Insulation Strategy

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).

Baseline Thermal Observations (Uninsulated)

Measured 2026-06-29, ~5:00 PM — IR (infrared) surface-temp gun, building still fully uninsulated

A hot, humid day (NWS Clare: ~83°F air at 9:55 PM with a 90°F heat index and 73°F dewpoint; daytime high ran above the 79°F seasonal normal). Surface temperatures taken with an infrared thermometer with the building in its as-built, uninsulated state — a useful before-picture to compare against once the envelope is closed in.

Location	Surface temp	Notes
Exterior wall, sunny face	93°F	Solar-heated cladding ≈ ambient + sun load
Garage floor (concrete slab)	~70°F	Consistent across the slab
Garage contents (tools, items)	~76°F (±1°F)	Tracking interior air, well below ambient
Loft floor	87°F	~17°F warmer than the slab below
Loft roof underside (interior)	~93°F (±2°F)	Essentially equal to the sunbaked exterior wall

What this confirms:

The lower level is naturally tempered by the slab. Even with zero insulation, the concrete floor held ~70°F and contents sat near 76°F — “notably much cooler” inside than out. The slab’s thermal mass and ground coupling (the same mass the radiant system heats in winter) is buffering summer heat. This is why the main level felt comfortable.
The uninsulated roof is the dominant heat path — by far. The interior roof underside read ~93°F, matching the sunbaked exterior wall. A bare metal roof transmits nearly all of its solar gain straight to the interior surface, which then radiates down into the loft. This is the single biggest argument for prioritizing the loft roof assembly — the depth-limited slopes and the condition-the-triangles / spray-foam decision are exactly about taming this surface.
Heat stratifies hard into the loft. The loft floor (87°F) ran ~17°F above the garage slab, and the loft was “significantly warmer” though not quite as hot as outdoors. Hot air rising plus radiant load off the ~93°F roof makes the loft the worst-case zone — reinforcing why R-49-equivalent roof performance and a tight air seal matter most up top, where today’s gain is concentrated.

This is a one-day spot reading, not an energy model, but it cleanly ranks the problem: roof first, loft envelope second, lower-level walls a distant third — consistent with the ROI Considerations note below.

Code Targets (Zone 6a - Michigan)

Walls (Main Garage)

Code Minimum (IRC Table N1102.1.2, Zone 6): R-20 cavity or R-13 cavity + R-5 continuous insulation (R-18 total)
Alternative: R-20 or 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 (code target)
Depth-limited exception (IRC R402.2.2): Where framing depth does not allow R-49, cavity must be completely filled; R-30 minimum commonly accepted — verify with inspector
This project: 2x8 top chord limits sloped sections — Option A: R-38 (batts + furring, DIY), Option B: R-50.5 (flash-and-batt + furring, meets R-49); peak area achieves R-49+

R-49 ceiling confirmed with the AHJ — Clare County (2026-07-01)

Verified by phone with Clare County Building Development and against their own permit packet: Table 402.1.1 lists Zone 6A ceiling = R-49. (Full table, inspection process, and the county’s permit/code handouts are archived locally under 90-Reference/Clare County Building Department/.) Two ways to comply: (1) Prescriptive — hit R-49 on the ceiling outright; or (2) System Analysis / REScheck — a whole-building UA trade-off that can let a below-R-49 ceiling pass if walls/windows/air-sealing over-perform. Run a REScheck before buying any extra insulation — the loft’s foam-tight, R-21-walled envelope may carry the quoted 5½” (R-38) on the trade-off for $0 added. Not guaranteed (the ceiling is the biggest surface), but free to check. Also note a mandatory blower-door test (≤ 4 ACH50) for all new construction — full-deck closed-cell foam passes this easily.

Update — installer input 2026-07-01 (Midstate/Eli): Clare County applies the R402.2.2 depth-limited cathedral exception and accepts the quoted 5½” (≈R-38) for this roof-plane assembly — it does not enforce a hard R-49 here (unlike e.g. Traverse City). So the quote is very likely code-compliant as-is and the R-49 upgrade is optional. Confirm directly with the building official for the record. See call notes.

