^^^ CJW I don't care about the math, in every Cobra-made board I have seen with major damage the big crease started from an area where the glass was not fully wet out. It is a fact we are paying for boards that are missing glue. What makes them strong is the laminated construction. If a bit of glue is missing I'd rather give it every chance not to have internal pressure fluctuations no matter how small. That little unglued bit is gonna get bigger and bigger and bigger....
To other who say it is too hard - Screwdriver lives in box next to mast base. Board comes out of car, reach for mast base and screwdriver every single time. = no forgetting.
You only forget once and you learn from it 
Exocet were clever : the vent plug is in front of the mast track, harder to forget when you insert the mast base !
When it gets bloody hot, sits on top of a dark coloured car, sits in a hot shed, sits in a hot car, when ever it flys. Heat wave on the way from WA, open those vents.
Deano
I know nothing about board construction. Agree, Delam under the feet is not a vent issue. It's where you stomp on it to jibe or should be. I have had many boards soft under foot but delaminate the bottom in cool and hot climates at a later date. Some lasting less than 12 months total. Driving hard into chop for hours on end & jumping will delaminate a light weight board, valve opened or not after use. You can even feel them go all squishy if they let go while sailing.
You have nothing to loose undoing the valve unless your forgetful 
Cut a few boards in half in several places after delam and you will see multiple construction issues as Mark suggested. Mine have always separated about 5 to 10 mm into the blank not at the contact point between lamination & foam. But I have seen egg shell thick rails not laminated to the foam not be the point of failure 
Deano
13N/cm^2 did seem a bit high to me but a long arvos sailing, due diligence was a low priority, thanks Ian, those factors of 10 are shady :) And yeah I just ignored free water for simplicity...as if you always leave your bung in you should have any ^_^ Point even more proven right. Right 
I asked my local board repairer this same question.
He fixed a cracked rail on a board that blew away from me when I turned away from it (I didn't use the board after it happened). A couple of weeks later when the board was ready, he said water was weeping from the crack and advised me not to have the vent open in the boardbag because of the high moisture content in the bag.
I use to open the vent before but not anymore, and definitly remove the screw if it's going on a plane.
Please choose an answer
Manufacturers put vent plugs in boards because
A. They want you to undo them so you forget to put them back in water log your board and they can sell you a new one
Or
B. They put a vent plug in to release pressure and dry board of any moisture that may have got in from hairline fractures you don't know about therefore prolonging the life off your valuable board.
I'd like to believe the option B. It costs manufacturers time and money to fit these plugs, why do it if they don't think it's necessary. It may only save them ten bucks a board to put it in but that's a lot off money over millions off boards.
Problem solved. Buy a Carbon Art, Plug is at the tail (Keeps Barn happy), No screw to undo (Keeps the forgetfull happy) and it breaths through the Gortex(Keeps board happy)
5kgs !!!,, why do they use open cell foam in the construction and not closed cell .. ?
Agree with barn that the vent screw setup is the worst system ever but dont know what else would be any better, maybe a ss set up where the oring wll only get minimum preload to seal and can not be over tightened and damaged.
When you pic up a new board you really need to check that the counter bore where the oring sits is flat with no lumps bumps and also that the thread in the board is square to the seat.
I had a new board that was so out of square that the oring was only sealing on one side and I was amazed how much water it sucked in first time on the water 
Also the last board I bought the thread was so tight it was hard to know how much pressure I was putting on the oring so I ran a 3/8 bsw tap in the hole to loosen the thread.
I always loosen the valve at home and then take it out completely at the beach and blow all the sand off and check the oring before doing it up.
My boards live in my van. I was a "leave the plug done up" guy until my good ol' Naish freeride board totally delaminated on the bottom..Now I'm a "undo the plug" kinda guy.
I remember that 15 years ago when I moved from a production board (bic saxo) to a custom, the new board was delivered without a vent.
I asked the shaper about it, he said "you want one ? no prob"
took his makita and inserted a screw in the board(no o ring).....
he told me "that's bull****, if the board is air thight there is no reason to put a bloody vent on it"
As a matter of fact I kept that board for 8 years in the van, never delaminated nor got heavier...
I'd rather think of the vent plug as an excuse for cheaper construction nowadays, in case things go wrong
After repairing and making hundreds of boards over the last 20 or more years. Here's my advice.
1 If you want to keep your board for years use the vent plug daily.
2 If you get a new board every year, don't bother using the vent! unless you take it on a plane or leave it in a really, really hot car. (don't buy a used board off this person)
3 If you use a vent plug daily and a board bag to store your board. Cut a hole in the board bag above the vent.
I have witnessed a mates board explode while driving up the volcano in Maui. Sounded like popcorn cooking as the sandwich skin was delaminating off the eps core. Some valve screws need to be removed completely ! Not just backed off.
The bottom of the board was as domed as the deck when we pulled it out of the board bag - haha
Had an old exocett ,great board,never opened bung, why bother,.... don't own that board any more,the bottom blew out, only a little at first till the next chop hop ,sounds really funny when a thin sheet of epoxy is slapping the water.was pissed, boards so expensive.phillips head always in car, vent always open when on dry land = less chance of delam. DONT FORGETT TO DO BUNG UP, ITS EASY, just like don't swim with crocodiles.works for me ,might renew o ring right now, cheers.
Confusing..going to buy exact size O rings Mon + order some Tabou ones..I noticed one of the plugs had the filed side and the other two didn't so they mustn't have been the original plugs..Filed the side down but I think I'll feel safer with original vent plugs..Cripes it was better when I was ignorant now I have more to worry about..
I had a small ding in the bottom of my new board, left the bung in and drove up & down the Wollongong escarpment on a very hot day . And the pressure change popped out the ding . Atleast I think that's how it happened .. winning!!!
I'm not sure what's right
But I do agree with barn that having no washer can pinch & twist o ring so it doesn't seal
& so why do vent plugs get made this way ? If it is so critical; the plugs would b designed not to chew up the o rings
My plugs have the one side that is flat but if the plugs have an O ring on how does the air escape the board when the plug is loose?
I learned the hard way.
I nearly always loosen the vent screw when packing up and tighten before going on the water. Only one board has a bag but I leave it unzipped so vapour pressure cannot build up.
Only once have I forgotten to loosen the vent screw before putting the board in my car (I was very tired and in a hurry to get home). The next time I went to get the board out, the bottom of the board was ballooned out and there was a 20cm crack in the bottom. My favourite speed board. Very sad, and very annoying to have been so stupid as to leave the vent plug in.
I am told that in England you never have to worry about vent plugs. In cars in WA summer you do.
