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

I have observed homes clad with vinyl lap siding (typically 15 - 25 years old) 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 (photos). 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 any 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 (under the siding) 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?

Unlike with the vinyl siding where the airflow appears to primarily come through the overlap of successive side-by-side sections of siding, the airflow with the fiber cement siding is not expelling from the butt joint (my interpretation based upon the visible patterns). Instead, the airflow seems to specifically expel out from the bottom edge of a course of siding right at the location of the (flashed) butt joint of the course below.

I'm curious why that is and why, for the fiber-cement siding, this pressure-pattern is from above the butt joints and not along the entire span?

Are their solutions to prevent this mostly cosmetic outcome?

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I think you are on the right track that it is air movement in, around, and behind the siding/cladding ... not "through" the building.

The way to answer that question definitively is to do a blower door test. While the blower door test is being done, go around the outside and check for air movement at the suspected areas (you would need to do this on a calm day with no breeze, otherwise you will not be able to tell what is air movement caused by the blower door test or even a slight breeze).

I suspect that you will find that it is not air moving "through" the house (unless it is an old house as old houses are rather "leaky" with regard to air moving "through" them, and by 'old house' I mean pre-1970s, maybe even 1960s).

Just some food for thought.

Thanks Jerry.

Any thoughts as to why the pattern of leakage on the fiber-cement siding is "around" the butt joints, but as I suspect, coming from up under the course above "in line with" the butt joint of the course below?

Any (reasonable) ways to prevent this, or are homeowners stuck with regular cleaning of one side of the house?

http://inspectapedia.com/exterior/Vi...ing_Stains.php

Btw Jerry is alluding to pressurizing the house, not your typical blower test where the house is depressurized.

Originally Posted by Raymond Wand

http://inspectapedia.com/exterior/Vi...ing_Stains.php

Btw Jerry is alluding to pressurizing the house, not your typical blower test where the house is depressurized.

No, I am talking about a typical blower door test which depressurizes the house - you can find air infiltration points that way as well as with pressurizing the house (I prefer the typical blower door depressurization over pressurization for most uses).

Thanks for the clarification. A smoke pencil is a good tool to use.

Originally Posted by Raymond Wand

Thanks for the clarification. A smoke pencil is a good tool to use.

Yes, a smoke pencil is good for checking small areas or for areas of suspected leakage, but if you need a lot of smoke, smoke bombs (like are used to test if corridors and stair towers are pressurized properly) make a lot of smoke and you can find the areas to check with the smoke pencil if needed.

I've never seen smoke bombs used with a blower door test, but I can see them working when the building for positive pressure testing as the building would fill with smoke and you could walk around the outside looking for smoke escaping. If someone has a blower door test setup, that would be an interesting test to do.

I think correlation with prevailing wind direction proves the flow that fouls siding does not pass through conditioned space. I imagine exterior walls not air tight to outside fully equalize with the distribution of outside static pressure. Large wall volumes then have air movement matching wind direction. A partial solution is in having a really airtight envelope to outside, under always-leaky siding. This stops wind washing of wall insulation. There can yet be circulation between siding and that outside barrier. An airtight envelope is achieved by methods and materials. A blower door test should be insensitive to the outside envelope and meaningless, if walls are airtight to inside, too.

Originally Posted by Phillip Norman

A blower door test should be insensitive to the outside envelope and meaningless, if walls are airtight to inside, too.

Precisely why a blower door test should be done - to confirm "if walls are airtight inside" or show that they are not.

A blower door is not a difficult test to do, and it is not very expensive considering that that it may eliminate an issue from consideration (no longer have to spend time or effort considering if that is part of the problem, the blower door test will show that it is not) ... or the blower door test may show that it might be an issue. Better to know than not know.

you can also use a commercial fog machine to find leaks.

- - - Updated - - -

you can also use a commercial fog machine to find leaks.

- - - Updated - - -

you can also use a commercial fog machine to find leaks.

http://www.greenbuildingadvisor.com/...ks-fog-machine

The fan blows inward- pressurizing the house.
During a conventional blower-door test, the fan is set up to blow outward, depressurizing the house. A fog test, on the other hand, requires the house to be pressurized, with the fan blowing inward. Fog tests are usually scheduled after a house has been insulated but before the drywall has been hung.


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The images appear to indicate the house wrap is deficient.
Many types of house wraps.
Building Paper. One of the most basic types of house wrap is building paper. ...
Plastic House Wrap. Another one of the most popular options on the market is plastic house wrap.
Moisture Permeable.
Tyvek
Permeability. It allows moisture to escape one way.

Did the attic give you any indications of the house wrap used and sealing methods?

Original post is almost a year old. Not likely the original poster has any new input.