Assembly Options (Walls & Garage Ceiling)

Main Garage Walls (2×6 @ 16” OC)

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

Material: Kraft-faced R-21 high-density batts (6.25” thick)
Cost: $0.50 - 0.70/ s q f t ($ 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: $1.00 - 1.50/ s q f t ($ 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: $3.15 - 4.00/ s q f t ($ 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 — Dense-Pack Through Drywall (RECOMMENDED — DECIDED ✅)

Material: Loose-fill cellulose (13-14” depth for R-49)
Method: Install Type X 5/8” fire-rated drywall on garage ceiling first (required for fire separation per IRC R302.6), then dense-pack blown cellulose through access holes in drywall. Drywall acts as containment for the cellulose.
Cost: $0.60 - 1.00/ s q f t D I Y ($ 576-960 for 960 sq ft)
Cost: $1.50 - 2.00/ s q f tp ro f ess i o na l ($ 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 @ $10 - 15/ ba g =$ 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: $0.70 - 1.00/ s q f t ($ 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: $3.50 - 4.50/ s q f t ($ 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 (~ $550 - 770) * * C e i l in g : * * Bl o w n ce ll u l ose t o R - 49 ($ 576-960 DIY or $1, 440 - 1, 920 p ro) * * A i r S e a l in g : * * D I Y s p r a y f o ama t cr i t i c a l a re a s ($ 150-250)

Total Cost: $1, 276 - 1, 980 (a ll D I Y) or$ 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)

2x8 Top Chord Depth Limitation

The A1 24’ attic trusses (Letherer Job #123907) have a 2x8 top chord (7.25” actual depth). After reserving 1-1.5” for a vent baffle air channel, only 5.75-6.25” remains for insulation in the sloped ceiling sections. R-49 is not achievable in the sloped sections with batts or cellulose alone — it requires 13.2” of cellulose. Two approaches are viable: (A) batts + furring (DIY, R-38), or (B) flash-and-batt with closed-cell spray foam (contractor + DIY, up to R-50.5). See Zone 1 options below.

✅ DECIDED: Zone-Based Approach (Revised 2026-04-01)

The loft has four distinct insulation zones, each requiring a different assembly based on available cavity depth:

                           Ridge
                            /\
                    Baffle /  \ Baffle
              ════════════/    \════════════
                         /      \
    ZONE 2: Peak area → /~~~~~~~~\ ← R-49+ (deep cavity)
                       /~~~~~~~~~~\
                      /════════════\
    ZONE 1:          /              \          ZONE 1:
    Sloped ceiling  /    LOFT ROOM   \  Sloped ceiling
    (R-38 to R-50) /                  \ (R-38 to R-50)
                  ║                    ║
    ZONE 3: ───→  ║  Knee walls ~3'7" ║  ←─── ZONE 3
                  ║  (R-13 to R-21)   ║
    ══════════════╩════════════════════╩══════════════
    ZONE 4: Triangular        ZONE 4: Triangular
    attic space (R-49         attic space (R-49
    on floor)                 on floor)
    ──────────────── GARAGE CEILING ─────────────────
                   (separate R-49 assembly)

Zone 1: Sloped Ceiling (Top Chord E→F and H→I) — Depth-Limited

Two options are viable for the depth-limited slopes. Both avoid cellulose, which has settling risk at the 10:12 pitch (≈40°) over decades.

Option A: R-25 Cathedral Batts + 2x4 Furring (DIY — BEST DIY VALUE)

Method: R-25 high-density cathedral ceiling fiberglass batts in the 2x8 cavity, with Raft-R-Mate baffles maintaining the vent channel. Add 2x4 furring below for additional R-13 batt layer.

Why batts instead of cellulose on the slopes:

No settling risk — batts friction-fit and stay put permanently, unlike cellulose at steep pitch
Easier DIY — same technique as garage walls, no blower needed for these sections
Better baffle contact — batts sit against the baffle face without risk of over-packing
Inspector can verify — full cavity fill is visible before drywall

Assembly (with 2x4 furring — RECOMMENDED if choosing this option):

    Roof sheathing
        /
       /  ← 1" vent channel (min per IRC R806.3)
      /═══════════════  ← Raft-R-Mate baffle (22.5" for 24" OC)
     /
    /  R-25 cathedral batt (6.25" in 2x8 cavity)
   /════════════════════════════════════════════
   ├── 2x8 top chord bottom face
   │
   │  R-13 batt (3.5" in 2x4 furring cavity)
   │
   ├── 2x4 furring (perpendicular to top chord, 24" OC)
   └── 1/2" drywall

   Total: R-25 + R-13 = R-38

Without furring: R-25 only (same assembly minus the furring and second batt layer). Not recommended — R-25 may not satisfy the inspector even under the depth-limited exception.

Headroom impact of furring: 3.5” lost on the sloped sections. The slope starts at ~3’7” knee wall height where you’re not standing at full height — acceptable trade-off for R-38 vs. R-25.

Code compliance (IRC R402.2.2): Where framing depth does not permit the required R-value, the cavity must be completely filled. Some jurisdictions accept R-30 minimum for depth-limited cathedral ceilings in Zone 6. R-38 with furring exceeds this. Verify with inspector before proceeding.

