By "writing houses" I suppose you mean an agent. It's impossible. Most agents don't take unsolicted manuscripts. It's best if you enter them into some of the screen writing competitions and see how you do. Slamdance actually offers critiques of submitted scripts, so this might be a good place to start. I've never sold a script, but have optioned a couple. I did it without an agent, simply because I know people (if you can call producers people. hehehehe...just kidding!). You might try sending the scipts to a producer who has made similar films to your script. I've had some minor success (meetings) this way. Before you submit them anywhere, make sure they are actually good, and that they are the best you can make them. Once an agent or producer reads your stuff and doesn't like it, they'll never read anything you submit again (they are just too busy).
Dan