As a licensed Architect and Real Estate Agent, my entire life revolves around the buildings I design, construct, buy, sell, rent, see, and experience everyday, like most of you I would assume. One of the most important and responsible things we can do as professionals in the building industry is to build right. The sad truth is that very few people do, not because they don't want to, but because they don't know how to. Just as importantly they often don't know that they don't know.
This is why while working on starting my own business in real estate and design, I jumped at the opportunity to work remotely for a company based in Massachusetts, but who works all over the world, called Building Science Corporation, www.buildingscience.com. There are a few companies like this around the country, but those who have met and know the founder of Building Science, Joseph Lstiburek, appreciate that there is no physicist or engineer quite like him. He's brilliant, matter of fact, and humorous. If anyone does know him, don't tell him I said that, it will go to his head. His writing style makes learning a lot of the very dry and boring parts of building construction fun and entertaining. It has already helped me immensely in my career and I hope that by sharing this blurb and the articles that he writes each month, it will also help more of you out there who are interested in learning how buildings are really supposed to go together.
A portion of this month's article, BSI-061: Inward Drive-Outward Drying, is shown below and a link to the full article is included at the end. I encourage everyone to read below and if you enjoy it and/or learn something, go check out the rest of the article and the other knowledge Joe shares.
BSI-061: Inward Drive - Outward Drying
Walter Payton does Permeance
Reservoir claddings are not easy especially with sun and air conditioning where it rains. Pretty much everywhere folks want to live… It is sunny in Minneapolis and we have air conditioning and it rains there. It is sunny in Miami and we have air conditioning and it rains there. It is sunny in Memphis and we have air conditioning and it rains there. Get the idea? My definition of where it rains is where we get more than 20 inches of rain a year. Check out Figure 1. Not much happens where it rains less than 20 inches of rain a year.
Figure 1 – Rainfall Exposure: Not much happens where it rains less than 20 inches of rain a year.
So what is a reservoir cladding? Anything that stores rainwater. Brick? Yes, of course. Stone? Yes, because of the mortar used to hold it together. Wood? Yes, unless you paint it or coat it on all six sides…and even then… Stucco? Come on, of course, even if you paint it. Synthetic stucco? Actually, not. The plastic “pookey” that is added makes it not store rainwater. Fiber cement? Yes, unless you paint it or coat it on all six sides…and even then. Sounds like wood, eh?
What is not a reservoir cladding? Vinyl siding. Aluminum siding. Metal panels. Glass curtain walls. Pretty obvious.
Wetting a reservoir cladding “charges” it. Think “moisture capacitor”. When the sun hits the “moisture capacitor” it discharges it. The heating of the stored water raises its vapor pressure. The warm water in the cladding is driven both inward and outward (Figure 2). Outward is good…unless the paint coating has too low of a vapor permeance causing it to bubble and blister and peel and otherwise not behave nicely. What is too low of a vapor permeance for a paint coating on the exterior of a reservoir cladding? Less than 10 perms. What is this based on. Experience. Lots, and lots of experience.
Figure 2 – Reservoir Cladding: Wetting a reservoir cladding “charges” it. Reservoir claddings behave much like “moisture capacitors”. When the sun hits the “moisture capacitor” it discharges it. The heating of the stored water raises its vapor pressure. The warm water in the cladding is driven both inward and outward.
.….Read the entire article at buildingscience.com.
 Walter Payton, arguably one of the greatest football players of all time, and according to Mike Ditka, one of the greatest human beings of all time, is known to his fans by his nickname “Sweetness”. In “Joe World” there is a “sweet spot” for vapor permeance – an optimum range for some types of wall assemblies….so it is inevitable that Walter “Sweetness” Payton would find the “sweet spot” for vapor permeance had he been an engineer rather than a Chicago Bear.
 Not just my experience, but lots of folks experience over many decades. Do not take a run at me with a bunch of hygrothermal simulations…because I can get any answer I want with a hygrothermal simulation. At the end of the day field experience “constrains” the simulation, not the other way around. Think peeling paint on stucco in Miami and what works to avoid it. Ask folks down there what works. Ask the ones with grey hair. They can be found drinking thick coffee eating squashed sandwiches….