Insulation Baffles and Eave Blocking Guide
A reference guide explaining insulation baffles, eave blocking, and why these components are critical for vented roof assemblies. Created to support the garage project’s R-49 blown cellulose insulation plan.
How Vented Roof Systems Work
A vented roof assembly relies on continuous airflow from the eaves to the ridge:
RIDGE VENT
(hot/moist air exits)
↑
/\
/ \
Cool air / \ Cool air
rises / → ← \ rises
/ ↑↑ \
/ \
/ LOFT \
/ ROOM \
/____(14' W)____\
/ \
/ GARAGE \
/________________________\
↑ ↑
SOFFIT VENT SOFFIT VENT
(cool air in) (cool air in)
The Ventilation Principle
- Air Intake: Cool outside air enters through vented soffits at the eaves
- Air Movement: Air travels UP through a channel under the roof sheathing
- Air Exhaust: Warm, moist air exits through the ridge vent at the peak
Why Ventilation Matters
This continuous airflow provides critical benefits:
| Benefit | What It Prevents |
|---|---|
| Moisture removal | Condensation, rot, mold in roof cavity |
| Temperature regulation | Ice dams in winter (snow melting unevenly) |
| Roof longevity | Premature shingle/sheathing deterioration |
| Insulation performance | Moisture-damaged insulation loses R-value |
The Problem: Insulation Blocks Airflow
When you install insulation (especially blown cellulose or fiberglass) in a vented roof assembly, the insulation naturally wants to fill the ENTIRE cavity — including the critical air channel.
Without Baffles (Incorrect Installation)
Roof sheathing → /
/ ← Insulation fills everything!
/ NO air channel remains
/~~~~~~~~~~~~~~~~~~~~~~
/~~~~~~~~INSULATION~~~~~~
/~~~~~~~~~~~~~~~~~~~~~~~~~~
/____________________________
↑
SOFFIT
(air BLOCKED - ventilation fails!)
Results of blocked ventilation:
- Moisture accumulates in roof cavity
- Wood rot and mold growth
- Ice dams form in winter
- Premature roof failure
- Insulation effectiveness reduced by moisture
The Solution: Insulation Baffles
Baffles (also called “vent chutes,” “rafter vents,” or by brand name “Raft-R-Mate”) are rigid channels that maintain an air gap between the insulation and roof sheathing.
What Baffles Look Like
┌─────────────────────────┐
│ ___________________ │ ← Fits between rafters/trusses
│ / \ │ (14.5" wide for 16" OC)
│/ \ │ (22.5" wide for 24" OC)
│ │ │
│ AIR CHANNEL │ │ ← Creates protected channel
│ (1-2") │ │ against roof sheathing
│ │ │
│\ /│ │ ← Insulation goes on
│ \___________________/ │ │ the room side
│ │ │
└─────────────────────────┘
Typically 4 feet long
(can overlap for longer runs)
With Baffles (Correct Installation)
Roof sheathing → /
/ ← 1-2" air gap maintained
BAFFLE → /___________________________
/ |
/~~~~~~~~INSULATION~~~~~~~~~~~|
/~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~|
/_______________________________|
↑
SOFFIT
(air flows freely UP into the channel)
Common Baffle Products
| Product | Material | Width | Notes |
|---|---|---|---|
| Owens Corning Raft-R-Mate | Foam | 14.5” or 22.5” | Most common, inexpensive |
| ADO AccuVent | Plastic | 14.5” or 22.5” | More rigid, durable |
| Durovent | Plastic | 14.5” or 22.5” | Good airflow capacity |
| Site-built | Rigid foam board | Custom | Cut from 1” polyiso or XPS |
Typical cost: $2-4 per baffle, need one per rafter bay
Eave Blocking: The Critical Foundation
Blocking is a piece of wood (or rigid foam) installed at the eave — where the roof meets the top of the wall. This is where baffles BEGIN and where the air channel STARTS.
The Eave Detail (Cross-Section)
ZOOMED VIEW AT THE EAVE:
Roof sheathing
/
/
/ ← Air channel (1-2")
BAFFLE → /==================
/ ‖
/ INSULATION ‖
/ ‖
BLOCKING → [████████]←── Gap for ‖
‖ ↑ airflow ‖
Top plate → [════════] ‖
| | ↓
| | Air flows UP
____| |____
| SOFFIT |
| (vented) |
What Blocking Does
Blocking serves four critical functions:
1. Provides Attachment Point for Baffles
The baffle needs to fasten to something at its bottom edge. Without blocking, the baffle hangs loosely and can shift or fall.
