Vinyl-Sided Buildings which originally had Light-Duty Aluminum Windows Pockets within the trim on vinyl siding are notorious for collecting gusts of rain, and funneling moisture right through onto the tarpaper protection behind it. With this in mind, envelope engineers will tell you that any exterior siding on a building is considered the secondary moisture barrier. The primary barrier is actually the entire tarpaper/flashing system behind the siding. Most window contractors will happily destroy an important part of this system by completely removing the original aluminum window frames. With that done, they can only provide a bead of caulk at the exterior surface of the vinyl siding (the secondary barrier) as your building's sole "protection" from driving rains. Not only does this caulk quickly break down in the sunlight, but it easily pulls away from the siding as soon as a baseball, soccerball, or strong gust of wind hits the flimsy material. Meanwhile, wind carries rainwater through the pockets in the trim, behind the siding, downward on top of the windows (behind the caulk), and right into the walls. That's why we like to keep the original aluminum frames intact, and encapsulate them with Stucco-Bar Insert Windows which rely on those frames as their four-sided flashing protection. Protective nail-flanges of the original frames are tucked behind the siding and woven in with the tarpaper (assuming the structure was build correctly). As you can see here, the nice sight-lines blend right in with the siding as if the new windows were the original thing.
Stucco Buildings which originally had Light-Duty Aluminum Windows Stucco, can be thought of as a big sponge, because any kind of cementitious siding is very porous. For this reason, you cannot completely remove the original aluminum window frames when replacing these windows. You must incorporate the existing frame into the installation as a four-sided intrinsic flashing with a Stucco-Bar Insert Window that encapsulates the aluminum entirely as you can see in the picture here. The original frames have protective nail-flanges that are hidden behind the stucco and tucked or woven in with tar paper so that wayward water cannot make its way to the wood framing and rot it out (assuming the structure was built correctly). If the original frames are completely removed, then your only moisture "protection" is the caulk between the outside face of the stucco and the new vinyl windows. That doesn't stop moisture from traveling through the porous stucco, right on past your caulk and into the framing at a breach point where the nail-flanged frame once protected! Also see Marblecrete Buildings below...
Marblecrete Buildings which originally had Light-Duty Aluminum Windows Marblecrete is about the same as stucco, except that it has a whole bunch of tiny crushed rocks stuck into it.  Those rocks look just like the ones in a fishtank.  One of our customers even bought some at a pet store to patch in some holes where she had sections of bathtub windows covered over, and they matched up just fine! If you have marblecrete siding, your new windows must be the Stucco-Bar Insert type and installed within the original aluminum frames.  In other words, if the existing aluminum frames are completely removed, then your building will most certainly leak.  It's a guarantee.  We've seen where window contractors have made this mistake again and again, and the story ends the same every time (serious water infiltration).  As you see here, the new window encapsulates the original aluminum frame.
Brick Buildings which originally had Light-Duty Aluminum Windows On brick buildings, most window contractors will completely remove the original window's aluminum frame and all of the protection that came with it. This leaves only an exterior caulk seal to protect the structure from a driving rain.  Caulk that's exposed to sun & other elements will always break down over time, leaving the building unprotected in just a few years. Furthermore, because these windows are recessed, there's only a sliver of frame for them to caulk to at the bottom where the weep-holes are. Worse yet, because their caulk must be applied to the face of the new window, it will eventually collect water at the bottom, directing it into the walls if the caulk first breaks away from the window while staying sealed to the exterior sill! The exterior envelope of multifamily buildings is a serious matter, and for that reason, we keep and encapsulate the original aluminum frame in our installations as our reliable four-sided flashing. As you can see here, our Stucco-Bar Insert Windows look great. With self-designed methods, redundant/hidden caulk seals, and original protection that comes with the existing aluminum frames, our windows are installed to last and last without the worries of water infiltration.
Wood-Sided Buildings which originally had Light-Duty Aluminum Windows Most contractors completely remove the original aluminum window frames when doing replacements on multi-unit buildings.  The only weather protection that this method allows for is an exterior bead of caulk which goes around the window perimeters. Exposed to the elements, this caulk soon breaks down and allows moisture from a driving rain right into the walls. That moisture probably won't be noticed, and can rot out the structural framing over time. If tenants do see signs of it, they might not even report the news. In contrast, we prefer to install Stucco-Bar Insert Windows around and within the original aluminum frames (with their original protective nail flanges still intact- hidden behind the siding). Our caulking is a hidden head-gasket seal that's protected from the sun & other elements. If Stucco-Bar Insert Windows were to leak, water's easiest path is straight to interior of the home, visibly down onto the windowsill or floor. This better-allows you to learn of a leak as soon as possible so that it can be fixed right away. As far as their appearance goes, you can see that our Stucco-Bar windows trim a building out nicely with simple, clean edges.
"New Construction" Installation The window replacement method that comes with the most protection happens to also be the most expensive. That is to first remove all of the original siding and all of the windows on a building, and then start over from scratch. Of couse, normally it's done in stages so that residents don't complain about all the extra fresh air for so many days. This method allows us (your window contractor) to apply a four-sided flexible-flashing system before installing the windows in a head-gasket bed of caulk. Then the siding contractor can come in behind us to apply more flexible flashing over the nail flanges with a metal cap at the top, and weave the entire assembly in with the new tarpaper on the wall before installing the new siding. Bulletproof (No, it won't really stop bullets).
Replacing Old Wood-Sash Windows When replacing old wood-sash windows, usually we only remove the existing sash (openers), and install new vinyl windows within the same space, reusing the interior wood trim.  This helps keep the original architectural look inside and out.  We employ many of our own fashioned techniques that help keep a super clean look around the perimeter while improving the integrity of the exterior seal.  Usually, traditional muntin or grid patterns in the glass can be closely matched in order to render an old-world look.
Metal Panning When replacing old wood-sash windows on brick buildings, we are often asked to cover the old exterior wood brickmould trim & sloped sills with a metal cap to avoid future painting.  As you can see here, it creates clean lines and nicely compliments the building. Heavy-gauge metal with some of the most durable finishes in the industry is what we offer. Here again, we use special methods we've created for rendering a clean look with a confident exterior seal.  Virtually every exterior-panned window installed by any other contractor might look good for a few months, but they always get ugly when the caulk they've applied begins to weather.  Walk around Queen Anne or Capitol Hill and see for yourself. They either look great, or they look like nightmares! The ones that look nice are most likely the ones that we installed.  It's a difficult installation that truly requires a skilled crew, the right products, and an experienced set of well-thought-out methods.