Building Science 101:: The Three Fundamental Rules of the House
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Three Fundamental Rules for Houses
One of my all time favorite books is Zen and the Art of Motorcycle Maintenance by Robert Pirsig. It’s really good on a lot of levels. But one thing that really stuck with me was the author’s discussion of how some people will tolerate things like a dripping faucet because of their fear of learning how it works.
Sound familiar? I’m a technically minded guy, but I occasionally succumb to this same fear. I’d been driving cars for seven years, for example, before I knew how an engine worked. I only learned then because I was given a 1961 VW bug that had thrown a rod, and I spent a month rebuilding the engine.
Most of us live in some kind of building, but how many actually know how buildings work? You might think that anyone who works on houses knows how houses work, but you’d be wrong. Builders and trade contractors know their part of building and repairing homes, but most lack knowledge of the fundamentals of building science.
The good thing is, it’s not rocket science. Yeah, you can study engineering or physics and go as far down the rabbit hole with this stuff as you want, but I’m going to boil it all down to three fundamental rules for you.
A House is a System
Put another way, this could be called the-hip-bone’s-connected-to-the-thigh-bone rule. And it’s the first thing that a lot of people who work on houses don’t grasp. A house is a system built out of a lot of interacting components: framing, electrical, plumbing, HVAC – each with its associated trade contractor. Mostly, the trade contractors look at a house with blinders on; they see what affects their work and not much else.
In terms of how a house performs, we can break it down into weather shell, building envelope, and mechanical systems. The weather shell keeps the elements out but isn’t usually the boundary between conditioned and unconditioned spaces. That would be the building envelope (or the thermal envelope), which comprises (i) a continuous air barrier and (ii) insulation that’s right up against the air barrier. There are many ways that the building envelope gets compromised, and even spray foam insulation isn’t a fix-all.
Editor’s Note: For more on how spray foam insulation can go wrong, see our article –
Mechanical systems have a huge impact on how a home performs. We know they increase the temperature difference between inside and out, but did you know that they also can create a big difference in moisture content and air pressure between inside and out? As you’ll see in the last rule below, those things can have a huge effect on comfort, durability, healthfulness, and efficiency. For best performance, you want heating and cooling systems that are properly sized and have distributions systems that are designed and installed for optimal efficiency.
Build for Your Climate
Ever pick up a home improvement magazine or watch a TV show about renovating your home? When they installed that vapor barrier, did they tell you what climate zones that works for and where you should never use one? I didn’t think so. As it turns out, you can’t build the same house to the same specifications in all climates. You actually have to pay attention to whether your climate is wet or dry, and whether it’s hot, mixed, or cold.
That vapor barrier, for example, may be a good idea in Minneapolis or Miami, but forget it in Memphis. It’s fine in a one-way climate, since the purpose is to limit the diffusion of water vapor from humid air into a wall cavity. In Minneapolis, the humid air is mostly inside the house, so the vapor barrier goes on the inside. Miami, however, it’s mostly outside, so it goes outside. In Memphis, if you choose a side for the vapor barrier, you’re going to be wrong for half the year.
Control the Flow of Moisture, Heat, and Air
Finally, we have to do as Homer Simpson does: “In this house, we obey the laws of thermodynamics!” Heat, air, and moisture all naturally flow from an area of more to an area of less. Moisture moves from wet to dry. Heat moves from hot to cold. Air moves from high pressure to low pressure. That’s from the second law of thermodynamics, and any attempt to violate the law reduces the integrity of the house.
You can see that all three of these rules are related. To control moisture, for example, you have to consider what kind of climate the house is in, which determines how you treat the building envelope and what kind of mechanical systems you install. Likewise with heat and air. The hip bone’s connected to the thigh bone, you know.
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Allison A. Bailes III, PhD is a building science pro, Home Energy Rating System (HERS) geek, irrepressible blogger and tweeter (@EnergyVanguard), builder of a super-green SIP home (that he no longer lives in), Mac lover, juggler, and Energy Vanguard chief. He currently lives in Atlanta, GA with his wife and cat in a condo. A textbook example of how to ignore the fundamentals of building science – and what goes wrong when you do.
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Note from the hosts: ” . . . you can’t build the same house to the same specifications in all climates.” << Roar and applauds! We’d like to thank Allison for this take on this and for taking time from his busy and demanding writing schedule to drop a little Building Science for us. The simple interrelatedness of things. Check him on his site >> his blog, the Tag Line: Building Science for a Better World. ~ jb
images via both Allison Bailes & Todd Vendituoli.
Excellent information, Allison! Thank you. One size does not fit all. Regionally specific advice and information is the key to a more educated population.
Glad you liked it, Tammy. Sometimes the simpler things are, the harder they are to grasp.
Allison,
Hands down…this is the most succinct and clear discussion of Building Science I’ve read. Imagine, if all homeowners viewed “homes as a system”, how much wiser our largest investments could become…real “sustainable” progress would naturally occur.
