The numbers that matter
U-factor measures heat loss, lower is better and it drives winter comfort. SHGC (solar heat gain coefficient) measures how much solar heat comes through, lower keeps summer heat out. Every legitimate quote should list both per window line.
In a heating-dominated climate the U-factor is the number to watch. Double pane with low-E and argon is the baseline, and triple pane can be worth it on exposed north and windward elevations if the frame and install are up to it.
- Low U-factor is the priority number, look for the NFRC label on every quote
- Double pane with low-E and argon as the baseline; triple pane on exposed elevations
- Insulated frame construction rather than hollow chambers
- Full perimeter insulation and air sealing of the rough opening
Rebates, credits, and what to verify
Federal energy-efficiency tax credits for qualifying windows have applied in recent years with annual caps, and some utilities run their own rebate programs. Rules change, so confirm current eligibility and the required performance ratings before you count on a credit, and keep the NFRC documentation with your receipt.
What the install does to the numbers
A high-performance window installed badly performs like a cheap one. Perimeter air sealing, correct flashing at the sill, and shimming so the sash operates square are what turn the label rating into real comfort. Ask how the crew seals and insulates the rough opening.
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Frequently asked questions
Is triple pane worth the upcharge here?+
Sometimes. In a heating-dominated climate triple pane meaningfully improves comfort next to the glass and cuts drafts on windward elevations. It also adds weight and cost. A common approach is triple pane on the coldest exposures and double pane elsewhere.
Will new windows pay for themselves in energy savings?+
Rarely on energy savings alone. The honest case for replacement is comfort, noise, operation, and eliminating rot or air leaks, with utility savings as a secondary benefit.
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