If the north wall of your house is the one wearing a gray-green film while the rest of the siding still looks fine, you are looking at mildew, algae, or both growing on the paint. It is the most common exterior complaint we hear in Coeur d'Alene, and the reason it keeps returning is not a mystery. Most people wash it off. Washing takes the stain away. It does not change the conditions that grew it, and plain water does not kill what is left behind in the coating.
Here is what is happening on that wall, how to tell mildew from algae from ordinary dirt, and what a north elevation actually needs so you are not scrubbing the same siding every summer.
Why the North Side Grows It and the South Side Does Not
Shade plus moisture. That is the whole answer. Biological growth needs a surface that stays damp, and the north face of a house in the Inland Northwest is the one surface that rarely gets enough sun to dry out.
The side that never fully dries
At roughly 2,150 feet in town the summer sun tracks high and hard, but it still swings south. A north wall in the Garden District or Fort Grounds picks up a little low-angle light in June and almost none from October through February. Morning fog off Lake Coeur d'Alene settles into the low ground and burns off the south and west faces first. The north wall stays damp two or three hours longer because nothing is drying it. Stretch that over most of the year and you get a surface that is wet more hours than it is dry.
Mildew, algae, or plain dirt
You can sort this out in about a minute. Dab a cotton swab of household bleach on a small stained spot and wait. Mildew lightens fast, usually inside sixty seconds. Algae fades more slowly. Dirt does not change color at all, which means you have a washing problem and not a growth problem.
Mildew reads black to dark gray and shows up speckled, worst in the band under the eaves and around window and door trim. Algae is greener, spreads in sheets, and streaks downward below gutter seams and roof edges where water runs the same path every storm.
The Local Conditions That Feed It
Ponderosa pine right up against the house
Ponderosa pine over a north elevation causes two problems at once. The canopy blocks what little sun that wall would have gotten, and it drops needles, pollen, and resin onto the siding all season. Resin is sticky. It holds organic dust, and that dust is what mildew feeds on. Older lots below Tubbs Hill and through the Garden District often have mature pines standing within a few feet of the wall, which is why those houses show growth years before a newer place out in Coeur d'Alene Place or Hayden Canyon does.
Freeze-thaw and the band under the eave
Our freeze-thaw cycles run from about November into March. Moisture works into hairline checks in the paint film, freezes, opens them slightly wider, then thaws and soaks in a little deeper. Meanwhile snow load sitting on eaves and trim melts slowly and drips onto the same eight to twelve inch band of siding for weeks. That band is almost always where the heaviest mildew shows up, and it is the first place to check when you walk your own house.
Sprinklers, gutters, and grade
A surprising share of north-wall growth is self-inflicted. A sprinkler head that clips the siding three mornings a week puts more water on that wall than the sky does. Out on the open ground toward Rathdrum Prairie the wind pushes irrigation spray sideways and it lands where nobody intended. Clogged gutters overflow at the corner and run down the same seam. Soil or bark mulch piled up against the bottom course wicks moisture into the siding from below.
Why It Comes Back After You Wash It
Rinsing is not killing
A pressure washer with nothing but water knocks the stain loose and leaves the roots and spores in the pores of the coating. The wall looks clean for a season. By the following summer the colony has repopulated from what stayed behind, and often it comes back faster than the first time because the wash roughened the surface. High pressure on wood or fiber-cement siding also drives water behind the boards, which creates a second problem you will not see until the paint starts lifting.
An aged coating holds growth in the film
High-altitude UV is brutal on south and west exposures here, and homeowners tend to judge the whole house by those two faces. The north wall ages differently. It does not chalk as fast, but the binder still breaks down, and once the film goes porous the growth is inside it rather than on top of it. That is the point where washing stops solving anything and the wall needs paint. If the coating is also flaking or lifting rather than just stained, the cause is usually moisture behind the film, and that is a different repair. We walk through that one in our guide on why exterior paint peels on Coeur d'Alene homes.
Flat paint on a shaded wall
Flat and matte finishes hold dirt and moisture in their texture. On a south wall that hardly matters because the sun bakes it dry by ten in the morning. On a north wall a flat sheen gives growth a head start. Satin holds up better on shaded elevations because it sheds water and washes clean without much scrubbing.
