Insulation and Air Sealing
Proper insulation and air sealing are the keys to a comfortable, energy-efficient ADU. While invisible once walls are closed, these investments pay dividends every month through lower utility bills and year-round comfort. Don't skimp here—it's nearly impossible to upgrade insulation later without major renovation.
Quick Summary
Timeline
2-4 days
Cost range
$2,000-8,000
Key inspection
Insulation inspection
Understanding R-Values
R-value measures thermal resistance—how well insulation resists heat flow. Higher R-values mean better insulation. Building codes specify minimum R-values for your climate zone, but exceeding minimums often makes economic sense.
Typical Code Requirements
Climate zone matters: Requirements vary by location. Cold climates need higher R-values than mild climates. Check your local energy code or ask your building department for specific requirements.
Insulation Types Compared
Fiberglass Batts
The most common and affordable option. Pink or yellow blankets that fit between studs. Easy to install but must be cut precisely to fit around obstacles. Gaps and compression significantly reduce effectiveness.
R-value per inch
R-3.2 to R-3.8
Cost installed
$0.50-1.50 per sq ft
Blown-In Cellulose
Made from recycled paper treated with fire retardant. Blown into attics or dense-packed into wall cavities. Excellent for filling irregular spaces and around obstacles. Good sound dampening properties.
R-value per inch
R-3.2 to R-3.8
Cost installed
$1.00-2.00 per sq ft
Spray Foam (Open-Cell)
Expands on application to fill cavities completely. Creates both insulation and air barrier. Open-cell is softer, more affordable, and allows moisture to pass through. Good for walls but requires vapor barrier in cold climates.
R-value per inch
R-3.5 to R-3.8
Cost installed
$1.50-2.50 per sq ft
Spray Foam (Closed-Cell)
Premium option with highest R-value per inch. Creates rigid insulation that also adds structural strength and acts as vapor barrier. Best for maximum efficiency or limited cavity depth. More expensive but performs exceptionally.
R-value per inch
R-6.0 to R-7.0
Cost installed
$2.50-4.00 per sq ft
Rigid Foam Boards
Solid panels of EPS, XPS, or polyiso foam. Used for continuous exterior insulation, basement walls, or under slabs. Eliminates thermal bridging through studs. Often combined with cavity insulation for best performance.
R-value per inch
R-3.8 to R-6.5
Cost installed
$1.50-3.00 per sq ft
Air Sealing: The Often-Missed Step
Insulation resists heat transfer through materials, but air leaks bypass insulation entirely. A well-insulated home with poor air sealing can waste 25-40% of heating/cooling energy. Air sealing must happen before insulation inspection.
Common Air Leak Locations
- • Top and bottom plates (where walls meet ceiling and floor)
- • Around electrical boxes, switches, and outlets
- • Plumbing and wire penetrations through framing
- • Window and door frames
- • Rim/band joist area
- • Recessed light fixtures (use IC-rated airtight cans)
- • HVAC boots and ductwork connections
- • Attic hatch or access door
Air Sealing Materials
Caulk
For gaps up to 1/4". Use fire-rated caulk around penetrations through fire barriers.
Expanding Foam
For gaps 1/4" to 3". Use low-expansion around windows/doors to avoid warping.
Weather Stripping
For operable windows, doors, and attic hatches.
Gaskets
Pre-formed foam gaskets behind electrical boxes and under drywall at top/bottom plates.
Installation Best Practices
Fill completely, don't compress
Batts should fill cavities fully without being squeezed. Compressed insulation loses R-value.
Split around wires and pipes
Don't push insulation behind obstacles. Split batts and fit around them for continuous coverage.
Cut precisely around electrical boxes
Gaps around boxes create air leaks and reduce insulation effectiveness.
Don't forget rim joists
The rim/band joist between floors is a major thermal bridge. Insulate and air seal this area carefully.
Maintain attic ventilation
Don't block soffit vents with insulation. Use baffles to maintain airflow from soffits to ridge vents.
The Insulation Inspection
The insulation inspection must pass before drywall can be installed. The inspector verifies proper installation and code compliance. Being present for this inspection helps you learn about any deficiencies that need correction.
What Inspectors Check
Common Insulation Problems
Missing Insulation
Small gaps and missed areas are common, especially in hard-to-reach spots like behind tubs, in narrow cavities, and at awkward angles. Each gap is a cold spot and potential condensation point.
Vapor Barrier Issues
In cold climates, vapor barriers on the wrong side trap moisture inside walls. In mixed climates, double vapor barriers (interior and exterior) prevent drying. Know your climate zone requirements.
Thermal Bridging
Wood studs conduct heat much better than insulation. Without continuous exterior insulation, studs create thermal bridges that reduce whole-wall R-value by 15-25%.
Unfaced Batts Where Facing Required
Some inspectors require faced batts with the vapor retarder facing the warm side. Using unfaced batts where faced are required can fail inspection.
Frequently Asked Questions
Should I exceed minimum code R-values?
Often yes. Code minimums are the bare legal requirement, not optimal performance. In extreme climates, upgrading from R-13 to R-21 walls or R-38 to R-49 ceilings typically pays back through energy savings within 5-10 years. The cost increase during construction is modest compared to retrofitting later.
Is spray foam worth the extra cost?
It depends on your situation. Spray foam excels in complex spaces with many penetrations, cathedral ceilings, or where you need both insulation and air barrier. For simple wall cavities in a straightforward ADU, properly installed batts or blown-in may be more cost-effective.
Do I need a vapor barrier?
Requirements vary by climate. Cold climates typically need an interior vapor barrier to prevent warm, moist interior air from condensing in walls. Hot-humid climates may need the opposite. Mixed climates often use vapor retarders (semi-permeable) rather than full barriers. Always follow local code requirements.
Can I install insulation myself?
Batts are DIY-friendly with proper technique—watch manufacturer installation videos. Blown-in cellulose can be DIY for attics (rent a blower from home centers). Spray foam requires professional equipment and training. Poor DIY installation can be worse than professional installation of a cheaper product.
Ready for the Next Step?
With insulation and air sealing complete, your ADU's building envelope is finished! You're now moving into the Systems phase, starting with rough electrical installation to power your new living space.