One of the beauties of vinyl siding is its ability to install smoothly over most substrates, provided they can be penetrated with traditional screws. Installing over masonite lap siding is simply a matter of levelling the playing field with a layer of fanfold foam insulation, in the same way you would for wood clapboard siding, before installing the vinyl. Vinyl siding is a flexible material that installs best when it is hung loosely rather than attached.
Mark a line along the wall where the bottom of the vinyl siding will run. Use a chalk line. Unfold ¼-inch fanfold foam insulation and cut it to fit on your wall, with its bottom edge along the chalk line. It typically comes in a folded bundle 4 feet high, by 25 or 50 feet long. Cut the insulation with a utility knife to the length of your wall.
Install the foam. Hold it in place, with a helper if possible, with its bottom line on the chalk line. Screw it to the wall with 1 ¼-inch treated deck screws every 16 inches. Run screws so that the heads are flush, but do not punch through the foam. Cut the foam away where it extends over doors and windows with the utility knife. Add rows of foam above this bottom row until the whole wall is covered. Trim to fit where needed with a utility knife.
Attach a starter strip for your vinyl siding along the bottom edge of the foam. Use the same treated deck screws. Run the screws in until they touch the vinyl, but do not dimple. Back out any screws that go too deep, to prevent creases.
Cut and install corner trim at each end of the wall to be covered in vinyl. Attach using treated deck screws every 12 inches along both edges. This trim provides a channel to hold the end of the siding and provide a clean finished look. Cut and install J trim on all sides of doors and windows, where they contact the siding area. Use the same screws to attach, one every 6 inches. Install an end-trim strip along the top of the wall to catch the top edge of the last row of siding.
Position the vinyl with its bottom edge directly beneath the starter strip. Lift up and press the bottom of the siding firmly up to snap the bottom lip of the siding around the lock bead, that runs along the bottom edge of the starter strip. Slide the end of the first piece over until it is in the channel of the corner trim. Screw the siding in place with one screw every 16 inches through the holes in the nailing strip along the top edge of the siding.
Install the next piece, overlapping the open end of the first piece by at least 1 inch. Lift up and snap it into place. Screw it down as for the first piece. Continue adding to the first row with full pieces as far as possible. Measure from the last piece to the corner trim and cut a piece, 1 to 2 inches longer, to fit this space. Use tin snips to make the cut. Snap it into the starter strip, tuck the end into the corner trim and fit the end overlapping the last piece, screwing it down as before.
Add the next rows in the same way, snapping the bottom of the siding into the lock strip running below the screws in the fastener strip of the row below. Start the second row with an offcut piece from the end of the first row to stagger the end joints. Cut out for doors and windows and tuck the ends into the channel of the J trim as you did with the corner trim.
Continue adding rows of full-height siding pieces as far as possible. Measure from the bottom of the lock strip on the last row to the bottom of the end trim at the top of the wall, adding ½ inch. Cut enough siding to this height to cover the last row. Snap it into the lock strip of the last full-height row and tuck the cut edge into the end-trim strip.