How do I determine if my older home's exterior walls need added insulation?

Short answer

You can tell if your older home’s exterior walls need more insulation by combining a few simple tests: check for cold wall surfaces and drafts on a cold day, look inside outlet boxes for insulation, scan walls with an infrared thermometer or thermal camera, and—if needed—open a small inspection hole to verify what’s in the cavity. A professional energy audit with blower-door and infrared imaging provides the most definitive answer.

Why this matters and what to look for

Older homes (pre-1980s especially) often have little or no wall insulation. Signs you may need more:
- Rooms with exterior walls feel noticeably colder than interior rooms.
- High heating or cooling bills compared to similar homes.
- Condensation or “ghosting” (gray dust lines) on exterior walls and along studs.
- Drafts at outlets, baseboards, and window trim.

Target R-values (typical):
- 2x4 wall: R-13 to R-15 (fiberglass/mineral wool), ~R-13 dense-pack cellulose
- 2x6 wall: R-19 to R-21 batts, ~R-20 dense-pack cellulose
Rule of thumb surface temps (winter test): If it’s 70°F inside and 30°F outside, a reasonably insulated wall interior surface should read ~60–65°F. If it’s below ~55°F, insulation and/or air sealing are likely lacking.

Step-by-step assessment

1) Take temperature readings (15–30 minutes)

• Use a non-contact infrared thermometer to compare interior surface temperatures:
  • Measure several points on an exterior wall (mid-height, near corners, around outlets).
  • Compare to an interior partition wall. A drop greater than ~5–10°F suggests poor insulation or air leakage.
• Do this early morning or after sunset to avoid solar warming.

2) Look inside an outlet box on an exterior wall (20–30 minutes)

• Turn off power at the breaker. Verify with a non-contact voltage tester.
• Remove the cover plate. Shine a flashlight or insert a slim borescope beside the box (between box and drywall) to check for fiberglass, cellulose, or foam.
• If you see bare sheathing, there’s likely no insulation.

3) Thermal scan for patterns (30–60 minutes)

• Use a thermal camera (a smartphone add-on works) or the IR thermometer to map cold spots, stud lines, and voids.
• Look for vertical stripes (studs) and colder bays—these are classic signs of missing or settled insulation.

4) Make a small, repairable test hole (30–60 minutes)

• Choose a hidden spot (behind a baseboard heater, inside a closet). Use a stud finder to avoid studs, wires, and pipes.
• Drill a 3/4" hole with a spade bit just through the drywall. Insert a borescope to inspect the cavity.
• Patch later with a drywall plug or patch kit.

5) Check air leakage (15–30 minutes)

• On a windy day, use a smoke pencil or incense near outlets, baseboards, and window/door trim.
• Movement indicates air leaks that should be sealed. Air leaks can mimic poor insulation by making walls feel cold.

6) Consider age and construction clues (10 minutes)

• Homes built before the mid-1970s often have empty wall cavities unless retrofitted. Look for filled holes/plugs on exterior sheathing (in older siding) indicating past dense-pack insulation.

Tools and materials

• Infrared thermometer or thermal camera (FLIR-type smartphone module)
• Borescope/inspection camera (USB or Wi-Fi)
• Non-contact voltage tester and outlet tester
• Stud finder with live-wire detection
• Drill, 3/4" spade bit, drywall patch kit, painter’s tape
• Flashlight, tape measure, notepad/phone for logging temps
• Smoke pencil or incense
• PPE: safety glasses, dust mask/respirator, gloves

Safety considerations

• Always de-energize circuits before removing outlet covers; verify with a tester.
• Homes pre-1978 may have lead paint. Minimize dust, wear a respirator, and follow lead-safe practices when drilling or sanding.
• Watch for asbestos-containing materials (plaster, vermiculite). If suspected, stop and test before disturbing.
• Use a stud finder and proceed slowly to avoid wiring and plumbing. If you have knob-and-tube wiring, do not add insulation around it—consult an electrician first.

Tips for best results

• Perform tests on a cold or hot day with at least a 25–35°F temperature difference between inside and outside.
• Avoid sunlit walls during thermal scans; sun skews readings.
• Pull back curtains and move furniture a few inches from walls before scanning.
• Differentiate air sealing vs insulation: seal leaks at outlets (gaskets), baseboards (caulk), and around windows/doors (low-expansion foam) before or along with insulation upgrades.
• Don’t forget rim joists and top/bottom plates—big leakage points.

Common mistakes

• Confusing drafts (air leaks) with missing insulation and ignoring air sealing.
• Misreading reflective surfaces with IR tools—take multiple readings and use matte tape on glossy paint if needed.
• Drilling in the wrong spot due to skipping wire/pipe scans.
• Adding insulation to walls with moisture problems—fix bulk water and flashing first to avoid trapping moisture.

When to call a pro

• You want a definitive assessment: an energy auditor can run a blower-door test with infrared imaging to pinpoint insulation voids and leaks. Typical cost: $300–$600, often rebated by utilities.
• You find moisture, mold, or suspected structural/rot issues in exterior walls.
• Presence of knob-and-tube wiring, asbestos, or complex claddings (stucco, brick veneer) that complicate dense-pack installation.
• Planning dense-pack cellulose or wall foam injection—pros have the equipment and know how to maintain proper density and moisture management.

Rough costs and next steps

• DIY assessment tools:
  • IR thermometer: $20–$60
  • Smartphone thermal camera: $200–$400
  • Borescope: $30–$100
• Professional energy audit: $300–$600 (check for rebates).
• Adding insulation (pro-installed dense-pack cellulose): roughly $2–$4 per sq. ft. of wall area, plus siding/patch work if needed.

If your tests show cold wall surfaces, visible voids in cavities, or pronounced thermal patterns, plan a two-pronged upgrade: air seal first, then add cavity insulation (dense-pack cellulose or properly fitted batts during renovations). This approach delivers better comfort, lower bills, and reduces the risk of moisture issues.