Minecraft – Cursed earth from extra utilities does not spawn mobs

Mobs don't spawn on glass, ice, half-slabs, and stairs. Your loft should be safe as per the minecraft wiki.

Spiders can climb glass walls, so you may want to add a small overhang or a row of ladders if there's an entrance.

I'm no expert at Java, so this isn't a decisive answer, but I figure it can't hurt to post the code here anyway for others to decipher. From SpawnerAnimals.java:

(Comments mine)

private static boolean canCreatureTypeSpawnAtLocation(EnumCreatureType enumcreaturetype, World world, int i, int j, int k)
    {
        if(enumcreaturetype.getCreatureMaterial() == Material.water)
        // If it's a water creature...
        {
            return world.getBlockMaterial(i, j, k).getIsLiquid() && !world.isBlockNormalCube(i, j + 1, k);
            // Make sure it spawns in water
        } else
        {
            return world.isBlockNormalCube(i, j - 1, k) && !world.isBlockNormalCube(i, j, k) && !world.getBlockMaterial(i, j, k).getIsLiquid() && !world.isBlockNormalCube(i, j + 1, k);
            // Makes sure the block it's spawning on is opaque, that it's not water, and that there are at least 2 blocks of air overhead.
        }
    }

It does not appear that it checks whether the mob is spawning underground or not.

Anecdotally, wandering around for a while with an Xray texture pack produced only surface mobs, so I'm not sure what other guards might be put in place to prevent underground spawns.