November 28, 2011

Air and Vapor

Let us quote Dr. Joe Lstiburek:

“Today walls [and roofs] need four principal control layers—especially if we don’t build out of rocks. They are presented in order of importance:

1. a rain control layer

2. an air control layer

3. a vapor control layer

4. a thermal control layer

A point to this importance thing here, if you can’t keep the rain out don’t waste your time on the air. If you can’t keep the air out don’t waste your time on the vapor.”  – Building Science Corp.  July 2010, Insight –The Perfect Wall

Pretty much everyone gets #1 – if you and your stuff gets wet it gets ruined and you get sick.  So while leaky buildings will always be with us no one is claiming they didn’t know they weren’t supposed to not let the water in.    #4 is also, for the most part, straightforward enough:  to stay warm  you need a good blanket of insulation.   Yet #2 and #3 – air and water vapor, are the subject of no end of debate and confusion. Air control and vapor control are the weakest link perhaps in our collective understanding of building science – and as such, an Achilles heel in achieving high-performance building.

We’ve all heard the chorus of misinformation:  “Buildings need to breathe – you’ll destroy them if you make them airtight”.   “In a cold climate, if you don’t put a vapor barrier on the inside of the  assembly you’ll rot the structure out – the vapor must be stopped!”

Of course this is almost exactly backwards: the air must be stopped and the vapor should breath.  So what is going on?   We’ll take a closer look in a series of posts on the subject – because, as it so happens, air and vapor control are of utmost interest  and concern to us at 475.

We are working closely with builders and building owners across the U.S. to responsibly address air and vapor control – making high-performance building not only possible but normal.

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  1. Air & Vapor, Part 2: The Importance of Air Tightness | Four Seven Five - February 6, 2012

    […] Part 1 of Air & Vapor we started our discussion noting the importance of air tightness and vapor control in building […]

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