Fiber cement panels as rainscreens

Third North_Exterior_Front
Photo courtesy Tom Witta and Schafer Richardson

By Carolina Albano
Moisture intrusion in a wall system can cause building defects and health ailments for occupants, making rainscreens a very important tool in water mitigation. There are several popular types of rainscreens that manage moisture infiltration in different ways. The two most common panelized systems are drained and back-ventilated, and pressure-equalized rainscreens.

The origins of moisture in wall cavities differ depending on climate, construction practices, and occupant lifestyles. Common sources present in all climates include moisture resulting from the construction processes either due to excess water evaporating from the building materials (e.g. curing concrete and drying lumber), or from exposed conditions during the construction process (e.g. uncovered material storage and rain-soaked wall assemblies).

Another source is elevated relative humidity (RH)—the amount of water in the air compared to the maximum amount it can hold at a given temperature. Cold air cannot hold as much water as warm air. When air comes in contact with a surface that reduces its temperature so the relative humidity reaches 100 percent, dewpoint temperature has been reached; building surfaces at or below dewpoint temperature will condense water onto the surface of building materials. Keeping the RH low keeps the dewpoint temperature low, reducing the potential for condensation.

Further precipitation and groundwater lead to bulk moisture intrusion. The cause might be faulty or non-existent flashing, poor site grading, improper or non-existent rainscreens behind exterior claddings, or non-existent or poorly maintained gutters and downspouts.

Water concentrates around window and door openings, the roofline, construction joints, and the base of exterior walls. The building envelope acts as the interface between the interior and exterior of buildings; to avoid moisture problems in extreme weather conditions, the building envelope must control water from all of these factors.

Long-term effects of moisture intrusion
Moisture in envelope assemblies can cause numerous problems affecting the indoor air quality (IAQ) of a building and the longevity of its components. Internal moisture degradation is a leading cause of premature failure of building envelopes. Persistent moisture can pave the way for rot, corrosion, and other forms of deterioration. Moisture-induced degradation could include reduced thermal resistance and decrease in the strength or stiffness of materials. Moisture also supports insect infestation, ranging from mites to cockroaches and ants.

Moisture traveling through building components can cause corrosion of components and dissolve water-soluble constituents, damaging the structure.

Some common moisture-related problems include:

  • structural wood decay;
  • high indoor humidity;
  • relative condensation;
  • expanding soil that can crack or undermine the foundation;
  • metal corrosion;
  • ice dams;
  • insect infestation; and
  • mold growth.

If elevated moisture levels persist on or inside a wall or roof assembly, it can lead to the growth of microorganisms such as mold and bacteria, which can create microbiological volatile organic compounds (VOCs) that adversely affect IQA and lead to health problems, such as asthma and lung disease.

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4 comments on “Fiber cement panels as rainscreens”

    1. Hi Frank, thank you! I am involved in several ASTM committees, including E06, C17, C26. Hope to see you at the next meetings in October in Tampa!

  1. Excellent article. Where would I be able to find more information about calculating the “right” amount of air space between the cladding and air barrier? Also, do you intend any sort of follow up article about the placement of insulation in relation to rainscreens?

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