How the two differ in kind
Both of these do the same three jobs, and it is worth naming them before comparing anything, because most arguments about weather barriers are really arguments about which job the speaker has in mind. The layer has to shed the water that gets past the cladding — and water always gets past cladding — down and out to a flashing that takes it back to the world. It has to stop air being pushed through the wall by wind and stack pressure, which is what carries far more moisture into an assembly than diffusion ever does. And it has to keep doing both while letting the sheathing dry, which is why almost everything sold for this position is vapour-open. A rolled sheet and a rolled or sprayed coating both satisfy that description. They satisfy it in structurally different ways, and the differences show up in places the product literature does not lead with.
The first difference is that one is draped and the other is bonded, and nearly everything practical follows from it. House wrap hangs on the sheathing, held by cap fasteners, with a plane of air behind it. That plane is why a leak at a window head can appear at a sill three metres away and two storeys down, why the sheet has to be sealed at the top, at the bottom and at every penetration before it counts as part of an air barrier assembly, and why wind can get under a lifted lap and strip an elevation overnight. It is also, in modest measure, a drainage path — and in a drainable wrap, a deliberate one. A fluid-applied barrier is adhered to the substrate across its whole face. Water that breaches it wets the sheathing directly underneath and goes no further sideways, which makes a leak diagnosable rather than a mystery; wind pressure passes straight into the sheathing instead of ballooning the layer; and continuity is what the system does by default rather than what a careful crew achieves. It also has no gap behind it at all, which is a real cost on a wall with no rainscreen cavity and no cost whatsoever on a wall that already has one.
The second difference is conditionality, and it is the one that decides jobs on site rather than on paper. A sheet goes on cold, damp and dusty sheathing, in almost any weather, and the building is dried in the hour it is fastened — it is the system that does not care what the forecast says. A coating asks for a substrate that is clean, sound and dry, above a stated minimum temperature, with a cure period before rain that belongs on the programme rather than in the small print. Give it those conditions and it turns an inside corner or a lumpy penetration with no folding whatsoever, and it works on masonry, concrete and exterior gypsum, where there is nothing to cap-nail a sheet into. Ask it to work in a wet fortnight in February and it is the wrong tool. Between those two facts sits almost every honest recommendation: what is the substrate, how long is the wall standing open, will the air-tightness be tested against a number, is there a cavity behind the cladding, and how much of this elevation is opening perimeter rather than plain wall.
The factors that actually differ
Show
| Mechanically fastened house wrap | Fluid-applied weather barrier | |
|---|---|---|
| Where it actually fails | At its lines. Every horizontal lap, vertical seam, fastener head and cut edge is a joint, and the whole sheet works only if all of it is shingled the way water runs. A single reversed lap at a head flashing is a funnel pointing into the wall, and it is invisible once cladding is on. | At its thickness. The field is genuinely seamless, so the weak points move to the film — pinholes where a rough substrate shades the roller, a pass laid thin at the end of a pail, an area skipped behind a ladder — plus the joints, corners and transitions that must be reinforced before the field coat goes anywhere near them. |
| Bonded or draped | Draped, with a plane behind it. Water that gets in travels laterally and surfaces somewhere else entirely, which makes leaks hard to trace and easy to misdiagnose. That same plane is why the sheet must be sealed at every edge and restrained against wind pressure before it belongs in an air barrier assembly. | Adhered across its whole face. A breach wets the sheathing directly beneath it and stops, so the damage stays where the fault is and the forensics are honest. Wind load goes into the substrate rather than into the fasteners, and there is nothing to billow, flutter or peel back. |
| Drainage behind the cladding | Only if you specify it. A plain flat wrap is no better placed here than a coating — water that gets past the cladding runs down the outer face of either one, in whatever gap the cladding's own profile happens to leave. The real split is not sheet against liquid but drainable against not: a wrap with a crinkle, weave or moulded spacer carries the gap in the barrier itself, and that is what lets a sheet serve behind a direct-applied cladding with no furred cavity. | None, by definition. A bonded film is the face of the wall, so the drainage has to be built — a furred rainscreen cavity or a drainage mat over the coating. Where the design already has that cavity this costs nothing; where it does not, the coating is relying on the back of the cladding for a gap it cannot supply. |
| Substrate and weather window | Indifferent to both. Cold, damp, dusty sheathing takes it, in most weather, and the building is dried in the moment it is fastened. The requirement it does have is something to fasten into, which means framing and sheathing rather than masonry. | Conditional on both. It wants a clean, sound, dry substrate above a stated minimum temperature and a cure period before rain, which puts it on the programme as a sequencing item. In return it is the one that bonds to concrete, block and exterior gypsum, where a mechanically fastened sheet has nothing to hold on to. |
