Guideline I.22019-08-16T15:44:56+00:00

      Guidelines Home     |     Project Team     |     v3.1     |     v3.0     |     Documents      |     v2.2     |     Previous Versions

Guideline I.2: Moisture and Water Control

Intent

To ensure a moisture-safe building envelope.

Required Performance Criteria

Guidelines apply to all New Construction projects and for Major Renovations with exterior envelope included in the project scope.

  1. Bulk Water Management
    Implement the following to ensure adequate control of bulk water on the site:

    1. Site grading at building perimeter: Ensure 5% slope away from the building for a minimum of 10 ft. in unpaved and non-pedestrian areas.
    2. Use downspout leaders, trench drains, and/or other methods to direct runoff from the building away from the perimeter.
    3. Ensure irrigation systems do not spray on the building enclosure.
  2. Moisture-safe design:
    Design the building envelope to manage moisture flow and maintain all layers of wall and roof assemblies at safe moisture levels by implementing items (1) and (2) below.

    1. For above-grade walls: Project teams must demonstrate safe moisture design by conducting a qualitative moisture analysis, and by conducting one of two quantitative moisture analysis options. Analysis must be performed for at least two wall assemblies or one wall assembly if it comprises 60% or more of the total wall area. Wall types following the principles of the “Perfect Wall” with all control layers (water, air, vapor, thermal) outboard of the sheathing (i.e. no cavity insulation) do not require the quantitative moisture analysis.Qualitative moisture analysis:
      This analysis shall be guided by the B3 Qualitative Moisture Analysis Worksheet for walls, discussing how the assembly manages liquid water, capillary drive, air leakage, and vapor diffusion, including location and type of each control layer: water, air, vapor, and thermal.Quantitative moisture analysis:

      • Option 1: Static temperature and vapor pressure profile calculation (Glaser method):Project teams are encouraged to use the B3 Glaser Calculator Tool. Results must show each surface layer through the wall section remains below the saturation vapor pressure. Test conditions for this analysis must use the average exterior winter temperature and humidity (average of the coldest three months) at or near the project site. Different buildings and space types will have varying interior temperature and humidity levels. To create a safety margin, this analysis should use challenging conditions; the highest interior temperature and humidity the exterior wall is likely to experience on a regular basis.
      • Option 2: Dynamic moisture simulation (i.e., WUFI software): An analysis following American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) 160 must show the wall’s total moisture content achieves a declining or stable moisture content pattern over three years while the sheathing meets the mold growth criterion of ASHRAE 160-2009. This criterion verifies that the highest 30-day running average surface relative humidity in the sheathing is below 80% when surface temperatures are between 41°F and 104°F.
    2. For roofs: Project teams must demonstrate successful moisture performance by conducting a qualitative moisture analysis. Analysis must be performed for at least two roof assemblies, or one roof assembly if it comprises 60% or more of the total roof area.
      • Qualitative moisture analysis:
        This analysis shall be guided by the B3 Qualitative Moisture Analysis Worksheet for roofs, and must discuss how the assembly manages liquid water, capillary drive, air leakage, and vapor diffusion, including location and type of each control layer: water, air, vapor, and thermal.
  3. Moisture-safe construction: Air leakage and resulting condensation is one of the primary causes of moisture damage in buildings. Construct the building to control air leakage. Compliance with this guideline can be achieved using one of the two following methods:
    1. Whole-building air tightness test: Test the building thermal envelope for air tightness using the Air Barrier Association of America (ABAA) Standard Method for Building Enclosure Airtightness Compliance Testing “operational enclosure test.” A pressurization test is not required, but may be performed in addition to the required depressurization test. The maximum air leakage rate allowed is 0.25 cubic feet per minute (cfm) per square foot enclosure area (6-sided) at 75 Pascal (Pa).[1] If the rate falls above this level, a diagnostic evaluation to find the primary sources of air leakage is required using smoke tracer or infrared imaging, followed by nondestructive remediation steps to reduce air leakage. A final air tightness test is required following remediation efforts. If the final leakage rate achieved is below 0.4 cfm per sq. ft. at 75 Pa, the building is considered compliant with this guideline.
    2. Third-party building enclosure consultant:The building enclosure must be designed and constructed with assistance from a third-party building enclosure consultant. This scope of work must include, but is not limited to:
      1. Regularly scheduled meetings and consultations with the design and construction team during each phase (Predesign through construction and project completion).
      2. Review of all technical plans, specifications, shop drawings, and material submittals relating to the building enclosure.
      3. Coordination, observation, and documentation of preconstruction training for enclosure contractors and construction managers, focused on proper installation methods for air and water barrier continuity. (The training may be conducted by a qualified third party such as a manufacturer/product representative.)
      4. Coordination, observation, and documentation of air and water leakage performance testing on window installation mockup and/or field installations. This performance testing may be conducted by a third-party testing agency.
      5. Field observation of critical enclosure details for quality assurance during construction.

