Short answer
Repair your existing wood windows if the frames and sashes are mostly sound, the rot is localized, and you’re happy with the look and size. Choose full-frame replacement if there’s widespread rot or water damage, persistent air/water leaks from poor flashing, you want a major efficiency upgrade, or you’re changing size/operation. A middle option—sash kits or inserts—works when the frame is solid but you want new operable parts and better sealing.
How to assess your wood windows
Walk each opening and note the following:
- Wood condition: Probe sills, lower rails, and exterior casings with an awl. Soft, punky wood deeper than 1/4–1/2 inch suggests significant rot.
- Water management: Look for staining under sills, failed drip caps, or peeling paint concentrated at the bottom corners—signs of flashing problems.
- Operation: Sashes that won’t stay up (broken cords), bind, or rattle may just need hardware/track work.
- Glazing and weatherseals: Cracked glazing putty, missing points, and hardened or missing weatherstripping cause drafts but are repairable.
- Glass: Single-pane with storms can perform surprisingly well after air sealing; cracked panes or failed insulated glass units (IGUs) are replaceable but add cost.
- Lead paint: Pre-1978 windows likely have lead; this affects how you approach sanding/stripping.
When repair makes sense
- Rot is confined to sill noses, lower sash rails, or exterior trim and can be scarfed or epoxied.
- Frames are square and solid, with no movement at the corners.
- You want to keep historic character/old-growth wood (often more durable than new lumber).
- Budget-conscious: Material cost often $50–$400 per window; DIY time 3–8 hours depending on glazing and epoxies.
- You can add or already have good storm windows; combine with weatherstripping for strong performance.
Typical repair tasks:
- Dutchman patches or epoxy consolidant/filler on sills and rails
- Reglazing panes and repainting
- New sash cords/balances, locks, and weatherstripping
When full-frame replacement makes sense
- Widespread rot in jambs, sills, or studs; sash rails crumble beyond practical repair.
- Chronic leaks due to bad flashing/housewrap integration that require opening the wall anyway.
- You’re changing size, style, or egress, or need tempered/laminated glass for code/safety.
- Severe lead paint buildup you don’t want to manage with RRP safe-work practices.
- You want factory-sealed, low-U-factor units and improved acoustics.
Expect $500–$1,200+ per window for DIY materials (mid-grade), plus trim and flashing; pro installs often $800–$2,000+ per opening depending on size and siding type.
A quick decision process
1) Probe the wood: If you can push an awl more than 1/2 inch into frame/sill in multiple spots, lean to full-frame.
2) Check square: Measure diagonals of the frame; over 1/4 inch difference indicates racking—harder to fix with repair alone.
3) Evaluate air/water control: If your biggest issues are drafts and rattles, repair + weatherstrip + storms is a great path.
4) Consider lead, time, and aesthetics: If you want a clean slate and new interior/exterior trim, full-frame fits.
Measurement basics for replacement
Rough Opening (RO):
- Measure width at top/middle/bottom; height at left/center/right. Use the smallest numbers.
- Target unit size ~1/2" smaller than RO in width and height for shimming.
Sill pan & slope:
- Create or use a sloped sill (6–10° or ~1/8" per foot) and install a back dam.
- Flash sides over the pan; head flashing laps over the WRB.
Tools and materials
Repair/restoration:
- Tools: Awl, 5-in-1 painter’s tool, glazing knife, heat gun or infrared paint remover, random-orbit sander with HEPA vac, miter saw (for patches), drill/driver, spring clamps.
- Materials: Wood consolidant/epoxy filler, kiln-dried wood for dutchman patches, glazing points/putty, oil or acrylic primer, exterior paint, bronze or silicone bulb weatherstripping, sash cord/balance kit.
Full-frame replacement:
- Tools: Oscillating multi-tool, reciprocating saw, flat/pry bars, impact driver, levels (2' and 4'), framing square, caulk gun, stapler for WRB, tin snips for drip cap.
- Materials: New window unit, pre-formed or site-built sill pan, flashing tape (flexible and straight), self-adhered membrane, backer rod, low-expansion window/door foam, exterior drip cap, shims, corrosion-resistant screws, interior/exterior trim.
Safety
- Lead-safe work: Use RRP practices—P100 respirator, plastic containment, wet methods, HEPA vacuum. Avoid open-flame paint removal.
- Glass handling: Wear cut-resistant gloves and eye protection.
- Ladder/fall safety: Stable footing and tie-offs for upper floors.
- Dust and fumes: Ventilate; use low-odor caulks/paints when possible.
Tips for best results
- For repairs, prioritize air sealing: weatherstrip meeting rails, add sash locks, and ensure storms fit tight with weep holes.
- For replacements, always use a sloped sill pan and integrate flashing with the WRB; add a drip cap under siding, above the head trim.
- Use low-expansion foam sparingly and backer rod + sealant at interior air seal for a flexible, durable joint.
- Compare performance: Look for U-factor ≤ 0.30 and SHGC suited to your climate (higher for passive solar in cold zones, lower in hot-sunny zones).
Common mistakes
- Foaming the cavity with high-expansion foam that bows frames.
- Capping/cladding over rotten wood rather than fixing the source of water.
- Skipping head flashing or blocking weep paths on storms.
- Measuring the old sash instead of the rough opening for a replacement.
- Painting too soon over fresh glazing putty—follow cure times.
When to call a pro
- Structural damage at the sill/rough opening, or significant framing rot.
- Masonry or brick veneer interfaces that need specialized flashing details.
- Large lead-paint projects where containment and cleanup scale up.
- Historic districts with approval processes and preservation standards.
- Second- or third-story installs without safe access.
Bottom line
If the frames are solid and you value the character of your wood windows, repair plus storms and weatherstripping often delivers comfort and efficiency at a modest cost. If the frames are compromised, you’re battling chronic leaks, or you want new operation/size and top-tier performance, a properly flashed full-frame replacement is the smarter long-term choice.