Tag Archives: insulation

Adding Insulation for Energy Efficiency part 2

The last time we checked in on the topic of insulation and insulating a house to the point where it wouldn’t need a furnace was back in December. Sheesh. The cold has broke here in the northern great lakes region, and while there is still a chill in the air some days, we seem to be headed right into spring. The good news is, insulation is not just a winter topic. Good insulation in your home will help keep it comfortable all year long. And keep your energy bills down. And so we forge ahead with adding insulation for energy efficiency.

Previously, I walked through the calculations to determine the payback period for adding insulation. Today let’s look at a couple of examples of how that might work our in practice.

  • R-value of the initial insulation (Ri)
  • R-value of the final insulation (Rf)
  • Cost of insulation (Ci)
  • Efficiency of the heat system (E)
  • Cost of energy (Ce)
  • Number of Heat Degree Days for the year (HDD)

And the equation looks like this:

P = (Ci * Ri * Rf * E) / (Ce * (Rf – Ri) * HDD * 24)

OK, take a deep breath. We’re about to do some math!

Example 1: Fiberglass Insulation Upgrade

For our first example, we’ll use the following situation: A house in Wisconsin is going to have its insulation upgrades. It currently has fiberglass batting with an R-value of 13, and will be upgraded to fiberglass batting with an R-value of 19. The cost of the new insulation is $0.41 per square foot. The house is heated by a natural gas furnace that is 85% efficient. The cost of natural gas in Wisconsin is $0.82 per therm, and 1 therm is equal to 100,000 Btu (British thermal units). The number of heating degree days for Wisconsin is 7499. We want to find the payback period for the new insulation.

So, breaking down our equation, we have:

Ci = $0.41 per square foot

Ri = 13

Rf = 19

E = 85% = 0.85

Ce = $0.82 per therm = $0.0000082 per Btu

HDD = 7499

P = (0.41 * 13 * 19 * 0.85) / ((0.0000082) * (19 – 13) * 7499 * 24)

P = 9.7 years

Wowza! That’s more time than I was expecting. So what are the key factors here that could cause this to payback period to go down? Well, first of all, with a little more looking, you might be able to find a better price on your insulation than a quick tour through the Home Depot website gave me. Also, natural gas in Wisconsin is pretty dang cheap right now, all things considered. But as more cities and states do things like ban fracking for natural gas, that cost could go up significantly, which would obviously bring the payback period down.

Example 2: Sprayed Foam Insulation – How much can we get?

What if instead of replacing all that R-13 fiberglass insulation with R-19 fiberglass insulation, we wanted to replace it with spray foam insulation?

Spray foam insulation has an R-value per inch of foam thickness. You can increase the total R-value by spraying a thicker layer of foam. There are tons of options available as far as spray foam goes, but for the sake of this example, we will use this Dow Froth Pack as our insulation. This spray foam provides R-6 per inch of thickness, so 1 inch has R-6, 2 inches has R-12, 3 inches has R-18, so on and so forth.

In this example, instead of calculating the payback period for the spray foam insulation, we’re going to see how thick of an insulation layer we can “afford” to apply, given the same payback period as the upgrade from R-13 to R-19 fiberglass. In other words, we are going to solve for Rf.

So, breaking down our equation, we have:

Ci = $1.01 per square foot

Ri = 13

Rf = x

E = 85% = 0.85

Ce = $0.82 per therm = $0.0000082 per Btu

HDD = 7499

P = 9.7 years

Through the magic of algebra, we can rearrange our equation to solve for Rf:

Rf = (P * Ri) – P – ((Ci * Ri * E)/(Ce * HDD * 24))

Which looks gross, but it’s really just a matter of plug and chug at this point:

Rf = (9.7 * 13) – 9.7 – ((1.01 * 13 * 0.85)/(0.82 * 7499 * 24))

Rf = 10.67, or about 1.75 inches thickness of the spray foam insulation.

So, for the same payback period as with the fiberglass insulation, we’d actually be downgrading from R-13 to R-10.67 with the spray foam. If we wanted to increase to the equivalent R-value, our payback period with the spray foam would be nearly twice as long!

But then what’s all the fuss about spray foam insulation? Why would anyone use it if the return on investment is apparently so low? Well, the R-value of the insulation isn’t telling you the whole story here. Remember the walls of your house are not just made out of batts of insulation. There is also the framing, the siding, the sheet rock, and all the other layers to consider. And those layers typically have small cracks and crevices where the heat can leak quite easily. One of the benefits of the spray foam insulation is that it fills in and seals all those leaky spots. So not only do you have the impact of the insulation layer, but you’ve increased the insulation abilities of all those other layers as well. Insulation can be one of those things were whole is greater than the sum of the parts.

Onward, Energy Efficiency Warriors. Next time we visit this topic we’ll get to the big finale: Can you insulate a house enough such that you don’t need a furnace???

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Adding Insulation for Energy Efficiency Part 1

adding insulation for energy efficiency

Untitled” by Jesus Rodriguez // CC BY

As we continue to explore the possibility of building a house that doesn’t require a heating and cooling system, the next step is to get to know the current standard for adding insulation for energy efficiency. This is a topic that involves a bit of math. In this post, I’ll walk you through the equations that are used to determine how much insulation to add. In the next post on insulation I’ll go through two simple examples of working out how much insulation to add.

