The Building Blocks of Building Science (PTC's Continuing Education Tour, Part 1)

March 11, 2021

The basic BS boils down to "stuff goes from more to less," more or less

Generally, when we talk about the building blocks of building science, we focus on heat, air, and moisture.

Before going very much further, let me say that I am not a physicist. I am not an engineer. And I am not a building official. I am a former framing carpenter-turned-remodeler-turned-editor when my knees and back started giving out and I saw an ad in Fine Homebuilding magazine looking for an assistant editor.

My first big score as an editor at Fine Homebuilding was getting an article by Dr. Joe Lstiburek, the mac daddy of BS. 

Through Dr. Joe, I have come to meet and work with some of North America’s leading engineers, architects, builders, remodelers, and trade contractors.

So, I’m not pretending to have figured this stuff out by myself, I just make friends with really smart people and get invited to their parties. 

And their jobsites.

Heat, air, and moisture follow the laws of physics, except when they don't

Heat, as you know,  works in a few ways, conduction, convection, and radiation. But really, all three are working together pretty much all of the time, so it’s a matter of balance.

Air movement, as you know, can be in any direction. Or multiple directions at the same time.

Moisture not only moves multiple ways, but it takes different forms. Ice cubes, water, and steam are everyday examples.

Ice dams, rain, humidity, and condensation are typical building science examples. 

And like heat, heat, air, and moisture do not work in a vacuum: they work together pretty much always.

They are controlled with what Dr. Joe calls Control Layers—

  • Water control layer
  • Thermal control layer
  • Air control layer

...and keeping the control layers continuously connected from the footing to the ridge.

 

First, we’ll look at heat flow

To do that, I’m going to use an episode of a podcast I produce called 7 Minutes of BS, but rather than make you sit and listen, I made the audio podcast into a video that we can watch it as a group.

So that’s the three ways that heat flows through a building enclosure.

Controlling the heat flow comes down to prioritizing the mechanisms. 

Most radiant heat flow is through windows. In Minnesota, you probably want the free heat that radiates through the windows in the winter, but you probably do NOT want the heat sucked out of your body when you sit next to that window in the evening. 

The best solution for northern climate people is usually triple-glazed windows with a coating that allows radiant heat to enter the house, depicted by a high SHGC rating on the window sticker.

Most convective heat flow is due to air leaks. because warm air rises, holes in the ceiling allow warm air to escape while sucking cold air in to the first floor. 

There is a really cool episode of 7 Minutes of BS about stack effect which explains why revolving doors were invented and a tower in Australia that sucks warm air in the bottom and has a wind turbine at the top.

Most conductive heat flow is from thermal bridging, where heat flows around the insulation rather than getting trapped in it. 

Let’s take a closer look at thermal bridging.

 

Now we’re going to look at air movement within and through building assemblies

Air generally moves in a house from one of three mechanisms: Fans or other mechanical equipment, wind, and stack effect, which is generally a really big deal in buildings taller than houses, but still a consideration in houses. 

Let’s look at fans. A simple scenario is a single bathroom exhaust fan sucking moist air out of the house. 

Exhaust Fans create negative pressure in the house, meaning that air is being sucked into the home. 

Where does the replacement air come from for the air that is sucked out? It is sucked in through air leaks. 

The air leaks may be in the floor between the radon-filled basement or crawlspace, it may be through the walls, where dead mice live, or it may be through leaky window openings.

When the fan is off and the forced-air HVAC system is running, the house in positively pressured, meaning that air is being pushed into the walls, roof, and floor through any leak it can find. 

This is where condensation can come from because humid air drops it’s moisture when it hits a cold surface inside the wall cavities.

Wind, obviously, can push cold air, or sideways rain into leaks in the outer envelope. 

Because wind can push against one side of the house, it ends up creating a negative pressure on that side, but it can also create a positive pressure on the opposite side, like how bicyclists draft behind each other. 

The negative pressure behind the front runner pulls the second biker along more easily.

Stack effect is a relentless pressure that increases dramatically with height. It is based on the principle that warm air is more buoyant than cold air, so it rises. The further it rises, the faster it pulls in replacement air. 

It’s why fireplaces do not really heat buildings.

Water is way cooler, I think than heat or air

Its molecular properties are different than any other liquid, allowing it to do stuff that other, mere mortal liquids, cannot do. 

Except for mercury. That stuff is unbelievable.

