Short #72 - Vapor Diffusion

HVAC School - For Techs, By Techs

Bryan Orr

Careers, Business, Self-improvement, Education

4.9 • 1K Ratings

🗓️ 17 September 2019

⏱️ 9 minutes

🔗️ Recording | iTunes | RSS

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Summary

In this short podcast episode, Bryan explains what water vapor diffusion is. He also explains why it matters to the everyday HVAC technician.

Vapor diffusion is the transfer and distribution of water vapor through a solid surface. New constructions sometimes have vapor barriers on the outside of buildings in hot, humid climates or on the inside of buildings in cold, dry climates. (Even so, vapor barriers are not 100% effective.) We are NOT referring to water or mist wicking through the buildings via capillary action; we are referring to water vapor.

Drywall, a common building material, is quite permeable and allows moist air to diffuse through it. The vapor diffuses through the drywall from the unconditioned attic to the conditioned living space. Of course, we have to focus on air sealing before anything else, but we also may need a vapor inhibitor or a means of controlling the attic dew point to prevent water vapor from moving through.

The driver that causes moisture to move into the space is surprisingly NOT relative humidity. Instead, we need to focus on the actual moisture content as a driver, and it would help us more to look at the dew point. Dew point is the key to controlling vapor diffusion. If the dew point in the unconditioned space is higher than that of the conditioned space, then you will get vapor diffusion into the conditioned space. However, if the dew points are the same in the conditioned and unconditioned spaces, then there won't be a differential that would cause vapor to move.

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Transcript

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0:00.0	Hey, this is the HVAC school podcast and this is a short episode. I'm your host, Brian
0:09.2	J-Oor Esquire the 14th. I apparently come from a long line of Brian J. Ors. So today's episode is going to be
0:16.4	about vapor diffusion, but first I've got to remind you of who our sponsors are and remind
0:20.4	you that you may want to check them out. They are the ones who have
0:23.4	decided to invest in this sort of education. And if you like this sort of
0:27.6	education then I think you're going to like these sponsors because these are
0:30.7	companies that I've worked with, companies I use, carrier and carrier.com.
0:35.2	Refrigeration Technologies at refriggedec.com, makers of the excellent Viper Cleaner
0:39.3	line and Nylog, Speed Clean and Speed Clean.com. They make the coil jet, the coil jet for cleaning,
0:45.8	condensers, and evaporative coils, even when it's hard to get water and electricity there.
0:50.6	Navac and Navac Global.com, hey, they make the battery-powered cordless vacuum pump, the NRP 8-DI.
0:57.0	If you haven't looked at this thing, it's just cute as a button, and it does a great job.
1:00.8	That's Navac and Navac Global.
1:02.1	And then finally, Field Piece and Field Piece
1:03.9	dot com they make the Job Link probes which is what I use for checking the
1:08.6	fridge the refrigerant charge checking the old antifreeze and the air conditioner I use
1:11.6	the JL freeze and even for charging I just use a
1:14.4	little charging tea they work great you can find most of these products by going to
1:17.9	truetech tools dot com and use the offer code get school to check out but I
1:21.1	digress what we're talking about is vapor diffusion, which is definitely heavily
1:24.8	in the building science category, but for any of you who have ever had moisture build up or the
1:30.4	creepy-crawly growth build up in a place you don't want it.
	...

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