Airflow "through" a building or "beneath" the exterior cladding

As a licensed home inspector in the southeast, I have observed residential structures (typically 15 - 25 years old) clad with vinyl lap siding that have particulate matter deposited at some of the overlapping seams of the siding (see photos). This is typically observed on one side - presumably the leeward side.

I've been of the impression that these “puff-like” markings are likely the result of dirt depositing on the siding from the movement of air in the space behind the siding due to changes in air-pressure on the building shell. When the wind blows on one side of a home and it creates a positive pressure on that windward side, the resulting negative pressure on the opposite (leeward) side “pulls” at the siding and the air behind the siding rushes out to fill the (pressure) void, bringing out any underlying particulate which may then deposit on the siding.

But I've secretly worried that this could instead represent airflow "through" the building envelope and not around it.

I've just recently observed this same phenomenon on a home constructed in 2012 with fiber-cement lap siding (with flashing at field butt-joints) that I believe was sheathed with the (Huber) ZIP System (additional photo). Done right, the taped seams of the Zip System create a reasonably tight air barrier.

I have consulted with 3 - 4 manufacturer's of fiber-cement siding hoping that they may have had a warranty claim on the issue (knowing full well it isn't a product defect), and could they offer insight as to the cause and/or cure. To no avail.

Am I on the right track, especially with this newer case of a home that I believe to be reasonably tight beneath the cladding? Is this occurring within the interstitial space between the backside of the siding and the exterior of the sheathing, then moving around the sides of the building and out the leeward side? Or could this possibly be representative of airflow through the building envelope and therefore represent more of an energy and IAQ concern for the occupants?

I'm also curious why, for the fiber-cement siding, the airflow seems to expel along the bottom edge of a course of siding specifically at the location of the (flashed) butt joint of the course below - why not along the entire span? The airflow is not expelling from the butt joint (based upon the pattern), unlike the vinyl siding, where the airflow appears to come through the overlap of successive side-by-side sections of siding.

Are their solutions to prevent this mostly cosmetic outcome?

Tags: Airflow, Building, Envelope

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Interesting observation. It sounds like your a good Home inspector.

It seems to me like the air movement is the  from the latter. The air is moving through the wall assembly caused by the Delta-P, and it is showing up where the leaks are located in the wall  assembly.

As far as a solution, I think it would depend on how much the homeowner wishes to pay (as usual). I would recommend pressuring the building with a Blower door and then scan with an IR camera to see for certain what the cause actually is, and then come up with a workscope. It may be that a more invasive evaluation may be needed, and that's where the added cost will determine what measures the homeowner is willing to take on.

I also wouldn't expect that newer houses are tighter unless they are Energy Star, and I see no claim under a warranty from a manufacture. I'm also surprised about the Zip System, they are usually somewhat effective and installed by contractors who understand its purpose.

The last picture with fiber cement - looks like the pattern you might see from water.

Is the fiber cement board on firring?  Blind nailing?  Did they paint the fiber board after installation and seal up the overlapping --- leaving only the end gaps for air to escape?  In general the manufacturers also recommend air gap at the bottom of the wall and the top of the wall for air circulation from bottom up... is that missing?

The purpose of the siding --- especially if it has firring behind it -- is a rain screen and  wind screen.  Hopefully the builder used other techniques to provide the air tightness.  Assuming that they did - air moving under the siding, round the outside of the building as wind blows is not really likely to cause a problem - provided the backsides of the siding was primed.  Installed as per manufacturer's instruction it shouldn't be a problem.

Did you do a blower door test?

The seams will also exhibit a capillary effect and hold moisture by them longer... which could help build up dirt. The horizontal pattern may be caused in part by moisture flowing along the seams when it rains...  what kind of climate is this in?


Looking at the first two pictures - that really looks like a pattern you might get from dirt collecting on the outside of a surface as its drying... with wind blowing mostly from one direction. The air will cling to the siding surface and at the joint there will be a slight increase in turbulence. It simply may leave that area wetter longer and allow for the dirt to cling to it and start building up. Look at the bottom of the joints - you can see a clear air gap on the bottom side and you don't see a dirt pattern coming from that air gap. But it does look like the edge of the sideing as the water would flow over it is dirtier... this looks like a normal build up of dirt in a climate that has some rain. With winds that blow frequently from one direction?

Just thinking.... out loud...

I'm suspecting the dust gets into the attic at the eaves where the largest pressure diff's are from the wind, from there down the wall outside the vapor barrier pumped through by gusts.

The barrier job sounds too worthy for blow-through to me and via the attic is something to check when you get dust in homes. I lived with strong winds in Phoenix & saw the attic as the usual source, having it exit under the siding is weird but the winds are really strong at times over the roof for pressure.

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