Payback Period

The typical plan for adding insulation for energy efficiency is to add to the point where you are able to cover the costs of the added material with the money that you will be saving in heating and cooling costs. The time it takes to recoup the money for energy efficiency upgrades is called the payback period. For the insulation of a residential building the average payback period that most people are interested in waiting is between 4 and 5 years. So, in order to figure out the payback period we need to consider the R-value of the insulation, and the cost of heating and cooling the house per year.

Calculating the R-Value

As you may remember from my last post on insulation, the R-Value is a numerical value given to insulation that tells you how much the insulation is going to resist the flow of heat. Determining the R-Value of an insulation material depends on a number of different factors:

  • Initial indoor temperature (Ti)
  • Outdoor temperature (To)
  • time (t)
  • surface area of the building (A)
  • The heat loss indoors (dQ)

And the equation looks like this:

R = (Ti – To) * A *t / dQ

The good news about R-Value calculations is that you usually don’t have to do them. Since the measurements to complete the calculation are done in a lab setting in a controlled environment, the insulation manufacturer provides that information for you when you choose your material.

Calculating the Payback Period

In order to calculate the payback period of adding insulation, we need to take into account the insulation and the heat system.  The payback period depends on the following features:

  • R-value of the initial insulation (Ri)
  • R-value of the final insulation (Rf)
  • Cost of insulation (Ci)
  • Efficiency of the heat system (E)
  • Cost of energy (Ce)
  • Number of days that require heat per year (t)

And the equation looks like this:

P = (Ci * Ri * Rf * E) / (Ce * (R2 – R1) * t)

You can find more information on calculating the payback period of adding insulation here.

I know looking at all these equations can be intimidating if you are interested in figuring out how much insulation to add to your house to meet the 4 – 5 year payback period. But hopefully after I work through a couple examples in my next post on insulation, it will seem manageable. Maybe you’ll even be inspired to add insulation to your own house to make it more energy efficient.

Insulation

couple weeks ago I posted an Autodesk Academy video of my hero, Amory B. Lovins talking about integrative design. In this video Amory talks about insulating a house to the point that a heating and cooling system are no longer needed. I’d like to dive deeper into this idea. To start, let’s talk about some of the basics of insulation.

"Insulation Roll" by Mark Evans // CC BY

Insulation Roll” by Mark Evans // CC BY

Why we insulate buildings

Here in the northern part of the country we typically think of insulation as necessary for keeping the heat in during the fall and winter (and sometimes spring) months. But the most basic purpose of insulation is to prevent the movement of heat. Both out of and into a structure. Insulation is also quite useful for keeping heat out of a building in the summer. So insulation can cut down on the need for both heating and cooling a building when more extreme temperatures hit. Having a well insulated home can reduce your energy use (and costs!) all year round.

R Values

Insulation materials are rated using an R Value. R value is a measure of resistance to heat flow, and is based on the temperature difference between indoors and outdoors, the area of the insulation, time, and heat loss. High R values provide better insulation than low R values. Now, walls and ceilings and floors are made up of multiple layers of different materials, and to find the total R value of the system, we add together the R value of each individual layer. Oak Ridge National Laboratory put together a recommended R value calculator based on zip code, heat source, and part of the building that is being insulated.

How buildings are typically insulated

If you rip open the drywall in one of your exterior walls chances are you will find fluffy, pink, fiberglass insulation. This is known as batting, or fiberglass batt insulation. Fiberglass batt has an R value of about 3 per inch of thickness.

Because heat rises, in colder climates it is important to have a well insulated roof or attic to keep that heat inside the building. Typically, houses have a blown insulation (loose insulation that is blown into a space to fill it), that can be 15 inches or more in thickness.

In warmer climates , where you want to keep the heat out of the house, sometimes the insulation batt has a shiny metallic side. This is put on the outer face of the wall to help reflect heat away from indoors.

Common Types of Insulation

There are many more types of insulation than the fiberglass batt or blown fiberglass.

Mineral Wool is a material that resembles matted wool, but is man made rather than sheep made. In batt form, its R value is equal to fiberglass batt, but as a blown insulation it has a larger insulation, making it slightly better for attic spaces.

Cellulose is a material made from plant fiber that can be used as a blown insulation. It also has a slightly higher R value than blown fiberglass. Cellulose can also be mixed with water, adhesive, and moisture retardant and used as a spray insulation, which has the advantage of being better able to get into nooks and crannies to seal up a space. Cellulose also has the benefit of being a vapor barrier, preventing the buildup of moisture, which can help prevent rot.

Foam board insulation can be made from polystyrene or polyurethane among other polys. The boards are made of dense foam that can be cut to fit into wall spaces, and provides a good amount of insulation for a small amount of thickness.

Foam spray insulation may be made from a variety of different man made materials such as polyurethane, and is sprayed into the walls and ceilings. Foam insulation is excellent at sealing up walls or ceilings that have small cracks and/or holes, however it is much more expensive than fiberglass insulation.

So, now we have a base understanding of insulation, next time we can talk about insulating for greater energy efficiency.


Want to read more about insulation? Here are some good sources:
Energy.gov articles on Insulation
Insulation R Value Chart