Northern Thermal Comfort
2379 Rux Rd.
Arbor Vitae, WI 54568

The Insulation Lab

Birds, Bees And Energies

Jeff Aderholdt - Friday, June 24, 2011

Birds Bess and Energies

In recent years, the construction scene has changed. With the increase of energy costs, as well as a growing concern for the environment, an effort has been made to make houses more energy efficient, and by extension, more environmentally friendly. With the booming economy comes larger and more elaborate constructions. These two forces effecting the construction industry has, at times, created a conflict, in particular, in the insulation and other energy related trades.

When conflicts arise, there is never a shortage of individuals ready and willing to jump in the middle and offer, or sell, advice on how theses issues should be resolved. Unfortunately, despite all good intentions, many such individuals offer ineffective and inaccurate advice resulting in increased cost and the conflicts not truly being resolved.


A common thread that seems to run through all those “would be advisers” is an obvious confusion of the basics. This is evident in how most view heat loss and vapor movement. Many times, well meaning individuals merge scientific laws and facts from one field and misapply them to another. The result is the birth of various myths, such as the myth of the “Perfect Vapor Barrier”. In order to identify and dis-spell such myths, it is at times necessary to review the basics. I will try to keep this simple.

The Basics of Heat Movement

A basic law of physics is that the universe does not like differences. The result is that when differences exist, the universe moves to normalize and eliminate the difference. What this means in heat movement is that if you have an area that contains more heat surrounded next to an area that contains less heat, the physical laws of the universe attempt to normalize the heat levels, transferring the heat from the hotter area to the cooler area until the mean average temperature is obtained.In attempting to normalize the temperature, different mechanisms are are in play. The four mechanisms or methods with which heat travels are conduction, convection, radiation and evaporation/condensation. Each of these mechanisms needs to be controlled in construction to achieve energy efficiency. The first step then is understanding each mechanism and how each one comes into play in construction. Then we can understand how to control them to our advantage (I will be limiting the information to the area to the area of heat).


Basically, conduction is heat that moves through matter. For example, you heat the bottom of the griddle, heat moves through the griddle making the top hot. You can now make pancakes.


Basically, convection is the heat carried along as fluid matter moves (by the way, air movement qualifies as fluid movement). For example, you go to your sink and turn on the hot side of your faucet. The water flows out. Eventually, the heat from your water heater is “carried along” with the water. You have hot water flowing out of the faucet. You can wash the syrup off of your hands.

A lot of forces effect convection, but pressure differences is the main force driving it.


This is a big one. All the energy we have on the earth is from radiation from the sun. Without radiation, there would be no life. The heat doesn't need to move through something (conduction) or be carried along by something (convection).

For example, after a hearty meal of pancakes, you retire to the living room and relax next to a warm, glowing fire. From a distance, you can feel the heat radiating.


Evaporation/ condensation is basically matter changing state from a gas to a liquid, or from a liquid to a gas. This is something that every heating / AC professional knows. Though this can involve any form of matter, for this discussion we will be focusing on water. To convert water from liquid to gas (evaporation) takes heat. A LOT OF HEAT! The heat is not lost, but stored in the water molecules. To convert the water vapor back to liquid (condensation) that heat must be lost. If the water molecule moves in between, the heat moves. For example, you boil water to make a cup of tea. You pour the steaming water in your cup on your teabag. You see the steam rise from your cup and move toward the window. The steam condensates on the glass. The result is that some of the heat has transferred from your teacup to the window glass.

Application in Residential Construction

Now that we have established the four ways heat moves (conduction, convection, radiation and evaporation/condensation), the question now is how can we put this information to use? To properly insulate a house, all four of these areas need to be addressed and controlled. Let's start with conduction.

Controlling Conduction

When people think of insulating a house, the first thing they think of is “R-Value”. R-value is a measurement that measures resistance to conductive heat transfer that starts at “1”, the lowest R-value, and goes to infinity (at time R-value equivalencies are given to non-conductive heat transfers. For example, gains created by controlling radiant heat transfer will be stated in R-value. This is an equivalency since R-value only measures conductive heat transfer). Pretty much all matter has an R-value, even rock (rock has an R-value of about 1 per foot). Building codes (See IECC) divide the country into several climate zones and then provide standards that meet the climate. The R-value is attained by installing insulation in wall cavities, attic spaces, or other areas that separate the outside of the house from the inside of the house. This insulation is usually in the form of batting, blown, or foam made from a variety of materials. To perform properly, each material needs to be installed properly in accord with manufactures recommendations.