Products:

Owens Corning R-25 Kraft-Faced 23” wide (for 24” OC trusses) — designed for cathedral ceilings
Johns Manville R-25 Cathedral Ceiling batt 23” wide
R-13 kraft-faced 23” wide batts for the furring cavity

Cost: ~ $350 - 525 f or ba tt s +$ 50-80 for furring lumber ≈ $400-605 total (~350 sq ft of sloped area)

Option B: 3.5” Closed-Cell Flash + Batt + 2x4 Furring (Contractor + DIY — BEST PERFORMANCE)

Method: Professional contractor sprays 3.5” of closed-cell spray foam directly against the roof sheathing (R-24.5). This eliminates the need for vent baffles on the sloped sections — the foam creates an unvented assembly per IRC R806.5. Remaining cavity and furring are filled with fiberglass batts (DIY).

Assembly (with 2x4 furring):

    Roof sheathing
        /
       /  3.5" closed-cell spray foam (R-24.5)
      /  ← Air seal + vapor retarder + no baffle needed
     /═══════════════════════════════════════════
    /  3.75" remaining cavity: R-13 batt
   /═════════════════════════════════════════════
   ├── 2x8 top chord bottom face
   │
   │  R-13 batt (3.5" in 2x4 furring cavity)
   │
   ├── 2x4 furring (perpendicular to top chord, 24" OC)
   └── 1/2" drywall

   Total: R-24.5 + R-13 + R-13 = R-50.5  ← EXCEEDS R-49

Assembly (without furring):

    Roof sheathing
        /
       /  3.5" closed-cell spray foam (R-24.5)
      /═══════════════════════════════════════════
     /
    /  3.75" remaining cavity: R-13 batt
   /═════════════════════════════════════════════
   └── 1/2" drywall

   Total: R-24.5 + R-13 = R-37.5

Why 3.5” minimum foam thickness:

IRC R806.5 (unvented assemblies in Zone 6) requires the air-impermeable insulation to provide at least R-25 of the total assembly R-value to prevent condensation on the sheathing
3.5” of closed-cell foam at R-7/inch = R-24.5, which approaches R-25 — some inspectors accept this, others may require 4” (R-28). Confirm with inspector.
Less than 3.5” (e.g., 2”) does NOT meet the Zone 6 vapor ratio and risks condensation

Pros:

Meets or exceeds R-49 code target with furring — no depth-limited exception needed
Superior air seal on the slopes — no gaps, no wind washing
No vent baffles needed on sloped sections (unvented assembly)
Foam contractor only needed for the sloped sections (~350 sq ft) — everything else remains DIY

Cons:

Requires a spray foam contractor — not DIY
Cost: ~ $1, 800 - 2, 450 f or t h e f o am p or t i o n +$ 200-350 for batts and furring ≈ $2,000-2,800 total
Creates a hybrid vented/unvented roof — slopes are unvented (foam), peak and attic spaces behind knee walls remain vented (baffles). The transition detail between vented and unvented zones must be air-sealed carefully.

Hybrid roof transition detail: The point where the unvented sloped section (foam) meets the vented peak area (baffles + cellulose) needs a continuous air seal. The spray foam contractor should extend the foam a few inches past the transition point to create an overlap with the baffled zone above.

Cut-and-Cobble Rigid Foam — Evaluated and Declined

DIY rigid foam board (polyiso) cut to fit each rafter bay and sealed with can foam was considered as a DIY alternative to spray foam. Declined because: (1) hundreds of foam-sealed joints are tedious and unreliable long-term as the building moves, (2) doesn’t bond to sheathing like spray foam, (3) inspector may not accept it as an unvented assembly, (4) marginal cost savings vs. professional spray foam for a small area (~350 sq ft).

Zone 1 Comparison

Approach	R-Value (no furring)	R-Value (w/ furring)	DIY?	Cost (~350 sq ft)	Vent Baffles?	Meets R-49?
Option A: Batts	R-25	R-38	Fully DIY	$400-605	Yes	No (needs exception)
Option B: Flash + batt	R-37.5	R-50.5	Contractor for foam	$2,000-2,800	No (unvented)	Yes (with furring)

Zone 2: Peak Area (F→G→H Around Point S) — No Constraint

Near the peak, the ceiling drywall runs approximately horizontal while the roof continues to slope up, creating a deep triangular cavity that easily exceeds 13.5” depth.

Method: Dense-pack blown cellulose to R-49 through drywall — same technique as the garage ceiling. No depth limitation here.

Zone 3: Knee Walls (~3’7” Vertical Walls Each Side)

The knee walls are formed by the truss web members at the 5’ and 19’ marks. Measure the actual web member dimensions on site — truss drawings show W1 as 2x4 and W3 as 2x6; which forms the knee wall determines batt size.