If compliance is achieved through a third-party building enclosure consultant, whole-building airtightness testing is not required as part of the process, though smaller scale assembly or panel testing may be appropriate as determined by the building enclosure consultant. Note that the building enclosure consultant may also perform the work required under Section B, Moisture-Safe Design. Note that a certified building enclosure consultant is preferred, though not required.

Moisture control is one of the most important functions provided by a building enclosure. It is vital to building durability and longevity, energy performance, and occupant health. Yet it remains one of the most challenging aspects of building design.

There are four key pathways for moisture to move through a building enclosure. Arranged in order of significance, these are 1) bulk water leakage, 2) capillary movement, 3) air leakage, and 4) diffusion. In general, the building industry has learned to control bulk water leakage using a large variety of materials and products, but relying on a few basic principles such as slope and drainage gaps, redundancy, and proper overlap of layers. Capillary movement can generally be controlled with rainscreens, drainage gaps, and capillary breaks installed in key locations.

Often failure to control bulk water and capillary drive is quickly evident, which is one reason why the building industry has been able to learn adequate methods of control for these wetting pathways. However, diffusion-driven wetting and especially air leakage remain difficult for the building industry to properly control. Modern enclosures incorporate less moisture storage and higher insulation values, which translate into less available heat to drive            off excess moisture. Increasingly moisture-sensitive materials may also be selected. As they become less tolerant to moisture, modern enclosures need to be paired with improved methods of air leakage and diffusion control. Better approaches to enclosure design, accessing enclosure expertise, setting air leakage targets, and testing performance have all been shown to improve enclosure durability, energy performance, and moisture safety.

A preferred method to control all four wetting pathways is through the use of an enclosure scheme called the Perfect Wall. This design approach can be safely used in any climate and for any part of the enclosure (wall, roof, slab on grade), as well as above or below grade. It also lends itself to easier installation and quality assurance inspections for the four essential control layers: water control, air control, vapor control, and thermal control. The Perfect Wall approach requires that all of these control layers be positioned outboard of the structure regardless of whether that structure is concrete, wood frame, steel frame, or other. Conveniently, the water, vapor, and air control layers can often be provided with a single product installed on the outside face of the sheathing or outermost structural surface. In this position, it can be installed continuously and inspected more easily than enclosure approaches using multiple layers installed variously inside, outside, and/or within the structure. The thermal control layer is applied outboard of these control layers, protecting them from damage functions and keeping them and the structure warm and dry as well. This outboard system may also be easier to repair or replace during the building’s lifespan. More information on the Perfect Wall is listed below under Additional Resources.

This Perfect Wall design approach is known as an inverted or protected membrane roof when applied to the top of the building. The waterproof roofing membrane is placed on top of the roof deck, providing air, vapor, and water control in a single layer. All of the insulation is placed on top of this, typically as rigid foam board, and weighed down and protected with gravel ballast or pavers. Although the roofing membrane is more difficult to inspect and access after construction is complete, it is also longer lasting because it is protected from sun, heat, ice, and physical damage. This type of roof design doesn’t create a cold side vapor barrier or double vapor barrier as most other roof designs do, leading to much greater drying potential and moisture safety. It also eliminates common air leakage pathways and associated moisture issues in truss roofs with dense pack insulation. Building enclosure consultants have found dense-packed truss roofs especially problematic due to the difficulty of creating a continuous, robust air barrier in the ceiling plane.

Adopting the Perfect Wall approach to enclosure design eases the requirements for conducting qualitative and quantitative moisture analysis, as these are considered less likely to have moisture issues. It will also help achieve the demanding air leakage requirement of 0.25 cfm/sq. ft. at 75 Pa, though use of this scheme is just one step toward achieving that target.

