25%
Of home heat loss
Through windows and doors
20 yrs
Insulated glass unit lifespan
Before seal failure becomes common
$700
Average window replacement
Per window, installed
3x
Energy improvement
Single-pane to triple-pane upgrade
Frame Materials & Glazing
Vinyl (PVC) frames
The dominant frame material in new construction. Vinyl is a good insulator, doesn't rot, doesn't require painting, and is cost-effective. Quality varies enormously — cheap vinyl windows flex under thermal stress, causing seal failure. Look for multi-chamber designs in the frame cross-section.
Fiberglass frames
Superior to vinyl in almost every performance measure — more stable, better insulator, stronger, and longer-lived. They cost 30-50% more than vinyl. Worth it for windows in high-sun, high-temperature-swing locations.
Double vs. triple pane glazing
Double-pane insulated glass units (IGUs) are the standard. The space between panes is filled with argon or krypton gas. Triple-pane adds a third pane and second gas-filled space. In cold climates (Zone 5+), triple-pane is worth the premium.
Performance Ratings
U-factor
U-factor measures how much heat flows through the window. Lower is better — a window with U-0.20 insulates twice as well as one with U-0.40. In cold climates, look for U-0.25 or lower.
Solar Heat Gain Coefficient (SHGC)
SHGC measures how much solar heat the window lets in. Lower SHGC is better for air conditioning-dominant climates. Higher SHGC is better for heating-dominant climates where you want passive solar gain in winter.
The fogging window problem
Failure Timeline
New window period
Annual inspection of weatherstripping and operation. Lubricate hardware. Inspect exterior caulk.
Watch phase
IGU seal failure begins, especially south and west facing units with high solar exposure. Weatherstripping degrades.
Active evaluation
Expect seal failures. Check for fogging in all panes. Evaluate whether IGU replacement or full window replacement makes more sense.
Full replacement window
Most vinyl windows in this age range have significant seal failure or frame degradation.
Inspection
The paper test is the most useful quick check. Close a window on a piece of paper. Try to pull the paper out. If it slides freely, the weatherstripping is no longer sealing. Check each window pane for fogging in bright daylight or with a flashlight held at an angle.
Replacement Guide
Questions to ask your contractor
"What is the U-factor and SHGC on the windows you're recommending?"
Any serious window contractor should be able to answer this immediately. If they can't, they're selling on price rather than performance.
"Can the IGU be replaced in my existing frames, or do I need full replacement?"
IGU-only replacement is significantly less expensive and is a viable option for good-condition frames with failed glass seals.
"Is this a full-frame replacement or a pocket insert?"
Pocket inserts fit into the existing frame — faster and less expensive but may reduce glass area. Full-frame is better if the existing frame is damaged.