The most imperative feature to look at with regards to frame construction, is how the windows are secured at the corners. There are three principal ways to fasten the corners jointly on your brand new replacement windows: Mechanical fastening, a glue or caulk-like filling, and fusion welding. The method of connection is of overwhelming significance, as this affects the products ability to keep air out of your home.

Mechanical fastening, as the moniker implies, is a method whereby the corners are screwed all together. This is the least air-tight as you might imagine, way to fasten corners since it leaves gaps. Most often, mechanical fastening occurs in aluminum windows, a number of low cost vinyl products, as well as a number of of the newer fiberglass and composite frames.

Window corners are occasionally glued together, similarly to filling a gap connecting two pieces of wood with caulk. As with caulk, these methods, while at first successful, often end up with problems, especially in more humid climates, as the material tends to break down leaving gaps. Most often this method is used in wood windows, although a number of the newer fiberglass/composite windows make use of this in conjunction with mechanical fastening.

These products capacity to resist air leakage or infiltration is of overriding consequence. A double hung window has 12 corners (4 in each sash and 4 in the frame); multiply that amount by the quantity of windows you are replacing and you can start to envision the benefit of a mode of connection that offers a entirely closed, and long lasting seal.

Fusion welding seems to be the finest overall selection when it comes to managing air. Only obtainable on vinyl replacement windows, fusion welding has to do with heating the corners to over 200 degrees and "fusing" the two corners simultaneously. Vinyl shares a pair of characteristics of steel, one being that wherever you fuse two pieces of vinyl simultaneously, it increases the overall strength of the frame, and provides the longest lasting seal.