Water Leaking Through the Wall or Around a Window in a Storm? How Wind-Driven Rain Gets Inside
By Bayron Alvarez, NYS Licensed Mold Assessor (#25-671AE-SHMO) and NYS Licensed Home Inspector (#16000145204) — Casablanca Environmental & Building Diagnostics, Southampton, NY
The September 2026 nor'easter was forecast to bring 4 to 6 inches of rain and gusts over 60 mph to Long Island. Walls and windows that stay dry in ordinary rain can leak in a storm like that. Some of the water shows up as a stain under a window or a wet spot on the floor. Much of it goes into the wall cavity and shows up nowhere. This post explains why storm rain gets in where ordinary rain does not, where it goes once it is inside the wall, how to find it without opening the wall, and when the wall has to be opened.
Summary
A storm puts far more water on a wall than ordinary rain, and wind pressure pushes it through joints that stay dry the rest of the year. In a nor'easter the north- and east-facing walls take most of it. A residential window is water-tested at 15 percent of its design pressure, with a 2.90 pound per square foot minimum, and the rating does not cover the joint between the window and the wall, where most window leaks start. Water that gets past the siding runs down inside the wall to the sill, the bottom plate, and the insulation at the base of the cavity, often without a visible stain. It is found with a thermal camera to locate suspect areas and a moisture meter to confirm them, against reference readings from an unaffected wall in the same house. The wall comes open when readings are not falling after several days, the insulation got wet, the drywall has swollen, or there is growth or odor. If mold growth is found over more than 10 square feet, New York Labor Law Article 32 applies to any contractor you hire. Drying does not prevent the next leak; correcting the flashing does.
Why a storm gets water in where ordinary rain does not
Two things change in a storm: how much water lands on the wall, and how hard the wind pushes it. In ordinary rain, most drops fall close to vertical and the roof overhang keeps much of the wall dry. In a storm, the rain is blown sideways. Building Science Corporation's research shows that the amount of rain deposited on a wall rises directly with wind speed, and that wind-driven rain deposits more water on the above-grade walls of a house than any other moisture source.
Which walls get it. The National Weather Service defines a nor'easter by its wind: the winds over the coast are typically from the northeast. The north- and east-facing walls take the most water. The wind shifts as the storm passes, so every elevation needs checking, starting with those two. On any wall, the upper corners and edges collect the most rain, and a roof overhang reduces what reaches the wall below it. Exposure matters as much as direction. In Building Science Corporation's worked example, an exposed wall received almost 20 times the rain of a sheltered one. A house on open water, open farmland, or a dune is an exposed house.
How hard it pushes. Wind pressure rises with the square of wind speed. The standard engineering relation is 0.00256 × V², in pounds per square foot with V in miles per hour. A 60 mph gust carries a velocity pressure of about 9.2 pounds per square foot. The pressure on a low windward wall is a fraction of that after height, terrain, and building shape are accounted for.
Compare that with how windows are rated. Under the North American Fenestration Standard, a residential window is water-tested at 15 percent of its design pressure, with a minimum of 2.90 pounds per square foot. A window rated PG 25 was tested for water at 3.75 pounds per square foot. A PG 50 window was tested at 7.5. Storm gusts can push a windward window at or above the pressure it was tested at.
The rating also covers only the window unit, tested in a laboratory. It does not test the joint between the window and the wall, the trim around it, or the flashing above and below it. In the houses we inspect, those joints are where most window leaks start.
Every wall lets some water past the siding. ASHRAE Standard 160, the design standard for moisture analysis, assumes by default that 1 percent of the rain reaching the exterior surface gets through it. Siding, shingles, and stucco are not waterproof. The water-resistive barrier and the flashing behind them are what keep the water out of the wall. In a storm, the volume reaching that barrier is many times larger, and the pressure pushes it through any lap, nail hole, tear, or seam that is not detailed correctly.
The usual entry points:
Window and door corners, where the frame joints and the sill meet the wall.
Trim and casing joints, and sealant that has cracked or pulled away.
Head flashing over windows and doors that is missing, too short, or lapped the wrong way.
Penetrations: hose bibs, dryer vents, exhaust hoods, light fixtures, and cable and line-set holes.
Roof-to-wall intersections without a kick-out flashing at the bottom of the step flashing.
Deck ledger boards fastened to the house without flashing above them.
Where the water goes once it is inside the wall
Water that gets past the siding runs down the face of the water-resistive barrier, the housewrap or felt behind the cladding. Where that barrier is intact and lapped correctly, the water drains out at the bottom of the wall. Where it is torn, cut short at a window, or lapped the wrong way, the water reaches the sheathing.
