Simple selection and mask

Open both images. Open the Layers palette also.

Drag the clouds layer from its Layer palette to the image f the building.

This action will place the Cloud layer above the background layer.

Drag the Cloud layer to below the House layer.
Ensure that the background layer is not locked.

Select the Magic Wand Tool from the Tools palette.

1. Enter Tolerance: 8. It may be necessary to enter a higher number if 8 proves not to be too low and not having the required effect. If so, then Enter 16 0r 24.
2. Tick both boxes: Anti-alias and Contiguous.
3. Leave Sample all layers un-selected unless you need to select from all the visible layers.

Ensure the the House layer is selected.

Using the Magic Wand select the sky on the House layer.

Hold down the Shift key with the Magic Wand in order to add to the selection. Note the + beside the Magic Wand which indicates that you are adding to your selection.
Hold down the Option key with the Magic Wand in order to subtract from the selection.

When your sky is completely selected, the selection will need to be inverted in order to mask the sky portion of the image.

Go: Select > Inverse.
This will invert the selected area for the unselected areas.

1. Ensure that the House layer is active.
2. Select the Add Layer Mask icon.

This will now create a layer mask on the House layer which will "remove" the sky from the image leaving only the building portion.

Check the mask edges to see if it needs further adjustments. Note the edge line around the chimney pots.

Open the Mask palette.
Select: Mask Edge . . .

Using the Refine Mask tools, adjust the masks edges to fit the building contours.

The mask edges should no longer be visible between the building and sky.

The composite should now be complete as a layer and masked image, and without any destructive image editing.