Vapor Barrier/insulation question
Hello all! Hoping someone can clear this up for me.
Up until recently, I was under the impression that vapor barriers always goes on the warm side of the wall. However, this Hyperfine question and video , and one question in Elif's PDD Quiz #2 say that the vapor retarder on inside face of insulation (shown in detail below for example) would work in BOTH warm and cool climates.
I am very confused how it's ok to have a vapor barrier on the inside face of insulation in a warm climate. Wouldn't the air pass through the insulation and condense inside the insulation? Thanks in advance!
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This is a great question Janet Ni
I think the confusion is coming from thinking that Weather and Vapor barriers are interchangeable, while they're not.
Vapor Barrier is impermeable material which function is to control the penetration of Vapor through the wall assembly. Vapor brings moisture inside of wall assembly and condensates into the water below the dew point.
Weather Barrier is impermeable material which function is to control the penetration of Air and Water through the wall assembly from outside.
So in short Vapor Barrier protects the assembly from INSIDE MOISTURE
Weather Barrier protects the assembly from OUTSIDE AIR & MOISTURE
Here is a great visual comparison of how much water is brought to the structure from inside and outside of the building into the wall assembly through the same area: https://youtu.be/FXXgjvOJcYI?t=315
To understand Vapor Barrier here is a great article from Building Science Corporation with your above detail explaining why it is the best wall for all regions: https://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers -
This is a super-complex problem with a super-layered explanation (1.5 hours of animated video on the topic in Amber Book). We will be covering this enclosure topic in our "40 Minutes of Competence" study session on October 14th (6pm ET). Zoom 83922907346. We've covered adjacent to it before on our previous Zoom call here.
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