Wall Condensation Calculator (Dew Point Method)
List the layers of a wall from inside to outside to see the temperature at each surface and where indoor moisture could condense in winter.
Method reviewed October 26, 2026. Version 1.0.0. How it works
This is an estimate. It uses a simplified method and typical assumptions, all listed below. Use it to plan and to check quotes; equipment should be sized from a full room-by-room load calculation (ACCA Manual J) by a qualified professional.
Result
Risk at layer 4
- Indoor dew point
- 44.6°F
- Inside surface temperature
- 66.4°F
- First surface below the dew point: outer face of layer 4
- 4
Condensation risk at the outer face of layer 4: 18.4 °F is below the 44.6 °F indoor dew point.
- If indoor air or vapour reaches the outer face of layer 4, it can condense there. An air barrier and a vapour control layer on the warm side of that surface, or insulation outboard of it, reduce the risk.
- Wall U-factor
- 0.088BTU/h·ft²·°F
- Total R-value
- 11.32h·ft²·°F/BTU
Temperature profile
- Outer face of layer 1
- 65.8°F
- Outer face of layer 2
- 55.2°F
- Outer face of layer 3
- 54.9°F
- Outer face of layer 4
- 18.4°F
- Outer face of layer 5
- 13.2°F
- Outer face of layer 6
- 10.9°F
Show the working
- Temperature drop across each layer is proportional to its R-valueΔT per unit R = 60 ÷ 11.32 = 5.3 °F
- Inside surface = indoor − drop across the inside air film70 − 3.6 = 66.4 °F
- Indoor dew point40 % at 70 °F → 44.6 °F
- Condensation risk where a surface is colder than the indoor dew pointouter face of layer 4 (18.4 °F)
The temperature profile
Under steady conditions the same heat flows through every layer, so the temperature drop across each layer is proportional to its R-value. Starting from the indoor temperature, the tool subtracts each layer’s share of the total indoor–outdoor difference to find the temperature at every surface.
The defaults reproduce the masonry wall in the National Research Council of Canada’s Building Digest 36: inside air film, plaster, concrete block, mortar, foam insulation, air space, brick and outside air film, total R-11.32. At 70 °F inside and 10 °F outside its interfaces are about 66, 54, 18, 13 and 11 °F.
Finding the risk
The indoor dew point comes from the indoor temperature and humidity. Any surface colder than it is a place where condensation could form if indoor air or water vapour reaches it. The tool reports the first such surface from the inside.
- An air barrier stops humid air from reaching cold layers
- A vapour control layer on the warm side slows diffusion
- Insulation placed outboard keeps inner layers warmer
What this method does not do
The dew point method shows where condensation could occur, not how much. It does not model vapour diffusion rates, air leakage, moisture storage, rain or drying. For critical designs use a hygrothermal analysis.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Indoor temperature
- 70 °F
- Indoor relative humidity
- 40 %
- Outdoor temperature
- 10 °F
- Inside air film R-value
- 0.68 h·ft²·°F/BTU
- Layer 1 R-value (innermost)
- 0.11 h·ft²·°F/BTU
- Layer 2 R-value
- 2 h·ft²·°F/BTU
- Layer 3 R-value
- 0.05 h·ft²·°F/BTU
- Layer 4 R-value
- 6.9 h·ft²·°F/BTU
- Layer 5 R-value
- 0.97 h·ft²·°F/BTU
- Layer 6 R-value (outermost)
- 0.44 h·ft²·°F/BTU
- Outside air film R-value
- 0.17 h·ft²·°F/BTU
Result
- Indoor dew point
- 44.6 °F
- Inside surface temperature
- 66.4 °F
- First surface below the dew point: outer face of layer 4
- 4
- Wall U-factor
- 0.088 BTU/h·ft²·°F
- Total R-value
- 11.32 h·ft²·°F/BTU
Condensation risk at the outer face of layer 4: 18.4 °F is below the 44.6 °F indoor dew point.
Assumptions
- Steady one-dimensional heat flow: the temperature drop through each layer is proportional to its R-value (NRC CBD-36).
- Dew point method: any surface colder than the indoor dew point is a possible condensation plane if indoor air or vapour reaches it.
Limitations
- Does not calculate vapour diffusion, air leakage, moisture storage, rain or solar drying; it shows where condensation could occur, not how much.
- Framing paths are colder or warmer than the insulated path; check both if they differ.
Frequently asked questions
Why does indoor humidity matter so much?
Higher indoor humidity raises the dew point, so warmer layers become cold enough to collect moisture. In winter, keeping indoor humidity moderate is one of the simplest protections.
Where do the air film values come from?
R-0.68 for still air on an interior vertical surface and R-0.17 for an exterior surface in wind, the same values used in the Building Digest example and in code default tables.
Should I check the studs too?
Yes. A wood or steel stud path has a different temperature profile from the insulated cavity. Run the tool once for each path.
Sources
- Canadian Building Digest CBD-36: Temperature Gradients Through Building Envelopes (Latta & Garden, 1962) — National Research Council Canada (archived copy at MIT). The temperature drop through each component is proportional to its thermal resistance; surfaces below the dew point of the air collect condensation; Table I example (70 °F in, 10 °F out): R 0.68, 0.11, 2.00, 0.05, 6.90, 0.97, 0.44, 0.17, total 11.32, interface temperatures 66, 54, 18, 13, 11 °F.
- Condensation in Exterior Walls (Technical Bulletin, Issue 77, June 2018) — National Institutes of Health, Office of Research Facilities. The dew point is the temperature below which condensation occurs; if it occurs within a wall the materials there become wet, reducing insulation R-value and causing mold; location is determined by dew point analysis.
- 2021 Seattle Energy Code, Appendix A (A101.3 air films; Table A101.4 compressed batts) — City of Seattle, Department of Construction and Inspections. Default air-film R-values: 0.17 all exterior surfaces, 0.61 interior horizontal heat flow up, 0.92 interior horizontal heat flow down, 0.68 interior vertical; fibreglass batts R-30 at 9.5 in and R-49 at 15.5 in standard thickness; an R-19 batt in a 3.5 in cavity is rated R-13.
- PsychroLib — psychrometric functions after ASHRAE Handbook—Fundamentals (2017) ch. 1 — PsychroLib (open source, MIT licence). Humidity ratio W = 0.621945·pw/(p − pw), wet-bulb, enthalpy and standard-atmosphere equations as published by ASHRAE.
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