Unvented (Hot Roof) Assemblies Under Metal Roofing: When They Work
An unvented roof brings the deck inside the thermal envelope. It is a legitimate assembly with real advantages — and it has no tolerance for gaps, guesswork, or a leaky ceiling.
An unvented roof — the trade calls it a hot roof — removes the vented air space entirely and brings the roof deck inside the insulated envelope. There is no soffit intake, no ridge outlet, and no attic. Done correctly, it is a sound assembly that solves problems a vented roof cannot. Done casually, it is one of the reliable ways to rot a roof deck from the inside, and the damage is hidden until it is substantial.
What Unvented Actually Means
In a vented roof, the deck sits outside the insulation, stays close to outdoor temperature, and has a stream of outdoor air carrying away any moisture that reaches it. In an unvented roof, the insulation is in direct, permanent contact with the underside of the deck, or above it, or both. The deck is now part of the conditioned side of the assembly, with no ventilation to remove moisture — so the design has to prevent moisture from arriving rather than rely on removing it.
Why Anyone Builds One
The reasons are practical. Complex geometry — multiple hips, valleys, dormers, skylights — makes continuous vent channels from soffit to ridge nearly impossible in every bay, and a partly vented roof is worse than a properly unvented one. Cathedral ceilings with shallow rafters lack depth for both a vent channel and enough insulation. Houses with no overhang have nowhere to put intake. Ductwork in the roof space is far better off inside the thermal envelope.
The Physics You Have to Respect
In an unvented assembly there is a first condensing surface — the coldest surface that indoor air can reach. In an all-cavity-insulation roof with no ventilation, that surface is the underside of the sheathing, and in a Southern Ontario January it will be well below the dew point of indoor air. Any humid indoor air that reaches it condenses, with no ventilation to remove the result and no drying path outward through a metal panel. The entire design problem is: keep indoor air away from that surface, or keep that surface warm enough that condensation cannot occur on it. Both accepted approaches do one or the other.
Approach One: Closed-Cell Foam at the Deck
Spray-applied closed-cell polyurethane bonded to the underside of the sheathing does several things at once. It is an insulation, it is an air barrier, and at sufficient thickness it is a vapour retarder. Because it is adhered with no gap, there is no air space for humid air to occupy against the cold surface — the condensing plane is moved to the interior face of the foam, which is warm. The requirements are that the foam is continuous with no gaps — at rafter edges, the ridge, the eave, penetrations and the wall junction — and thick enough for the climate. Gaps are the failure mode, because a gap is a channel where humid air reaches cold sheathing.
Approach Two: Exterior Rigid Plus Cavity Insulation
The hybrid puts a layer of impermeable or semi-impermeable rigid insulation above the deck, with air-permeable insulation such as batts or blown fibre filling the rafter bays below. The exterior layer keeps the sheathing warm from above so that its temperature stays above the dew point of the air in the cavity below. This only works if enough of the total insulation is on the exterior side — that is the whole basis of the approach.
- ✓The colder the climate, the higher the share of total insulation that must be above the deck. The ratio is climate-dependent and is set out in code and in published guidance; it is not a number to estimate.
- ✓Confirm the required ratio for your location with your municipal building department or the designer of record before ordering material.
- ✓The cavity insulation below must be in full contact with the underside of the deck — no air gap, because a gap lets cavity air circulate against the sheathing.
- ✓The interior side needs an air barrier. A hybrid assembly with a leaky ceiling defeats the calculation, because the moisture load reaching the sheathing is no longer what the ratio assumed.
- ✓The interior side should generally not have a low-permeance vapour barrier in this assembly, because inward drying is the drying path available. Check what the specific design calls for.
Airtightness Is Not Optional
A vented roof tolerates a mediocre ceiling air barrier because ventilation removes the moisture that leaks in. An unvented roof has no such safety valve. Every unsealed pot light, every open top plate, every bath fan housing pushes humid air directly into an assembly with no exit. This is why an unvented roof should not be treated as a shortcut on a house with a leaky ceiling — it is a higher-performance assembly that demands higher-quality execution below it, not a way to avoid dealing with the ceiling plane.
The Drying Question
Every assembly should be able to dry in at least one direction, because sooner or later moisture gets in — a flashing detail, a leak, construction moisture, an ice dam. An unvented roof with a metal panel above and closed-cell foam below dries in essentially neither. That is tolerable only when the assembly is genuinely airtight, which is why detailing standards are higher. A vented batten cavity above the deck restores an exterior drying path.
Do not convert a vented attic to an unvented one by blocking the soffit vents and adding foam partway up the roof slope. A partly foamed, partly vented roof creates cold sheathing above the foam line with attic air still circulating against it, and the transition point becomes a reliable condensing surface. If an assembly is going to be unvented, it has to be unvented from eave to ridge.
Ontario Realities
Three practical points for our climate. An unvented roof is warmer at the deck than a vented one, which can increase snow melt — so ice damming control depends even more on carrying insulation continuously out over the exterior wall and the overhang. Snow retention attachment still has to reach structure, and where exterior rigid insulation is present the fastener path through it has to be designed. And you lose the ability to inspect the deck from an attic, so eave and valley waterproofing and flashing quality carry more weight.
When We Suggest Not Doing It
Where a vented assembly is achievable with reasonable effort — a simple gable roof with adequate overhang, decent heel height, and attic access — venting is the more forgiving and more repairable choice. Unvented is the right answer when geometry, ceiling type, or mechanical layout makes venting impractical. It is not a way to skip the work of opening blocked soffits.
Frequently Asked Questions
Is a hot roof allowed in Ontario?
Unvented roof assemblies are a recognized construction approach, and the requirements that apply to a specific project — including insulation ratios and vapour control — are enforced by your municipal building department. Confirm the details with them or with the designer of record rather than assuming a rule of thumb applies.
Can I spray foam my existing attic roof deck?
It is done frequently, and it converts the attic into conditioned space. The important conditions are that the foam is continuous from eave to ridge with no gaps, that the existing soffit venting is closed rather than left half open, that the deck is dry and sound before foam is applied, and that any combustion appliance or ductwork in that space is assessed for its new conditions.
Does an unvented roof void a metal roof installation?
Panel systems are installed over a range of assemblies, and manufacturers publish requirements for underlayment, deck type, ventilation and clearances that vary by product. The assembly should be discussed before ordering so the specified details match what the manufacturer requires for that system.
Are ice dams worse on a hot roof?
They can be if the insulation is not continuous out over the exterior wall and the overhang, because the deck over the heated space runs warmer than a vented deck would. Where the exterior insulation or foam layer continues past the wall line and the overhang is detailed correctly, the melting pattern is more uniform. The eave detail is what decides it.
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