What Actually Stops It
Soft wash with a mildewcide, then let it dry
The fix is chemistry rather than pressure. Low pressure, a cleaning solution carrying a mildewcide, enough dwell time for it to work, then a full rinse. Done correctly the wall comes clean without forcing water into the siding, and the growth is dead instead of relocated. Then the wall has to dry. We hold a full 24 hours between washing and any coating, longer if the weather is cool or the wall is deeply shaded, because paint over damp siding fails early. There is more detail on that step in our post on pressure washing before painting, and the service itself is covered on our pressure washing page.
Spot prime what is stained, not the whole house
After the wash, any area where the stain shadow is still visible gets a stain-blocking primer before topcoat. This is usually the eave band, the trim returns, and a few feet up from grade. Priming the whole elevation is rarely necessary and adds cost without adding life.
Use a coating with a real mildewcide package
Quality exterior acrylics carry mildewcide in the can. Contractor-grade economy paint often does not, or carries so little that it washes out in two seasons. On a north elevation the difference shows up fast. Two coats of a good exterior acrylic in satin, applied over a dry and primed surface, will typically hold a shaded wall clean for six to eight years here. Our breakdown of the exterior paints that survive high-altitude UV and freeze-thaw covers which product lines earn the upcharge. Full exterior work, prep through topcoat, is on our exterior painting page.
Fix the water and open up the shade
None of the coating work matters much if the wall keeps getting soaked. Cut branches back to leave three feet of air between the canopy and the siding so the wall can dry after a rain. Turn or replace any sprinkler head that reaches the house. Clear the gutters above that elevation and run the downspout discharge well away from the foundation. Pull mulch and soil down so there are six inches of clearance below the bottom course of siding.
What This Costs in Coeur d'Alene
Wash only
A soft wash of the full exterior runs about $300 to $850 in the Coeur d'Alene area, depending on square footage, stories, and how hard the north side is to reach. If the coating is still sound and the stain is sitting on the surface, this is all the wall needs. Plan on repeating it annually.
Wash plus repainting the north elevation
Exterior painting here runs roughly $3.00 to $5.50 per square foot of wall. One north elevation on a typical Coeur d'Alene home is somewhere between 400 and 700 square feet, so washing, prep, spot priming, and two coats on that single face lands in the neighborhood of $1,200 to $3,800. Worth knowing before you commit to one elevation: a fresh coat next to twelve-year-old paint on the adjoining walls will not match, even with the original color code, because the old paint has faded. Some homeowners accept that on a face nobody sees from the street. Others use it as the reason to do the whole house.
Full exterior repaint
A whole-house exterior in this market generally falls between $5,500 and $14,000 depending on size, siding type, story count, and how much wood repair turns up during prep. If the north wall has gone porous and the south and west walls are chalking under years of high-altitude UV, doing all four faces at once is usually the better value per year of service.
A Rhythm That Keeps the North Wall Clean
Wash once a year inside the dry window
Our dry summer window runs May through September, and that is when a wash actually dries. June and July are ideal because the wall has time to dry fully and you still have months of warm weather ahead. Washing in October leaves a damp north wall heading into the freeze, which is the worst possible timing.
Walk the house every fall
Before the first hard freeze, look at the eave band, the window trim returns, and the bottom two feet of siding on the north and northeast faces. You are checking for new speckling, open caulk joints, and any spot where the film has gone dull and chalky. Catching a failing caulk joint in September is a tube of sealant. Catching it in April is a rot repair.
When to stop washing and start painting
The rule we use on estimates is simple. If the growth returns within one season of a proper mildewcide wash, the coating is finished and the wall needs paint. If it stays clean for two or three years between washes, keep washing and leave the paint alone.
If you want a second opinion on which side of that line your north wall is on, we walk exteriors across Coeur d'Alene, Hayden, Post Falls, and Rathdrum and will tell you if a wash is all you need. Any painter you consider should be registered with the state, and you can check an Idaho RCE (Registered Contractor Entity) number in a few seconds before anyone sets a ladder against your house. Request a free estimate and we will take a look at that elevation.