How to properly seal a vinyl retrofit window

These days a lot of homeowners are replacing their old windows with vinyl windows using the retrofit style of window frame. This is particularly true in the west, and specifically, in California. The number one arguement that I have heard against using the retrofit method, is that it is susceptible to water leaks. Well, that's true if you don't do it properly. But, if you do a complete tearout of your old window down to the studs, you're going to have water leak issues there as well if you don't install the new window properly. So I think that arguement is, well, all wet. So, let me tell you the best way to install your retrofit windows that will ensure that water cannot get in.
There is an old song that goes, "It never rains in California, but girl don't they warn ya, it pours, man it pours". For those of you in California, you know how true this is. While California doesn't get a lot of annual rainfall, when it does rain, it can come down in buckets due to the close proximity to the ocean. So, you want to be sure that your windows are well sealed. If you are installing retrofit frames against a stucco house, you want to put a thick bead of sealant right on the outside face of the old window frame, all the way around. Latex caulk should work fine, but if you want to spend a little more to get the best sealant available, use 100% silicone. Depending on the number of windows you will be doing, this extra cost can add up. You pay approximately $1 for a tube of acrylic latex caulk, and $4 or more for a tube of 100% silicone. You are going to use 1-3 tubes per window, depending on the size. So you can see how it could add up. Here is a trick that I used to do to save a little money; The most vulnerable part of your installation is the top of the window, because gravity will have the water running down from the roof to the ground. It's not likely that water is going to find it's way through the sides or bottom. So, I used to carry two caulking guns, and load one with the silicone, and the other with the acrylic caulk. I would run the silicone accross the top of the old frame, and caulk the sides and bottom. Then, put your new window into the opening and have a helper hold it firmly in place while you plumb and level it, then screw it into place.
After you have the window completely installed, your final step should be to caulk where the retrofit lip meets the stucco. Here again, I used to use white silicone on the top, and caulk on the sides and bottom. You now have a double barrier against water infiltration. After about a week, check the sealant around each window for signs of cracking. Because stucco is usually uneven, there could have been gaps that were larger in some areas than in others. If you don't force the caulk into the gap to completely fill it, the caulk can sag before drying, causing a crack to form. Simply recaulk over any cracks that you see. You can check the silicone on top as well, but because silicone dries like a rubber substance, you shouldn't see any cracks there. OK, what if the replacement windows are going between wood trim surrounding the opening? If you are using the retrofit lip, and trimming it to fit between the wood, then you still apply the heavy bead to the old frame before installing the window. But, instead of sealing where the retrofit lip meets the stucco, you seal where it meets the wood. Then, you want to be sure to seal above the window, where the top piece of wood meets the stucco. Again, use silicone up there. Now, no water can run down the stucco wall and get under the top piece of wood.
Sometimes, though, you might decide not to use a retrofit style frame between the wood, choosing a block replacement frame instead. If you choose to do it this way, you have to add trim to the outside. You still want to apply the sealant to the old frame, then apply your trim so it contacts the new window as well as the sealant on the old frame. If you follow these procedures, you won't have to worry about any water penetrating into your home, I don't care how hard it pours!