there is another way you can use layer blending modes to add a sky that will work round the edges of intricate shapes like trees and leaves, and it doesn't involve use the magic wand, although you might need to do some masking afterwards.
If you have a very light/white sky in your image, and the objects in the foreground are a lot darker, then you can use the Darken Blending mode to put a sky in.
Here's basically what you do:
1. open image with white sky
2. open sky image
3. drag sky layer into white sky image and position as required. The sky layer should be at the top.
4. change the blending mode of the sky layer to Darken (Multiply can also work)
5. Now any pixel in the sky layer that is darker than the whie sky layer below it will show through.
6. apply a layer mask to the sky layer and mask out anything that's overlapping things it shouldn't.
You should notice that it does a good job of getting right up to the edges around intricate shapes.
You could also, instead of the magic wand tool, make a more accurate selection around the house and plant etc by using a channels method of selection where you:
1. pick the most contrasty colour channel (usually blue)
2. duplicate it
3. use levels to black it out as much as possible leaving a silhouette of the shape you want to select
4. fill in any gaps caused by highlights with the black brush
5. Ctrl + mouse click on the layer inset thumbnail part to select everything (you may need to inverse selection to get the black bit)
6. then copy and paste into the sky image and position, or use the selection for an accurate mask.
But you all knew that didn't you