In the scene right after they board up all the windows Mel Gibson comes out of a room and closed the door toward him. He then boards up that door. The problem is that the door doesn't open out it opens in so if the aliens were in that room they could open the door and climb through the board. What good would that do in stoping them? Not a darn bit!
You must remember that Graham and Merrill were panicking. They probably didn't give a thought to the fact that the door opened inward, they were just trying to cover every entrance. There is another area they forgot about - the door to the attic. This is included on the DVD under deleted scenes. They only think of it when they hear the aliens banging on the attic door . . .
A character being an idiot is not necessarily a mistake in the script. The Graham Hess character is indeed an idiot; the film shows repeatedly that he doesn't think things through (like his oscillations regarding faith), and after all, he is in a panic. He could have tried to board up the house with crepe paper or aerosol pudding -- it certainly wouldn't have worked, but his mind doesn't function properly, so he tries anyway. Actually, deterring the aliens with aerosol pudding would have been a much more convincing ending than deterring them with H2O.
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