Homeowner happy with choice of cellulose insulation

Homeowner happy with choice of cellulose insulation

Thirteen years ago, Gary Conway built his home with geothermal heating and cooling. He shares his journey with ProudGreenHome.com.

When first planning my home, one of the primary concerns was the ongoing costs associated with the home environment. I researched insulation and narrowed it down to two choices: fiberglass and cellulose.

As with most things in life, there are trade-offs. With regard to insulation, you can spend more money up front and spend less over time, or you can spend less up front and spend more over time. If you spend more money on insulation at the beginning, you will spend less money conditioning the air in the home over time. If you spend less money on insulation, you will most assuredly spend more money on conditioning the air ever after.

This is a seemingly simple quandary with both implications and ramifications that will last the entire lifetime of your home and your monthly budget.

After some research, I determined that my goals for insulation were as follows:

  • Vapor barrier
  • Moisture barrier
  • Heat barrier

One of insulation's goals in life is to keep the temperature outside the home from coming INSIDE the home and vice-versa. It should provide a barrier that refuses to telegraph temperature variations from one side of the wall to the other. In a word, it's the house condom; after all, you pay good money to  either heat or cool the air inside the home, it's not very cost effective to heat or cool the neighborhood as well, I don't care how much you like your neighbors.

Temperature is transmitted in several ways. One is by directly heating (or cooling) one side of a surface and then having that temperature migrate through the surface's interior to the other side. This is called "conduction."

Another way is to make holes in the surface and let air from one side blow through to the other side. This is called “convection.” Home insulation's job is to severely limit or stop this activity. Fiberglass insulation does a pretty good job of accomplishing the first; however, it does a terrible job of stopping airflow, which is why most manufacturers add paper to one side of the insulation.

CAUTION: geekisms ahead! (but I'll keep it minimal)

To fully understand insulation, it's role and how that is accomplished, it's useful to understand exactly what temperature is.

We describe temperature primarily using two words, hot and cold. In engineering terms, we are talking about thermodynamics, which is the study of the flow of heat and the entropy of systems.

That's a mouthful, so you can just forget I mentioned it.  Likewise, I will not mention that studying the insulation techniques in homes requires an understanding of quantum mechanics, because entropy has everything to do with the energy levels of matter. You may also forget I mentioned that as well.

Temperature is all about HEAT and WHERE it's going. Heat always migrates toward things that are not heated. For all you thermodynamics students, this is corollary to the second law of thermodynamics. Insulation’s job is to prevent this migration.

Cold is the absence of heat; therefore heat and cold are simply words used to describe which way the heat is going. When we are heating an object, we are putting heat INTO that object. When we are cooling an object, we are REMOVING heat FROM that object. Fourier's law tells us that the rate of heat transfer is proportional to the surface area and the temperature differential on the opposing sides. I can hear you saying, “so what.”

In English, this means to us as homeowners, that the larger the surface area of our outside walls, the greater the transfer of heat. That's seems pretty logical.

Additionally, the greater the difference in temperatures between outside and inside, the greater the heat transfer. Insulation’s job is simply to prevent the ebb and flow of heat. It needs to have a low thermal conductivity, which is defined as: the heat flow per unit area per unit time when the temperature decreases by one degree in unit distance.

Another mouthful, however, simply put, thermal conductivity is directly affected by how large the wall is and how big the temperature differential is between outside and inside.

Thermal conductivity is the wall’s proclivity for conducting heat. We want walls that don't conduct or convect  any heat at all (in my ideal world).


Our assault on this problem is twofold. First, we need to prevent the movement of air from one side of our exterior wall to the other. Secondly, we need to prevent the wall from conducting heat. Obviously, the primary barrier between outside air and inside air is threefold: walls, doors and windows.

The largest of these, that is, the one encompassing the larger amount of surface area is more than likely the walls, so it seemed to me that this was a good place to start my analysis. It also seemed to me that the most important place to spend insulation dollars would be on the walls first, because that would yield the most return in years to come.

I found that the true goal of insulation in homes is to trap as much air between the walls as possible, because air is an excellent insulator. Fiberglass insulation does exactly that, it traps pockets of air within the walls, which serves to keep the temperature outside the walls from creeping INSIDE the walls.

The problem with fiberglass though is that it doesn't hold the trapped air captive. With fiberglass, air can still move, and moving air  means transferring heat energy from one side of the wall to the other. That's not a desired outcome if you want your electric bill to be as low as possible.

What is R-value anyway? Simply put, R-value is the measure of resistance to heat flow. The higher the R-value, the better the resistance to heat flow (not airflow).

Fiberglass  has an R-value range of 3.0-4.0 per inch. Cellulose has an R-value range of 3.6-4.0 per inch, so for a 3.5-inch (nominal 2 inch x 4 inch) stud, this comes out to having an R-value of 12.6 to 14, depending on how well packed the wall is.

Cellulose is about three times as dense as fiberglass, which means it also helps to deaden sound transmission as well as limiting heat exchange. Cellulose insulation takes its name from the fact that the material is made up of cellules and glucose (sugar).

This type of insulation also contains borates (boric acid) which act as an insect repellent, which means no cockroaches in those walls. This borate also gives the cellulose a very high class in fire safety ratings. Typically, a vapor barrier is not needed with cellulose insulation because by nature, it IS a vapor barrier, as opposed to fiberglass which needs the kraft paper on one side to form the vapor barrier.

There are several green benefits to cellulose:

  • It requires 10 to 40 times less energy to produce by the manufacturer. It is made almost entirely from recycled materials, eg. newspaper
  • By utilizing recycled paper, greenhouse gases formed from the decomposition of the paper is avoided totally
  • Cellulose insulation is recovered and recycled on site, fiberglass and foam go to a landfill and neither decomposes. Cellulose is regionally produced, using local recycling programs

And the answer is...
I chose the cellulose. The R-value is not quite as good as fiberglass; however, as I said in the beginning, life is about trade-offs, and here's one of them. I traded the lower R-value of the cellulose for the additional features of being an excellent vapor barrier, fire retardant, sound deadener and roach killer. After 13 years, I think I got a good deal.

Have you faced the insulation question in your home? What did you decide? Let us know in the comments.

For more information, see our Building Green Research Center.

Topics: Building Green, Insulation

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