Garage Construction Project

Insulation Strategy

6 min read

Context

24’×40’ garage with loft (A-frame), vented soffits + ridge vent, metal roof, radiant floor heating (heated space). Targeting code in Zone 6a (Clare, Michigan).

Code Targets (Zone 6a - Michigan)

Walls (Main Garage)

  • Code Minimum: R-13 cavity + R-5 continuous insulation (R-18 total)
  • Alternative: R-19 cavity insulation (fills 2×6 framing)
  • Heated Garage Recommendation: R-21 (2×6 walls with high-density batts)

Ceiling (Garage Ceiling / Loft Floor)

  • Heated Garage Below: R-30 to R-38 minimum
  • Recommended: R-38 to R-49 (if loft will eventually be heated)

Loft Ceiling (Attic/Sloped Roof)

  • Attic/flat ceiling: R-60
  • Cathedral/sloped roof: R-49

Assembly Options (Walls & Garage Ceiling)

Main Garage Walls (2×6 @ 16” or 24” OC)

Option 1: R-21 Fiberglass Batts (RECOMMENDED - Best DIY Value)

  • Material: Kraft-faced R-21 high-density batts (6.25” thick)
  • Cost: 550-770 for 1,100 sq ft walls)
  • Installation: DIY friendly
  • Performance: Exceeds code (R-21 vs. R-19 minimum)
  • Air sealing: Spray foam at rim joists, top/bottom plates, penetrations
  • Vapor barrier: Kraft facing toward warm side (interior)
  • Time: 2-4 weekends
  • Pros: Cheapest, proven, DIY-able, good R-value
  • Cons: Itchy, labor-intensive, gaps if not careful

Option 2: Dense-Pack Cellulose

  • Material: Blown cellulose @ 3.5 lbs/cubic ft
  • Cost: 1,100-1,650 for walls)
  • Installation: DIY possible but challenging (requires drilling holes in drywall)
  • Performance: R-20 to R-21 (R-3.6 per inch × 5.5”)
  • Air sealing: Excellent (fills all gaps and voids)
  • Vapor control: 6-mil poly or smart vapor retarder on interior
  • Time: 1-2 weekends with helper
  • Pros: Best air sealing, sound dampening, eco-friendly
  • Cons: Messy, requires patching drywall holes, equipment rental

Option 3: Flash-and-Batt (Professional Only)

  • Material: 2” closed-cell spray foam (R-12) + R-8 to R-10 batt = R-20 to R-22
  • Cost: 3,465-4,400 for walls)
  • Installation: Professional contractor required
  • Performance: Excellent (best air sealing + vapor barrier)
  • Time: 1 day professional install
  • Pros: Best performance, excellent air/moisture control
  • Cons: EXPENSIVE (5-6× cost of batts), contractor required, overkill for garage
  • Verdict: Not recommended for garage application (diminishing returns)

Garage Ceiling (2×10 Joists @ 16” or 24” OC)

Option 1: Blown Cellulose to R-49 (RECOMMENDED)

  • Material: Loose-fill cellulose (13-14” depth for R-49)
  • Cost: 576-960 for 960 sq ft)
  • Cost: 1,440-1,920)
  • Installation: DIY friendly (easier than walls) or hire contractor
  • Performance: Excellent coverage, fills all gaps
  • Equipment: Insulation blower rental $45/day (often free with material purchase)
  • Materials: ~37 bags cellulose @ 370-555
  • Time: 3-6 hours with helper
  • Pros: Best coverage, fast, fills gaps, eco-friendly, excellent ROI
  • Cons: Messy (dust), requires helper, need baffles at eaves

Option 2: R-38 Fiberglass Batts

  • Material: R-38 unfaced batts (12” thick for 2×10 joists)
  • Cost: 672-960 for ceiling)
  • Installation: DIY friendly
  • Performance: Good (meets code minimum, slightly under R-49 target)
  • Time: 1-2 weekends
  • Pros: Clean, no equipment rental, straightforward
  • Cons: Harder to avoid gaps, lower R-value than blown, slower install

Option 3: Flash-and-Batt Ceiling

  • Material: 2” spray foam (R-12) + R-30 batt = R-42
  • Cost: 3,360-4,320 for ceiling)
  • Verdict: Not recommended (blown cellulose gives R-49 for 1/4 the cost)

Hybrid Approach (BEST VALUE RECOMMENDATION)

