Building Science 101

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building science zen and motorcycle maintenance

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 - and learning through hands-on experience what it really meant to 'throw a rod.'

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 enclosure, 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 enclosure (also known as building envelope or thermal envelope), which comprises (i) a continuous air barrier, (ii) insulation that’s right up against the air barrier (or is the air barrier, like spray foam), and (iii) water and vapor control layers. There are many ways that the building enclosure gets compromised, and even spray foam insulation isn't a panacea.

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 OK 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. In Miami, 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 says: “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 go in the opposite direction must be done with the utmost care.

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 enclosure and what kind of mechanical systems you install. Likewise with heat and air. The hip bone’s connected to the thigh bone, you know.


If you think you've seen this article before, it's possible that you're right. I originally wrote it as a guest post over at the Building Moxie blog. Photo by MonsieurLui, from, Creative Commons license.


Terry Hill
Jan 14 2011 - 8:45am

I am surprised that you include a "continuous air barrier" as a component of the building envelope. I know that is becoming more common but is it required in any of the codes?

Allison Bailes
Jan 14 2011 - 9:00am

Terry, yes, a continuous air barrier is now in the energy code (IECC)! In fact, in Georgia, where I live, new homes now have to be tested with a Blower Door and the house has to come in at 7 ACH50 or less. Without a continuous air barrier, some of the insulation will have air moving through it, and that pretty much kills R-value.

David Butler
Jan 21 2011 - 2:17am

Another great article Allison!

Carl D. Clark
Jan 24 2011 - 8:31am

I enjoy reading your articles, you have a gift for making any subject matter worth reading 
Keep it up

Allison Bailes
Jan 24 2011 - 8:35am

Thanks, David and Carl! I enjoy writing and am happy to know that you readers find it helpful.

Pareto Patrick
Apr 26 2012 - 9:57am

Great post! Your discussion on vapour barrier is good and true, but many building codes do require vapour barriers so while the building science supports not having them, the legislation doesn't. 
Terry, continuous air barriers are required where I live too!

Ralph Puig
Apr 27 2012 - 9:23am

Great article! I just received my BPI BA & BE professional certification and while I am new to the industry, it's great to see best practices reinforced. Thank you, I am in the NYC area and eager to get it right. Thank you for sharing an excellent synopsis.

Allison Bailes
Apr 27 2012 - 9:34am

Pareto P.: Here's a quote I posted on our Facebook page a few days ago:  
"Moisture control snuck into building codes in the 1940s. Ever since that time, architects and builders turn to building codes for direction to ensure dryness in buildings. Building codes have never been a good source of that information." 
~ Bill Rose, from Water in Buildings
To be fair, though, the codes are way better than they used to be on this issue. 
Ralph P.: Glad you liked it! Enter your email address at the top of the page, and you'll get an email notification every time we publish a new article. And be sure to dig around in here, because we've got hundreds of articles here.

Swede White
Nov 5 2012 - 11:33am

Very good article here. Codes are the minimum standard. We as building professionals should strive for better.

Swede White
Nov 5 2012 - 11:35am

Also, a continuous air barrier is essential to preventing moisture issues. Here in south Louisiana, we run into this problem constantly.

Bryan Kim Turner
Feb 24 2014 - 9:00am

Well written. Easily understood by both civilians and those of us working hard to bring your words to a reality. I am looking forward as always to your next post. Thanks.

Erik Beall
Aug 12 2014 - 12:48am

Allison great article! Another part is the bit about quality "you know it when you see it" was as good as he (the protagonist) could define it. 
I think affordable thermal imaging will allow more people to "see" quality problems in buildings, and thats why I created the Hema-Imager, which I think is a strong competitor to the FLIR ONE. By a 3-person startup in Northeast Ohio.  
Please check us out, we're raising funds now at (sorry, shameless plug but feel it really is relevant)