Assembly (two-part — both required):

Part A: Knee Wall Face (loft side)

If 2x4 webs (3.5” cavity): R-13 or R-15 kraft-faced batts, facing toward loft interior
If 2x6 webs (5.5” cavity): R-21 kraft-faced batts (same as garage walls)

Part B: Rigid Air Barrier on Attic Side (CRITICAL)

Wind Washing — #1 Knee Wall Failure

Without a rigid air barrier on the attic (back) side of the knee wall, soffit air blows directly through the batt insulation (“wind washing”), destroying its effective R-value. This is the most common insulation failure in attic truss construction.

Install 1” rigid foam board (XPS or polyiso) on the back side of each knee wall
Seal all edges and seams with spray foam
Adds ~R-5 to the knee wall assembly
Alternative: OSB/plywood sheathing sealed at edges (air barrier only, no added R-value)

Zone 4: Attic Floor Behind Knee Walls (Triangular Spaces)

The triangular attic spaces on each side (from knee wall to eave, ~5’ deep) should have the floor insulated with blown cellulose to R-49. Plenty of depth on the flat bottom chord. This insulation sits on top of the garage ceiling assembly.

Method: Blown cellulose to R-49 on the attic floor — same material and blower as the garage ceiling and peak area.

Requires:

Raft-R-Mate baffles in every rafter bay from eave blocking up through this zone
Eave blocking between truss bays at top plate (prevents wind washing while allowing baffle airflow)
Access door (~22”×30”, insulated and weatherstripped) through each knee wall — required by IRC R807.1 for these spaces, and needed to inspect the Zone 3 rigid air barrier long-term. See 2026-04-23 decision. No hatch to the peak cavity (Zone 2) — dense-pack fills it and clear height is <30”.

Alternative Under Consideration (2026-06-23): Condition the Triangles for Storage

This reopens the decided Zone 3 / Zone 4 design

The Zones 3 & 4 above keep the triangular spaces outside the envelope (vented attic, kneewall is the boundary). The owner is weighing the opposite: move the thermal/air boundary from the kneewall out to the roof slope, bringing both triangles inside the conditioned envelope so they become usable conditioned storage. This is a real envelope change — documented here as an evaluated option; not yet decided. It must be settled before insulation/drywall (pre-insulation gate).

Why consider it: Conditioning both perimeter triangles (~5’ deep × ~40’ each long wall) reclaims roughly 280–400 sq ft of usable low storage wrapping the loft — a large gain in a 560 sq ft unit for an office / home-theater / future-rental. The idea originates from the future-apartment thinking: the kneewall depth becomes a real bedroom reach-in/wardrobe closet (the “1BR” definition) plus conditioned living-area overflow — extra blankets, books, electronics like game controllers, etc. This is the key distinction driving the decision: those items are exactly what a cold vented attic ruins, so the closet is only worth building if the space behind the kneewall is conditioned, not just vented-and-stored. It also eases the small loft-bathroom storage crunch (real conditioned shelving along the adjacent kneewall) — see Loft Bathroom Waterproofing. Connects the already-planned kneewall closet and kneewall built-ins to the envelope decision that determines how good they can be.

What changes: Delete the kneewall insulation (Zone 3) and the triangle-floor cellulose (Zone 4); instead insulate the roof slope continuously down to the eave. The kneewall stays as a finish wall (wrapping the structural truss webs) with cabinet/door openings into the conditioned space behind — but it is no longer the air or thermal barrier; the slope is. Fire separation is unaffected — the Type X garage ceiling under the triangle floor stays exactly as designed; this is purely a thermal/air-barrier move.

Two ways to insulate the slope-to-eave:

	3a — Vented cathedral to eave (recommended)	3b — Unvented hot-roof to eave
Assembly	Raft-R-Mate baffle → vent channel at deck → batt below; triangle conditioned underneath	Closed-cell flash-and-batt (or full CC) on the deck, no vent channel
Keeps the confirmed vented assembly?	Yes — extends it down the whole plane	No — reverses it to unvented (IRC R806.5)
Eave performance	Weaker — vent gap eats depth where it’s already pinched	Better — foam gives more R/inch in the pinch
Materials cost vs. current plan	~Cost-neutral (see below)	+$2,000–3,500 (foam area grows ~500 sq ft)

Cost (3a is roughly a wash): You stop buying ~400 sq ft triangle-floor cellulose + kneewall batts/rigid air barrier (≈ $670-1, 100 s a v e d) an d * in s t e a d * b u y 500 s q f t s l o p e ba ff l e + ba tt (\approx$ 600–900). Materials roughly cancel; the real costs are labor (insulating an awkward low slope) and the eave detail. Only full-foam 3b adds significant dollars. (Estimates — needs a real area takeoff.)

