how long do you intend to foster cats? that fact that you are not planning on keeping this house, and intend on selling, means as long as you keep fostering cats you're just going to have to do this all over again when you go to sell. if its me, I don't put any flooring down. I paint the concrete with Kilz oil based primer, and then paint a color I can stand to live with. Just my opinion, im sure theres something else out there, but its like Mold, just by cleaning the surface wont solve your problem, you need to get eliminate the source.

The short description by cat mom is accurate. To generalize this: A series of rods sticking out of the wall go into holes perfectly aligned to receive them. 1./ Any metal rod sticking out of the wall, made any way. 2./ Any very well aligned hole in your chosen shelf material Here is an example from five years ago. Jackww had metal rods welded onto a flat bar. Then he bolted the flat bar into the wall. This is one way to get bolts into the wall studs. Probably he grooved out the backside of the shelf so that the flat bar would be hidden. His shelves were massively heavy oak used normally as countertop material. See http://www.ikeafans.com/galleries/index.php?n=803 and http://www.ikeafans.com/forums/yes-s-ikea/5498-masterpiece-abstrakt-white.html The strongest method is Metal rods. Other methods work too. They all involve attaching something to the wall which then gets covered by the shelf after the shelf gets slid over it or onto it. This is why most floating shelves are thick. In the Ikea web site and in Ikea stores, see Lack and Besta floating shelves. There are others too. Here is a link that might be useful: LACK floating shelf for $15 at Ikea

These photos and your suggestions are so helpful! I really appreciate it. We have a very large book collection, and I'd like to be able to fit in as many shelves as possible. I'm hoping for two, with the bottom one starting right above the baseboard, but was worried it would look too crowded or strange starting so low to the ground.

This is a 'mini makeover' I did for a client. She painted her bases and I replaced her uppers and did the tilework. Her floating shelves are 12" deep. She did this as a temporary fix until she could remodel the entire kitchen. Although she spent more than she imagined for a makeover, she now loves it so much, she's not ever going to re-do it. So in the end, the makeover saved her a ton of $! click on the link (more info) to see her 1990s before kitchen!