Dew point and vapor
Check where condensation could form in the wall in winter or summer, and whether the interior vapor retarder fits the climate zone.
The Dew point & vapor panel walks temperature and vapor through the assembly, layer by layer, and says whether condensation is predicted at the design conditions. It uses a simplified vapor method, so treat it as a planning aid and have it checked by a licensed professional.
Winter or summer #
Pick a season at the top of the panel:
- Winter: warm, moist inside air driving outward. This is the default.
- Summer: hot, humid outside air driving inward toward an air-conditioned interior.
The panel and the drawing's analysis overlay both follow the season you pick.
Set the design conditions #
Each season has four boxes: Inside and RH (inside temperature and humidity), Outside and Out RH.
In winter, picking a Location or using the project site sets Outside to the coldest-month mean temperature, which is the right basis for condensation. A note under the boxes says where the number came from and shows the 99% design-day temperature for comparison. Typing your own Outside value replaces it.
Summer starts with typical conditions. Edit them for your site.
Read the results #
- Interior air dew point (or Outdoor air dew point in summer).
- Vapor control layer: the layer you marked VCL and its temperature. It turns red when it sits on the cold side.
- The verdict, green or red, with any advice under it. For example, the tool flags a vapor barrier that sits colder than the dew point.
- The boundary list: the temperature at every boundary between layers, from outside to inside. Red rows are colder than the dew point. A drop marks a boundary where condensation is predicted.
If the assembly has no layers yet, the panel says Add layers to see the temperature and vapor profiles.
Vapor retarder check #
The Vapor retarder section checks the interior vapor retarder against the code's rule for frame walls. It cites IRC R702.7 for the residential codes and IBC 1404.3 for the commercial code.
The tool finds the interior retarder (the lowest-perm layer inside the insulation), works out its class, and checks it for the climate zone:
- Zones 1–2: no Class I retarder, and a Class II only if it's a smart retarder.
- Zones 3 and 4: no Class I retarder. Class II and III are allowed, with conditions for Class II.
- Zones 5–8 and Marine 4: Class I or II is required. Class III is allowed when the wall has enough insulated sheathing, or vented cladding over the right sheathing.
The rule doesn't apply to floors, mass walls, or assemblies with no framing layer. The section says so when that's the case.
The rows are transcribed from the code. Check the code text yourself and have it verified by a professional engineer.