Exterior Window Cleaning — North-Facing Elevation
On a shaded, north-facing elevation, the glass had become home to Klebsormidium — a hardy green alga that takes hold wherever moisture lingers. Along the silicone seals, the familiar dark staining was Aureobasidium, a pigment-producing organism, not "dirt". None of this is neglect; it is simply biological colonisation responding to a damp, sheltered microclimate. Left unmanaged, the moisture it traps begins to work against the seals and frames it clings to.
Residential
Northamptonshire village · Nene Valley humidity plume · north/east elevation
Soda-lime float glass · uPVC frames · silicone perimeter seals
1930s semi-detached residential (non-listed)
Residential · Exterior Window Cleaning
The Pathology
On a shaded, north-facing elevation, the glass had become home to Klebsormidium — a hardy green alga that takes hold wherever moisture lingers. Along the silicone seals, the familiar dark staining was Aureobasidium, a pigment-producing organism, not "dirt". None of this is neglect; it is simply biological colonisation responding to a damp, sheltered microclimate. Left unmanaged, the moisture it traps begins to work against the seals and frames it clings to.
χ Drag Factor
χ — demonstrative: elevated (sustained high time-of-wetness, shaded aspect)
Biomass / Taxon
Klebsormidium flaccidum (green biofilm); Aureobasidium pullulans (dark seal colonisation)
Atmospheric Log
High time-of-wetness (≈ BS EN ISO 9223 T3–T4); partial shade; Nene Valley humidity
The Method
We treated the elevation as an asset to be protected, not a surface to be attacked. A neutral surfactant lifted the biofilm, applied by hand through a soft-bristle pole brush — never abrasives, never high pressure near the perimeter seals, which would only drive water into the frame cavity. A pure-water rinse left no residue to feed re-colonisation. The seals were addressed gently, respecting the silicone rather than stripping it.
Specification / Dataset
North-facing glass rarely sees direct sun, so it stays damp for longer after rain or dew. In building-science terms its time-of-wetness is high, and moisture is the single variable biological colonisation needs most. That is why one elevation can look pristine while another, metres away, greens over. Understanding this is reassuring: the staining reflects aspect and microclimate, not how well a home is kept.
The green film is Klebsormidium, a filamentous alga that binds to the microscopically hydrated surface layer of the glass using a natural adhesive. It is living, not loose, which is why a quick wipe never fully clears it and why it returns so readily. A correctly chosen surfactant releases that bond and lifts the organism cleanly, leaving the glass surface itself undamaged and slower to re-colonise.
The dark line along the silicone is Aureobasidium, an organism that produces protective dark pigments and thrives where moisture and a little organic matter meet. It favours the seal because that junction stays damp longest. It is harmless to treat correctly, but aggressive bleaching or scraping damages the silicone and shortens the seal's life — the opposite of asset protection.
It is tempting to think more force means a better result, yet directing pressurised water at perimeter seals risks pushing moisture past the silicone and into the frame cavity behind. On timber that invites decay; on masonry it invites trapped damp. Gentle, hand-controlled cleaning protects the very assembly the clean is meant to preserve, which is why we never reach for that approach near glazing.
A clean window is the visible result; the real value is quieter. By removing biological colonisation and the moisture it holds against seals and frames, the elevation is left drier and more stable. That is environmental stewardship of the building fabric — clarity restored today, and the home's glazing protected for the seasons ahead, verified rather than assumed.
This case study is a demonstrative model of our method and standards. It is not a price list; every property is assessed individually, and any figures shown illustrate approach rather than cost.
Why does only one side of my house go green?
Almost always aspect. North- and east-facing elevations stay damp far longer after rain, and that sustained moisture is exactly what algae need. The sunnier sides dry quickly and colonise much more slowly. It reflects microclimate, not how the home is kept.
Will the green come straight back?
Not at the same rate. Lifting the organism with the right surfactant and a pure-water rinse leaves no residue to feed regrowth, so re-colonisation is slower. A gentle maintenance interval — guided by your elevation's exposure — keeps it comfortably in check.
Isn't a pressure washer faster?
Faster, but risky on glazing. High pressure aimed at seals can force water into the frame cavity and damage silicone, causing problems far costlier than the clean. Hand-controlled pure-water cleaning protects the frames and seals while still fully restoring the glass.
Is the black stripe on my seals mould?
It's Aureobasidium, a dark-pigmented organism rather than household mould. It settles where seals stay damp. Treated gently it clears safely; harshly bleached or scraped, the silicone is damaged. The aim is to clean the organism without harming the seal.
Do you use harsh chemicals near my plants and pets?
No. The approach centres on a neutral surfactant and a pure-water rinse, chosen to be effective on the organism and considerate of the surroundings. Anything stronger is reserved only where genuinely needed and applied with full care.
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