John, there are several approaches; here's one:
1. Open your building photo in Elements; I'll assume it's a one-layer photo at this point.
2. In the Layers palette, double-click on the layer, which should be named "Background." When you do that, you should get a pop-up dialog box called New Layer, which proposes to change the name of the background layer to Layer 0. Hit OK.
3. Open up your background sky photo. Do a Select>All, Edit>Copy.
4. Return to your building photo. Do an Edit>Paste and now the sky will be showing and you won't see the building except ...