The eave pinch — the make-or-break detail (building durability / property damage)

At 10:12 pitch the deck drops to near-zero depth at the eave, so a vent channel + batt can’t hit much R-value in the last ~12–18″ — leaving a cold, under-insulated eave with conditioned, humid air now behind it. In a Michigan winter that risks condensation on the cold sheathing → mold/rot and warm-deck ice damming. Mitigations (all required if pursued):

Closed-cell foam the last 12–24″ at the eave — more R/inch where depth pinches, doubles as the air seal (a small targeted application even in the 3a vented scheme).

Continuous baffle soffit-to-ridge + eave blocking between truss bays so soffit air can’t wash the batt.

Unbroken air barrier up the slope — now doing the job the kneewall back used to; every gap is a condensation path.

Measure the eave heel depth on site before committing. If these aren’t raised-heel trusses (likely not), the pinch is real and the eave-foam detail is non-negotiable.

Re-confirm the assembly with the inspector — vented was confirmed once; extending/reversing it (esp. any R806.5 unvented portion) should be re-blessed.

Recommendation if pursued: 3a (vented to eave) + closed-cell foam at the eave pinch — preserves the vented strategy, ~cost-neutral on materials, hazard fully mitigable. Fall back to 3b only if on-site eave depth is too tight for any baffle+batt.

Contractor input 2026-06-29 — Midstate Spray Foam Insulation LLC (favors 3b, full-foam) · quote received 2026-07-01: $7,864

An insulator (Midstate Spray Foam Insulation LLC, owner Eli Coblentz — the Hershberger referral; name corrected 2026-07-01 from the quote letterhead — earlier notes called this “Mid Michigan,” a mishearing) walked the loft on 2026-06-29 and recommended spray-foaming the entire roof and conditioning the kneewall triangles as storage — i.e., the 3b unvented hot-roof variant applied to the whole plane. Their position: spray foam is the only way to get this roof to code, which tracks with the 2×8 top-chord depth limit (R-49 unreachable with batts/cellulose on the slopes). Caveats to weigh against the in-house 3a recommendation above: (1) full-deck foam reverses the confirmed vented assembly to unvented (R806.5) — re-confirm with inspector; (2) it disables the already-installed soffit/ridge vents; (3) full-roof foam is a larger, pricier scope than slopes-only. Quote (received 2026-07-01): $7, 864.00 * * — roo f * * 5½" c l ose d - ce ll$ 6,590 + gable ends & dormer walls ** $1, 274 * * . A t R - 7/ in c h, 5½" \approx * * R - 38 o n t h e d ec k * *, co m f or t ab l yc l e a r in g R 806. 5^{'} s R - 25 ai r - im p er m e ab l er a t i o (v s . t h e b or d er l in e 3.5"/ R - 24.5 t ha t s l o p es - o n l y Opt i o n Bw a scos t e d a t) . T hi s i s a * * f u ll - roo f * * n u mb er — co m p a re a g ain s tOpt i o n B^{'} s$ 2,000–2,800 slopes-only on total assembly performance + the conditioned-storage gain, not $/sq ft. See quote summary, 2026-06-29 - Site Visit - Midstate Spray Foam Insulation, and Insulation Contractors.

Gating open item: measure available eave/heel depth on site → then decide. Tracked in Considerations Backlog. Midstate’s full-roof quote ($7,864) is now in hand; get ≥1 competing full-roof quote before settling 3a-vs-3b.

Why DIY foam doesn't pencil out here — pro is cheaper than doing it yourself (analysis 2026-07-01; full quote scope)

Counter-intuitively, self-installing the closed-cell foam would cost the same or more than Midstate’s quote — the rare system in this project where DIY loses. Volume take-off matching the full quote scope (24×40, 10/12 pitch; dormer profiles per Job #123907 = TX/T1 6’ commons at 6/12 with 2×4 walls; gable ends = 24’ base × 10’ rise triangles):

Surface Area Depth Board-feet
Main roof plane (both slopes, eave→ridge) ~1,250 sq ft 5.5” 6,875
Dormer roofs (2×, ~6’×8’, 6/12) ~100 sq ft 5.5” ~550
Dormer walls (2×4, fronts + cheeks) ~75 sq ft 3.5” ~265
Gable-end triangles (2×) ~240 sq ft 3.5” ~840
Total ~8,530 bd-ft

(Dormer projection ~8’ and wall heights are estimated — but dormers + gable walls are only ~19% of the job, so even ±50% on them moves the total ~±9%; the conclusion holds.)

Path $/bd-ft Cost
Midstate (pro, installed — full quote) $0.92 $7,864
DIY kits — best case (bulk system, ideal yield) ~$1.05 material ~$9,000
DIY kits — realistic (+25% waste + PPE/access) ~$1.10-1.40 $12, 000 -$ 16,000

DIY loses by ~ $1, 100 (b es t c a se) t o$ 8,000 (realistic) on the identical scope, and you’d still do all the labor and carry the risk.