| Corners, penetrations and transitions | Cut, fold, tape, in the right order. Every rough opening, pipe, bracket, ledger and change of plane is a folded sequence with a correct sequence and several plausible wrong ones. The more items on the elevation, the more chances to shingle something backwards. | Poured into shape. A liquid turns an inside corner or wraps a messy penetration with no origami at all, which is why it wins hardest on the busiest walls. The trade is compatibility: every tape, primer, sealant and flashing that touches it has to be one the system says will bond to it. |
| Permeance, and who controls it | A factory property. The sheet's permeance is printed on the roll, does not change on site and is nearly always well into the vapour-open range — the installer cannot make the wall drier or wetter by working differently. | An applied property. Products come both vapour-open and vapour-closed, and permeance moves with the thickness laid down, so a heavier-than-specified build makes the wall dry more slowly than the specification assumed. It is the only one of the two where the person holding the roller changes the assembly's drying behaviour. |
| What inspection can see | Visible and durable evidence. Lap direction, fastener type, tape coverage and the flashing sequence at every opening can be read from the ground on any day before cladding, by anyone who knows which way water runs. | Visible only during application. Wet build is checkable at the moment it is laid with a film gauge; the day after, a thin coat looks exactly like a correct one. Some systems tint the two coats differently so the second pass proves it covered the first — worth specifying when you are not the one holding the gauge. |
| Standing open before cladding | Flaps. Wind finds a lifted lap and can take an elevation off overnight, and the sheet carries a manufacturer's exposure limit in months after which it is degraded material rather than a barrier. | Stays put. Nothing to tear, nothing to catch the wind, and the wall is weathertight while the cladding waits. It has its own stated exposure limit and some products chalk or lose bond strength if left uncovered well past it — but its failure mode is ageing, not departure. |
| Where the money lands | Bought by area. Roll coverage against wall area with a lap allowance, and that quantity barely moves when the specification is tightened. Labour is fast and universally understood, and the hours concentrate at the openings, in tape and flashing. | Bought by volume against a specification. Area multiplied by the data-sheet wet film thickness and the number of coats, so a heavier build raises the material bill in direct proportion, and the reinforced detail bands are a second quantity measured in running length. Labour is slower and more specialised, and the cure window is a schedule cost the sheet does not have. |
Which one, and when
Choose mechanically fastened house wrap when…
- Wood-framed walls in changeable weather, where the value of drying the building in the hour the sheathing goes on outweighs everything else on this page.
- Cold or wet season work, or a programme with no room for a clean dry substrate and a cure window on the critical path.
- Direct-applied cladding with no rainscreen cavity, where a drainable sheet supplies drainage the adhered alternative cannot provide at any thickness.
- Ordinary crews and ordinary inspection: a system almost any siding crew can install correctly and almost any inspector can check from the ground before it is covered.
- Small or simple elevations, where the mixing, masking, gauging and cure logistics of a coating never earn back what they cost in time.
Choose fluid-applied weather barrier when…
- Masonry, concrete or exterior gypsum sheathing — substrates a mechanically fastened sheet was never meant for, and where a bonded film is the conventional answer rather than the ambitious one.
- An air-tightness target that will actually be tested, where you want continuity to be the system's default state rather than the reward for a careful week.
- Busy elevations: many penetrations, parapets, changes of plane and transitions to other materials, all of which a liquid handles without folding anything.
- Walls that will stand open for a long time, or an exposed windy site where a fastened sheet is a liability before the cladding arrives.
- A rainscreen cavity already in the design, which removes the coating's one structural disadvantage and leaves only its advantages.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Which one is cheaper?
- They are shaped so differently that a single answer would be a fiction, and this site does not publish one. The sheet is a material bought by area: roll coverage against wall area with a lap allowance, a quantity that barely changes when the specification is tightened, installed fast by crews who all know it. The coating is a material bought by volume against a number on a data sheet — area multiplied by wet film thickness multiplied by coats — so specifying a heavier build raises the material bill in direct proportion, and spray application on an exposed elevation loses a real fraction of it to the air. Both then carry a second quantity that scales with perimeter rather than area: flashing tape and sill pans on one side, reinforced detail bands on the other, measured along every sheathing joint, corner, transition and opening. That is why the elevation that is mostly glass costs disproportionately more in either system, and much more in the coating. Then weigh the two costs nobody lists. Cure time is a schedule item with no counterpart on the sheet side, and a rain event mid-cure is rework rather than a delay. And unopened rolls go back to the yard, while a part-used pail has a shelf life and an opened one has a pot life. Run both calculators against your own elevations and your own quotes, and price the detail lengths before the field area, because that is the number that surprises people.