WITHOUT BLOCKING: WITH BLOCKING:
Baffle → /==== Baffle → /====
/ | / |
/ | / |
/ ↓ / |
/ (falls!) / |
/ / [████] ← Blocking
[════] Top plate [════] Top plate
2. Stops Insulation from Falling into Soffit
Blown cellulose is loose and fluffy. Without a barrier, it falls into the soffit cavity.
WITHOUT BLOCKING: WITH BLOCKING:
/~~~~~insulation~~~~ /~~~~~insulation~~~~
/~~~~~~~~falls~~~~~~~~ /~~~~~~~~STOPS~~~~~~~~
/~~~~~~~~~~↓~~~~~~~~~~~ /~~~~~~~~~~HERE~~~~~~~~
/___________↓___________ /________[████]________
↓ ↓ ↓ ║
[ SOFFIT ] [ SOFFIT ]
(full of insulation!) (clear for airflow)
3. Creates Defined Air Inlet
The blocking is positioned (or has gaps/holes) to allow air through while stopping insulation.
Insulation stops here
↓
/~~~~~~~~~~~~~~~~~~~~~~~
/~~~~~~~~~~~~~~~~~~~~~~~~
/~~~~~~~~~~~~~~~~~~~~~~~~~
/___________[████]_________ ← Blocking with gap above
↑
Air inlet
↑
From vented soffit
4. Defines Boundary Between Zones
Creates a clear separation between:
- Soffit zone (below blocking) — uninsulated, for airflow
- Insulation zone (above blocking) — filled with insulation
Why Blocking Must Be Done During Framing
The Construction Sequence
CONSTRUCTION TIMELINE:
1. FRAMING (trusses installed)
│
├──→ BLOCKING GOES HERE ← Easiest time!
│ - Wide open access from above
│ - Access from sides (no soffits yet)
│ - Framing crew has tools & skills
│
↓
2. ROOF SHEATHING INSTALLED
│
├──→ Now harder: Can only work from inside
│
↓
3. ROOFING INSTALLED
│
├──→ Can't access from above at all
│
↓
4. SOFFITS INSTALLED
│
├──→ Very hard: Inside access only, can't see from outside
│
↓
5. INSULATION INSTALLED
│
├──→ Too late! Would need to remove insulation
│
↓
6. DRYWALL INSTALLED
│
└──→ Nearly impossible without demolition
Access Comparison
| Timing | Access Quality | Labor Multiplier |
|---|---|---|
| During framing | Easy - open access all sides | 1x (baseline) |
| After roof sheathing | Hard - inside only | 2-3x |
| After soffits | Very hard - blind work | 4-5x |
| After insulation | Extremely hard - remove first | 10x+ |
| After drywall | Near impossible | Major demolition |
Physical Access Challenges
For attic trusses like the garage project:
ATTIC TRUSS CROSS-SECTION:
Peak
/\
/ \ ← Easy access in center
/ \
/ \
/________\ ← Loft ceiling
/| |\
HARD TO / | Loft | \ HARD TO
ACCESS → / | Room | \ ← ACCESS
/ |________| \
/______| |______\
↑ ↑
EAVE AREAS
(blocking location)
The eave areas are:
- At the very edge of the building
- Minimal headroom (heel height may be only 6-12”)
- Triangular spaces that get tighter toward the edge
- 5-8 feet from any practical access point
- Must be done for EVERY rafter bay (20+ per side)
Attic Truss Considerations
For buildings with attic trusses (like the garage project with 14’ loft room):
Where Baffles and Blocking Are Needed
ATTIC TRUSS DESIGN:
Peak
/\
/ \
SLOPED SECTION / \ SLOPED SECTION
← Baffles here / \ Baffles here →
/________\
/| |\
KNEE WALL / | Loft | \ KNEE WALL
AREA / | Room | \ AREA
/ | (14') | \
/____|________|____\
↑ ↑
BLOCKING BLOCKING
LOCATION LOCATION
Sloped sections above loft room:
- Need baffles to maintain ventilation channel
- Need blocking at eaves to support baffles
- Will be filled with R-49 blown cellulose
Knee wall areas (triangular sides):
- May be left as storage space
- May be insulated at the knee wall itself
- Access through small doors for maintenance
Heel Height Matters
The “heel height” is the depth of the truss at the eave:
HEEL HEIGHT COMPARISON:
SHALLOW HEEL (problematic): RAISED HEEL (better):
/ /
/ /