Why the curve inverts: contractors buy closed-cell as matched 55-gal A/B drum sets at ~ $0.40 - 0.55/ b d - f t * *; D I Y * * ki t s * * re p a c ka g e t h e * s am ec h e mi s t ry * in t o d i s p os ab l ecy l in d ers a t * *$ 1.00-1.50/bd-ft — a 2-3× material markup that alone exceeds the pro’s whole labor bill. Their truck-mounted, heated, ratio-controlled rig also sprays faster with far less waste. Net: ** $0.96/ b d - f t in s t a ll e d i s a l o w n u mb ere v e nb y p ros t an d a r d s * * (p roc l ose d - ce ll i s u s u a ll y$ 1.25-2.00 installed) — a price a kit can’t touch.

DIY also carries risks the quote silently absorbs: isocyanate exposure (wants a supplied-air respirator, 24 hr re-occupancy), exotherm/fire if 5½” isn’t built up in ≤2” lifts, and off-ratio foam if the garage is outside ~65-85°F (crumbly, permanently off-gassing, scrape-out-and-redo — and drum kits aren’t returnable). On an assembly sealed behind drywall forever, “great work the first time” has real dollar value — hence the Amish shops’ stellar-reputation-at-a-low-price combination is the right buy.

Levers to actually spend less are assembly, not who-sprays-it: the DIY batts+furring (Option A, ~ $400 - 605) an d * * f l a s h - an d - ba tt * * (Opt i o n B,$ 2,000-2,800) paths above are far cheaper — but neither delivers the conditioned-triangle hot-roof. That specific goal points at full-depth foam, and full-depth foam points at hiring it out. Before treating $7, 864 (or t h e$ 6,590 roof-only line) as the benchmark for competing bids, confirm 5½” is a firm spec (R-38 on the deck) and that the inspector accepts it on an unvented deck — if they want closer to R-49, the depth and every quote grow.

Surface	Area	Depth	Board-feet
Main roof plane (both slopes, eave→ridge)	~1,250 sq ft	5.5”	6,875
Dormer roofs (2×, ~6’×8’, 6/12)	~100 sq ft	5.5”	~550
Dormer walls (2×4, fronts + cheeks)	~75 sq ft	3.5”	~265
Gable-end triangles (2×)	~240 sq ft	3.5”	~840
Total			~8,530 bd-ft

Path	$/bd-ft	Cost
Midstate (pro, installed — full quote)	$0.92	$7,864
DIY kits — best case (bulk system, ideal yield)	~$1.05 material	~$9,000
DIY kits — realistic (+25% waste + PPE/access)	~$1.10-1.40	$12, 000 -$ 16,000

Reaching R-49 (only if you choose to) — add foam, NOT flash-and-batt (rev. 2026-07-01)

First: R-49 is likely not required here. Per the installer, Clare accepts the 5½” (see the Code Targets callout) — so this whole section is probably moot and the quote stands as-is. It matters only if you choose to upgrade for comfort/resale margin. The as-quoted 5½” closed cell ≈ R-36–38. The two ways to reach R-49 on the ~1,350 sq ft roof plane, purely on cost:

Option What Added cost Trade-off
A — Flash-and-batt keep 5½” foam + unfaced R-13 batt in 2×4 furring below ~ $1, 100-1, 350 D I Y * * (ba tt$ 700–900 + furring ~ $400) — or * *$ 700–900 if the sloped-ceiling furring is going in anyway your labor + ~3.5” slope headroom
B — Thicker foam bump foam 5½” → 7” **~ $1, 850 p ro * * (+ 2, 025 b d - f t @$ 0.92/bd-ft) turnkey; ~1.5” headroom only

**Cost-wise flash-and-batt is ~ $550-850 c h e a p er — b u tt h e in s t a ll ers t ro n g l ys t eers a w a y f ro mi t, an d sos h o u l d w e . * * M i d s t a t e u se d t o in s t a ll f l a s h - an d - ba tt an d * * s t o pp e d * * : w a r min t er i or ai r mi g r a t es t h ro ug h t h e ai r - p er m e ab l e ba tt, co n d e n seso n t h eco l df o am f a ce, an d * * se v er a l o f t h e i r in s t a ll s f ai l e d w i t hina f e w ye a rs * * ([[40 - M ee t in g s /2026 - 07 - 01 - C a ll - M i d s t a t e Sp r a y F o am R - 49 an d Fl a s h - an d - B a ttG u i d an ce ∣ c a ll 2026 - 07 - 01]]) . T ha t^{'} s t h ec l a ss i c * u n d er - f l a s h e d * co n d e n s a t i o nm o d e; a 5½" f l a s h (R - 38 \approx 78$ /bd-ft drops with thickness — the rig’s already set up, so the 5½”→7” bump costs less than the ~$1,850 linear figure above), if you upgrade at all, add foam — don’t batt. Ask Eli for the incremental-thickness price. Verdict: flash-and-batt is off the table.

