Green Film on the North Side of a House

Why it is always the north wall

North elevations get the least direct sun, so they stay damp the longest after rain, fog or dew. Algae needs moisture and shade, and the north side of a Treasure Valley house supplies both for months at a time in winter. The sunny walls dry out too quickly for it to establish.

It is alive, which changes everything

Algae anchors into the surface texture of siding. Removing the visible layer without addressing the organism leaves what regrows, which is why a wall that was blasted clean in April is green again by August. The growth was never actually removed, just the part you could see.

Why pressure washing is the wrong tool

Beyond leaving the root structure behind, high pressure forces water behind siding. On vinyl it goes up under the courses, on stucco and hardie it penetrates the surface, and the trapped moisture creates a worse problem months later. On an already-damp north wall this is the last thing you want to add.

What soft washing does instead

Soft washing applies a cleaning solution at low pressure that kills the growth itself, then rinses gently. Because the organism is treated rather than just the visible layer, the result lasts through a season rather than weeks. It runs $0.18 per square foot and is safe on vinyl, stucco, hardie and painted surfaces.