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An unvented attic is defined by its roof-ventilation strategy, not by where the insulation sits. Put insulation at the roof deck and the attic can become part of the home’s thermal boundary—a conditioned unvented attic. Put insulation at the ceiling line and the attic remains outside that boundary—an unconditioned unvented attic. Both arrangements can work, but each needs a deliberate solution for moisture at the underside of the roof sheathing.
Joseph Lstiburek’s BSI-119: Conditioned Unconditioned, published November 15, 2020, lays out the choices. The practical decision is not simply whether to vent the roof; it is how insulation, air control, vapor movement, rain control, climate and combustion safety work together.
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1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minuteWhat “unvented” and “conditioned” actually describe
These are two different labels for two different properties of an attic:
| Term | What it describes | Typical insulation location | Relationship to the house |
|---|---|---|---|
| Unvented | The roof/attic has no conventional vented air channel from soffit to ridge or gable. | Roof deck, above the roof deck, ceiling line, or a combination. | Does not by itself say whether the attic is inside the thermal boundary. |
| Conditioned unvented attic | The attic is inside the home’s thermal and air boundary. | At the roof deck, or in an assembly that places the boundary at the roof. | Attic air is thermally connected to the occupied space. |
| Unconditioned unvented attic | The attic is outside the home’s thermal boundary even though conventional vents are sealed or omitted. | At the ceiling line. | Attic air remains outside the conditioned space. |
Calling an attic “unvented” therefore does not tell you where insulation belongs. A roof-deck assembly can be unvented and conditioned; a ceiling-insulated assembly can be unvented and unconditioned. The enclosure must be designed around that distinction.
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The moisture problem the assembly must solve
Roof sheathing can accumulate damaging moisture when warm, humid air reaches a cold underside. Lstiburek states the central principle plainly: “The key to unvented attic and roof assemblies is to control condensation or moisture accumulation on the underside of the roof sheathing.” The objective is to keep the sheathing warm enough, dry enough and protected from uncontrolled air and vapor movement.
BSI-119 discusses a roof-sheathing temperature target above 45°F (7°C), using 68°F (20°C) indoor air and the monthly average outdoor temperature of the three coldest months in its calculations. Its model-code discussion assumes approximately 35% indoor relative humidity at 68°F in winter. Those are the article’s design assumptions, not a universal guarantee for every building. A project with higher indoor moisture, unusual occupancy or a different climate needs its own hygrothermal analysis.
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The article also gives moisture-content limits drawn from its discussion of field experience: keep sheathing below 20% by weight during the coldest part of winter and below 16% during summer and fall. Treat those figures as the article’s stated control limits, not as a promise that any product or detail will automatically achieve them.
Four ways to arrange an unvented roof
1. Insulation at the roof deck: a conditioned unvented attic
Insulating and air-sealing at the roof deck brings the attic into the thermal boundary. Ducts, air handlers and other mechanical equipment in the attic can then be inside the enclosure rather than exposed to outdoor temperature swings. A continuous roof-deck boundary can also simplify air-sealing compared with trying to seal every ceiling penetration.
Comfort and ice-dam performance may improve when the roof deck is kept warmer, but neither result is automatic. Thermal bridges, discontinuous air control, inadequate insulation or snow and weather conditions can still create cold areas. The complete roof assembly—not the word “unvented”—determines performance.
2. Insulation at the ceiling line: an unconditioned unvented attic
With insulation and air control at the ceiling, the attic stays outside the home’s thermal boundary. This can suit a building where ducts and equipment are not in the attic, but the ceiling plane must be continuous and airtight. Plumbing, electrical boxes, recessed fixtures, access hatches and top-plate joints are common leakage paths.
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In hot-humid climates, BSI-119 describes sealing or omitting soffit vents and using a vapor-diffusion vent or port near the upper attic or roof. The approach is discussed in connection with duct sweating and buried ductwork. It is an assembly-specific strategy: an ordinary open vent is not an equivalent substitute, and an incorrectly detailed port can admit rain or uncontrolled air.
