House Insulation Basics: How Heat Moves

Principle #1, Equilibrium Everything in nature moves towards balance. Wherever there is an imbalance things will shift and flow until there is balance. This is called the law of equilibrium. When we want to heat a house we are fighting against the law of equilibrium. Pure equilibrium can be hard to see in nature because most things are shifting and changing, this is called dynamic equilibrium. Remember this in your house, you are often adding or taking away heat with a furnace or AC unit. But your house will come to balance if you shut those things off and leave for several days.

Principle #2, It takes work to hold off equilibrium It takes work (also called “energy” in physics terms) to create and maintain an imbalance in nature. In your house, this means that you need to burn fuel (use up energy) in order to have it warmer inside than it is outside (imbalance) and walls and a roof (boundaries) in order to help keep heat inside.

Insulation Guide - Nature moves towards balance and it takes work and boundaries maintain an imbalance. Our warm house in the middle of winter is an example of an imbalance that we want to maintain. Now,

Principle #3, How heat moves Heat moves in 3 ways: conduction, convection, and radiation.

Conduction: Heat moves by direct contact. When one object touches another, and there is a difference in temperature between them, heat will flow from the warmer object to the cooler object until they are equal. The rate of conduction from one object to another is dependent upon the density, thickness, and type of material. It is important to note that you may be in direct contact with objects that you don’t even think of, air and water for example.

Insulation Guide - The rate of conduction from one object to another is dependent upon the density, thickness, and type of material but not on the reflectivity or the air tightness of the material. R-value, or resistance to heat flow, is a measure based mostly on heat flow by conduction.

Convection: Heat moves by fluids. A fluid is anything that flows. Fluids do not have to be liquids, they can be gasses and semisolids too. Water, air, pudding, carbon dioxide, and oil are all examples of things that flow. Simply put, warmer fluids become less dense and float, cooler fluids become more dense and sink. When warm fluids float up and away from a heat source, cool down, and then sink this is called a convection loop or convection cycle. In your house this is called the stack effect or chimney effect. If you have a tight boundary (walls and roof) the convection cycle stays internal to the house and pretty much the same air gets heated and cooled over and over again. If you have a boundary that is leaky, warm air rises and goes right out the holes in the roof, cool air comes in through holes in the low parts of your house. You have to heat up new (and very cold) air from outside more often. Having the convection loop happen externally is great for ventilation but poor for energy efficiency but we will discuss that more when we get to modern building practice. Convection is also called air leakage in the construction world.

The stack effect is demonstrated very well in the photographs below. This masonry staging is being heated inside to help the mortar cure for the new brick. The tenting around the staging is being pushed tight against the staging by cold dense air at the lower levels of staging. The cold air is entering to replace the warm air that is floating up and pushing out of the tenting at the top of the staging. If the walls of buildings were flexible buildings would look like this in the winter. Instead, cold air enters through holes down low and warm air exits through holes up high. Block the holes on either end and you will reduce the flow.


Convection also happens when fluids are pushed around by other forces like the wind from the weather or from a motorized fan. This is called forced convection. Fluids pick up heat (by conduction) as they flow past warmer objects. Conversely, fluids can deposit heat as they flow past cooler objects.

Insulation Guide - The rate of convection is controlled by sealing holes and making things airtight, not by the thickness, type, or reflectivity of materials.

Radiation: Heat moves by electromagnetic waves. Light, radio waves, microwaves and infrared heat waves are all versions of the same thing: electromagnetic waves. Like light, infrared heat waves need no medium to move, but they can be blocked and reflected. As long as one object is warmer than the other, and they are in line of sight of each other, heat can from the hotter object to the cooler one by radiation. This is how earth gets its heat from the sun.

Insulation Guide - Heat transfer by radiation is controlled by the color and reflectivity of materials not by their thickness or air tightness.

Insulation Guide - In reality, it is often hard to see heat moving in just one isolated way, in most cases is moving in all 3 modes at the same time. This is certainly true in your home. When considering insulation or window upgrades it is important to identify which mode of heat transfer the proposed product is most likely to influence and how significant is it to reduce that mode of heat transfer. Also, when someone is talking above your head about insulation think back to the modes of heat transfer and try to identify which mode they are talking about, it will make things simpler.

Principle #4 – There are more things moving besides heat

The good news is these things don’t have 3 different modes of movement. The bad news is, there are more things to worry about, namely water vapor. The relative humidity (RH) in your home is constantly changing based on the weather, the number of occupants in the home, the inside temperature, the outside temperature, and what you may be adding or taking away with a humidifier or dehumidifier. The relative humidity is just a ratio of how much water vapor the air is storing to the total amount of water vapor it can store at a given temperature. So 40% relative humidity (which is typically most comfortable for humans) just means that the air is storing 40% of its full capacity at that temperature. 100% RH means the air is holding the maximum amount in vapor that it can at that temperature, any more is just going to have to stay as a liquid. Not hard to understand. The key here is the phrase “at that temperature” because when the temperature changes so does the air’s ability to store water. The rule is simple, the cooler the air the less its ability to store water vapor. If air losses its ability to store water vapor the vapor turns back into liquid on the nearest surface (called a condensing surface, you’ve seen them before, think cool glass of lemonade on a hot summer day, why do the sides get wet?) Air that is 40% RH at 68 ºF can be 100% RH if it is cooled to 43 ºF. If you set your thermostat to 68 ºF can you even find parts of the house at 43 ºF? Oh yeah, I’ve seen them with an infrared camera, plenty.

Going back to heat transfer, convection is heat movement by fluids, in your house that fluid is air. So if we move a pocket of air at 40% RH to a new location where the air is allowed to cool it can rapidly reach 100% RH. At 100% RH it is holding all the vapor it can, any cooler and it will have to “let go” of that vapor as liquid water on the nearest surface. Sometimes that surface is readily visible, the cool surface of your basement wall for example. Other times its a surface of your wall that’s behind sheetrock. Imagine the amount of water on the outside of a glass of lemonade sitting hidden inside your wall.

Another example, conduction through steel studs can create condensation right on the wall. Some homes are framed with steel studs. Heat conducts through them very well. So the warmth of the home conducts to the outdoors through those studs faster than through the insulation between the studs. Along comes the warm air in the room. Its at 40% RH in the middle of the room, 40% RH near the insulated part of the wall but 100% RH when it gets near those steel studs. Over time small amounts of water are deposited in the location of the studs and the sheetrock starts to have a different color there, it’s called ghosting or shadowing. Don’t get me wrong, I have nothing against steel studs, the same thing may happen on wood studs and often happens on the surface of windows. This just makes a great example for conduction and water vapor.

Insulation Guide -I don’t get hung up on what materials are good or bad, they all have strengths and weaknesses. The key is knowing what those strengths and weaknesses are and choosing the materials that have the strengths in areas where you need it most and checking to make sure the weaknesses won’t cause a problem.

Water vapor, like heat, is subject to the same law of equilibrium. So where there is more water vapor in the air (called vapor pressure) it is going to flow to where there is less water vapor in the air. Vapor pressure also moves from warm areas to cold areas. To stop it, you guessed it, takes energy and a boundary.

Principle #5, your house is a dynamic system Your home is a very dynamic system. In addition to the heat you are actively adding with a furnace (or removing with AC) there’s heat from the building occupants and appliances. During the day your house is taking up heat from the sun through the roof, windows and some walls. At night it is radiating heat though those same surfaces. Some surfaces, like concrete, thick plaster, and stone store heat longer than other surfaces like glass and plywood. With all that, heat is moving in and out of your home in 3 different ways. Taken together, its surprising that we don’t have more problems with our houses.

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