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Mulling the Magic (Er, Science) of Evaporation
by Greg Johns
Where does water go when it dries? Though this may sound like a replacement line for “Lake of Fire,” a song written by the Meat Puppets and famously covered by Nirvana, it’s a serious question. Where does it go?
As home inspectors, building consultants, homeowners and trades professionals, we all seemingly have a magical understanding related to things drying. But what really happens when something dries, and how might this process be important to our understanding of its effects on modern homes and buildings? Well, strap on your thinking cap because we’re going on a Reading Rainbow journey!
The Drying Process
Let’s take drying clothes as an achievable example. If we pull clothes from the washing machine and hang the delicates to dry, how does this happen? We can feel the clothes are wet when they come out of the washing machine drum, so we tangibly know the water is there. But, then we hang them and … poof … eventually the water is no longer within the clothes and isn’t anywhere else we can see.
This is what we call evaporation, but simply saying the water evaporated doesn’t adequately explain what occurs. Let’s think of it this way: Evaporation is the process in which a tiny mouse wizard comes behind us when we leave the room, waves around a wand, chants some very indiscernible words and creates just enough energy (think megajoules per kilogram) to disperse the bonds of the water molecules near the surface. These molecules are henceforth not a liquid, but a gas and, then like all well-meaning gases, leave to play pickleball, go see an over-priced movie or get their hair blown out. Eventually, the molecules will get lonely and recongregate into a nefarious fluffy cloud and phase change back into a liquid. They could also reappear as condensation (See Figure 3), depending on several variables.
As a side note, as the water molecules nearest the surface change phase and evaporate, water molecules left behind take their place (more on this below) at the surface, and then, they too evaporate and disperse if conditions are proper. And thus, eventually, we get “dry” (See Figure 1).
Image created by Greg Johns via Dall-E (AI-generated)
Drying Potential
Ok, so let’s maybe ditch the magical mouse and wand bit, but the energy part is true. The thing is, the required amount of energy to “dry” depends on several factors which we don’t have room to dig into here. The key for those of us who deal with building environments is what often gets phrased as “the drying potential.” While we may not need to worry about how our plasticized briefs and bras air dry (there’s a reason many of our undergarments come with warnings to avoid excessive heat), we should be concerned with the drying potential for moisture in the materials that comprise homes and other structures.
For example, in my glorious 1940 home, any water that adsorbs or absorbs (See the companion article Let’s Absorb Adsorption Together) into the board sheathing will diffuse readily along the grains (wood in particular) and eventually evaporate/dry out with minimal effect because my home has great drying potential due to being poorly air sealed and insulated. Though this may be advantageous for drying, it’s far less so on my utility bills!
But, in some homes of various vintages, inclusive of modern builds and commercial applications, there is less drying potential due to modern configurations of wall assemblies and building materials. In other words, some areas that get wet don’t necessarily have access to the energy needed for evaporation because energy is being properly sequestered within the structure’s interior habitable enclosure.
Figure 1: Evaporation (change of phase from liquid to gas) technically can occur without diffusion, but diffusion is often necessary to disperse gaseous molecules. Image created by Greg Johns via Dall-E (AI-generated).
This is not a bad thing, if you care about the status of the environment or your bank account. However, it is a bad thing if you want your walls to not be a terrarium of mush that can’t stand up to wind and pests and you build without paying heed to moisture movement.
Think about all the local news stories, social media posts and leaflets dropped from above that focus on toxic mold in homes. These “highly credible” news sources should be focused on how these fungi showed up to party in the first place. If we managed our assemblies better with a mind toward drying potential (read: evaporation), then it would be less frequent for these spores to rudely take over. But, I digress.
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Diffusion and Osmosis
OK. Let’s get back to the discussion at hand. So, evaporation occurs when water molecules near the surface gain enough energy (heat energy, temperature) to free their oppressive bonds. Then, diffusion (think about perfume being sprayed in the air) via physics disperses these from high to low concentration and, hence, they “disappear.” (See Figure 2.) Again, remember that diffusion is also the reason one concentrated wet area spreads to less-concentrated dry areas during the stage of what we’ll call “water wetting” (this is named thusly because there’s another chemical process referred to “wetting” that I don’t want to confuse with our current topic). Without diffusion in the product/material, evaporation would not be possible, but we’re talking about diffusion in a liquid phase first, then in a gas phase post-evaporation.
Meanwhile, back at the farm, if your environment is really wet, or cold, the moisture in our materials, be they clothes or building supplies, will not evaporate as quickly because there isn’t enough potential energy and there may not be enough dry air for diffusing from wet to dry/more to less.
There’s also osmosis. Though this technically isn’t the same as water drying, it plays a role in the exchange of water from Point A to Point B. We won’t get deep into osmosis because it technically reverses the always high-to-low principle and is reliant on solutes to dilute water. As an aside, though, osmosis can be the force behind the death of brick and other masonry materials; the force is strong with this one! So, the next time you see spalling brick and efflorescence, you can thank me.
Figure 2: An example of simple liquid diffusion. Diffusion can occur in liquid, gas or solid states, and is mostly always more to less. Image created by Greg Johns via Dall-E (AI-generated).
Why It Matters
Why does this truly matter for home inspectors, builders and consumers? Well, if you have a failure in a wall, roof or floor assembly (use the same mental image of a rectangle, just rotate for each position), whether that failure results in catastrophic damage and fungal growth partially depends on the drying potential, which happens via diffusion (liquid phase), evaporation and diffusion (gaseous phase). The longer organic-based materials (and non-plasticized composites) stay wet, the greater potential for degradation.
In many regions, some trade professionals remain ignorant to building with a mind’s eye toward the physics of drying, so moisture gets “trapped” in the assembly components. If you read the fine print from many manufacturers that make building materials, you will see they assume their products will get wet. After all, they are exposed on the exterior, which is outside. They provide instructions, configurations and videos explicitly showing how to make sure their products can be protected and/or can dry by not trapping water.
Figure 3: Condensation is the complicated result of evaporation, diffusion, concentration, dew point and phase change. Image created by Greg Johns via Dall-E (AI-generated).
So, if you find evidence of something wet, it has at least one source, which may or may not be readily obvious. Remember, water, heat, energy and pressure move from high to low, more to less. If you see something deteriorated from being wet, you can assume it doesn’t have the potential (energy) to dry via evaporation and diffusion because there are nearby variables askew for healthy conditions (or it was exposed to moisture in a way that it was never designed to be, such as composite cement cladding in direct contact with a roof/ground surface).
Once you find the askew variables and can name “it” and “them,” you can put the puzzle pieces together, inform your client and help the right people answer, “So, what now?”
About the Author
Greg Johns is the owner of a home performance consulting company as well as a bustling home inspection business. He works hard to keep the humor, advocate for consumer protection and help industry professionals grow their own building science understandings. He can be reached at greg@tnergyservices.com.
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