Direct answer
For a 100+ year-old house, the most cost-effective way to improve insulation and reduce drafts is to prioritize air sealing (especially in the attic and basement rim joists) and then add attic insulation—typically blown-in cellulose—followed by weatherstripping doors and windows. Air sealing first delivers the biggest comfort and energy savings per dollar, and in old homes it also prevents moisture and ice-dam problems.
Why this works in older homes
Many older houses have balloon framing, leaky top plates, open chases, and unsealed rim joists that let heated air escape up and out (stack effect). Sealing those leakage paths cuts drafts and heat loss immediately. Once leaks are controlled, adding attic insulation to modern R-values keeps the heat where you want it.
Step-by-step plan (weekend-friendly)
1) Diagnose the leaks
- Hire an energy audit with a blower door and IR scan if you can (often $0–$200 after utility rebates). DIY checks also help: on a windy day use a smoke pencil/incense stick and feel for drafts along baseboards, around outlets, attic hatches, and basement rim joists.
2) Attic air sealing (most impact)
- Target: gaps around wiring/plumbing penetrations, recessed lights, bath fan housings, chimneys/flues, top plates, and open chases.
- Seal small cracks with caulk; larger gaps with low-expansion spray foam. Around metal flues/chimneys, use mineral wool (rock wool) and high-temperature fire-rated sealant or sheet metal flashing with fire-rated sealant.
- Weatherstrip and insulate the attic hatch; add a rigid, insulated cover for pull-down ladders.
3) Add attic insulation
- Blow in cellulose to reach modern R-values. Install soffit baffles first to keep ventilation paths open and prevent wind washing at eaves. Use insulation rulers to hit target depth.
Attic targets (typical):
- Cold climates: R49–R60 (~14–18" cellulose)
- Mixed climates: R38–R49 (~11–14" cellulose)
Cellulose R ≈ 3.2–3.7 per inch; Loose-fill fiberglass R ≈ 2.5–2.9 per inch
4) Weatherstrip exterior doors and windows
- Add quality door sweeps and compression weatherstripping; adjust strike plates so doors latch snugly. Caulk exterior window/door trims where needed.
- If you have historic windows, add interior storm panels (magnetic or compression-fit acrylic) to cut drafts without replacing the sash.
5) Seal the rim joist/basement band
- Use rigid foam board (1–2") cut to fit, sealed with foam, or apply closed-cell spray foam kits. This area is a huge leak path in old homes and greatly reduces floor drafts.
6) Bonus: Seal ducts and bath/kitchen fans
- Mastic-seal duct seams. Ensure bath and kitchen fans vent outdoors, not into the attic.
Tools and materials
- Air sealing: caulk gun; acrylic latex or siliconized acrylic caulk; low-expansion spray foam; fire-rated sealant; mineral wool; sheet metal/flashing; backer rod; putty knife; utility knife
- Attic work: blower (rental), cellulose bags, soffit baffles, insulation rulers, staple gun, measuring tape, straightedge
- Weatherstripping: door sweep, adhesive/compression weatherstrip, V-strip, exterior caulk
- Diagnostics: smoke pencil/incense, infrared thermometer or IR camera (rental)
- Safety: P100 or N95 respirator, safety glasses, gloves, headlamp, knee pads, stable ladder, crawl boards
Gap sealing guide:
- Hairline–1/4": paintable caulk
- 1/4"–3/4": backer rod + caulk
- 1/8"–3/8" around window/door frames: low-expansion foam
- 3/8"–1": standard spray foam
- Around chimneys/flues: mineral wool + fire-rated sealant (no foam)
Safety and old-house cautions
- Knob-and-tube wiring: Do not bury active knob-and-tube in insulation. Have an electrician upgrade first.
- Vermiculite in attic: May contain asbestos. If present, stop and call a pro for testing/abatement.
- Lead paint: Avoid dry-scraping/ sanding. Use safe practices when working around painted surfaces.
- Combustion safety: After major air sealing, ensure boilers/furnaces/water heaters draft properly. A pro can perform a combustion safety test.
- Recessed lights: Only cover IC-rated fixtures; use fire-rated covers or maintain clearances per manufacturer.
- Ventilation: Keep soffit and ridge vents open with baffles. Seal bathroom fan housings and vent them outdoors.
Cost and time
- DIY attic air sealing materials: $150–$400; 4–10 hours depending on house size and access
- Blown-in cellulose (DIY): $700–$1,500 materials for ~1,000–1,500 sq ft; blower often free with purchase
- Weatherstripping: $10–$25 per door; $5–$15 per window for materials
- Rim joist: $150–$400 with rigid foam; $300–$600 for a small spray foam kit
- Audit: Many utilities subsidize blower door audits; check local programs
- U.S. tax credit: The Energy Efficient Home Improvement Credit (25C) can cover a portion of insulation/air sealing costs; many utilities also offer rebates.
Tips for best results
- Start at the top: Seal the attic before adding insulation. Then tackle the rim joist.
- Use baffles at every soffit bay before you blow insulation.
- Mark buried junction boxes with flagging tape for future access.
- Photograph penetrations before and after sealing for reference.
- Match sealant to substrate: paintable acrylic for interior trim, polyurethane/silicone for exterior joints.
Common mistakes to avoid
- Skipping air sealing and only adding insulation
- Blocking soffit vents with insulation
- Using spray foam around hot flues or non-IC lights
- Overfilling door/window gaps with high-expansion foam that warps frames
- Ignoring the attic hatch—it’s often the single leakiest spot
When to call a professional
- You suspect vermiculite/asbestos or see knob-and-tube wiring
- You want dense-pack wall insulation (old balloon framing may need fire blocking and experienced installers)
- Significant ice dams, moisture staining, or attic mold
- You have combustion appliances and want a blower door/combustion safety test
- Full spray foam projects or tricky cathedral ceilings
Tackle air sealing and attic insulation first, then weatherstrip and rim-joist sealing. This approach delivers noticeable comfort, lower bills, and protects your old house from moisture issues while preserving its character.