- Is a fluid-applied barrier automatically the better air barrier?
- It is the better default, which is not the same claim. A bonded film is continuous where it is applied, transfers wind pressure into the substrate instead of billowing, and needs no seam to be taped for the field of the wall to be airtight — so the ordinary result is close to the good result. A sheet can reach the same performance and routinely does: the materials themselves are tested for air permeance, and complete assemblies are tested for leakage with the seams, penetrations and terminations included, which is the number that matters. The difference is how much has to go right, and how much of it is still right in five years — every lap taped, every penetration sealed, every edge terminated, and the sheet held against wind pressure so the fasteners are not doing the work alone. If an air-tightness figure is being tested and someone has to answer for the result, that asymmetry is worth paying for. If it is not, a properly taped and terminated wrap is not the weak link people imply.
- Do I still need a drainage gap behind a fluid-applied barrier?
- In almost every case, yes, and it is the honest counterweight to the seamless argument. An adhered film has no gap behind it by definition, so anything that gets past the cladding runs down the face of the coating and is in direct contact with the back of the cladding all the way. With a furred rainscreen or a drainage mat that is a good wall and the coating costs nothing in drainage terms. Without one, an absorptive cladding sitting hard against a bonded membrane is the assembly that gets people into trouble — which is also why many jurisdictions require a defined drainage layer, or a second layer of barrier, behind cement stucco and adhered masonry veneer regardless of which barrier is underneath. The practical reading: if a rainscreen cavity is already in the design, that argument disappears entirely; if the cladding is going straight onto the barrier, a drainable sheet is doing something real that the coating cannot.
- Can I put a coating over old house wrap, or wrap over a coating?
- Neither, and for different reasons. A fluid-applied barrier is a bonded system — it needs a sound substrate to adhere to, and a loose fastened sheet is not one; a film over a wrap is a film over something that moves, tears and holds air behind it, and the manufacturer's warranty ends at the first sentence. Wrap over a cured coating is not dangerous, just redundant and awkward: you would be cap-nailing through a barrier that is already continuous, putting holes in the good layer to hold up a second one. What is legitimate, and common on real buildings, is using both on the same building in different places — a coating on the masonry podium or the exterior gypsum, a sheet on the framed floors above — in which case the entire question becomes the transition detail between them, drawn before either goes on, using flashing products both systems accept. Mixing brands within one transition is where those details usually fail.
- How do windows and penetrations integrate with each?
- Same principle, different craft, and both are the place the wall is most likely to leak. Every opening needs a sill pan with an upturn and end dams, jambs lapped over the sill, and a head flashing lapped over the jambs, so that each layer sheds onto the one below in the direction water travels — the sequence is the detail, and no product substitutes for it. With a sheet, that sequence is executed by cutting and folding wrap into the opening in a specific order and taping it in another; it is well documented, widely understood, and easy to do backwards under time pressure. With a coating, the reveals and returns are simply more surface to coat and the corners need no folding at all, which is a genuine advantage on a busy elevation — but every flashing membrane, tape, primer and sealant that touches the barrier has to be one the system says will bond to it, and an incompatible sealant against a cured membrane is a de-bond waiting for the first thermal cycle. Whichever you choose, the pan goes in before the window and the head flashing goes over the flange, not under it.
- What if the wall has to stand open for weeks?
- That question alone settles more of these than the technical argument does, and it settles it toward the coating. A bonded film has nothing to catch the wind, so a windy exposed site or a long wait for cladding costs it nothing structurally; it ages against a stated exposure limit and, past that limit, may chalk or lose bond strength, which is a material question with a defined answer. A fastened sheet in the same situation is a sail. Wind gets under a lifted lap, works the cap fasteners, and an elevation can be stripped overnight or, worse, left looking intact with its laps opened and its tape peeled. Its exposure limit is also real and stated in months, after which UV has left degraded material on the wall rather than a barrier. If the cladding is following within days, this factor barely registers. If the programme has the wall open through a season, weigh it heavily — and if the sheet is what you are using anyway, that is the argument for battening laps and getting the cladding on rather than trusting the tape.