Short answer
Yes. You can insulate a drafty exterior wall from the inside without creating mold problems by first stopping air leaks, then adding the right insulation, and finishing with a smart vapor control layer that allows the wall to dry. The safest DIY paths are dense-pack cellulose through small holes, or opening the wall to air-seal and install mineral wool batts with a smart vapor retarder. Avoid interior polyethylene in most homes.
Why moisture problems happen
Moisture trouble in walls is driven more by air leaks than by vapor diffusion. Warm, humid air leaking into a cold wall (winter) or hot, humid outdoor air pushed into a cooled wall (summer, in humid climates) can condense inside the cavity. The cure is:
- Air seal first (stop air movement),
- Use appropriate insulation (fills the cavity without gaps), and
- Control vapor, don’t trap it (allow drying to at least one side).
Climate quick guide
- Cold/Mixed (IECC 4–8): interior smart vapor retarder or Class II VR; no interior polyethylene unless very cold and assembly is designed for it.
- Warm-Humid (1A–3A): avoid interior vapor barriers; focus on airtight drywall and/or interior rigid foam + latex paint.
Options that work
Option A: Drill-and-fill dense-pack cellulose (minimal demo)
Best for finished rooms where you don’t want to remove drywall.
Steps:
1. Map studs with a stud finder; remove baseboards as needed.
2. Drill 2–2.5 in holes between studs near the top (and mid-height for tall cavities).
3. Blow cellulose to 3.5–4.0 lb/ft³ density to stop convection and reduce air movement.
4. Patch holes, tape, mud, and finish with vapor retarder (Class III) latex paint in cold/mixed climates.
5. Add foam gaskets behind outlet/switch plates.
Pros: Excellent air retardancy, minimal disruption, cost-effective. Cons: Requires blower and technique to hit proper density. Estimated cost: DIY ~$1.25–$2.00/sf (rental blower + material); Pro dense-pack ~$2.50–$3.50/sf. Time: 1–2 rooms/day.
Option B: Open the wall: air seal + mineral wool batts + smart membrane
Best for major refresh or if you suspect hidden leaks.
Steps:
1. Remove drywall carefully; check for signs of bulk water entry and fix exterior issues first (flashing, siding, gutters).
2. Air seal:
- Seal top/bottom plates to sheathing and subfloor with polyurethane caulk or high-quality acrylic.
- Foam small gaps and penetrations (wires, pipes). Use fire-rated foam near flues; maintain clearances.
- Seal around windows with low-expansion foam and flexible tape.
3. Install mineral wool batts (R-15 for 2x4, R-23 for 2x6). Trim for a friction fit; no compression or gaps.
4. Staple a smart vapor retarder membrane (e.g., variable perm) on the warm-in-winter side; tape seams and seal at edges.
5. Optional performance boost: add 1/2–1 in rigid foam (EPS/XPS/polyiso) over studs for a continuous thermal break; tape seams, then install furring strips and drywall.
Pros: Highest control over air leaks and wiring/plumbing. Cons: More labor and finish work. Cost: Materials ~$1.50–$3.50/sf (+$0.50–$1.50/sf if adding interior foam). Time: A weekend per room for an experienced DIYer.
Option C: Spray foam
- Closed-cell spray foam (2 in+) both insulates and creates a Class II vapor retarder, excellent for cold climates or problem areas. Ensure it’s covered with 1/2 in drywall as a thermal barrier.
- Open-cell foam air seals well but is vapor-open; pair with a smart vapor retarder in cold/mixed climates.
Pros: Superior air sealing; high R/inch. Cons: Highest cost; pro application recommended. Cost: $4–$7/sf of wall.
Tools and materials
- Stud finder, inspection mirror or borescope, utility knife, oscillating multi-tool
- Drill/driver with hole saws (2–2.5 in), cellulose blower (rental), stapler
- Sealants: polyurethane/acrylic caulk, one/two-part spray foam, backer rod, tape (acrylic sheathing/VR tape)
- Insulation: dense-pack cellulose, mineral wool batts, or spray foam
- Vapor control: smart vapor retarder membrane or vapor-retarder latex paint
- Optional: rigid foam boards, furring strips, outlet/switch foam gaskets
- PPE: P100/N95 respirator, gloves, eye protection, long sleeves
Safety and code considerations
- Homes pre-1978: test for lead paint before drilling or demolition.
- Suspect asbestos in vermiculite insulation or old joint compound—stop and test.
- Knob-and-tube wiring cannot be buried in insulation—have an electrician update it first.
- Maintain required clearances from chimneys/flues; use fire-rated materials where required.
- Foam plastics must be covered by a code-approved thermal barrier (typically 1/2 in gypsum).
Tips for best results
- Fix exterior water first: flashing, siding gaps, grade, and gutters. Insulation won’t stop rain leaks.
- Control indoor humidity: target 30–50% RH in winter; use bath/kitchen fans vented outside.
- Use an IR camera or smoke pencil to find leaks before and after work.
- Don’t over-stuff batts. Gaps, compression, and mis-cuts lower R-value and increase moisture risk.
- Seal electrical boxes with putty pads or gaskets; tape membrane to boxes with compatible tape.
Common mistakes to avoid
- Installing interior polyethylene in most climates—often traps moisture.
- Skipping air sealing and relying only on insulation.
- Ignoring the top and bottom plates—major air paths.
- Covering up bulk water problems (leaks) with insulation.
When to call a pro
- You see signs of bulk water intrusion, rot, or complex flashing issues.
- You want dense-pack done to spec or are considering spray foam.
- Presence of lead, asbestos, or outdated wiring.
- Masonry or stucco wall assemblies that need tailored moisture design.
By focusing on airtightness, correct insulation choice, and smart vapor control, you can make a drafty wall warm and quiet while keeping the assembly dry for the long haul.