Project teams may wish to hire a building enclosure consultant as well, particularly if the project team or contractor has never designed a building that has to achieve this level of airtightness. Building enclosure consultants can help project teams design and construct an air barrier system that maintains continuity even at locations that have traditionally been problematic, such as parapets, windows, and projections like balconies and overhangs. They can suggest products, materials, and approaches that have been proven to perform better in the field or are easier to install in an air- and watertight manner. Building enclosure consultants may also suggest conducting preliminary performance tests around installed components such as windows and doors, or testing the tightness of certain wall sections or building zones. Finding and remediating discontinuities in the air barrier is easier and cheaper early in the construction process (rather than during or after the final blower door test).

Mechanical engineers and contractors also should be aware of the air leakage requirements of these guidelines. Specifying higher quality louvers and dampers that can close fully during normal operation has been shown to significantly improve commercial-scale buildings’ air tightness results. A reduced level of air leakage may also impact heating and cooling load calculations and improve mechanical air distribution and pressure management.

WUFI modeling for compliance with the quantitative moisture analysis should be done by experienced professionals. Currently, there are no widely-followed certifications for this particular modeling skill, though many building enclosure consultants and firms do employ people with WUFI modeling experience. It is highly recommended to follow expert guidance for WUFI modeling techniques. Thorough guidance can be found in the Building Technologies Office Strategy Guideline: Modeling Enclosure Design in Above-Grade Walls, 2016.

Predesign:

  • 2C: Determination of which method the project is pursuing. If the project team is using a third-party building enclosure consultant, add the consultant to the B3 Project Team in the B3 Tracking Tool and submit a description of the consultant’s contractual scope of work.

Design:

  • 2B: Complete and submit the B3 Qualitative Moisture Analysis Worksheets for the proposed primary roof and exterior above-grade wall assemblies. Submit either 1) completed B3 Glaser Calculator Tool with calculations for the proposed wall assembly, or 2) WUFI simulation results for the proposed wall assembly including documentation of the modeled wall materials and properties, simulation settings, and moisture content or relative humidity graphs at the sheathing surfaces, showing compliance with ASHRAE 160-2009.
  • 2C: If project team has determined to use a third-party building enclosure consultant, submit minutes and/or reports from the Design Development-phase meetings with the enclosure consultant.

Final Design:

  • 2A: Submit site plans documenting proper slope next to the building and location and direction of water flowing through downspout leaders and/or trench drains. Submit an irrigation plan identifying location and coverage of spray irrigation or sprinkler heads.
  • 2B: If changes have been made to the primary roof and exterior above-grade wall assemblies, complete and re-submit the B3 Qualitative Moisture Analysis Worksheets and Glaser calculation or WUFI simulation documentation.
  • 2C: If the project team has determined to use a third-party building enclosure consultant, submit minutes and/or reports from the Construction Documents-phase meetings with the enclosure consultant.

Closeout:

  • 2A: Confirmation that site grading provides proper slope next to the building. Confirmation that installed downspout leaders and/or trench drains lead water away from the building perimeter. Confirmation that all spray irrigation or sprinkler heads do not spray on the building enclosure.
  • 2C: Submit results of the depressurization air leakage test, done in accordance with listed standards and given in terms of cfm/sq. ft. enclosure area at 75 Pa. If the test results are above 0.25 cfm/sq. ft. at 75 Pa, submit documentation from the diagnostic evaluation, air leakage remediation efforts, and the final test results following remediation. If the project team selected to use a third-party building enclosure consultant, submit minutes and reports from the Construction Administration-phase meetings with the enclosure consultant as well as results from the window performance air and water leakage testing.

Appendix I-2: Glaser Calculator

ABAA Standard Method for Building Enclosure Airtightness Compliance Testing: http://www.airbarrier.org/technical-information/whole-building-air-tightness-testing-2/.

Building Enclosure Consultant Certification and Training Programs: http://rci-online.org/pro-registrations/registered-building-envelope-consultant-rbec/ and http://besinstitute.com/besi-certifications/.

Building Science Corporation (BSC) Explains the Perfect Wall Concept: https://buildingscience.com/documents/insights/bsi-001-the-perfect-wall.

Dynamic Moisture Modeling Guidance from the Department of Energy (DOE) Building Technologies Office, Strategy Guideline: Modeling Enclosure Design in Above-Grade Walls: https://www.nrel.gov/docs/fy16osti/65479.pdf.