From there it moves down. It collects where the wall stops it:
The window sill and the framing below it. A window without a sill pan drains straight into the wall below it.
The bottom plate and the rim joist. Water that runs down the inside face of the sheathing ends up at the bottom of the wall, and so does the evidence.
The insulation at the base of the cavity. Fiberglass batts hold water at the bottom of the bay and keep it against the sheathing and the plate.
The bottom edge of the drywall. Gypsum board wicks water up from a wet plate. The paper facing is the material mold grows on first.
A wet wall cavity dries slowly. The siding covers it on the outside, and drywall and paint cover it on the inside. There is little air movement inside it. EPA's guidance is that wet materials need to be dried within 24 to 48 hours to prevent mold growth. A closed wall cavity will not meet that on its own. The USDA Forest Products Laboratory gives 20 percent moisture content or below as the margin of safety against fungal damage in wood. That is the number the framing and sheathing have to get back under.
Stucco walls. Stucco absorbs water and holds it. After a long storm it holds that water after the rain stops and can drive it inward toward the sheathing, which is why it is called a reservoir cladding. The code requires a water-resistive barrier equivalent to two layers of Grade D paper behind stucco on wood sheathing, or one layer with a drainage space. Stucco installed directly over a single layer, or with the flashing lapped wrong, fails at the windows first. The damage is behind a surface that often shows nothing until the sheathing is badly deteriorated.
Cedar shingle walls. The shingles shed most of the rain, and the water that gets past them runs down the barrier behind. The failures are the same ones: a barrier that is missing at the window rough opening, head flashing that is too short, and trim joints that were caulked instead of flashed.
Spray foam walls. Where closed-cell foam fills the stud bay, water that reaches the sheathing is trapped between the sheathing and the foam. It cannot dry to the inside and it cannot be seen from the inside. The sheathing dries only outward, through the siding.
Signs inside the house, and why there are often none
When wind-driven rain gets into a wall, these are the signs to look for:
A stain, drip line, or bubbled paint at the lower corners of a window, or on the wall below the sill.
A window stool or apron that has swollen or separated, or baseboard that has lifted or opened at the joints.
Damp carpet, or a stained tack strip, along an exterior wall.
Floorboards that have started to cup near an exterior wall or below a window.
In a finished basement, water or staining at the top of the wall below a window or door on the floor above.
A musty odor in one room, or one closet on an exterior wall, days to weeks after the storm.
From the outside, look below the window corners for staining on the siding or trim, paint that has started to peel, and cracks or dark streaks in stucco.
Often there is nothing to see. The water runs down the sheathing and the back of the drywall, inside the cavity, not across the painted surface. Paint slows what reaches the face of the wall. A stain can take days or weeks to appear, and in a wall filled with closed-cell foam it may never appear on the inside at all. A dry-looking wall after a storm is not evidence that the wall is dry. On a house that took the storm on an exposed north or east side, the walls below the windows on those sides are worth checking even when nothing shows.
How to find it without opening the wall
Most of this can be answered with a thermal camera, a moisture meter, and a set of reference readings. The wall comes open only where the readings say it has to.
Thermal imaging shows where to look. It does not show water. A thermal camera measures surface temperature. Wet drywall usually reads cooler than dry drywall because water is evaporating from it. The camera finds that pattern; it cannot detect or confirm moisture on its own. Right after a storm, with the house humid and little temperature difference between inside and outside, a wet wall and a dry wall can image the same. Missing insulation, air leaks, and framing also show up as temperature patterns that look like water. Every thermal finding needs a meter reading before it means anything.
The meter confirms it. There are two kinds, and a proper survey uses both:
A pinless meter reads through the drywall surface without marking it. It is fast for scanning the base of a wall and the area below a window, and it reads only a shallow depth into the material.
A pin meter drives two pins into the material and measures the electrical resistance between them, which drops as wood moisture content rises. It reads wood directly: the bottom plate behind a baseboard, a window stool, the rim joist, the sheathing.
Reference readings decide what "wet" means. The restoration industry standard, IICRC S500, calls this the dry standard: the moisture level of the same material in an unaffected area of the same house. An interior partition wall, or an exterior wall on the side the storm did not hit, gives the baseline. A reading means something only against that baseline, taken with the same meter on the same material.
Where we take readings after a wind-driven rain event:
Below both lower corners of every window and door on the exposed elevations, at the base of the wall.
The bottom plate, through the baseboard or behind it.
The rim joist and sill plate from the basement or crawl space, directly below the windows and doors on those walls. Often this is the fastest way to confirm that water came down the inside of the wall.