Its surface tension properties allows it to stick to itself, and to build up actual height, which is how it travels from the earth to the top of a redwood tree.

The basic rules of plumbing, “Shit flows downhill and payday is on Friday,” illustrate gravity. 

Carpenters use levels to get things flat, plumbers use levels to get things not-flat. 

Rain falls from the sky. The water seeps down into the ground. 

That’s gravity. 

 

Water also goes up, as in the redwood tree example, through capillarity. When the size of the tube is smaller than the meniscus of the liquid, the liquid climbs. 

That’s not really anti-gravity, but it is not not anti-gravity.

Water also exists in vapor form, like when you boil water in a tea kettle, you can see steam shooting out of the whistling hole. But water doesn’t need to be at the boiling point temperature to make vapor. 

Water in a glass will evaporate into room temperature air if the air is dry enough, which it almost always is.

Water molecules are mixed with the air at almost all temperatures. At least at all temperatures that living things are exposed to.

Warm air can hold more moisture than cold air and that’s one thing that the psychrometric chart illustrates.

It’s also illustrated by condensation on a single-pane window, or frost on the underside of roof sheathing in the attic.

So that’s how water moves in buildings: up, down, and sideways. The basic rules of plumbing, “Shit flows downhill and payday is on Friday,” rain falls from the sky; the water seeps down into the ground. 

That’s gravity. 

 

Water also goes up, as in the redwood tree example, through capillarity. When the size of the tube is smaller than the meniscus of the liquid, the liquid climbs. 

That’s not really anti-gravity, but it is not, not anti-gravity.

Water also exists in vapor form, like when you boil water in a tea kettle, you can see steam shooting out of the whistling hole. But water doesn’t need to be at the boiling point temperature to make vapor. 

Water in a glass will evaporate into room temperature air if the air is dry enough, which it almost always is.

Water molecules are mixed with the air at almost all temperatures. At least at all temperatures that living things are exposed to.

Warm air can hold more moisture than cold air and that’s one thing that the psychrometric chart illustrates.

It’s also illustrated by condensation on a single-pane window, or frost on the underside of roof sheathing in the attic.

So that’s how water moves in buildings: up, down, and sideways.

 

Unfortunately, heat, air, and moisture conspire together 

Stack effect is an example of heat driving air movement, basically convection. 

Not only can this make a house drafty, but it can dry out the air inside because all of the moisture is getting sucked out, and cold dry air is getting sucked in.

Another good example of all three working together is cold windows in warm rooms. 

Warm air in the middle of the room can hold more moisture than cool air at the surface of the window, so the air sheds water molecules as it comes into contact with the window, which turn into droplets and fall under their weight due to gravity.

So those are examples of heat driving air movement, which can cause comfort problems and high heating bills.

Just like heat causes air movement, Air moves water around.

And as the air gets warmer, it can hold more moisture. 

If air were a bucket, a warm bucket would be bigger than a cold bucket.

If a five-gallon bucket is full of water (99% full), and someone turns the heat up, the bucket gets bigger. Now it’s only 75% full, but it still has 5-gallons of water in it.

That’s what relative humidity is.

This is a psychrometric chart. It shows graphically how moisture and air behave at different temperatures. 

The image is from EPA Water Management Guide

The outer curve indicates where the vapor/liquid boundary is, and it is labeled the saturation curve on this image. It is what we call the dew point. 

Condensation happens when moisture in the air (vapor) hits a cool enough surface to convert the vapor into water. The moisture condenses from a gas to a liquid, and then it drips into a puddle, where it grows into a problem.

Dew point is the practical application of RH: the temperature at which moisture in the air becomes water on a window. 

Or a beer glass. 

Or your supply ducts.

One interesting relationship between RH and dew point is that as the RH goes up, dew point squeezes itself closer to the air temperature. 

This is illustrated on the chart by the lines getting closer o the graph, but you cal also just picture the bucket, as the water level approaches the top the point at which it will overflow gets a lot smaller. 

In the real world, at 90% RH, the dew point is a three degrees cooler than the air temperature. If the temperature shifts 3-degrees,  the air may shed a lot of water inside the walls.  

Three-degree shifts in temperature are pretty common pretty much everywhere in the world, so high-humidity situations need to be taken seriously. 

Especially if you couple it with high temperature. Like just about anywhere in climate zone one and two and some of climate zone three some of the year. 