Each material has it's strengths and weaknesses. Because of this, a product may perform strongly in a particular application or area, but may be weak and less desirable in another. For example, closed-cell spray foam has the strengths of having a high R-value per inch, creates an air-seal, and is a vapor barrier, but also has the weakness of being very, very expensive. If you have an area that has a limited space, cannot be air sealed, or cannot be vapor sealed, it is a strong choice (an example of this would be the box sill area of the floor system.). On the other hand, if space is not an issue, and air and vapor can be controlled by other means, it quickly becomes too expensive (an example of this is in a standard 2” X 6” framed wall.).A few other things to remember about R-value. The accumulated effect is exponential, not linear. What does that mean? Doubling the R-value does not double the effect. The first R-value will stop x percent of heat flow, the next R-value will stop the same percentage, but only of what is not stopped by the previous R-value. This is what is called the “law of diminishing returns”. Now, let's put this in application. An area (wall, ceiling, or floor) that is insulated to R-20 will stop 95% of conductive heat transfer. That leaves only 5% to potentially stop. If we double the R-value to R-40 we stop 97.5%. R-50 stops 98%, R-100 stops 99%. The returns of adding greater and greater R-Values diminishes. The question then is, when does the investment cease to be a value? The current recommendation for climate zone 7 (that is for far northern Wisconsin and most of the UP of Michigan, is R-21 walls and R-49 attics. Though it is possible to increase the R-Value in these areas, the added cost just doesn't justify it, even if energy costs go many times what they already are. There are better, more cost effective techniques that can be used to provide better results.Another thing to remember about R-values, remember the weakest point. When we talk about R-value for a given area (ceiling, walls, etc.), it is usually stated as center of cavity. For example, one will insulate a 2” X 6” wall cavity with R-21 fiberglass batting and refer to it as an R-21 wall. Pretty much all insulators would call it an R-21 wall. I would! But it is technically not. Though it has R-21 batting in the cavity, on an average, it is less than R-21. The weakest point for such a wall be where we have direct conduction through the wood, where, depending on how things are built, we probably have a R-value of 6-8. This would be at each stud, as well as the top and bottom plates. We could, with very expensive spray foam, fill the wall cavities and have R-35, at center of the cavity, but the weakest points would still be only R- 6-8. Money would be better spent to improve the weakest points.

One more thing. Think minimization. If you can minimize the amount of conductive surface area, you will minimize the amount of conduction. This can be done by working with the framing carpenters to redirect a few areas to create a smaller thermal footprint. A big way to minimize the surface is not to add to it. There are still some HVAC contractors that think running the ductwork or other mechanicals outside the thermal envelope, like in the attic (this is wrong in so many ways!), is a good idea. Among other issues this creates, it makes for a more conductive surface area.

Controlling Radiation

Probably the most common area in construction were we can control radiation is in the windows. A reflective film is applied to the glass, suspended between panes of glass, or both. The result is that heat from the summer sun is reflecting out, reducing the air-conditioning requirements. Heat inside during the winter is also reflected back in, reducing heat-loss. Some window manufacturers have windows that have a center of glass R-Value equivalency of about R-9 (This is center of glass, at the edges, where everything is connected, conduction takes over, leaving an R-Value on 1-2. That is why in the wintertime moisture condenses along the edges of the glass. There are some semi-obscure manufacturers that have focused on what is called “Warm Edge” windows. (Some of these get real expensive).You may consider minimization. If you reduce the area of negative radiation, you minimize the negative effect. This is tough because minimizing windows has a definite aesthetic effect. It can also have a negative energy usage effect. (Without natural lighting, you may need to use artificial lighting more).

There are other areas that radiation control plays a part. Darker colored materials absorb and radiate heat at a faster rate than lighter colored, or reflect, materials. In application, a lighter colored roof will not absorb as much heat, keeping the attic cooler and reducing cooling costs. In warm climates, a reflective, radiant barrier is sometimes installed in an attic to slow heat transfer to the ceiling.In heating climates, it's common to use foil faced foam board. If installed correctly, with the reflective, unlettered side of the foam towards a dead air-space), you can gain an R-Value equivalency of about R-2.5-2.7.