Below any roof-to-wall intersection, deck ledger, or wall penetration on those elevations.
Any closet or cabinet on an exterior wall, where there is no air movement.
Take them more than once. One set of readings shows where water is. Readings repeated over several days show whether it is leaving. A wall that is falling toward its baseline is drying. A wall that is still well above baseline a week after the storm is not drying on its own and needs to be opened or dried from inside the cavity.
A small inspection hole behind a baseboard, or a borescope through an electrical box, can confirm the condition of the insulation and the sheathing before any larger opening is made. From the outside, removing a few shingles or one piece of trim at the suspected entry point often shows the flashing defect and the sheathing condition directly.
When the wall has to be opened
The wall comes open when the readings, the materials, or the growth say it will not recover closed. Any one of these is enough:
The readings are not falling. Plate, sheathing, or drywall readings that stay well above the baseline after several days of drying mean the cavity is not drying on its own.
The insulation behind it got wet. EPA's water damage table gives one instruction for wet fiberglass and wet cellulose insulation: discard and replace. A batt that is wet at the bottom of a stud bay holds that water against the plate and the sheathing, and it will not dry in a closed wall.
The drywall has swollen or the seams have opened. The same EPA table allows wallboard to be dried in place only if there is no obvious swelling and the seams are intact.
There is visible growth or a localized musty odor. Either one means the drying window has already passed for some of the material.
The wall is stucco and the sheathing reads wet. That investigation starts from the outside, at the window, and the stucco has to come off to see the sheathing.
How it should be opened. Open only as far as the wet readings go, plus a margin into dry material. On an interior wall face that usually means a cut a short distance above the highest wet reading, from the window down to the plate. Where the siding or trim has to come off to fix the flashing anyway, opening from the outside can do both jobs at once. Wet insulation comes out. Framing and sheathing are dried to below 20 percent moisture content and back near the baseline before anything is closed. A wall should never be closed over wet framing, and never before the leak that wet it has been fixed.
The 10 square foot line. If the opened wall shows mold growth and the work area is larger than 10 square feet, New York Labor Law Article 32 applies to any contractor you hire. The work needs a written remediation plan from a licensed mold assessor, a separately licensed remediation contractor, and a passed clearance from the assessor before the wall is closed. The same company cannot do both the assessment and the remediation on the same property. A homeowner working on their own house is exempt from the licensing requirement.
Fix the leak, not only the wall
Drying the wall does not stop the next storm. The sequence is: find the entry point, correct the flashing and the water-resistive barrier, dry or replace the wet materials, then test the repair before the wall is closed.
What the code requires. The Residential Code of New York State is based on the International Residential Code. Its flashing section (R703.4 in the IRC) requires corrosion-resistant flashing applied shingle-fashion so that water cannot enter the wall cavity. It requires flashing at window and door openings, at wall and roof intersections, and where porches, decks, or stairs attach to the house. Where the window manufacturer provides no flashing details, the code requires a pan flashing at the sill, sealed or sloped to drain water to the outside, with flashing at the head and sides. At the bottom of a roof-to-wall intersection, the roofing section (R903.2.1) requires a flashing that diverts water away from the wall where the eave meets the sidewall, the kick-out.
Sealant is not flashing. Caulk at a trim joint or a window perimeter is maintenance. It breaks down with sun and joint movement, and a caulked joint over a missing head flashing will leak again in the next storm. The repair that lasts is a flashing lapped over the barrier below it and under the barrier above it.
Test before closing. A repaired window can be water-tested in the field before the interior is closed. ASTM E1105 is the standard field test for installed windows. A careful hose test works on the same principle: start at the bottom of the opening, wet one area at a time, and work upward, so a leak can be traced to the joint that caused it.
Document it. Photograph the entry point, the flashing defect, the wet materials, and the meter readings before anything is repaired or removed. That record is what the contractor, the adjuster, and a future buyer all work from.
What we do
We perform mold assessments, moisture investigations, and post-remediation verifications on the East End. After a storm, that means a thermal survey of the exposed walls, pin and pinless meter readings against reference readings from unaffected walls in the same house, and a written finding of which walls are wet, where the water got in, and which walls have to be opened. Where mold growth is present, the report includes the remediation plan the contractor has to follow. By law we do not perform remediation on any property we assess. Call or text (631) 655-9855 or email bayrona@casablancaebd.com.
