Frame

Frame materials for residential windows are usually wood, vinyl (pvc), or fiberglass.
Each has it's own advantages and disadvantages. Wood frames can look nice but have the inherent problems of often needing paint every 2 or 3 years, they can split and crack and, ultimately, rot.

Fibreglass frames sections are strong as linear sections, however, when assembled  into window structures they are, typically, very weak compared to welded (fused) corners of a pvc window. The corners of fibreglass windows are mechanically joined with riveted or screwed sheet metal and will often allow water to penetrate through that corner and into the wall of the house, causing rot. Fibreglass frames will usually require painting every few years.

Most PVC frames are extremely strong, never need paint and, if you like the appearance of wood, can be finished on the inside with wood jamb and casings and even real wood blinds. So, no maintenance and look good...inside and out.

Insulated Frame

Some companies are now making PVC frames with the core of the frame filled with polyurethane insulation. Smart! Most manufacturers make a completely hollow frame and rely on dead air space for insulation. You'd never put insulation in a exterior wall and then purposely leave out a spot, would you?


Frame Seals

These are the seals that provide a wind and water seal between the frame and the sash of the window, and also between the sealed unit and the frame. It's very important that you find a frame where the seals are continuous through the corner of the frame. Some companies make a frame with the seals already in place when they weld together the corners of the frame. The welding process will often cause hard knobs of vinyl in the corners which prevent the sash from closing tight against the seals and, therefore, air leakage occurs.

Hardware

Hardware is pretty basic yet very important. Most window manufacturers don't make their own. Watch for quality. The last thing you need is for hardware to fall apart or attaching screws to come loose (in many frames, once the screw has come out, the hole is often too big for it to hold that screw again. Look for hardware that is secured to the frame with metal reinforcement. These are strips of metal that slide into the frame and all of the hardware screws go through the frame and bite into the steel to provide a much stronger attachment than a simple screw into bare PVC. A world of difference here...much stronger and more reliable method and also makes hardware adjustable at a later date. Otherwise, look for smooth operation, tight fit and a good warranty. Hardware can cause lots of headaches in years to come and be quite expensive to fix.