The black streaking on the shaded side of a house is mildew, and the green film is algae. Neither is dirt. Blasting them with pressure removes the visible surface and leaves the organism, so the wall looks clean for a month and then does not. A house wash kills it, and that is why professionals clean siding at low pressure with a lot of chemical.
Which makes this the page that matters more than any machine page for anyone with siding.
How these differ
Oxygenated siding cleaners — the Oxy Solve House and Siding formulation is aimed specifically at mildew streaks on vinyl, which is the one job a general-purpose cleaner routinely fails. Reasonable on planting, needs real dwell time.
Stronger biocidal concentrates — Home Armor and 30 Seconds work on growth the oxygenated cleaners leave behind and work visibly faster. More care required around plants, metal and fabric, and both must be rinsed.
No-rinse treatments — Wet & Forget is in the picks here even though it is not a pressure washing product at all. You spray it on and leave it; results appear over weeks. For a shaded north wall, a fence, or anywhere you cannot safely reach, it is often the correct answer.
Multi-purpose concentrates — Krud Kutter is the cheapest competent option and will do a decent job on moderate growth. Beaten by the specialists on heavy mildew.
Why low pressure, specifically
Vinyl siding survives a surprising amount of pressure from the front. What it does not survive is water driven upward under the laps, where it enters the wall cavity and stays. That is a far more expensive problem than dirty siding, and it is the single rule to take from this page.
So: apply cleaner through the black soap tip, let it dwell, then rinse with the white 40-degree tip from a sensible distance, angled slightly downward. The full method is in how to pressure wash vinyl siding, and the general principle in soft wash vs pressure wash.
The sodium hypochlorite question
Most professional house washing uses diluted sodium hypochlorite — bleach — with a surfactant. It is faster and more effective on heavy mildew than anything on this page. It also kills plants, corrodes metal, bleaches fabric, and must never pass through your pump.
It is a legitimate option with real care, and the honest account of what that care involves is in bleach and pressure washers. For a first house wash, start with a product on this page.
Getting it onto the wall
Three routes. An onboard tank if your machine has one. A downstream injector drawing from a bucket, which is the most flexible. Or a pump sprayer for application with the pressure washer used only to rinse — which is what many professionals do, and is the safest option on a delicate wall.
For an upper story, use an extension wand from the ground. Never a ladder.
Protect the planting, properly
Pre-wet every bed under the wall before you start, cover anything precious, and rinse everything thoroughly afterwards. Run-off concentrates at the base of the wall, which is exactly where the planting is. This applies to the gentle products too.
