Correctly specified and correctly installed spray foam generally improves moisture control in a Puget Sound house rather than causing problems. The wrong product in the wrong assembly can absolutely trap or hide water, and our climate changes which of those mistakes actually matter.
That second sentence is why this page exists. Most of what you will find searching this question was written by contractors in Texas, Florida, or Georgia, and their moisture problem is not ours. Apply their rules to a Seattle house and you will spend money defending against a mechanism that is weak here while ignoring the ones that are strong. So: how water actually moves through a building, what a dew point is in practical terms, and what both of those mean for a house in this region.
The short version
Water moves three ways: as bulk water, carried by moving air, and by vapor diffusion. Air movement is by far the biggest, and air sealing is exactly what foam does. Our regional risks are wet ground, a long wet winter, drainage, and cold surfaces in January, not humid summer air condensing indoors. Foam helps with most of that. It hurts when it hides a leak, when the wrong product is used in an assembly that cannot dry, or when it is sprayed over a water problem nobody fixed. Fix bulk water first, always.
The three ways moisture moves through a building
Nearly every argument you will read online is really an argument about which of these three a person is thinking about. Once you can name them, the whole subject gets much calmer.
1. Bulk water: the biggest by an enormous margin
Rain, roof leaks, a downspout dumping at the foundation, a lot sloping toward the house, groundwater under a crawl space, a supply line weeping in a wall. Liquid water, arriving in gallons. No insulation product is a defence against it and none is meant to be: bulk water is a roofing, flashing, drainage, and plumbing subject. If you have it, the rest of this page is beside the point until it is fixed.
2. Air transported moisture: the one that actually damages houses
Air carries water vapor with it, so whenever air moves through a gap, moisture rides along. A house is a chimney in winter: warm air leaks out the top, which pulls replacement air in at the bottom through the crawl space hatch, the rim joist, the can lights, the plumbing chases. That moving air is the main way indoor moisture reaches a cold surface where it can condense.
The scale difference is what surprises people. A well known building science illustration compares a heating season of moisture diffusing through a sheet of drywall, a fraction of a quart of water, against the same season of air leaking through one square inch of hole, which is on the order of thirty quarts. The assumptions move the numbers, but the ratio is not close. Air movement is the mechanism that wets buildings.
This matters because air sealing is precisely what spray foam is good at. Both open-cell and closed-cell stop air movement when properly installed. That is the single largest moisture benefit foam gives you, and almost nobody warning you that foam causes moisture problems ever mentions it.
3. Vapor diffusion: the smallest, and the one everyone argues about
Vapor diffusion is water vapor pushing its way slowly through a solid material, driven by a difference in vapor pressure from one side to the other. No air movement required, no hole required. It happens through drywall, through wood, through open-cell foam, and it happens slowly.
Diffusion is real and worth designing for, particularly at a roof deck where the receiving surface is cold. It is also the smallest of the three by a wide margin, which is why it is odd that it takes up most of the internet’s attention. The vapor barrier and perm rating fight is a fight about the smallest term in the equation.
| Bulk water | Air transported | Diffusion | |
|---|---|---|---|
| Relative volume | Gallons | Quarts | Ounces |
| Driven by | Gravity and rain | Pressure differences | Vapor pressure |
| What stops it | Roofing, flashing, grading, drainage | An air barrier | A vapor retarder |
| What spray foam does | Nothing. Do not ask it to. | Stops it. This is the big win. | Closed-cell retards it, open-cell does not |
| How much it matters here | Most | A lot | Assembly by assembly |
Dew point, in practical terms
Dew point is the temperature at which air has to let go of the water it carries. The useful way to hold it in your head: condensation is a surface temperature problem, not a humidity problem. Water forms where a surface is colder than the dew point of the air touching it, and nowhere else.
Put real numbers on it. A comfortable Seattle living room in January, say 68 degrees with 45 percent relative humidity, has a dew point somewhere around 46 degrees. That means every surface in that house colder than about 46 degrees is collecting water out of your indoor air. A single pane window. An uninsulated corner behind a bookcase. A sheet of sheathing with a gap in the insulation behind it. The air is not the problem. The cold spot is.
That one idea explains most of what follows. Insulation that warms a surface lifts it above the dew point and the condensation stops. Insulation with a gap in it leaves a cold spot exactly where moist air can reach it and concentrates the problem there. Same material, opposite outcomes, and the difference is entirely the quality of the install.
Why the Southeast playbook does not apply to a Seattle house
Here is the regional part, and it is the reason this page is worth your time.