Common questions about storm leaks in walls and windows
Why does my window only leak when it storms? In ordinary rain there is little water on the wall and little pressure behind it. In a storm, wind drives far more water onto the wall and pushes it through joints. A residential window is water-tested at 15 percent of its design pressure, and gusts can exceed that. Most storm leaks at windows come through the joint between the window and the wall, which the window's rating does not cover.
Can I just caulk around the window? As a temporary measure until the storm passes, yes. It is not a repair. If the head flashing is missing or the sill has no pan, caulk will not stop the next storm, and it can trap water that would otherwise drain.
How do I know if water got inside the wall? Look for stains or bubbled paint at the lower corners of the window, swollen trim or baseboard, and damp carpet along the wall. The absence of those signs does not mean the wall is dry. A moisture meter reading at the base of the wall, compared with the same material on an unaffected wall, is what answers it.
Will the wall dry out on its own? Sometimes, if only the surface of the sheathing got wet and the insulation stayed dry. A closed cavity dries slowly, and EPA's window to prevent mold is 24 to 48 hours. Readings taken over several days show whether it is drying. If the insulation got wet, it has to come out.
Do I need a mold test? Not to find out whether a wall is wet. That is a moisture investigation, done with meters and thermal imaging. Sampling is useful when the result will change the plan, such as whether growth has spread beyond the wet area or whether a room is fit to occupy.
Who fixes the leak: the window company, the roofer, or the siding contractor? Whoever is responsible for the flashing at that joint. A leak through the window unit itself is a warranty question for the manufacturer. A leak around the window is an installation and flashing problem. A leak at a roof-to-wall intersection is usually the step or kick-out flashing.
East End questions
Our house is on the bay and the north side took the storm. Do we need to check every window? Start with the north- and east-facing walls, and on those, the windows and doors nearest the upper corners and any that sit below a roof-to-wall intersection. An exposed waterfront site receives far more driving rain than a sheltered lot. The fastest confirmation is often in the basement or crawl space: meter readings on the rim joist and sill plate directly below those windows.
The house is closed for the season. Could a wall be wet with nobody there to see it? Yes, and a closed house is where it goes unnoticed longest. There is no one to see a stain, and there is little air movement to dry the wall. If the house took the storm on an exposed side, have the walls below the windows on that side checked before it is closed for the winter. Our post on thermostat and humidity settings for an unoccupied house covers the rest of the closed-house conditions.
We have a new house with cedar shingles and closed-cell spray foam. Can water still get in? Yes, at the same places: window and door openings, trim joints, penetrations, and roof-to-wall intersections. The foam changes how you find it and how it dries. Water trapped against the sheathing cannot be seen from inside and dries only outward through the shingles. Readings are taken from the outside, under a shingle or behind a piece of trim at the suspected entry point.
Our stucco has dark streaks below the window corners after the storm. Is that a problem? It can be. Stucco holds water, and staining below a window corner is a common sign that water is running behind it at that corner. The sheathing behind stucco cannot be read from the surface. The standard method is invasive probing: small holes drilled through the stucco at the suspect areas, probes read at the sheathing, and the holes sealed afterward.
Sources and further reading
NBC New York, Nor'easter wind advisory and track update, Long Island and New Jersey (September 2026 storm forecast).
National Weather Service, Nor'easters.
Straube, J., Building Science Corporation, BSD-148: Simplified Prediction of Driving Rain on Buildings (2010).
Fenestration and Glazing Industry Alliance, Performance Class Overview (AAMA/WDMA/CSA 101/I.S.2/A440 water test pressures).
ANSI/ASHRAE Standard 160, default 1 percent rain penetration, as quoted in 14th Canadian Conference on Building Science and Technology proceedings (2014).
Fine Homebuilding, Rainscreen Systems for Stucco.
U.S. EPA, A Brief Guide to Mold, Moisture and Your Home; Mold Remediation in Schools and Commercial Buildings, Table 1: Water Damage — Cleanup and Mold Prevention.
USDA Forest Products Laboratory, Wood Handbook, Chapter 14: Biodeterioration of Wood; Electric Moisture Meters for Wood, FPL-GTR-6.
The Snell Group, Basics of Using IR for Building Moisture Inspections; Fluke, Restoration Contractors Profit from Thermography.
Restoration & Remediation Magazine, What Is Acceptable Dry? (IICRC S500 dry standard).
International Code Council, 2015 International Residential Code, Chapter 7: Wall Covering (R703.4 flashing, stucco water-resistive barriers).
U.S. DOE Building America Solution Center, Step and Kick-Out Flashing at Roof-Wall Intersections (IRC R903.2.1).
New York Labor Law, Article 32: Section 930 (definitions), Section 933 (exemptions), Section 936 (prohibited conflicts).