Light Spots on Vinyl Siding

[Jim Katen]

As a companion to "Rust Spots on Vinyl Siding"

I sometimes see this effect. The light spots are always directly above nail heads. Sometimes, if there's a gap in the fanfold foam, there's a light stripe immediately over the gap.

The first picture is untouched. The second picture is after scrubbing with detergent and a brush, rinsing with water, and drying with a towel.

The third and fourth pics show the surface of the vinyl and what's going on just behind it.

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[Rob Amaral]

Could be heat being retained by the nail-head...concentrating.. and somewhat changing the vinyl???

An IR shot would be a good idea to figure this out..

Sort of like when we see roofing nail heads 'cuting thru' asphalt on roofs that are poorly-ventilated... ?

[Jim Katen]

Could be heat being retained by the nail-head...concentrating.. and somewhat changing the vinyl???

An IR shot would be a good idea to figure this out..

Sort of like when we see roofing nail heads 'cuting thru' asphalt on roofs that are poorly-ventilated... ?

I think it's more like the thermal bridging we see on interior drywall and the dark spots associated with it. I suspect that water condenses on the outside of the vinyl less readily at the nail spots and at the spots where there are gaps in the insulation. That water probably has an effect on the color of the vinyl over time.

IR investigation would show that at some times of the day, the nail-head spots are warmer than the surrounding vinyl and at other times of the day, they're colder. No big help there.

The marks are permanent. Either you paint the vinyl, replace the vinyl, or live with it. It would be interesting to figure out the mechanics of why this is happening, but sort of pointless. I suspect that the vinyl manufacturers are well aware of it and have, possibly, altered their formulations to account for it.