If you are using the buttons given to you by the housing I am confident they will NOT come with the metal spacer. This allows the button to press against the cable, here's an example:
http://www.ebay.com/itm/Power-Button-Metal-Spacer-Volume-Metal-Spacer-Shim-Set-for-iPhone-5-/151351742918?pt=US_Cell_Phone_Replacement_Parts_Tools&hash=item233d4451c6
What I like to do is clean the old button up and reuse it, but you can take the spacer off the old one and glue it to the new one.