Check a wall assembly for condensation inside the wall with the Glaser method. List the layers from inside to outside; the tool compares the vapor pressure at each layer with the saturation pressure there.
| Point | Temp °F | Saturation kPa | Vapor kPa |
|---|---|---|---|
| Inside surface | 67.4 | 2.29 | 1.00 |
| Between gypsum board and fiberglass batt | 66.4 | 2.21 | 0.97 |
| Between fiberglass batt and osb sheathing | 22.5 | 0.39 | 0.94 |
| Between osb sheathing and housewrap | 20.8 | 0.36 | 0.26 |
| Outside face of housewrap | 20.8 | 0.36 | 0.24 |
How the Glaser method works
The method treats the wall as a stack of layers. Each layer’s thermal resistance sets the temperature at its faces, and each layer’s vapor resistance (sd = μ × thickness) sets how fast vapor pressure falls through it. Where the actual vapor pressure would rise above the saturation pressure at that temperature, condensation is predicted.
Worked example
A 2×4 wall with drywall, 3.5 in fiberglass batts, 7/16 in OSB and housewrap, with 70°F / 40% RH inside and 20°F outside, predicts condensation on the back of the OSB. The OSB is cold and resists vapor far more than the drywall and batts in front of it. Add a vapor retarder on the warm side, or move insulation to the outside, and the risk clears.
Why restorers use it
It explains moisture found inside walls with no plumbing or roof leak: cold-weather condensation on sheathing. It also checks a rebuild before the walls close, for example whether adding exterior foam or a poly layer moves the wet spot.
Frequently asked
What is the Glaser method?
A steady-state calculation (used in standards such as ISO 13788) that compares vapor pressure with saturation pressure through each layer of a building assembly to predict interstitial condensation.
What are the limits of the Glaser method?
It ignores moisture stored in materials, liquid water movement, rain, solar heating and air leakage, and it assumes steady conditions. It is a screening tool; use hygrothermal modeling such as WUFI for design decisions.
What does sd value mean?
sd is the equivalent air-layer thickness in meters: a layer’s vapor resistance factor μ times its thickness. A higher sd means a tighter vapor retarder; polyethylene sheet is around 15 m, drywall well under 0.1 m.
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RestoredOS runs the whole job on the same field knowledge behind these tools — drying logs, moisture mapping, scope, and billing.