3. Rigid insulation above the roof deck
Exterior insulation raises the sheathing temperature by placing thermal resistance above it. Boards may be vapor-impermeable or vapor-permeable; the article names rigid mineral wool, rigid fiberglass and other rigid-board options. The material choice does not remove the need for continuous air and water control.
The air-control layer belongs below the exterior boards. BSI-119 describes either a fully adhered membrane or airtight roof sheathing with taped or sealed seams. Multiple board layers with offset joints are recommended so that joints do not line up through the entire thickness and create three-dimensional airflow paths. The roof covering and flashing must still provide a water-shedding layer.
4. A hybrid assembly above and below the sheathing
Insulation can be split between the exterior and interior sides of the roof sheathing. The exterior share keeps the sheathing warmer; the interior share supplies the rest of the required thermal resistance. The necessary exterior proportion depends on climate zone and interior moisture load, so a ratio suitable for one building may be inadequate for another.
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Climate ratios in the 2015 IRC-based table
The table in BSI-119 is adapted from Table R806.5 of the 2015 International Residential Code. It defines the ratio as the exterior rigid-board or air-impermeable insulation R-value divided by the total insulation R-value. These are figures reported by the 2020 article and are not a statement of the code currently adopted in any particular locality. Verify the edition and amendments enforced by the building department for the project.
| Climate zone | Exterior rigid-board or air-impermeable insulation | Code-required total R-value in the cited table | Exterior share of total insulation |
|---|---|---|---|
| 1, 2 and 3 | R-5 | R-38 | 10% |
| 4C | R-10 | R-49 | 20% |
| 4A and 4B | R-15 | R-49 | 30% |
| 5 | R-20 | R-49 | 40% |
| 6 | R-25 | R-49 | 50% |
| 7 | R-30 | R-49 | 60% |
| 8 | R-35 | R-49 | 70% |
For example, the table’s 50% entry for climate zone 6 means the exterior rigid-board or air-impermeable portion is intended to provide half of the assembly’s total insulation R-value under that cited code framework. It does not mean every climate-zone-6 roof can use the same thickness, because product R-values, framing, interior humidity and the locally adopted code still control the design.
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How to control air, vapor and rain at the roof deck
Make the air-control layer continuous
Air leakage can transport far more moisture than slow vapor diffusion. Seal roof-deck panel seams, penetrations, perimeter joints and transitions to walls. If exterior boards are used, the layer beneath them must remain airtight; simply pressing boards against a leaky deck is not an air-control strategy.
Use exterior boards as a coordinated system
Install more than one board layer when needed, staggering and offsetting joints between layers. Tape, seal or otherwise detail joints according to the chosen assembly. Coordinate fasteners, curbs, skylights, valleys, eaves and wall intersections so that the air-control and water-control layers remain continuous.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchUnderstand a vapor-diffusion port
A vapor-diffusion port is not a generic hole and not an ordinary attic vent. BSI-119 describes it as vapor-open, airtight and watertight: it permits water vapor to diffuse outward while limiting air movement and rain entry. It is used for certain fibrous-insulation assemblies and must be detailed as part of the roof system. Substituting an open grille or an unsealed penetration defeats the stated purpose.
Can an unvented attic use fiberglass?
Yes, but only conditionally. BSI-119 discusses fiberglass and mineral wool in unvented assemblies; it does not say that installing a batt or loose-fill product by itself makes an attic safe or code-compliant. Fibrous insulation generally needs a properly designed air-control layer and a moisture-control strategy appropriate to the climate.
- At a roof deck, pair the fibrous insulation with a continuous, airtight boundary and the required condensation-control approach.
- In a hybrid roof, calculate the exterior share needed to keep the sheathing within the design moisture limits.
- At a ceiling line, make the ceiling plane airtight and address any hot-humid vapor-diffusion-port detail specified for the assembly.
