Wall Calculator for R-values and U-factors including checks for moisture control

This wall calculator is a tool to help coordinate energy code thermal insulation compliance and building code water vapor control compliance for a proposed wood frame wall assembly on a commercial or residential building.

It performs the following two design checks for a user inputted wall assembly:

Computes the assembly U-factor (and effective R-value) and compares it to code minimum thermal performance requirements (maximum U-factors) found in 2015 IECC Tables C402.1.4 and R402.1.4 (IRC Table N1102.1.4) which are climate dependent. An R-value of 0.17 and 0.68 are assumed for exterior and interior air films, respectively.

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The U-factor calculation is based on the “parallel path method” in accordance with 2013 ASHRAE Handbook of Fundamentals. The calculated effective R-value for a wall assembly is reported as 1/U which reflects actual performance, not a simple summation of only insulation component rated R-values. The calculator may also be used to investigate the effects of different materials on a wall assembly that otherwise complies with the prescriptive wall insulation component R-values for wood-framed walls found in IECC Tables C402.1.3 & R402.1.2 (IRC Table N1102.1.2).

Conducts a water vapor control check as an aid to help determine if the proposed wall assembly also complies with minimum building code requirements associated with various interior vapor retarder options which are dependent on climate and other factors such as insulation amount and location or sheathing type/permeance and cladding ventilation.

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To supplement building code requirements and make them more complete, the check also integrates building science recommendations or best practices to control water vapor by way of (a) properly using cavity and exterior continuous insulation materials to control water vapor (e.g., minimum insulation ratios) or (b) specifying the water vapor permeance of materials on the exterior side of assemblies in coordination with vapor retarder options (i.e., minimum permeance ratios). These practices also vary by climate. Thus, the moisture control check is based on ABTG Research Report No. 1410-03, which integrates 2015 IBC, 2015 IRC, and 2010 NBCC requirements as supplemented by relevant building science research, data, and practices.

The user is encouraged to confirm the suitability of this tool for the intended application, verify all outputs and recommendations, and seek professional advice as determined necessary.

Additional Moisture Control Considerations:

Inward Moisture Movement into Walls through High-Perm Exterior Coverings

RECOMMENDATIONS: In moist, warm/mixed climates (e.g., Marine and 1A – 4A), use one of the following options:

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  • non-reservoir cladding (e.g., vinyl siding, aluminum or steel siding, etc. )
  • a reservoir cladding that is back-vented (e.g., brick with vented air-space, adhered veneer with drainage/vent layer, cement siding on furring, etc.)
  • WRB layer that has a water vapor permence of not greater than 10 perm (ASTM E96 wet cup) – some wraps and many insulating sheathing materials meet this requirement.
  • Source: ABTG Research Report No. 1410-03

Balancing Wetting & Drying Potential

RECOMMENDATIONS: Limiting wetting potential is the primary means of protecting walls against water damage in wet, wind-driven rain climates.

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Increasing drying potential will not necessarily resolve wetting problems or eventual damage from poor WRB and flashing installations. In fact, too much drying potential can result in too much wetting potential at different times of the year (see item 1 above). In any climate with moderate-to-severe wind-driven rain, use all of the following practices is recommended:

  • Inspect all flashing and WRB continuity prior to concealment with cladding and consider conducting a water-hose spray test of the WRB and flashing prior to concealment.
  • Use pan (sill or threshold) flashing at all unprotected window and door openings.
  • Air-seal the interior perimeter of all window and door frames.

In severe wind-driven rain climate (31.5 inches or greater per map), some additional measures are recommended:

  • Provide clear drainage path (drainage space) behind cladding (e.g., use a rainscreen cladding material or cladding installation method with furring.)
  • Design the wall assembly using this calculator for use with a Class II smart vapor retarder (e.g., Kraft paper) or Class III vapor retarder (e.g., latex paint) on the interior to maintain or maximize inward drying potential.

For additional information and guidance, refer to:

DISCLAIMER: While every effort has been made to ensure the accuracy of the information presented, the actual design, suitability and use of this Information for any particular application is the responsibility of the user. Where used in the design of buildings, the design, suitability and use of this information for any particular building is the responsibility of the Owner or the Owner's authorized agent, and this information shall be submitted to the Building Official and reviewed for code compliance.

Wall Assembly Inputs

1. Building / Energy Code & Year
2. Climate Zone and Heating Degree Days

Enter Heating Degree Days (HDD) if you want the minimum Insulation Ratio (Re/Ri) to be based on heating degree days, rather than strictly on the Climate Zone minimums. Values outside the range shown will be ignored. Heating degree days option is only available for some climate zones. HDD values are on 65°F basis.

3. Exterior Continuous Insulation
4. Cavity Insulation
5. Cladding
6. Exterior Sheathing

If using a structural insulated sheathing, select "None" for Exterior Structural Sheathing and enter the R-value under Exterior Continuous Insulation.