Controlling Evaporation / Condensation

This is something usually not thought of by anyone other than HVAC people, but there are areas of concern in home construction. The first area to control is convection. This will be discussed a little later. Closely related to convection is to control vapor movement. A properly installed vapor barrier will accomplish this.

Another way is to actively control the humidity level. As a general rule, too little moisture in a house built to modern standards is not an issue, unless it is empty for extended periods of time. Living in a house creates lots of moisture through cooking, showers, breathing, etc. Too much moisture and getting rid of it, is the problem. In the summertime, air-conditioning, or even opening the windows helps to minimize, or at least equalize, the indoor humidity. In the wintertime, use of exhaust fans will lower the humidity and help minimize condensation on windows and other areas. Of course, just exhausting the air doesn't save and recover the energy in the air in in the water molecules. A central heat-recovering ventilator can help to recoup the energy.Again minimization is a possibility. A few lifestyle changes, like reducing the length of showers, can reduce the amount of moisture in the air, thereby reducing the potential for condensation and the need for mechanically removing the moisture. Also minimizing the potential surfaces where condensation can occur can reduce the heat lost.

Controlling Convection

I have saved this one for the end. Not because it is less important, in fact it is the most important to control. Our mothers taught us this when we were young. When we would look outside at the fresh snow, we couldn't wait to get outside and play in it! We would grab our coat and head for the door. Before we could get out the door, our mother would say “be sure to zip up your coat!”. Mothers are pretty smart! She knew that we could have on the best arctic parka, but if we didn't “zip it up”, it wouldn't work right and keep the heat in, keeping us warm. The same is true with our home. We need to “zip it up”. How well we control convection determines how much of the air that we just paid to heat stays inside and also determines how much moisture goes where we don't want it to.

I break down convection into two basic types, what I like to call Open Circuit & Closed Circuit. This is my terminology and not necessarily the terms used by others.

So, in a house, what does this mean? An “Open Circuit” convection would be a direct penetration through the thermal envelope, allowing air to move the inside to the outside, or, from the outside to the inside. These are usually referred to as “by-passes”, since the air can by-pass the thermal envelope. One of the most common places this is observed is through electrical boxes in exterior walls. A “Closed Circuit” convention is a enclosed area with temperature differences, side to side or top to bottom. As the air inside the enclosed area changes temperature, the air circulates, warm air rising and the cold air dropping. A convective loop is created, and will continue until the temperature has normalized. The entire inside of a house is an example of “Closed Circuit” convection. The warm air moves to the highest point inside the house, up to the tallest ceiling, and the cold air drops to the lowest point, down to the lowest floor.How do we control the convection? In the case of the “Open Circuit” convention, meticulously seal the holes, inside and out. Little details make big differences!!! Because some insulation is more given to air moving through it than others, the choice of insulation type in open areas, such as the attic, can make a difference. But, what about the “Closed Circuit” convection? Where does this apply? By installing insulation in such a way so as to minimize this type of convection. Studies have shown a significant performance loss in exterior wall cavities where the the fiberglass batt insulation has not been installed completely and neatly. Yes, neatness counts! Some installers fail to push the batting completely to the outside of the cavity creating an air channel on the outside. Some do not bring the interior of the batting flush to the inside creating an air channel on the inside. As a result of the air channels created, the air starts to circulate. The warm air on the inside rises to the top and pushes its way to the outside. The cold air on the outside drops, pushing its way to the inside. The result is heat loss, and a loss of performance. This happens more than one may think. It takes time to install the batting right and time is money.“Closed Circuit” convection can also be controlled by using insulation materials for wall insulation that are less given to “Closed Circuit” convection. Spray-foam insulation stops it well, but, among other concerns, it is very, very expensive. Expensive to the point that, expect for specialized areas, it is cost prohibitive. Other methods can perform comparably for a lot less money, making them a better value.

Cellulose insulation, in particular dense-pack cellulose, is an excellent alternative to batting in walls. Though it is not an air barrier, when packed into a wall cavity, it virtually eliminates convection through it (Open Circuit) as well within it (Closed Circuit). It is also value priced, a little more than fiberglass and a lot less than foam.


Well, I tried to keep it simple. It's not easy because thermal control in building science is a very complex system. In addition, there are new things frequently being learned and new techniques are being tried. If you would like to look at some of this information in greater detail, check out some of the more technical information on this website, or follow some of the links.

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