- Do not confuse a vapor-open insulation with an assembly that is open to air or rain.
A rigid mineral wool board or rigid fiberglass board can be a component of an exterior-insulation design, but a retail product alone does not establish compliance, durability or combustion safety.
A practical design sequence
- Identify the intended boundary. Decide whether the attic will be inside the thermal boundary. If ducts or mechanical equipment are there, a roof-deck boundary may be appropriate; if not, a ceiling-line boundary may be simpler.
- Confirm the climate and code basis. Determine the project’s climate zone and the code edition adopted locally. Use the 2015 IRC-based ratios above only as the historical framework reported by BSI-119, not as current local requirements.
- Establish indoor moisture assumptions. Account for winter relative humidity, occupancy, ventilation and any process moisture. The article’s approximately 35% RH at 68°F is a model assumption, not a substitute for project conditions.
- Select the condensation-control method. Choose insulation above the deck, a permitted split above and below the deck, or the vapor-diffusion-port strategy where the assembly and climate support it.
- Draw the air and water layers continuously. Mark every seam, penetration, eave, ridge, valley, wall connection and roof opening. Specify membranes, tapes, sealants, flashings and board-joint offsets.
- Check sheathing temperature and moisture risk. Use a qualified hygrothermal calculation or building-science review to test the coldest winter conditions and summer drying. Compare the result with the article’s stated 45°F temperature target and moisture-content limits, recognizing that they are design references rather than guarantees.
- Coordinate the roof covering and trades. Exterior insulation changes nail lengths, flashing geometry and roof height. Resolve those details before installation rather than cutting through the air-control layer later.
- Verify combustion safety. Locate fuel-burning equipment and confirm that the proposed attic configuration meets the sealed-combustion requirement described below.
Combustion-appliance warning
BSI-119 warns that the unvented attic configurations and retrofits it discusses require sealed-combustion appliances. Gas furnaces and water heaters in the attic should be sealed-combustion types, with the appliance and enclosure reviewed by qualified professionals. Do not assume that an ordinary atmospheric-combustion appliance is safe merely because the attic has insulation or because the roof is unvented. Changing the enclosure can alter pressure relationships and combustion-air availability, creating a carbon-monoxide and code risk.
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Common failure modes and how to avoid them
“Unvented” is treated as a complete design
Closing soffit vents without relocating insulation, sealing the boundary or providing the required moisture-control detail can leave cold, wet sheathing. Define the thermal boundary first, then design the roof layers.
Exterior boards are installed over a leaky deck
Rigid insulation above the deck cannot compensate for an unsealed air path below it. Tape or seal the sheathing, or install the specified fully adhered membrane before the boards.
Board joints line up
Continuous joints through multiple layers can form airflow channels. Use multiple layers with offset joints and detail fasteners and edges so the air-control layer remains intact.
A diffusion port is replaced with an open vent
An open vent allows air and potentially wind-driven rain, while the described port is vapor-open but airtight and watertight. Use only a designed, weather-protected port assembly.
Fiberglass is installed without an air barrier
Fibrous insulation does not stop air leakage by itself. Seal the deck or ceiling plane and verify the vapor and drying paths for the climate.
Atmospheric-combustion equipment remains in the attic
Remove or replace it with an appropriate sealed-combustion appliance, and obtain a professional review before enclosing the space.
What the 2020 article does—and does not—establish
BSI-119, dated November 15, 2020, presents design alternatives and a table adapted from the 2015 IRC. The publisher’s archive lists it on November 16, 2020, as Issue 123; the archive is available at Building Science Corporation’s newsletter archive. The article does not establish current code adoption in a locality, a universal insulation ratio for every project, or a guarantee that any unvented roof will save energy or prevent ice dams.
For a real building, the designer must reconcile the adopted code, climate, interior moisture load, roof covering, structural details, air and water control, and appliance safety. The most reliable assembly is the one whose layers are continuous on paper and verifiable in the field.
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