What goes wrong in the humid Southeast
Their classic failure is a summer failure. In Houston or Atlanta, July outdoor air carries a dew point in the mid seventies, while air conditioning holds indoor surfaces in the low seventies or cooler. Warm humid outdoor air gets driven inward toward surfaces below its dew point and condenses inside the assembly. That single mechanism writes their whole rulebook: vapor drive goes inward in summer, keep the vapor control toward the outside, do not trap a cooled surface behind a vapor closed layer.
What goes wrong here
Puget Sound summer dew points generally sit well below normal indoor temperatures. That is why a Seattle August feels pleasant, why so few of us have air conditioning, and why that inward summer vapor drive is much weaker here than it is in the Southeast. The mechanism exists on a hot humid stretch, but it is not what is wrecking houses in this region.
Our moisture problems are different and mostly cold season problems:
- Wet ground. Soil under a Seattle house holds water most of the year and releases it upward as vapor into the crawl space, day after day, for most of the year.
- A long wet winter. Not violent rain, persistent rain. Assemblies here rarely get a hot dry month to recover in, so anything that gets wet stays wet longer.
- Bulk water and drainage. Gutters, downspout discharge, grading, site water. The number one moisture source in the houses we get called to, and not an insulation problem.
- Winter condensation on cold surfaces. Moisture from showers, cooking, laundry, and people, carried by leaking air to a cold rim joist, sheathing panel, or window, over a season that runs October to April.
What that means when you are reading advice online
Advice written by a Texas or Florida contractor is not wrong for Texas or Florida. It is answering a question you do not have. The tell is the season: if the failure story happens in summer with the air conditioning running, it is not describing your house. If it happens in February on a cold surface, it is.
So around here the mistakes that matter are the cold season ones. Leaving a cold spot in the insulation. Letting indoor moisture diffuse through to a cold roof deck. Sealing a space whose ground is still wet. Building an assembly with no direction it can dry in. Those are our failure modes and they are what we design against.
Where spray foam genuinely helps in this climate
Four things, and they are all direct answers to the list above.
- It stops air transported moisture. The biggest damaging mechanism in a cold wet climate, shut off by the same product that is doing the insulating. See air sealing for what that involves on its own.
- It keeps crawl space ground moisture out of the house. Combined with a sealed ground barrier, foam on the foundation walls cuts off the path from wet soil to your floor framing and your indoor air. That is the subject of crawl space insulation.
- It warms surfaces so they stay above the dew point. A rim joist at 60 degrees instead of 42 degrees is a rim joist that cannot collect water. Insulation is condensation control, when it is continuous.
- Closed-cell adds vapor control where it is wanted. At sufficient thickness it acts as a vapor retarder as well as insulation and air barrier, which is why it is our default against damp foundation walls and cold roof decks. See closed-cell spray foam insulation.
Where spray foam genuinely causes or hides trouble
We are not going to pretend this list does not exist. Every one of these is real, and every one of them is a specification or installation failure rather than something wrong with foam itself.
It bonds to roof sheathing, so a leak goes unseen
This is the honest one. In a vented attic a roof leak announces itself as a brown ring on a bedroom ceiling. Foam against the underside of the deck removes that warning, and a small leak can work on the wood for seasons before anyone knows. A different foam does not fix that. Fixing the roof first does. The whole argument is in will spray foam rot my roof deck.
Open-cell in the wrong assembly lets vapor reach a cold surface
Open-cell air seals but stays vapor open. In many walls that is fine, and the vapor openness is useful because the assembly can dry inward. Under a roof deck, where the receiving surface is cold and one layer away from a wet November night, is where the diffusion term stops being trivial. That is why closed-cell is our default there. Full comparison: open-cell vs closed-cell spray foam.
Sealing a crawl space over an unresolved water problem
This is the one we refuse jobs over. Standing water, a downspout discharging at the footing, a lot draining toward the house: seal that space and you have put the water in a container with your floor framing. It will smell, it will grow things, and you will have paid us to make it invisible. The mechanism and the repair order are in why Seattle crawl spaces get moldy.
Drying direction, which is the subtle one
Every assembly gets wet occasionally. What decides whether that matters is whether it can dry, and in which direction. A wall that can dry inward or outward is forgiving. A wall closed to vapor on one side can still dry the other way, which is usually fine. A wall closed on both sides, which is what you get when foam is added without checking what is already in there, cannot dry at all. Then water that gets in stays in.
Older Seattle houses make this live rather than theoretical. Board sheathing, old asphalt impregnated felt, poly sheeting a previous owner stapled up in the 1980s, kraft faced batts, a foam panel added during a re-side. Any of those may already be the vapor control layer, and adding closed-cell on the other side of the cavity without knowing it is how you build a sandwich that cannot dry. Working out what is already in the wall is part of the job, and a real part of why we want to look at the house.
