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FSC Wall Calculator: I-Code Differences

The FSC Steel and Wood Wall Calculator tools provide water vapor control guidance to the user. The technical basis for the water vapor control checks has served as the basis for recent IBC and IRC code editions, including the forthcoming 2024 editions of the IBC and IRC. While the calculator strives for code consistency, it has maintained a more conservative position or provided additional features that are omitted from the codes. The reasoning and relevant building science for each case is outlined below as it relates to the 2021 editions of the IBC and IRC.

  1. The 2021 IBC and IRC provisions do not differentiate minimum insulation ratios for wood and steel frame walls. Thus, the calculator maintains a slightly more conservative insulation ratio for steel frame walls as supported in the available research reported in ABTG Research Report No. 1410-03.
  2. The 2021 IRC did not adopt a minimum required insulation ratio for application to all of Climate Zone 4 when using a Class III vapor retarder and it limited the insulation ratio requirement to only Climate Zone 4C – Marine. However, the 2021 IBC did adopt an insulation ratio consistent with the technical basis for this calculator in all cases, including all of Climate Zone 4 when using a Class III vapor retarder. In this regard, the calculator remains unchanged and is consistent with the 2021 IBC and applies this also to buildings within the scope of the IRC.
  3. Again, for use of exterior continuous insulation with Class III vapor retarders, the 2021 IBC and IRC adopted more stringent insulation ratios for Climate Zones 7 and 8, but not quite as stringent as this calculator and the underlying research reported in ABTG Research Report 1410-03.
  4. The 2021 IBC and IRC (and earlier editions) continue to allow the use of a Class I vapor retarder (e.g., 4-mil poly) in Climate Zone 4C-Marine whereas this calculator continues to indicate not permitted for all of Climate Zone 4 favoring instead the use of Class II and Class III vapor retarders based on the underlying research reported in ABTG Research Report 1410-03.
  5. The 2021 IBC and IRC now provide specific guidance for use of a Class II “smart” vapor retarder (e.g., coated Kraft paper on fibrous cavity insulation) where applied on walls with exterior foam plastic insulating sheathing and apply insulation ratios consistent with this calculator and its technical basis.  Notes have been added to the calculator’s moisture control check results to reinforce this requirement to use a Class II “smart” vapor retarder and also require a more conservative minimum wet-cup permeance value of 1.5 perms for the Class II smart vapor retarder to ensure it is at least equivalent to Kraft paper (the 2021 IBC/IRC definition requires a minimum 1 perm wet cup permeance). However, due to a rounding simplification in the 2021 IBC/IRC provisions for Class II vapor retarder in Climate Zone 4, this calculator has been revised to include a consistent minimum insulation ratio for this condition rather than the prior requirement for “R-2ci minimum”.
  6. The 2021 IBC and IRC now allow a Class I or II “smart” vapor retarder to be used in any climate zone and define these as having a wet cup permeance that is greater than 1 perm.  This calculator does not adopt this change and continues to not permit any Class I or II vapor retarder in the warmer climate zones (1-4 for Class I; 1-2 for Class II).  This action is based on ABTG Research Report No. 1410-03 where reviewed evidence suggest a minimum wet cup permeance of about 3 perms should be required in the hot/humid climate zones. 
  7. The “smart” vapor retarder change to the 2021 IBC/IRC described in #6 above and taken together with the change described in #5 above also could be taken to mean that a Class I vapor retarder can be used on walls with foam sheathing irrespective of minimum insulation ratios used in this calculator.  Thus, notes have been added to this calculator to reinforce that where a Class I vapor retarder is permitted for use with foam plastic insulation (exterior continuous insulation) it must “pass” the insulation ratio check and the Class I vapor retarder also must be a “smart” vapor retarder with a greater than 1.5 perm by wet-cup measurement (which again is more conservative than the minimum 1 perm wet cup value required in the 2021 IBC/IRC for reasons described in #5 above). Note that a “smart” vapor retarder will be defined as a “responsive vapor retarder” in forthcoming 2024 editions of the IBC/IRC. Also, the ability to use a Class I responsive vapor retarder on walls with foam sheathing has been further clarified in the forthcoming 2024 editions of the IBC and IRC in a manner consistent with this calculator.
  8. The 2021 IBC and IRC (and prior editions) continue to omit requirements for a so-called “perfect wall”, that is, a wall where all control layers, including the vapor control layer, are placed on the exterior side of the wall assembly.  The ability to design this type of wall is maintained by this calculator as a “No Interior VR” case and is based on the underlying research in ABTG RR No. 1410-03 and RR No.1701-01. Where user input cavity R-value is less than R-5, a minimum R-5 is conservatively used to determine the insulation ratio to account for effective R-value of potential building materials, components, and air spaces to the interior side of the exterior insulation. A note to the moisture checking table output has been added to clarify where to place the water vapor control layer on the exterior side of the assembly and its required properties. The forthcoming 2024 editions of the IBC and IRC will include the “No Interior VR” option consistent with this wall calculator and the ABTG research reports.
  9. The 2021 IBC and IRC (and prior editions) and the forthcoming 2024 editions omit requirements pertaining to water vapor control for assemblies that do not included exterior continuous insulation and rely extensively on the ratio of permeance of interior and exterior sides of the assembly to control wetting and drying by vapor diffusion. Therefore, this calculator continues to provide a separate vapor control check (noted as “Option 2”) for these types of wall assemblies using permeance ratios as they vary by climate based on ABTG Research Report 1410-03.
  10. Finally, the 2021 IBC and IRC added an exception whereby no vapor retarder is required in Climate Zones 1, 2, and 3. The calculators do not apply this exception in performing vapor control checks; however, the users should be aware of this exception and apply it with judgment appropriate for their specific project conditions and climate zone.

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