PLRB Test Your Claim Knowledge: Condensation and Air Leakage Property Claims

Article originally published by Property Loss Research Bureau (PLRB). Reproduced by permission from PLRB.

Question:  Why do those stains on the siding re-appear every year and why is that siding buckling?

Background:  A recurring type of claim that we have been involved with concerns stains that occur on hardboard siding every year.  The insured would first hire a roofer thinking it was a roof leak, so the roofer would of course find the leak, “caulk and walk”, only to have the stains re-appear the following winter.  The insured then blames ice dams and files a claim.  This claim can also involve siding that buckles, creating a wavy appearance that spurs a claim to their insurance company, or buckled siding may be reported without associated staining.


These photographs depict the stains on the hardboard siding that come back every year even though the insured paints the wall and hires a roofer who fixes “roof leaks”.

These photographs depict wavy or buckled hardboard siding, with and without stains.

Part 1 of this series discusses the various ways condensation occurs in buildings, with water vapor migration via air current leakage being a common culprit.  In these claims, after interviewing the insured, we will take a look at their furnace and around the house for a humidifier and other sources of water vapor generation.  Humidifiers can be attached to the furnace or they can be standalone, such as in a bedroom. In addition, common sources of water vapor generation are aquariums, plants, pools/hot tubs/Jacuzzis, cooking, showering, laundry, groundwater migration in basements (damp basements), to name a few.  In the southeast, a significant source of water vapor inside the building may be from outside air!  It is good to get an idea of how much water vapor is being generated in the subject building.  For example, the homes with stained siding can have a humidifier on the furnace, three teenage girls (i.e. long showers), plants, etc. which means there is a high water vapor load in the building.

Next, if possible, we remove one or two of the siding boards to look at the backside.  This is what we commonly see:

claims82Notice how the stains on the siding align with joints (openings) in the foil-faced sheathing boards?  If you look closely at the photograph on the right, you will notice that the staining on the backside of the board aligns with an errant puncture in the sheathing.

In the case of the warped and buckled siding below, the location of the stain was first marked on the underlying housewrap (Tyvek®) before the housewrap was cut to observe the joint in the sheathing.


The fact that the stains align with “holes” or gap in the sheathing is a strong indication that water vapor is migrating through the exterior wall assembly via air leakage and condensing on the backside of the siding.  In some cases with certain siding compositions or manufacturers, this water will cause swelling and buckling, other times it causes the saturants in the siding to leech out and stain the front face.

To prevent these stains and buckling from occurring in the future, two things must happen:

  1. Stop air leakage
  2. Reduce interior water vapor levels

Stopping air leakage, either air exfiltration (indoor air migrating outward) or air infiltration (outdoor air migrating inward) can be accomplished using visual means or most effectively by fan pressurization combined with infrared thermography (IRT) and smoke tracer methods.  The latter method clearly verifies where air leakage is occurring and what should be sealed to prevent future air leakage.  The photographs below show typical fan pressurization and infrared/smoke diagnostic methods to find air leakage in buildings.


This IR image shows warm air exfiltrating at a roof to wall parapet interface where condensation is possible in cold climates.


This IR image shows warm air infiltration around a duct penetration at exterior wall where condensation is possible in warm, humid climates.

Stopping air leakage in buildings is a staple for weatherization projects and is becoming prevalent in commercial building energy saving techniques.  Considerations must be given to indoor air quality and combustion venting appliances if these conditions relied on uncontrolled air infiltration before the retrofit.  If sufficient make-up air is not available to create a natural draft for combustion appliances, backdrafting can occur.

Reducing interior water vapor levels involves the use of spot ventilation (i.e. bathroom and kitchen exhaust fans), smart controls of any humidification system, or the best case scenario is a controlled whole-building ventilation system.  Exhaust fans should be used at all times when moisture is generating and furnace humidifiers should be set low during cold winter months.

Vapor retarders are materials that are required by certain building codes to prevent water vapor diffusion through exterior walls and ceilings.  However, vapor retarders alone do not address the issue of air leakage.  Air barriers on exterior walls or interior walls are becoming mandated by some codes to address the issues contained in this article.

Hopefully the information presented herein gives the reader a broad overview of the issues of air leakage and condensation.  The topics are broad such that textbooks have been written on them, but the “condensed” information here should give the reader a better understanding when handling a moisture claim where air leakage and condensation are suspected.

Test Your Claim Knowledge

  1. The movement of air from inside to outside the building is referred to as:
    1. Infiltration
    2. Exfiltration
    3. Osmosis
  2. Condensation via air leakage only occurs in cold climates in the winter months.
    1. True – the cold exterior surfaces are the cause, in warm climates there are no cold surfaces.
    2. False – buildings in warm climates are also susceptible to condensation via air leakage.
  3. Infrared thermography, especially when combined with fan pressurization, can be effective in identifying which of the following (select all that apply):
    1. Diffusion
    2. Thermal bridges
    3. Air leakage
    4. Missing insulation
    5. Water intrusion
    6. Mold
  4. When retroactively air sealing a building, why should combustion appliances be analyzed?
    1. Old, naturally venting appliances often rely on air infiltration for make-up air.
    2. Without sufficient make up air, a draft may not be developed and backdrafting can occur.
    3. The insured may be looking to replace old appliances anyways.
    4. Backdrafting is a real and significant safety concern.
    5. All of the above.
  5. Which is true of vapor retarders (select all that apply)?
    1. They are always installed perfectly
    2. They reduce the drying ability of wall assemblies
    3. They can also serve to function as air barriers
    4. They lower the indoor relative humidity

Answers to above:
1. b
2. b
3. b,c,d,e
4. e
5. b,c

Click here to read the PLRB PDF article

Additional sources of information:

WholeBuildingDesign Guide – Moisture Dynamics

Air Barrier Association ofAmerica

American Society of Testing and Materials (ASTM)

Moisture Control in Buildings:  The Key Factor in Mold Prevention

American Society of Heating and Refrigeration Engineers (ASHARE)

Humidity Control Design Guide for Commercial and Institutional Buildings

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