/ /
/ /____
/ Only 4" / | 10-12"
/_____ ← Heel / | ← Heel
[═════] [═══════]
Less room for insulation More room for insulation
+ airflow at eave + airflow at eave
Typical heel heights:
- Standard: 6-8” (workable)
- Raised heel (energy truss): 10-14” (better)
- Shallow: 3-5” (may need special solutions)
The Complete System
Full Eave Detail (Cross-Section)
THE COMPLETE VENTILATION SYSTEM AT THE EAVE:
Roof sheathing ─────────────→ /
/
1-2" air channel ───────→ / ══════════════════
/ ║
Baffle ─────────────────→ / ════════════════════╣
/ ║
Blown insulation ────→ /~~~~~~~~~~~~~~~~~~~~~~~║
(R-49 cellulose) /~~~~~~~~~~~~~~~~~~~~~~~~~║
/~~~~~~~~~~~~~~~~~~~~~~~~~~║
Blocking ──────────→ [████████] ║
(with air gap above) ↑ ║
│ Air gap ║
Top plate ───────────→ [════════════] ↓
| |
Wall stud ───────────→ | | Air flows UP
| | from soffit
Soffit ──────────────→ ___| |___
| VENTED |
| SOFFIT |
↑
Outside air IN
Component Checklist
For a properly vented roof assembly, you need:
- Vented soffits — Perforated or slotted aluminum soffit panels
- Blocking at eaves — Wood blocking between each rafter/truss bay
- Air gap in blocking — Space above blocking for air to enter
- Insulation baffles — One per rafter bay, starting at blocking
- Proper insulation depth — Filling cavity without compressing baffles
- Ridge vent — Continuous exhaust at the peak
Questions to Ask Your Contractor
When verifying ventilation provisions with your framing contractor:
-
“Will there be blocking installed at the eaves for insulation baffles?”
- Confirms blocking is in scope
- If no, request it be added
-
“What is the heel height of the trusses at the eave?”
- Determines how much space exists for insulation + airflow
- Less than 6” may need special attention
-
“How will airflow be maintained from soffit vents to ridge vent?”
- Open-ended question to hear their ventilation approach
- Good contractors will have thought about this
-
“Are the trusses designed to accommodate vented insulation?”
- Truss engineer may have already specified provisions
- Check engineering documents for notes
Cost and Labor Estimates
If Done During Framing (Recommended)
| Item | Materials | Labor | Total |
|---|---|---|---|
| Blocking (lumber) | $50-100 | Included in framing | $50-100 |
| Baffles (40-60 pieces) | $100-200 | DIY or incl. in insulation | $100-200 |
| Total | $150-300 |
If Done After Framing (Retrofit)
| Item | Materials | Labor | Total |
|---|---|---|---|
| Blocking (lumber) | $50-100 | $300-600 (difficult access) | $350-700 |
| Baffles (40-60 pieces) | $100-200 | $200-400 | $300-600 |
| Soffit removal/reinstall (if needed) | $50-100 | $200-400 | $250-500 |
| Total | $900-1,800 |
Key insight: Doing it during framing costs 1/6th to 1/10th of retrofit pricing.
Related Documents
- Insulation Strategy — Overall insulation approach and R-values
- Insulation Execution — Step-by-step insulation installation plan
- Critical Pre-Insulation Requirements — Pre-insulation checklist
- 2025-12-01 - Call - Marcus WRB and Venting Confirmation — Contractor verification call
- Truss delivery packet — Truss engineering documents
References
- Fine Homebuilding: “Insulating a Roof Properly” — https://www.finehomebuilding.com/project-guides/insulation/five-cathedral-ceilings-that-work
- Building Science Corporation: “Vented Roof Assemblies” — https://www.buildingscience.com/
- Owens Corning Raft-R-Mate Product Info — https://www.owenscorning.com/
Created: November 26, 2025 Purpose: Reference guide for garage project insulation planning