Three Options for Replacement Windows

1 The cheapest:
REPLACE ONLY THE SASH
If the existing frame and sill are in good condition, you can swap the sash for an energy-efficient upgrade.   Remove the sash and parting stops, then install jamb liners against the sides of the window frame.  The liners form a tight seal with the new sash.  Many manufacturers have replacement-sash kits designed for their older frames.

Important installation details

• Measure according to manufacturer’s specifications.
  
• Check that window frame is square.
  
• Check that sill is flat, not crowned.
  
• Jamb liners and sash must slope of sill precisely; kinks allow air leakage.
  
• Finish air-sealing with foam plugs at top and bottom of jamb liners.

Pros

• Easy installation.
  
• Energy-efficient upgrade.
  
• Maintains the window’s original glass area.
  
• No need to disturb existing casings.
  
• Least expensive replacement-window system.
  

• The finish product might be leakier than Options 2 and 3
• Hard to fit properly in old openings that might be out of square or crowned.

2 The easiest :
INSERT A FRAME AND SASH
The most common system comes as a complete unit that slips into the existing window jambs against the exterior (or interior) window stops.  Use low-expanding foam to seal the perimeter gap and ensure an airtight installation.

• Verify opening for square; measure accordingly.
  
• Air-seal the perimeter with low-expanding foam, not fiberglass batts.
  
• Fill header and sill extensions with low-expanding foam insulation.
  
• Protect against wind-driven rain with highest-quality sealant at exterior stops.

Pros

• More-dependable energy ratings than Option 1 because the sash and frame are tested as a unit
  
• Less invasive than whole-window replacement.   The process takes one hour.
  
• Won’t disturb existing casings, siding, or wall coverings.

3 The best:
COMPLETE  REPLACEMENT
When water damage is occurring, the window and rotten framing should be replaced and the source of the leak tracked down and stopped.  A rotted window may be the symptom, not the source, of the leak.  You can integrate a new window fully into the wall with new head and pan flashings, low-expanding foam, flexible flashing membranes, and high-quality sealants.

Important installation details

• Protect against wind-driven rain with head flashing and high-quality sealant along top and side flanges; uncaulked  bottom  flange allows water to escape.
  
• Ensure against rot with pan flashing.
  
• Integrate flashings shingle style into building paper.
  
• Maintain air seal with low-expanding foam along inside perimeter.

Pros

• Most versatile because you can change the window’s size.
  
• The most energy-efficient system because the window is fully integrated into the wall.
  
• Most durable because you can upgrade the flashing details.

Cons

• The most invasive method.