A tighter house needs deliberate ventilation
Here is the part that gets shouted about online, stated calmly. Air sealing reduces the accidental ventilation a house used to get through its gaps. That leakage was random, uncontrolled, and worst exactly when you least wanted it, and it was also quietly removing some of the moisture your household produces. Take it away, put nothing in its place, and indoor humidity can climb.
This is not a reason to avoid air sealing. It is a reason to finish the job. The old draughty house was not healthy, it was leaky, and leaky is not the same thing as ventilated. Sealed on purpose and ventilated on purpose gives you control over the air you breathe instead of leaving it to the weather. In rough order of cost:
Use the fans you have
Bath fans that vent outside rather than into the attic, used during and after showers, and a kitchen hood that goes outdoors. Cheapest humidity control there is.
Fix the obvious sources
A dryer venting under the house, a bath fan ducted to nowhere, a crawl space with bare soil. Removing a source beats ventilating it away.
Add controlled fresh air
In a house tight enough to need it, a continuous low speed exhaust fan or a balanced ventilator brings air in on purpose. An HVAC contractor sizes it.
Watch a hygrometer
A twenty dollar meter tells you more than any argument online. If indoor relative humidity sits high all winter, you have a source or a ventilation gap to find.
We are insulation contractors, not HVAC contractors, and we will say so. What we will do is tell you when the work we are proposing gets a house tight enough that you should be talking to somebody about ventilation, rather than letting you find out in February.
The rule that runs through all of this: bulk water first
If you remember one thing from this page, remember the order. Bulk water, then air, then vapor. Gallons, then quarts, then ounces. Deal with the biggest source before you spend a dollar on the smaller ones.
Foam sprayed over a drainage problem does not solve it. It covers it, hides it, and makes it more expensive to find later. We will say that on the estimate rather than take the deposit, because in 15 years of this work in this climate the jobs that came back were never the ones where we said no first. If your house needs a downspout extension, a regrade, and a drainage contractor before it needs us, that is what goes in writing, and we are happy to come back afterwards.
Common questions about spray foam and moisture in the Pacific Northwest
Does spray foam trap moisture in my walls?
It can, if the assembly is specified without thinking about how it dries. Foam does not create water, but it does change the paths water has for getting back out. The risky version is a cavity with a vapor closed layer on both sides, which happens when closed-cell is added to a wall that already has old poly sheeting, kraft facing, or exterior foam in it. The safe version is an assembly that can still dry in at least one direction. That is why we want to know what is already in your walls before quoting, and why “just spray it all” is not an answer we give.
Is our climate actually less risky for spray foam than the Southeast?
It is different rather than simply safer. The classic Southeast failure, hot humid outdoor air driven inward and condensing on air conditioned surfaces, is much weaker here because Puget Sound summer dew points generally sit well below indoor temperatures. What we get instead is a long wet season, wet ground under most houses, and months of cold surfaces for indoor moisture to condense on. Our failure modes are cold season and ground related, so advice written for a humid Southeast climate does not transfer in either direction.
Do I still need a vapor barrier if I have spray foam?
It depends on the product, the thickness, and the assembly, and it is one of the few places we will not give you a general rule on a web page. Closed-cell at sufficient thickness performs as a vapor retarder on its own, which is often the reason it gets chosen. Open-cell stays vapor open and the assembly has to handle vapor some other way. What is required for your specific job is something we confirm against the manufacturer data for the actual product and with your local building department, not something anybody should assert to you from a website.
Will sealing up my house make the inside damp?
Only if you seal it and stop there. Air sealing removes the accidental ventilation the house used to get through its gaps, and that leakage was removing a little moisture by accident. The fix is deliberate ventilation rather than deliberate draughts: bath fans that vent outdoors and get used, a range hood that goes outside, no dryer venting into a crawl space, and in a genuinely tight house a fresh air system sized by an HVAC contractor. A cheap hygrometer tells you where you stand. We flag it when a job is tight enough for this to be worth planning.
My crawl space has standing water. Can you just foam over it?
No, and we will not. Sealing a space that still has liquid water arriving in it puts that water in a closed container under your floor. Grading, downspouts, discharge lines, and footing drainage come first, and those are drainage and general contracting work rather than ours. After the water is handled the sequence is cleanout, a sealed ground barrier, perimeter air sealing, then insulation. We would rather hand you that list and lose the job this month than do it in the wrong order and hear from you in two years.
Is closed-cell always the safer choice in a wet climate?
It is our default where moisture is part of the problem, which here means crawl space walls, rim joists, and roof decks: it resists liquid water, retards vapor at sufficient thickness, and warms the surface behind it more per inch. It is not universally correct though. In an assembly that needs to dry inward, a vapor open product can be the better answer, and in a generous attic cavity with no moisture concern closed-cell is just a more expensive way to buy the same R-value. We would rather put you in the cheaper product and be right.