Chapter 5 Key Takeaways: Windows and Doors

Window Anatomy Essentials

  • The IGU (insulated glass unit) is the sealed double- or triple-pane glass assembly. When the seal fails, the window fogs — but the frame and sash may still be in good condition.
  • Low-e coatings are metallic films on the glass surface that reduce radiative heat transfer. Their placement (which surface of the IGU) matters by climate.
  • Frame material matters: aluminum (without thermal break) = poor insulation. Vinyl = good and low-maintenance. Wood = good, high-maintenance. Fiberglass = best, most durable.
  • Argon fill provides modest R-value improvement over air fill; works with low-e for meaningful whole-unit gains.

The Three Performance Numbers

Rating What It Measures Better =
U-factor Rate of heat transfer through whole assembly Lower
SHGC Fraction of solar radiation passing through Higher (cold climate, south) or Lower (hot climate)
VT Fraction of visible light transmitted Higher (more daylight)
  • Always use the NFRC whole-unit U-factor — not the center-of-glass R-value, which overstates performance.
  • SHGC should be matched to your climate zone AND window orientation. South windows in cold climates want higher SHGC; east/west windows want lower SHGC regardless of climate.

Diagnosing Window Problems

Symptom Cause Correct Fix
Foggy/hazy glass between panes Failed IGU seal Replace IGU only (not whole window)
Draft at meeting rail Worn weatherstripping Replace meeting rail weatherstripping ($10–$30)
Draft behind interior trim Unsealed rough opening gap Remove trim, seal with low-expansion foam
Condensation on interior glass surface High interior humidity and/or high U-factor Reduce humidity first; address window second
Sash won't stay up Failed sash balance mechanism Replace balances ($15–$40/window DIY)

The Window Replacement Reality Check

  • Windows typically account for 10–20% of total heat loss in older homes — not the largest source.
  • Replacing double-pane with better double-pane saves roughly $150–$300/year for a typical home.
  • Full replacement projects at $12,000–$20,000 have payback periods of 50–100 years for energy savings alone — far beyond the windows' service life.
  • IGU replacement (glass unit only) costs $100–$300 per window and is the correct fix for fogged glass.
  • Window replacement is financially justified when: frames are structurally failing, windows are true single-pane, or as part of a comprehensive renovation where rough opening access is already open.

When Restoration Beats Replacement

  • Pre-1940 old-growth wood windows are often more durable than modern replacements.
  • Professional restoration (reglaze, repaint, re-rope, weatherstrip): $150–$400 per window.
  • An interior storm window added to a restored single-pane achieves R-3 to R-4 effectively — comparable to a budget replacement double-pane.
  • In historic districts, replacement windows may violate preservation requirements.

Proper Installation is Critical

  • Head flashing must lap over the drainage plane, not behind it. Water running down the wall must shed outward — over the window, not behind the frame.
  • The flashing sequence: Bottom pan first, sides second, window installed, head flashing last (lapping outward).
  • Always use low-expansion "window and door" foam — not standard expanding foam — in rough opening gaps. Standard foam can bow vinyl frames.
  • Full-frame replacement allows inspection of the rough opening; pocket/insert replacement does not.

Noise and Windows

  • Standard double-pane windows have STC 26–28 — barely better than single pane for noise.
  • Effective acoustic windows require: laminated glass (STC 35–42), wide air gap between panes, and asymmetric glass thicknesses.
  • Air sealing always comes first. Sound follows air paths — a leaky frame undermines any glass acoustic performance.
  • Interior storm windows create a wide air gap that provides significant acoustic benefit at a fraction of replacement cost.

Door Essentials

  • Exterior doors must be solid-core. Hollow-core doors should not be used at exterior locations.
  • The door between attached garage and living space is a fire door — it must be code-compliant (typically 20-minute rated). Do not substitute a hollow-core door.
  • Full perimeter weatherstripping replacement: $20–$40 in materials, 45 minutes. Do it every 7–10 years.
  • Garage door insulation upgrade: $150–$300 DIY, meaningful savings in cold climates.

Cost-Effectiveness Hierarchy for Window and Door Improvements

  1. Weatherstripping (doors and windows): payback typically 1–3 years
  2. Door threshold seal replacement: payback under 2 years
  3. Rough opening air sealing: payback typically 2–5 years
  4. IGU replacement for failed seals: payback 5–10 years
  5. Interior storm windows: payback 5–12 years
  6. Full window replacement for true single-pane: payback 15–30 years
  7. Full window replacement for existing double-pane: payback typically exceeds window service life