The fiber cement siding has flashing installed at the butt joints. Consider that maybe the metal flashing is creating a temperature differential where it is installed at the butt joints. A temp differential might cause a surface moisture condition at those locations. The extra moisture can let additional dirt stick to the surface and cause the staining patterns.

Just a guess but maybe ....huh?

John, difficult question to answer without being on site. Location to prevailing winds, atmospheric grime, tress, shrubs etc. Like ghosting effects on interior walls.

I zoomed in on what looks like wood or cement board time. The ob

Robert I tried to quote your post #12 but for some reason not identifying my space bar. I am trying to understand for sure as this can be difficult for me to comprehend sometimes it seems. I am pretty sure house wrap is a water resistive barrier and allows air flow/vapor to flow both directions. i.e. it breathes and has a larger range of perm values. Maybe I am incorrect and need to go back in my notes. But Tyvek house wrap does prevent liquid moisture to penetrate past. And allows vapor to pass out (Per Tyvek). But I don't see how vapor and air can only go one direction and typically vapor is in the air. Unless this is due to vapor drive. Thus in my opinion air and vapor can go both directions through the microscopic holes.

Vapor barriers placed on the warm side of wall in cold climates have a very low perm rating to prevent moisture from traveling both directions. I don't have my perm value information with me right now.

In regards to the discoloration it might be possible if there is a temperature differential then it is condensation related due to flashing temperature lower than dewpoint of exterior moist air(More Likely? Lack of interior or deficient interior vapor barrier in winter months allowing warm moist air to escape and come in contact with cold outside air and metal flashing below dewpoint(Less Likely)?

Originally Posted by Paul Douglas

Robert I tried to quote your post #12 but for some reason not identifying my space bar. I am trying to understand for sure as this can be difficult for me to comprehend sometimes it seems. I am pretty sure house wrap is a water resistive barrier and allows air flow/vapor to flow both directions. i.e. it breathes and has a larger range of perm values. Maybe I am incorrect and need to go back in my notes. But Tyvek house wrap does prevent liquid moisture to penetrate past. And allows vapor to pass out (Per Tyvek). But I don't see how vapor and air can only go one direction and typically vapor is in the air. Unless this is due to vapor drive. Thus in my opinion air and vapor can go both directions through the microscopic holes.

Perm barriers, vapour retarders, vapour barriers.
Ray posted Green building Advisor.

The attic is the best clue for house-wrap/weather/perm/vapour/barrier/retarder, and insulation. I also have a tool to lift and reset the butt lock to see what's under siding.
Seen entire homes with only TenTest acting as the weather barrier.
Built a Modern Traditional North American Platform Wood Framed (4) Four season home in 1984 with steel wind braces wrapped in Tyvek. Taped every lap personally. I was the lead on that job.

Paul, do not feel confused. It is difficult subject.
Most of what is posted on MB is from Google or other browser engines and saved articles.

Raymond Wand post a great resource, Daniel Friedman's InspectAPedia. A great overall resource and bonafide investigator. Fiber Cement Wall Siding.

I applied siding and opened countless veneers throughout my 35 years in the trades.
Atmospheric Grime/microscopic particulate (in) the assembly falls, shakes loose, and deposited (on) the exterior siding through openings.
Water on the siding acts likely the catalyst for the staining patterns. In an indirect way, think of efflorescence only the particulate is not soluble, well some might be.
I suspect that is where you are drawing your dewpoint hypothesis from.

Originally Posted by Paul Douglas

Vapor barriers placed on the warm side of wall in cold climates have a very low perm rating to prevent moisture from traveling both directions. I don't have my perm value information with me right now.

I concur.
Paul, if weather barrier are not sealed well, what occurs? Where does the interior atmosphere go?

Originally Posted by Paul Douglas

In regards to the discoloration it might be possible if there is a temperature differential then it is condensation related due to flashing temperature lower than dewpoint of exterior moist air(More Likely? Lack of interior or deficient interior vapor barrier in winter months allowing warm moist air to escape and come in contact with cold outside air and metal flashing below dewpoint(Less Likely)?

As for a temperature differential, materials, and dewpoint.
There will be a temperature differential with dissimilar materials. Unavoidable.
Dewpoint requires what?
Atmospheric conditions.
Stretching, but if all the stars are aligned, and there is reasonable testing, it can happen.

IMO, particulate in the wall assembly is dislodged, falls out openings, attaches to the exterior surface.
The patterns may well be an indication that the home is not sealed adequately.

If the home had a HVAC system a good thermographer would detect the leaks.

... more to ponder ...

http://structuretech1.com/qa-siding-...do-houses-cry/
http://inspectapedia.com/exterior/Vi...ing_Stains.php
http://www.inspectionnews.net/home_i...ng-stains.html