Our Southern Homes and Water Vapor

Prologue

Writing, for me, is synthesizing ideas, concepts, and facts into understandings of my own words. This creates meaning for me as a professional and, thus, helps me to be a better professional for my clients.

Writing helps create meaning…which has to be read…to create meaning

Recently, I was re-reading Dr. Joseph Lstiburek’s 2004 article, “Insulations, Sheathings, and Vapor Retarders.” In my best moments, I try to touch one of his articles once per week. His material is dense, but accessible and witty. It really satiates my brain. But, I find that the information does not self-retain in my noggin – there is no adsorption or absorption for all of what Dr. Lstiburek freely disseminates. So, I have to re-read things continually to overcome my mutton-headed ways.

In the above-mentioned article, vapor in walling (and technically, roof and floor) assemblies is discussed in terms of thermodynamics and chemistry. At one point, the article talks about how vapor will diffuse to the lower vapor pressure and/or temperature side between two sides, regardless of air pressure. For our southern, humid climate this means that the water vapor that is outside in the warmer months will migrate inward for two reasons – our air conditioning “usually” is pulling moisture from the air inside our homes while simultaneously cooling interior surfaces. So, the inside has less vapor pressure (fewer water molecules inside than outside) and cooler surfaces. Vapor moves from high concentration to low concentration (inward in warmer months in our region) and from warmer to cool (inward in warmer months in our region).

Our Market

Now, the most common wall assembly configuration in our market is 2x4 studs, OSB wall sheathing, and a weather-resistive-barrier from DuPont (although competing brands are starting to show more frequently, but they mostly perform the same). These WRBs are not air barriers if not fully sealed (they never are in our market) and they are not vapor barriers as they are usually semi-permeable. Basically, they are designed to manage bulk water, but can’t do much for water in gaseous form (if you live here, you know we have a ton of water in gaseous form for at least 7 months each year).

A representative example for homes in our region which usually look modern-pretty from the cladded finish side

So, our wall assemblies are deplorable, historically, at managing water vapor. In older homes, like one I was in last year, this results in saturated fiberglass battens and some fungal growth on the battens’ paper-facing and the backside of the finished walling. The assembly configuration is uber common, even in new homes here – brick veneer, a banal WRB, sheathing, battens, interior wall finish (paneling and drywall in this instance). Now, this home’s walls had not rotted, but insulation was ruined and there was the fungal growth. So, what gives?

The walling was disassembled from the interior for renovation during the summer months. The previous owners had kept the home at 68 degrees F. up until it sold to the new owners. When we got into the walls, we saw thermodynamics at work – the interior temperature of the walling materials were at least 20 degrees cooler than outside temperatures. That undeterred, hot and humid air was like Garfield at a lasagna dinner – “Gimme, gimme, gimme!”. So, the vapor moved inward to less density and cooler surfaces whereby we happened to hit dew point due to Delta T and, Ta-da! Wet stuff.

Where’s the Mush?

So, why wasn’t it all mush? Why aren’t all our homes mush? Well, honestly, my best guess is that because we spend almost no time on air sealing, the walls are drying out – in the summer from heat and convective air movement; in the summer from vapor moving into the interior and the A/Cs reducing some amount of said moisture load (hello, utility costs…!?!); in the winter from being generally more dry outside while also being dry inside (a equilibrium of sorts). Inefficiency has saved us. Quick, ditch all that damn wall insulation…inefficiency saves us!

Except, maybe not. Utility costs continue to rise (TVA announced plans to raise their rates by 4.5% this fall). Building materials are not coming down in price to pre-COVID numbers. Home prices and mortgage rates also remain elevated. Oh, and let’s not forget the changing of weather that adds more strain to our structures. Maybe inefficiency shouldn’t remain our rule of thumb.

The flipside scenario also is not the answer – in several new homes in the last two years I’ve been called out related to moisture (relative humidity) being too high inside and under the home. Usually, these homes were built with modern materials and slightly better air management details. Slightly. We haven’t increased the R-value of our walling beyond an R19 (not regionally enforced), nor are we sealing our sheathing or WRBs, nor air sealing dissimilar assembly joints. We are starting to use integrated air/moisture management systems, but not properly executed. So, water vapor is still getting in via diffusion and air penetrations. And then it gets trapped with nowhere to go from the inside of the home (except the attic sometimes…not good in winter…unless we also are leaking heat to dry the moisture (heat from the sun and heat from our houses)…inefficiency is such an efficient cycle).

We Are Not In The Clear

What we can’t see within pretty exterior and interior finishes may pose issues for our younger homes in years to come…

In none of these newer homes have we been allowed to perform surgery to see what the innards look like. I’m hopeful, but not delusional. What creates deficiency in one direction can create it in the other direction, seasonally speaking. As most newer homes are drywall and latex-based painted, unsealed penetrations for conditioned winter air to move outward are going to be somewhat limited to man-made holes for electric and finish features. Vapor diffusion, moving outward, shouldn’t pose an issue due to proper paint finish on the interior side; plus, during the day the exterior materials are warmer which may escape the dew point (except shaded sides) and vapor isn’t trapped by vapor semi/permeable sheathing and membranes which is mostly what we still use. So, our winters mostly should harbor fewer issues than summers when warm air and higher vapor concentrations hit the cooler/dryer backside of drywall.