Windows and Exterior Doors
Windows and doors transform your ADU from a shell into a livable space. They provide natural light, ventilation, emergency egress, and security. Quality windows and proper installation significantly impact energy efficiency, comfort, and long-term maintenance costs.
Quick Summary
Timeline
2-4 days
Cost range
$3,000-8,000
Critical code
Egress window required
Egress Window Requirements
Building codes require at least one egress (escape) window in every bedroom. This is a life-safety requirement that cannot be waived. The window must be large enough for occupants to escape and firefighters to enter in an emergency.
Minimum Egress Requirements (IRC)
Clear opening area
5.7 square feet minimum
Minimum opening height
24 inches
Minimum opening width
20 inches
Maximum sill height
44 inches from floor
Important: The clear opening is measured with the window fully open—not the window size. A window that appears large enough may not meet egress requirements if the opening mechanism reduces the clear opening. Always verify clear opening dimensions before purchasing.
Window Types and Considerations
Single-Hung and Double-Hung
Classic style with vertically sliding sashes. Single-hung has a fixed top sash; double-hung allows both to move. Traditional look, good for egress, easy screens.
- • Double-hung easier to clean from inside
- • Good for egress if properly sized
- • Less airtight than casement windows
Casement Windows
Hinged on the side, opening outward with a crank mechanism. Excellent for ventilation (entire window area opens) and the most airtight window type.
- • Best energy efficiency (seal tightly when closed)
- • Full opening great for egress
- • Cannot have screens on outside
- • May conflict with patios or walkways when open
Sliding Windows
Horizontally sliding sashes, essentially a sideways single-hung. Modern look, good for wide openings, but only half can open for ventilation.
- • Modern aesthetic
- • Check egress carefully (only half opens)
- • Easy operation
Fixed (Picture) Windows
Non-opening windows for light and views. Cannot be used for egress. Often combined with operable windows for balanced design.
- • Least expensive
- • Most energy-efficient (no seals to fail)
- • No ventilation or egress
Energy-Efficient Window Features
Windows are thermal weak points in your building envelope. Energy-efficient glazing dramatically reduces heating and cooling costs. Most jurisdictions require minimum energy performance that exceeds basic double-pane windows.
Double-Pane (Minimum)
Two layers of glass with insulating gas (usually argon) between. Required by code in most areas.
Low-E Coating
Microscopically thin metal oxide layer reflects heat. Keeps heat inside during winter, outside during summer.
Argon or Krypton Gas Fill
Inert gas between panes insulates better than air. Krypton is slightly better but more expensive.
Triple-Pane (Premium)
Three glass layers for maximum efficiency. Worth considering in extreme climates; adds 25-50% to window cost.
Look for ENERGY STAR: ENERGY STAR certified windows meet EPA efficiency guidelines for your climate zone. This simplifies selection and may qualify for rebates or tax credits.
Exterior Door Selection
Your ADU needs at least one exterior entry door. The door affects security, energy efficiency, and curb appeal. Consider both function and aesthetics.
Steel Doors
Most affordable and secure option. Insulated steel doors offer good energy efficiency. Can dent and may rust over time without proper maintenance.
Cost: $200-600 for door slab
Fiberglass Doors
Best balance of durability, efficiency, and aesthetics. Can mimic wood grain. Won't dent, rust, or warp. Low maintenance and good insulation value.
Cost: $400-1,500 for door slab
Solid Wood Doors
Beautiful and traditional but requires regular maintenance. Can warp or swell with humidity changes. Best protected under an overhang.
Cost: $500-3,000+ for door slab
Proper Installation is Critical
Even the best windows and doors will fail if installed incorrectly. Proper flashing and sealing prevent water intrusion—the leading cause of building envelope failures.
Installation Checklist
Common Window and Door Problems
Water Intrusion from Poor Flashing
Most window leaks occur because flashing wasn't properly integrated with the weather barrier. Water gets behind the window and into the wall cavity, causing rot and mold.
Wrong Size Rough Opening
Openings too small require cutting framing (expensive). Openings too large require shimming (potential air leaks and settling issues).
Condensation Between Panes
Indicates seal failure. The insulating gas has escaped. Window needs replacement. Better-quality windows have longer seal warranties (20+ years).
Doors That Don't Close Properly
Usually caused by out-of-square installation or settling. Doors should be shimmed and installed level and plumb, with proper clearance around all sides.
Frequently Asked Questions
Vinyl, wood, or aluminum frames—which is best?
Vinyl offers the best value—low cost, low maintenance, good insulation. Wood is beautiful but requires maintenance and costs more. Aluminum is durable but conducts heat (less efficient). Fiberglass is premium—strong, efficient, and low maintenance but expensive.
Should I match the main house windows?
Not necessarily, but the ADU should complement the main house. Matching window style (single-hung, casement, etc.) creates cohesion. Matching exact profiles is often impossible if your main house has older windows. Focus on similar proportions and colors.
What about sliding glass doors?
Sliding doors bring in lots of light and provide easy outdoor access—great for small ADUs. Modern sliding doors can meet energy requirements. Consider impact on furniture placement and privacy. A good slider costs $1,000-3,000+ installed.
Do I need a storm door?
Storm doors add weather protection and can improve efficiency of older doors. For a new, well-insulated entry door, a storm door is optional. They're most valuable in harsh climates or where the door is exposed to direct weather.
Ready for the Next Step?
With windows and doors in place, your ADU is enclosed! Next comes exterior siding and trim to complete the weather-resistant envelope and give your ADU its final look.