Walls: DIY R-21 batts (~576-960 DIY or 150-250)

Total Cost: 2,140-2,940 (pro ceiling) Performance: Exceeds code, excellent ROI, 85% of flash-and-batt performance at 20% of cost Why It Works:

  • Batts in walls are acceptable and economical for garage
  • Blown ceiling is faster with better coverage than batts
  • Focused air sealing gives most of flash-and-batt benefits
  • Saves $4,000-6,000 vs. flash-and-batt for similar performance

Assembly Options (Loft Sloped Roof Sections)

  • Fur-down rafters + batts/cellulose: lowest cost; adds depth and labor.
  • Flash-and-batt: 2–4” closed-cell spray foam for air seal, fill rest with fluffy insulation; cost/performance balance.
  • Full closed-cell spray foam: highest cost; meets R-49 in 2×10; excellent air seal.
  • Exterior rigid foam (only if reroofing): continuous insulation; strong, but affects roof assembly.

Cost Ranges (per sq ft)

Walls

  • R-21 fiberglass batts (DIY): $0.50-0.70/sq ft
  • Dense-pack cellulose (DIY): $1.00-1.50/sq ft
  • Flash-and-batt (professional): $3.15-4.00/sq ft

Garage Ceiling

  • Blown cellulose to R-49 (DIY): $0.60-1.00/sq ft
  • Blown cellulose to R-49 (professional): $1.50-2.00/sq ft
  • R-38 batts (DIY): $0.70-1.00/sq ft
  • Flash-and-batt (professional): $3.50-4.50/sq ft

Loft Sloped Roof

  • Full-depth spray foam to R-49: ~$8–12/ft²
  • Hybrid flash-and-batt: ~$4–8/ft²
  • Batts/cellulose alone: ~$3–6/ft² (requires furring for depth)

Air-Sealing Priorities (CRITICAL for Performance)

Impact: Air sealing is 50% of your thermal performance! Gaps and air leaks can reduce insulation effectiveness by 30-50%.

Priority Areas:

  1. Rim joists (where floor meets foundation) - Use spray foam
  2. Top plates (where walls meet ceiling) - Spray foam or caulk
  3. Bottom plates (where walls meet floor/subfloor) - Spray foam
  4. Electrical boxes - Use foam gaskets or putty pads
  5. Penetrations - Spray foam around all pipes, wires, ducts
  6. Knee-wall backs (loft) - Air barrier critical
  7. Attic hatch/access - Weatherstrip and insulate

Materials Needed:

  • Great Stuff Pro spray foam cans (gun-style): 6-12 cans
  • Acoustic sealant or caulk for smaller gaps
  • Foam gaskets for electrical boxes
  • Cost: $150-250 total

ROI: Spending 5,000 on spray foam.

Open Questions

  • Vented vs unvented cathedral assembly preference by local inspector.

ROI Considerations

  • If garage doors remain around R-9, upgrading wall assemblies to very high R-values may yield limited ROI. Prioritize air-sealing and door performance (insulation and weatherstripping) before chasing marginal wall R increases.

Decisions

Building Envelope Strategy ✅ DECIDED: Complete Both Levels

  • Sequencing: Insulate both main garage AND upper level before drywalling either level
  • Conditioned zones: Both garage and loft will be finished and conditioned
  • Approach: Option A - Complete building envelope insulation in one phase for efficiency

Rationale:

  • Buy all insulation at once (bulk pricing, save $100-200)
  • Single blower rental covers both levels (vs. two separate rentals)
  • One insulation inspection vs. two
  • Maximum heating efficiency during construction and long-term
  • Complete messy work (insulation) before clean work (drywall)

Main Garage (Lower Level) - Immediate

  • Wall insulation method: R-21 batts (DIY) - DECIDED
  • Ceiling insulation method: Blown cellulose R-49 (DIY or pro) - DECIDED
  • Air sealing approach: DIY spray foam - DECIDED
  • Budget tier: All DIY ($1,300-2,000 main level only) - DECIDED
  • DIY ceiling vs. hire contractor (decide when ordering)

Upper Level (Loft) - Immediate (Same Phase as Main)

  • Sloped-roof approach: Blown cellulose R-49 vs. flash-and-batt vs. batts
  • Any knee walls: R-21 batts (same as main level walls)
  • Timing: Immediately after main level (same material order)

References

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