Option	What	Added cost	Trade-off
A — Flash-and-batt	keep 5½” foam + unfaced R-13 batt in 2×4 furring below	*~ $1, 100-1, 350 D I Y * (ba tt$ 700–900 + furring ~ $400) — or * $ 700–900 if the sloped-ceiling furring is going in anyway*	your labor + ~3.5” slope headroom
B — Thicker foam	bump foam 5½” → 7”	*~ $1, 850 p ro * (+ 2, 025 b d - f t @$ 0.92/bd-ft)	turnkey; ~1.5” headroom only

Loft Insulation Summary Table

Zone	Assembly	Method	R-Value	Settling Risk
Sloped ceiling — Option A (batts + furring)	2x8 + baffle + 2x4 furring	R-25 + R-13 batts	R-38	None
Sloped ceiling — Option B (flash + batt + furring)	3.5” CC foam + batt + 2x4 furring	Spray foam + R-13 batts	R-50.5	None
Peak area	Deep cavity + baffle	Dense-pack cellulose	R-49+	None (flat)
Knee walls	Web cavity + air barrier	Batts + 1” rigid foam (back)	R-13–R-26	None
Attic floor behind knee walls	Flat on bottom chord	Blown cellulose	R-49	None (flat)

Options Previously Evaluated and Declined

Dense-pack cellulose on slopes: Settling risk at 10:12 pitch over decades; batts are more reliable for sloped cavities
Full closed-cell spray foam (entire cavity): R-47 in 7.25”, nearly meets R-49 — but costs $3, 800 - 5, 075 f or t h es l o p es a l o n e v s .$ 2,000-2,800 for flash-and-batt with furring that exceeds R-49. The full-fill approach is 35-80% more expensive for a marginally lower R-value.
Cut-and-cobble rigid foam (DIY): Tedious, unreliable long-term air seal at joints, inspector may not accept as unvented assembly
Exterior rigid foam (only if reroofing): Not applicable — roof is already complete

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 (All Zones Combined)

Sloped ceiling (Zone 1) — two options:

Option A — Batts + furring (DIY): ~ $400 - 605 f or 350 s q f t ($ 1.15-1.75/ft²)
Option B — 3.5” flash + batt + furring (contractor + DIY): ~ $2, 000 - 2, 800 f or 350 s q f t ($ 5.70-8.00/ft²) Other zones:
Knee wall batts + rigid foam air barrier: ~$1.50-2.50/ft²
Attic floor cellulose (behind knee walls): ~$0.60-1.00/ft²
Peak area cellulose: included in garage ceiling blower run Declined:
~~Full-depth closed-cell spray foam (7.25”): ~$10.85-14.50/ft²~~ — flash-and-batt achieves R-50.5 for less
~~Cut-and-cobble rigid foam: ~$1.50-2.00/ft²~~ — unreliable long-term air seal

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:

Rim joists (where floor meets foundation) - Use spray foam
Top plates (where walls meet ceiling) - Spray foam or caulk
Bottom plates (where walls meet floor/subfloor) - Spray foam
Electrical boxes - Use foam gaskets or putty pads
Penetrations - Spray foam around all pipes, wires, ducts
Eave blocking between truss bays at top plate - Prevents wind washing while allowing baffle airflow
Knee-wall backs (loft) - Air barrier critical
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 $200 o nai rse a l in gg i v esyo u 80$ 5,000 on spray foam.

Open Questions

~~Vented vs unvented cathedral assembly preference by local inspector.~~ RESOLVED: Vented assembly confirmed — Raft-R-Mate baffles in all rafter bays, continuous soffit-to-ridge airflow.
Verify depth-limited exception with inspector: If choosing Option A (batts only, R-38), confirm the inspector will accept R-38 on the sloped ceiling sections where the 2x8 top chord prevents achieving R-49. Reference IRC R402.2.2. If they won’t accept R-38, Option B (flash-and-batt, R-50.5) meets code without exception.
Get spray foam quote for slopes (if considering Option B): Only the sloped ceiling sections (~350 sq ft) need spray foam — 3.5” of closed-cell. Confirm the contractor can do a small-area job and ask about the vented/unvented transition detail at the peak.
Confirm foam thickness with inspector for Option B: IRC R806.5 Zone 6 requires air-impermeable insulation to provide R-25 of the total assembly R-value. 3.5” closed-cell = R-24.5 (borderline). Some inspectors may require 4” (R-28). Clarify before contracting.
Measure knee wall web member dimensions on site: Truss drawings show W1 (2x4) and W3 (2x6) — determine which member forms the knee walls to specify the correct batt width.
Choose Option A vs. Option B for slopes: May depend on inspector’s position, spray foam quote, and budget. Option A is $400 - 605 (D I Y, R - 38, n ee d se x ce pt i o n) . Opt i o n B i s$ 2,000-2,800 (contractor foam, R-50.5, meets code).

