If there is any rot in the substrate around the window area it should be repaired prior to siding installation. If not, water will be trapped and will rot the frame and wood.
Start at the bottom of the wall and work your way up, overlapping each piece as you go.
Building Paper
Building paper is the first layer of protection on your window siding project. It keeps water vapor that might get past the siding from saturating the sheathing. This helps prevent rot, mildew and mold. It also inhibits air leaks from entering and causing drafts. Its breathable nature allows it to regulate temperatures inside your home and lower energy bills.
In the past, tar felt paper was used for this purpose and was commonly referred to as building “felt”. It was black in color and could be bought in different strengths depending on its thickness (15lb or 30lb). However, today’s Tyvek and Typar house wraps are much lighter weight and much wider than traditional tar felt. They are more breathable than building paper and roof felt, which makes them even better choices for protecting your clapboard, vinyl or masonry sided walls from moisture and air infiltration.
When properly installed, a layer of building paper (and a waterproof flashing) will ensure your new window is watertight and secure. It is important to overlap the building paper by six inches horizontally and vertically on the sill plate, and to use a waterproof caulking along all penetrations to keep out moisture.
Although this is a great way to prevent water infiltration, it has its drawbacks. Tears and rips can occur in the paper, which diminishes its ability to function as a weather resistant barrier. It also creates a barrier that blocks oxygen from reaching the wood sheathing which can cause the structure to deteriorate.
Using the proper installation techniques for both the building paper and the flashing will help to protect your windows, as well as your entire home from damage. The best method for trimming the edges of the building paper and flashing is to use tinner’s snips. These are small scissors with a pointed end that can be used to trim any sections of the materials you don’t need. Alternatively, you can score the material with a utility knife at the point where you wish to cut it and snap it off along the scored line.
Flashing
Flashing is a sheet of impervious material used to prevent water penetration into walls and to direct moisture away from the wall. It is especially important at junctions, such as roof hips and valleys, joints between the roof and vertical walls, dormers and chimneys, vent pipes, window openings, and so on. Flashing is typically made of galvanized steel or aluminum, and is installed on top of the building paper. It is installed in a manner that is consistent with the construction of the roofing system to ensure it will seal the area effectively.
Before installing the siding, it is important to remove any wood trim that is damaged or rotted. It is also helpful to remove any downspouts, moldings, lights, decorations, or other attachments that might get in the way of your work. Lastly, make sure to scrape all old caulk off the window and door frames before attempting any new installation.
To begin the window installation process, first apply a bead of sand-resistant silicone sealant around the frame. Then install a piece of flashing, which should extend up and over the nailing fin and be secured to the wall. The flashing should be cut from a 5-inch strip and should overlap the row of siding that is below it by 2 inches. This will help to keep moisture out of the walls, and it will also allow any water that makes its way inside the siding to be directed to the weep holes that are designed for this purpose.
If you are installing a new dormer window, or a window in a wall that projects out from your home, it may be best to use step flashing, which is typically installed as multiple rectangles of sheet metal bent into an L-shape and layered together like fish scales. This will help to create a solid waterproof barrier that is durable and that will not allow for any gaps.
After the flashing is installed, a drip edge should be attached to the bottom of the window, which will help to further protect it from moisture and that will allow water that has made its way through the substrate and weather barrier to drain away from the wall. A type of flashing called Z-flashing is usually used above door and window frames, which has one end of the flashing held tight against the trim, and the other hanging slightly over it.
J-Channel
A J-channel is a piece of trim that has a shape that resembles the letter “J.” It is used to conceal cut edges of your home’s soffit lengths and fascia boards. It also acts as a water channel to prevent moisture from getting into your home.
Using a J-channel is important when installing vinyl siding around your windows and doors. It will keep the siding from cracking or fading and will create a smooth, clean appearance. It is also a good idea to use it when installing soffit panels on open eaves. This will help prevent the soffit from being clogged with debris, which could lead to rotting or leaks.
To install the J-channel, first cut it to the appropriate length with a miter saw or tin snips. Then, put on your gloves and place the J-channel in a vise or clamp. Using your tin snips, cut the J-channel along the line you marked, creating a flap on both ends. Next, fold each of the end pieces down to the bottom of the J-channel. Afterwards, nail the bottom piece of J-channel to the fascia board, making sure that it laps over the side pieces of the channel.
Now you can start to nail in the rest of the J-channel. Make sure that you nail at the slot centers to allow for expansion. If you are installing a soffit on an open eave, nail one band of J-channel every 8 to 12 inches. If you are installing a soffit over an enclosed eave, then you may need to add a second band of J-channel at the corners of your roof.
You should also consider adding a layer of substrate over the J-channel to protect your home from moisture damage. A popular material to use for this is oriented-strand boarding, which is similar to plywood. Once the substrate is in place, you can install a weather barrier on top to further protect your home from moisture. A weather barrier is a great way to increase the longevity of your vinyl siding and will help you avoid costly repairs down the road.
Undersill Trim
Many home improvement contractors overlook the importance of substrate and a weather barrier, especially around windows. Without them, moisture can seep into the wall and cause rot and damage. If you’re considering having your siding replaced, be sure to look for a contractor that uses them. If your siding contractor claims to use them but doesn’t show proof, that should be a red flag.
When working with vinyl siding, there are a few special trim pieces that hold everything together or hide the edges and ends of your panels. These include undersill trim, J-channel, and metal head flashing.
Undersill trim — also known as cellwood vinyl finish trim — secures the trimmed end of siding panels under windows and other horizontal projections. It’s similar to the fascia trim that’s installed at the eaves, and both should be stained or painted to match your home’s exterior.
Building paper is a layer of waterproof protection that prevents water from getting behind the siding and damaging the wood framing beneath. It’s important to install a full sheet of it at the bottom of your wall, with upper pieces lapped over lower ones and taped over any tears or holes. Then, the undersill trim should be secured to the underside of the siding panel and the nailing fin with flashing tape.
Once you’re ready to install your siding, you should start at the bottom and work your way up. This keeps the rain and wind from penetrating your house’s structure and causing damage to the walls and insulation. It also provides the best coverage to your window trim and sills.
Before you begin, make sure to power wash the entire area and remove any caulk that’s holding your siding in place. This will ensure that any paint or stain you’re using doesn’t trap dirt and residue underneath, which can lead to uneven or bubbling. Then, before you begin working with the vinyl, you’ll want to apply a primer and two coats of stain or paint. This will provide a final layer of protection and make it easier to maintain.