Installing new windows

[gregs]

I will be removing some old wood sliding windows on my house and replacing with new vinyl sliding windows.The house is sided with hardboard siding. After removing the old windows I will have appx. a 1-1/2"-2" gap around the new window on each side. I was thinking of filling each side up with plywood to bring the depth even with the siding. Since the siding will no longer butt against the new windows on all four sides, I was considering putting up shutters on the sides to cover up the plywood. And build a window flower box to cover the plywood on the bottom. Does this sound like a good idea? Any thoughts on what I could use to cover the gap on top? I also do not know what I will find under the siding. If there is nothing under the siding should I slip some roofing paper behind the siding, and if so, how far behind the siding? Or can I just use flashing tape directly on the wood?

[Pulpo]

Home Depot sells some vinyl casing in long pieces, about 1" x 4" I used them to box windows on the outside along with caulking. Flower boxes and shutters are a lot of work.

[Just Bill]

I hesitated giving a reply. IMHO, you are going about this backwards. You should order the windows to fit the opening, not make the opening to fit the windows. If you are buying stock sized windows they are probably not worth the time/effort to install them. Proper flashing around the nailing fin and sealing air gaps is critical to keeping out water and wind. Otherwise, you may as well leave the windows open all winter.

I don't mean to overcritical, but windows are a major expense, and should be done seldom, but done wrong can be expensive is energy costs, and possibly damage to the house framing.

[XSleeper]

Padding a rough opening down to fit a smaller window is not usually a big deal, if it's within reason. Ideally, after you pad the opening down, you want about 3/8" - 1/2" of space around the perimeter of your new window to insulate. So you pad the rough opening in accordingly based on that. It sounds like you might be needing to add a 2x4 to each side of the opening or something like that. I like to run a bead of sealant between the original rough opening and any additional framing that is added so that you don't have any chance of air infiltration between the layers.

Once the rough opening has been resized, you may need to add some additional sheathing (plywood) onto the surface of the framing so that your window will be installed at the same level as the existing sheathing. Once that's done, you can apply some strips of felt paper, Tyvek, or a flashing tape such as Protecto-wrap, Grace Vycor, Tyvek Straightflash, etc. to the sheathing. This should be incorporated into the existing water-resistive barrier (WRB- your felt paper or housewrap- if you have such) that is protecting your existing sheathing so that the window will not leak, and so that your sheathing stays dry. Exterior trim is applied over this to cover the 1 1/2" - 2" gaps, but you would NOT just use plywood for your exterior trim, nor would you want to imagine that shutters or a flower box is going to help seal up the window opening and prevent it from leaking or rotting. Selecting something that does not need maintenance would be a better idea. Most big box stores sell PVC trim- either 2" brickmould, or 1x4 or 1x6 stock that can be cut down to the exact size you need with a table saw on site. If you don't mind painting, you could use white wood or cedar or such, but plywood just doesn't work well as a finished exterior trim.

After all that has been done, if you still want shutters or a window box, that's all fine and good. But your primary means of sealing the window, making sure it doesn't leak water or air (like Just Bill mentioned) is primarily your WRB, with some help from your exterior trim and head flashing.