Moisture vapor can come from inside or outside the house—usually both, it just depends on the season which is the main driver.

Interior moisture sources are predominantly from people and the stuff they do: cook, wash floors, wash clothes, dry clothes …

Exterior sources are predominantly from the water cycle: Rain, ground water, floods, ice storms

Damp basements and crawl spaces can add a lot to the indoor moisture load, too.

You can manage indoor moisture vapor mechanically with dehumidifiers, air conditioners, and bath fans. You can also open a window if the air outside is not more humid than the air inside.

All of that stuff brings us to the practical information: how to control it so that it is not a problem for you or your customers.

At the beginning I mentioned Dr. Joe’s Control Layers. He also has a game that he plays with young architects and engineers who want to join his firm. It’s called the Pen Test.

In it, you have to look at a set of your own blueprints and draw the water control layer, the air control layer, and the thermal control layers with different colored pens. 

If you have to lift the pen from the page, you fail. The point is that the control layers must be continuous.

So here’s a video I made illustrating the concept.

The example comes from a builder who is relatively local to your area, Michael Anschel, of OA Design+Build in Minneapolis.

The core concept, is that if you cannot draw where the boundary is, it does not exist. 

In this animation I use red lines to show heat leaks and blue lines to show water control leaks, and purple to show air leaks.

In addition to not being an engineer to a piste, I’m also not an art director.

 

Insulation is the thermal control layer

If it is between studs, it is not continuous. Continuous is better for everything.

 

The air control layer can be anything or many things in the assembly that stop air.

Spray foam between studs stops airflow, but, again, the top and bottom plates and the studs are thermal nosebleeds.

Plywood wall sheathing is a great air control layer as long as there are not a bunch of gaps and holes.

Sometimes when a porch roof is laid over an existing roof, framers will leave out a few sheets of plywood from the main roof because the porch roof will cover it, but that creates a huge hole in the air barrier.

It’s also important to think about an air control layer at the slab because soil gasses can get sucked into houses, and that can make people sick.

The water control layer is most likely the siding and roofing, but underground it needs to be some sort of paint-on damp-proofing or gravel. Like under this slab, the rigid foam is waterproof and will stop capillarity, but the gravel underneath it will prevent the foam from ever seeing capillary action.

The concept is simple, but it is a little trickier than it looks.

It will certainly be less tricky to work it out on paper than to work it out in the field.

That takes us to the end of the first section, the Building Blocks of BS.

Recap:

To recap, heat moves in three ways, but almost never in only one of those ways at once. It is usually a combination. Radiant floor heat actually heats floors through convection (pumping liquid through a tube) and conduction, with the tubes in contact with the subfloor. 

The floors then radiate heat into the room, warming all surfaces it can see, or touch. The warm surfaces cause air movement, which make convective loops in the room.

When you walk on it barefoot, you are experiencing conductive heat flow, not radiant flow.

Moisture is tricky because there are different forms, subject to different laws of physics. Vapor does not obey gravity and neither does liquid if the tube is small enough. 

Most of the moisture to worry about in houses is bulk water leaks, (which is usually cured with flashing) and humid air leaks dumping moisture in cool wall and roof cavities.

 

Air also moves three ways, from fans, wind, and natural pressures, like stack effect. 

These mechanisms cause pressure differentials on each side of a wall or roof, which can suck moisture through that wall or roof, and lead to bugs, mold, and rot. BMR

The point is that they work together following the laws of physics, specifically that when there are two bodies of different temperature close to each other, they will tend toward equilibrium, or the hot gets cooler and the cool gets hotter until they are both warm.

Building scientists like Dr. Joe simplify it even more: heat goes from more to less. and for the same reason, water does that too (because water is carrying heat).

An example is when bricks get rained on. The surface moisture wicks inward toward the dry bricks. Especially if it is warmer inside than outside. If the sun comes out, the moisture moves outward, partly because of the temperature gradient and partly because the outer surface is now dry.

If there’s air conditioning inside the brick wall, it makes some really crazy stuff happen.

Basically they all operate differently, together, and they change their behavior when conditions shift.

 

—This is part 1 of a five-part continuing education program that Protradecraft worked on with Northstar Associates, who does builder training in Minnesota, Wisconsin, and Iowa, I think. If you are interested in bringing in a continuing education class for your jurisdiction or builder group, please contact John Miller

 


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