7. Stud & Framing Factor
8. Interior Finish

Net Permeance Calculator for Exterior Material Layers

When complying by way of Option Two (see compliance check table), a minimum net water vapor permeance (WVP) of all exterior material layers must be achieved or exceeded instead of a minimum insulation ratio as required by Option One. This calculator assists in determining the net WVP of all exterior layers including sheathing, water-resistive barrier, siding, etc.

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The permeance values inputted should be based on ASTM E96 wet cup Procedure B, but use of dry cup values are permissible and generally give a conservative result for materials that have dynamic vapor permeance (e.g., changes with ambient moisture/humidity conditions.) These values should be sought from the respective material manufacturer and verified. Some resources offering permeance values for various materials are linked below.

For this calculator, Net Perms is calculated as follows:
Net perms = 1 / [1/perm1 + 1/perm2 + 1/perm3 + etc.]

Permeance Value inputs for Exterior Layers (Leave blank or 0 to ignore layer)

1. Exterior Sheathing
2. Water-Resisitant Barrier

If the Exterior Sheathing is the water-resistive barrier, leave Water-Resistant Barrier blank and enter the value under Exterior Sheathing.

3. Exterior Continuous Insulation

If the Exterior Sheathing is the continuous insulation, leave Exterior Continuous Insulation blank and enter the value under Exterior Sheathing.

4. Cladding
5. Other

Net Perms: {{ (perms = cc.netPerms(cc.permeance)) == 'Infinity' ? '' : perms }}


Energy Code Thermal Check

U-Factor Method

Factor Proposed Wall Code Requirement Compliance Check
Effective R-value1 for the opaque assembly
U-factor of opaque wall assembly

R-Value Method

Factor Proposed Wall Code Requirement Compliance Check
*R-value for wall assembly2
1 Based on U-Factor Method
2 Based on R-Value Method

Building Code Water Vapor Control Check

Option One
Insulation Ratio (Re/Ri) Method
Interior Vapor Retarder Class1 Proposed Minimium Required
Class I2 {{ cc.moisture.proposed | noval }} {{ cc.moisture.required[0].display }} {{ cc.moisture.required[0].ratio ? (!!cc.moisture.proposed ? ((cc.moisture.proposed.toFixed(2) >= cc.moisture.required[0].value) | check) : '') : ((cc.data.continuousInsulationRValue > 0) ? ((cc.data.continuousInsulationRValue >= cc.moisture.required[0].value) | check ) : '' )}}
Class II3 {{ cc.moisture.proposed | noval }} {{ cc.moisture.required[1].display }} {{ cc.moisture.required[1].ratio ? (!!cc.moisture.proposed ? ((cc.moisture.proposed.toFixed(2) >= cc.moisture.required[1].value) | check) : '') : ((cc.data.continuousInsulationRValue > 0) ? ((cc.data.continuousInsulationRValue >= cc.moisture.required[1].value) | check ) : '' )}}
Class III4 {{ cc.moisture.proposed | noval }} {{ cc.moisture.required[2].display }} {{ cc.moisture.required[2].ratio ? (!!cc.moisture.proposed ? ((cc.moisture.proposed.toFixed(2) >= cc.moisture.required[2].value) | check) : '') : ((cc.data.continuousInsulationRValue > 0) ? ((cc.data.continuousInsulationRValue >= cc.moisture.required[2].value) | check ) : '' )}}

Option Two
Net Permeance (perms) of all
Exterior Material Layers Method5
Interior Vapor Retarder Class1 Proposed Minimum Required
{{ '(Zone ' + cc.data.climateZone.name + ')' }}
Class I2 {{ (perms == 'Infinity') ? '-' : perms }} {{ cc.moisture.permMinimum[0].display }} {{ (perms == 'Infinity') ? '' : ((perms >= cc.moisture.permMinimum[0].value) | check) }}
Class II3 {{ (perms == 'Infinity') ? '-' : perms }} {{ cc.moisture.permMinimum[1].display }} {{ (perms == 'Infinity') ? '' : ((perms >= cc.moisture.permMinimum[1].value) | check) }}
Class III4 {{ (perms == 'Infinity') ? '-' : perms }} {{ cc.moisture.permMinimum[2].display }} {{ (perms == 'Infinity') ? '' : ((perms >= cc.moisture.permMinimum[2].value) | check) }}
1 Per ASTM E96 dry cup Procedure A
2 Class I:    0.1 perm or less.  (e.g., 4 mil poly)
3 Class II:   0.1 < perm ≤ 1.0.  (e.g., kraft paper)
4 Class III:  1.0 < perm ≤ 10.  (e.g., sp. latex paint)
5 The permeance values used for each exterior layer should be based on ASTM E96 wet cup Procedure B, but use of dry cup values are permissible and generally give a conservative result for materials that have dynamic vapor permeance (e.g., changes with ambient moisture/humidity conditions.