Many thanks!
Mike
Thanks, Mike! I’m glad you liked the article. This is the result of years of work and thought and teaching about the fundamentals.
This is a really great article, Allison!
I recently heard of an adage sometimes used by roofers and builders regarding protection against water infiltration, which is simply to “think like a rain drop”.
Perhaps “think like a volume of air/heat/moisture under the range of local, seasonal climatic variations” would likewise be a useful sanity checker for anyone concerned with building performance! :-)
Great summary, and thanks for the links to relevant postings on your blog, as well!
~John
Thanks, John. We definitely need sanity checkers, so maybe in all contracts for work on new and existing homes, we could include a sanity clause. But, as Fiorello said in Night at the Opera, everyone knows there isn’t a sanity clause.
Now I know enough to understand that I should NEVER be allowed to work on anything involving the house. Although I totally understand systems theory, but from the counseling perspective. I’m one of those with a healthy fear of understanding how something works. Why? Because I am afraid that if I understand it, the building/maintenance of it will become my chore. Pfffffft on that!
Prudent on your part, Alexandra. Once upon a time, I had a boss who liked to say “power goes to where knowledge is”, but he neglected to say that the same is probably also true of responsibility. So having the knowledge also means you have the responsibility to fix something when the proverbial shite hits the fan. Like the domestic water tank I’m currently applying high-temp. epoxy to. Right this very second, in fact (was just taking a momentary blogging break!… :-)
I meant to say “domestic hot water tank”. Typing not engaged with brain :-D
Gosh, you sound so sexy and manly when you talk about high-temp epoxy and hot water heaters. I think you should come over and check my dryer ducts!
See, Allison, how it works? If John hesitates, I mention my red hair and offer to buy him a dozen donuts w/sprinkles!
Peter Parker, aka Spiderman, got it right: “With great power comes great responsibility.” Then he said, agreeing with Alexandra, “This is my gift, my curse.”
Great point, Alexandra. But another way to look at it is that if you don’t know at least something about how it works, you won’t know whom to call when it breaks.
Allison, that was great. Seldom do I see so much information given, so accurately and in so few words.
Thanks, Dan! Sometimes it just flows, and this was one of those times. I sat down to write, not knowing which direction it would take, and it all just poured out of my fingertips.
Allison – excellent way to think about a house as a system. Is it all air related? What about structure, the gravity items? or lighting – natural and artificial? and of course, convenience, delight, identity – the human issues? You explained the air and protection from the elements systems so elegantly. Then I think about the poetry and use of the place, the structure and protection. I love Pirsig’s Motorcycle book too, a favorite. Cindy @urbanverse
Thanks, Cindy! Air is very important, but so are heat and moisture. You have to control all three for the house to work well. Structure, gravity, and lighting are all important, too, but I wanted to keep this article as simple as possible and focus on the most fundamental aspects of building science. Delight, human issues, and poetry are good, too, but I’ve learned to leave that to architects and designers who are much better at that than I. It IS a wonderful book, and I should go back and reread it again. It’s been a while.
Some good basics here, and it looks like just enough to drive home the point that people shouldn’t muck around with their homes, however I don’t think that most builders or remodelers grasp how a structure performs well enough to advise on how best to build or remodel it.
I think we can break this down another way that might be more useful by referring to the assemblies that make up a home rather than the layers . The components of an assembly are not interchangeable and too often I see confusion and mistakes made as a result.
I understand why, but the automatic inclusion of HVAC systems in the building science discussion without a conversation about materials, human activity, and details of attached structures and room use falls short.
I should point out that Vapor Barriers are not encouraged in good building. Vapor retarders are what we are designing today. Very few places have humidity strictly limited to one side of the assembly or the other. Bulk water intrusion further complicates the issue making two-way vapor retarding assemblies (Breathable Walls) ideal. (Note that breathable walls, like PassivHaus sound like something they are not.)
All that said, nice post Bailes
Thanks for your comment, Michael. I disagree with your first point, though. I think it’s fine for people to ‘muck around with their homes,’ as long as they do their homework and have the skills needed for what they’re doing. Yeah, it’s possible to make it worse, but what they inherit from builders & remodelers (with the exception of people like you who know what they’re doing) is often pretty bad to start with.
After that, you raise a lot of really good points, but they’re beyond the scope of this article. Layers are part of assemblies, and getting into that is Building Science 102, not 101. Same goes for materials & occupants. Same goes for vapor barriers vs. vapor retarders. I’m focusing on the big picture here.
I actually had occupants in my list of major components that affect a building’s performance (weather shell, building envelope, & mechanical systems) and took it out because I decided to focus just on the fundamentals. Accounting for occupant behavior is a higher order effect.
These are all great points for discussion, though.
Count me in on Building Science 102! Good job Allison. (this dude is turning up everywhere I look!)
gs
Dang, Gary! Now that you’ve asked, I going to have to write it. (Yeah, I get around. Someone recently told me that I do social media ‘promiscuously.’)