Vapour Barriers and Air Barriers: They Are Not the Same Thing
One controls diffusion through materials, the other controls air flowing through holes. They are made of different things, tested differently, and fail differently — and the roof needs both.
These two terms get used interchangeably on job sites, and the confusion causes real damage. A vapour barrier and an air barrier do different jobs, obey different physics, and are judged by different criteria. A material can be one, the other, both, or neither. If you only take one thing from this article: the air barrier does most of the work.
Two Different Jobs
Vapour control deals with diffusion — individual water molecules migrating through solid materials, driven by a difference in vapour pressure between one side and the other. It is slow, steady, and spread evenly over the whole surface. Air control deals with bulk flow — air moving through holes, gaps, and cracks, driven by pressure differences from stack effect, wind, and mechanical systems, carrying its water content along for the ride. Diffusion is a trickle across an entire ceiling; air leakage is a stream through the openings in it.
Permeance and Vapour Retarder Classes
How much vapour a material lets through by diffusion is its permeance. Materials are grouped into broad classes: essentially impermeable materials such as polyethylene sheet, sheet metal, and glass; semi-impermeable materials such as many foam boards and vapour retarder paints; and semi-permeable to permeable materials such as gypsum board, most housewraps, and many synthetic underlayments. The classes matter because the design question is never simply barrier or no barrier — it is how much vapour should be allowed to pass, and in which direction the assembly needs to be able to dry.
Vapour Drive Direction in a Cold Climate
Over a Southern Ontario year the net vapour drive is outward. Winter is long and the difference between warm humid indoor air and cold dry outdoor air is large, so vapour pushes from inside toward outside for months. Summer produces a weaker inward drive for shorter periods. That is why the conventional detail here places the vapour retarder toward the warm-in-winter side of the insulation, and why the assembly is generally designed to dry toward the exterior. Putting a low-permeance layer on the cold side of insulation in a heating climate is how assemblies get trapped.
Air Barriers: Continuity Is Everything
The performance of an air barrier is not proportional to its coverage. A ceiling that is ninety-eight percent airtight is not ninety-eight percent as good as one that is fully sealed, because the leakage concentrates through the remaining openings and the driving pressure does not change. One open chimney chase can move more air than a large expanse of well-detailed ceiling. This is why air barrier work is fundamentally about finding and closing every hole, in a continuous plane, rather than about the material chosen.
- ✓Interior wall top plates: the drywall stops at the wall and the cavity is open to the attic unless the plate is sealed.
- ✓Recessed light housings, unless they are rated for insulation contact and sealed at the ceiling.
- ✓Attic access hatches and pull-down stairs, which need weatherstripping and an insulated cover.
- ✓Plumbing stacks, wiring penetrations, and duct boots where they pass through the ceiling.
- ✓Bathroom fan housings, which leak around the body of the unit as well as through the duct.
- ✓Chimney and flue chases, which need firestopping with the correct materials, not spray foam against a hot flue.
- ✓Bulkheads, dropped ceilings, and cabinet soffits that are framed but left open at the top.
- ✓The junction between an addition and the original house, which is often left completely open in the attic.
Polyethylene in Ontario Attics
Poly under the drywall has been standard practice in Ontario construction for decades, and it does two things at once when installed carefully: it is an excellent vapour retarder, and when it is lapped, sealed at the perimeter, and taped at penetrations, it also acts as the air barrier. The problem is that on most existing houses it is neither continuous nor sealed. It is stapled up, cut around boxes, torn during drywall, and left open at the top plates. In that condition it still slows diffusion but does almost nothing for air leakage — which is the mechanism that carries most of the moisture. Finding poly in an attic is not evidence that the ceiling is airtight.
The Airtight Drywall Approach
The alternative that has aged well is to make the drywall itself the air barrier, gasketed or sealed at every edge, with sealed electrical boxes and sealed penetrations, and to handle vapour control with a vapour retarder primer or a smart membrane. It is easier to inspect than hidden poly, easier to repair during renovations, and it allows some inward drying in summer. Either approach works; what does not work is an air barrier that exists in intention only.
Do Not Build a Double Vapour Barrier
If an assembly has a low-permeance layer on the interior and another on the exterior, any moisture that gets between them — during construction, from a small leak, or from an air leak — has no way out in either direction. This is the classic wet roof, and it is why detailing matters more as assemblies get tighter. Every assembly should have a designed drying direction. Decide which way it dries, then make sure nothing in the stack-up blocks that path.
A metal roof panel is, by itself, a vapour barrier. That is fine when the deck below it can dry — into a vented attic, into a vented batten space, or through a permeable underlayment. It becomes a problem when the panel is combined with an impermeable layer on the warm side and nothing in between can dry. What sits under the panel is the design decision, not the panel itself.
Where This Lands on a Re-Roof
During a metal re-roof we are working on the exterior side, so the vapour retarder at the ceiling is usually out of reach. What is in reach is meaningful: the underlayment choice and its permeance, whether a vented batten space is added above the deck, ridge and eave venting, and access to the attic for air sealing at the top plates, hatch, and penetrations while the crew is on site. Those decisions get made before the panels are ordered, because some of them change the eave and ridge geometry.
Frequently Asked Questions
Is a vapour barrier required under a metal roof?
The panel is not where vapour control belongs. Vapour and air control belong at the warm side of the insulation — the ceiling plane in a vented attic, or the interior face of the insulation in a cathedral assembly. What goes directly under the panel is underlayment, and its permeance is chosen to suit the assembly below it.
Can I add poly to my attic ceiling from above?
It is not practical or advisable. Laying poly on top of existing insulation puts a vapour barrier on the cold side of the insulation, which is the wrong side in a heating climate and can create a condensing surface. Air sealing the penetrations from above is the useful work that can be done from the attic.
Which matters more, air sealing or insulation?
They work together, but air sealing is generally the higher-value first step for moisture problems, because it stops the transport mechanism that carries the most water. Insulation added over a leaky ceiling still lets humid air stream through it, and blown insulation is not an air barrier.
What permeance should my roof underlayment have?
It depends on the assembly. Over a well-vented attic, a wide range works. Over an unvented or cathedral assembly, or where a batten space is used, the permeance of the underlayment becomes part of the drying strategy and should be chosen deliberately with the rest of the stack-up rather than by habit.
Get a Free Metal Roofing Quote.
We measure your roof remotely by satellite and deliver a written, itemized quote — same day, no obligation.