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 ceiling general approach (REVISED 2026-04-01): Batts or flash-and-batt — cellulose declined for slopes due to settling risk at 10:12 pitch - DECIDED
Sloped ceiling method — choose one:
- Option A (DIY): R-25 cathedral batts + 2x4 furring + R-13 batt = R-38 (~$400-605)
- Option B (contractor + DIY): 3.5” closed-cell spray foam + batt + 2x4 furring = R-50.5 (~$2,000-2,800) — meets R-49 code target without exception
- Decision depends on: inspector’s position on depth-limited exception, budget, and whether R-38 vs. R-50.5 matters for the loft’s intended use
Peak area: Dense-pack cellulose R-49 through drywall (deep cavity, no depth limitation) - DECIDED
Knee walls: Batts (R-13 or R-21 depending on web member size) + rigid foam air barrier on attic side — measure actual web dimensions on site. ⚠️ UNDER RECONSIDERATION (2026-06-23): may be deleted entirely if the triangles are conditioned instead — see Conditioned-Triangle Alternative.
Attic floor behind knee walls: Blown cellulose R-49 ⚠️ UNDER RECONSIDERATION (2026-06-23): the prior R-49-floor / vented-triangle decision may be superseded by conditioning the triangles (boundary moves to the slope). See Conditioned-Triangle Alternative. Gated on on-site eave-depth measurement.
Timing: Immediately after main level (same material order) - DECIDED

Envelope Tightness → ERV Pre-Rough

The split-thermal-envelope strategy above (flash-and-batt sloped sections + dense-pack cellulose + R-49 attic floor + taped sheathing + sealed-combustion boiler) intentionally produces a very tight loft envelope. The mini-splits do not move outside air. Without an active fresh-air strategy, ASHRAE 62.2’s ~30 CFM-continuous baseline for the 560 sq ft loft + 2 occupants is met only through uncontrolled envelope leakage — exactly the failure mode this insulation strategy is designed to prevent.

The HVAC plan therefore includes a pre-rough for a Panasonic Intelli-Balance 100 ERV during framing (duct chases, exterior wall penetrations, condensate stub, capped 120V outlet), with the unit purchase and install deferred until apartment-conversion trigger (5+ years out, uncertain). The pre-rough preserves the option cheaply (~ $200) w hi l e l e tt in g c u rre n t o w n er - u se d e f er t h e$ 1,100-1,300 unit purchase.

See ERV Pre-Rough (Install Deferred) and 2026-05-15 — Loft ERV Pre-Rough Now, Install Later for the full rationale and pre-rough scope.

References

Five cathedral ceilings that work: https://www.finehomebuilding.com/project-guides/insulation/five-cathedral-ceilings-that-work
Related chat summary: Garage_Insulation_Summary

Garage Construction Project

Explorer

Insulation Strategy

Context

Baseline Thermal Observations (Uninsulated)

Code Targets (Zone 6a - Michigan)

Walls (Main Garage)

Ceiling (Garage Ceiling / Loft Floor)

Loft Ceiling (Attic/Sloped Roof)

Assembly Options (Walls & Garage Ceiling)

Main Garage Walls (2×6 @ 16” OC)

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

Hybrid Approach (BEST VALUE RECOMMENDATION)

Assembly Options (Loft Sloped Roof Sections)

✅ DECIDED: Zone-Based Approach (Revised 2026-04-01)

Zone 1: Sloped Ceiling (Top Chord E→F and H→I) — Depth-Limited

Option A: R-25 Cathedral Batts + 2x4 Furring (DIY — BEST DIY VALUE)

Option B: 3.5” Closed-Cell Flash + Batt + 2x4 Furring (Contractor + DIY — BEST PERFORMANCE)

Zone 1 Comparison

Zone 2: Peak Area (F→G→H Around Point S) — No Constraint

Zone 3: Knee Walls (~3’7” Vertical Walls Each Side)

Zone 4: Attic Floor Behind Knee Walls (Triangular Spaces)

Alternative Under Consideration (2026-06-23): Condition the Triangles for Storage

Loft Insulation Summary Table

Options Previously Evaluated and Declined

Cost Ranges (per sq ft)

Walls

Garage Ceiling

Loft (All Zones Combined)

Air-Sealing Priorities (CRITICAL for Performance)

Open Questions

ROI Considerations

Decisions

Building Envelope Strategy ✅ DECIDED: Complete Both Levels

Main Garage (Lower Level) - Immediate

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

Envelope Tightness → ERV Pre-Rough

References

Graph View

Table of Contents

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