Dew point in wall
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In this article by Martin Holladay from 2017 https://www.finehomebuilding.com/project-guides/insulation/exterior-rigid-foam-thin (see attached image), he shows a diagram with R19 batt and min R 11.25 outside with the dew point landing in the exterior insulation. However, my calc for temp at back of sheathing puts the dew point inside the wall assuming R12 outside. Am I getting this wrong? Can someone check my math?
70 deg – [45 delta x 19/31] = 42.42 deg at back of sheathing.
Just trying to wrap my head around the calcs and concepts and want to make sure I’m not misunderstanding!
Thanks
Brad
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Replies
Your math is correct. What many people miss about the prescribed ratios of interior to exterior insulation is that the sheathing does drop below the dewpoint for some amount of time each year. It's not for a long time, maybe a week or two at a time in most cases, and the required Class 3 vapor retarder on the interior is meant to limit how much moisture gets into the wall. As Dr. Joe Lstiburek often says, wetting isn't the problem; it's when the rate of wetting exceeds the rate of drying that there's a problem.
Understood, that is extremely helpful. Think I was being pretty absolutist about this - wet or dry - but what you are saying makes perfect sense.
Thanks!