Quick answer
You can spot poorly insulated exterior walls by checking for cold or hot wall surfaces, drafty outlets, uneven room temperatures, and high energy bills. Low-cost fixes start with air sealing: add foam gaskets behind outlet covers, caulk gaps at trim and baseboards, weatherstrip doors and windows, and seal penetrations with caulk or spray foam. For a bigger impact without major renovation, consider blown-in cellulose into wall cavities (often DIY-friendly with a rented blower) and add interior window film and thermal curtains.
How to tell if your exterior walls lack insulation
Fast checks you can do today
- Feel test: On a cold day, touch exterior walls. If they feel much colder than interior partitions, insulation may be minimal or missing.
- Outlet test: Remove a few exterior wall outlet/switch covers. If you see bare sheathing or nothing but an air gap, that cavity likely isn’t insulated. If you see fluffy fiberglass or dense gray fibers, there’s some insulation present.
- Temperature differences: Rooms with exterior walls that are hard to heat or cool, or where the thermostat runs often, suggest heat loss through walls.
- Condensation and cold spots: Persistent cold patches and winter condensation on exterior corners point to thermal bridging or empty cavities.
Better diagnostics (low cost)
- Infrared thermometer ($25–$50): Scan exterior walls on a cold morning. Uninsulated bays will read several degrees colder than adjacent studs.
- Thermal camera (smartphone attachment or rental, ~$25–$40/day): Reveals cold bands that line up with empty cavities.
- Borescope inspection ($30–$80): Drill a 3/8–1/2 in hole in a discreet spot (closet), insert the scope, and look for insulation.
- Utility history: Compare usage to similar homes; unusually high kWh/therms can corroborate poor insulation.
Safety for outlet and borescope checks:
- Turn off the circuit at the breaker and verify with a non-contact voltage tester before removing covers.
- Avoid drilling above/below outlets or near plumbing; use a stud finder with AC wire detection.
Typical wall R-values
- 2x4 cavity: R-11 to R-15 fiberglass or cellulose
- 2x6 cavity: R-19 to R-23 fiberglass or cellulose
If you find near-zero cavity fill, you’re missing most of this thermal resistance.
Low-cost fixes that make a real difference
Start with air sealing. It’s inexpensive, DIY-friendly, and often yields the best bang for the buck.
1) Air seal outlets, trim, and penetrations (1–3 hours, $20–$80)
Tools/materials:
- Foam outlet/switch gaskets and child-safety plugs
- High-quality paintable caulk (acrylic-latex with silicone) and caulk gun
- Low-expansion spray foam (window/door formulation) for larger gaps
- Putty knife, utility knife, rags
Steps:
1. Turn off power to targeted outlets/switches; add foam gaskets behind cover plates; use safety plugs on unused outlets to cut drafts.
2. Caulk along baseboards and window/door trim where you see cracks.
3. Spray-foam larger gaps around plumbing, cable, and HVAC penetrations. Don’t overfill; foam expands.
Tips:
- Use low-expansion foam around windows/doors to avoid frame distortion.
- Smooth caulk with a damp finger or caulk tool for neat lines.
Common mistakes:
- Over-foaming cavities and bowing trim.
- Sealing weep holes on brick walls—leave those open for drainage.
2) Weatherstrip doors and windows (1–2 hours/door, $10–$25 each)
Tools/materials:
- Adhesive-backed foam weatherstrip, V-strip, or silicone gasket kits
- Door sweep for exterior doors
- Scissors, measuring tape
Steps:
1. Clean surfaces, apply weatherstrip to stop light/air gaps.
2. Install an adjustable door sweep to seal thresholds.
3) Interior window film and thermal curtains (1–3 hours, $20–$50 per window)
- Apply shrink film kits to reduce drafts and convective losses.
- Hang lined, floor-length thermal curtains to limit night-time heat loss.
4) Target the rim joist and top/bottom leakage paths (2–4 hours, $30–$100)
Even if walls are weak, sealing the building “shell” helps a lot.
- Rim joist: Use rigid foam cut to fit and seal edges with spray foam in the basement/crawlspace.
- Attic penetrations: Seal wire/plumbing holes above exterior walls with foam or caulk to stop stack-effect air movement.
5) Blow-in cellulose for walls (weekend project, ~$300–$1,000 DIY)
If you’re ready for a moderate project, dense-pack cellulose can significantly improve wall performance without removing drywall.
Tools/materials:
- Cellulose insulation bags (estimate 2–3 bags per 100 sq ft of wall; check bag coverage)
- Insulation blower (often free rental with purchase at big-box stores)
- 1 1/2–2 in hole saw, drill, long fill tube, plug patches
- Drop cloths, PPE (N95 mask/respirator, eye protection)
Steps (interior method):
1. Mark stud bays with a stud finder. Drill a hole near the top of each cavity between studs.
2. Insert fill tube to the bottom, start blower, and backfill while withdrawing to achieve dense pack.
3. Cap holes with wood/plastic plugs and patch/paint.
Safety and cautions:
- Verify no knob-and-tube wiring in walls before dense-packing.
- Watch for signs of moisture issues; fix exterior leaks and flashing first.
- Wear a respirator and eye protection.
Best practices and practical tips
- Pick a cold or hot, calm day for IR scans to maximize temperature contrast.
- Tackle air sealing first; then reassess with the IR thermometer.
- Start in the most uncomfortable room; improvements are immediately noticeable.
- Keep receipts—many utilities offer rebates for air sealing and insulation.
When to call a pro
- You suspect asbestos (e.g., vermiculite in walls/attic) or lead paint in areas you’d drill or disturb.
- Knob-and-tube wiring or uncertain electrical conditions exist.
- Masonry walls or complex assemblies (stucco, double brick) that need specific fill strategies.
- You want a blower door test and professional thermal imaging to pinpoint leaks and verify results.
Cost snapshot
- Outlet gaskets/caulk/foam: $30–$100 for a small house zone
- Weatherstripping and door sweeps: $10–$40 per opening
- IR thermometer: $25–$50 (keeps paying off for diagnostics)
- DIY cellulose dense-pack: $300–$1,000 depending on area; pro install typically $2,000–$5,000 for a whole house
With a weekend of targeted air sealing and window treatments, many homeowners cut drafts dramatically. If diagnostics show empty wall cavities, stepping up to blown-in cellulose is a strong value move before any major remodel.