash, but has a gore-tex vent built in.) Available here:

http://greenlightsurfsupply.com/ventedleashplug.aspx

The Gore Tex fabric allows air to pass but not water. They have been tested on surfboards and proven to work. Just add one to each bulkhead and you will be fine. My suggestion, however, is to utilize all that space by at least building a hatch in the rear bulkhead (easier to reach than the front). Deck hatches for boating are easy to install in flat bulkheads. You will want to open the hatch when storing the kayak to let pressure and moisture out.