Tag Archives: heating

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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Introducing The Energy Efficiency Project

Once upon a time, husby and I thought, what if we bought a house in the small town we’ll be moving to this spring. About a year ago, when I first starting envisioning this whole building earth project, I thought a good progression would be to have a house to demonstrate some of the energy efficiency, green building, and sustainable design ideas that I’ve been writing about. Not to mention we were ready to start investing in our own place inside and out.

One of the projects that I’m super excited about starting in regards to our new house is this series on The Energy Efficiency Project. Each month I’m going to explain what things we’ve done to reduce our houses energy use: upgrades, downgrades, or behavior changes. And then I’ll share the nitty-gritty with you: our monthly energy bill, and the costs, and pros and cons of the changes we’ve implemented. My goal is to be as transparent as possible in how we use energy and how we are attempting to save energy. My hope is to show how small changes can add up to significant energy savings, and maybe you’ll be inspired to adopt some of the same changes yourself.

power lines: the energy efficiency project

So first, let me share some details about our new home to give you the lay of the energy use land.

Size: 1,026 square feet. Single story, with an unfinished basement, rafter attic for insulation.
Energy using appliances: refrigerator, stove, washer, dryer, hot water heater, gas furnace, central air, garage door, coffee grinder, exhaust fan
Electronics: computer, cell phones, alarm clock, seedling starter heating pad
Light fixtures: 21 bulbs worth
Windows: approximately 100 square feet, most of which are fairly new with aluminum sills
Insulation: I’m not sure exactly, since I haven’t looked inside the walls yet, but I’m pretty sure it’s just your basic fiberglass batts. The attic has about 3-4 inches of blown insulation covering the house.
Occupants: 2 adults, one wee tender babe

The Energy Efficiency Project: Month 1

We’ve been living in the new house for about a month now. Long enough to get our first energy bill! So we have a bit of a baseline to start with.

For December 17th through January 13th this is what our energy usage looked like:

Over 27 days we used 379 KWH (kilowatt hours) of electricity. We are part of the Alliant Energy Second Nature renewable energy program, at the 100% level. (In this program you can choose the amount of your energy use that you want to be matched in renewables, and we chose 100%.) So the cost of our electricity is $0.13 per KWH, for a total of $49.62.

We also used 85 Therms of natural gas heat energy. The natural gas market fluctuates in Wisconsin, so there is not an easy dollar per Therm number to give you, but during this billing period we paid $72.90 for our gas use.

Our energy bill also provides these numbers for helpful comparison:

Electricity used this month last year: 834 KWH (!!! what did the former owners have plugged in that sucked more than twice as much electricity as we used?)

Gas used this month last year: 96 Therms. Average temperature this month: 20° F. This month last year: 14° F. So last year was a bit colder than this year which explains the higher gas usage.

Degree Days this month: 1211, vs this month last year: 1698. Degree days are the number of degrees below 65° F in one day, all added together for the total 27 days of the billing period

Now let’s see where we can go from here!

P.S. Interested in seeing a picture tour of our new house? You can check that out over on macnamania.com!

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.

Amory B. Lovins on Integrative Design

I’m going to give you a little bit of homework before we get into the meat of this post. Watch this video:

(I’ve probably posted that before. I’m a wee bit obsessed with Mr. Lovins and his work)

Now let’s talk a little bit about integrative design. Integrative Design is a method of design based on working from the top down. Basically you look at the entire system – the entire car, the entire house, the entire factory, with the intention to make it as energy efficient as possible. By looking at design from the top down you ask how to make the best holistic design by intertwining the functions of the different components.

Integrative Design is different from traditional design methods which focus on optimizing each individual piece of the system and then fitting them together and adjusting how they interact. This traditional method creates the most optimized walls and plumbing and HVAC. But the integrative design approach allows you to say, what if we didn’t need the HVAC at all  (or at least not our idea of the most optimized HVAC) because we change the way we build the walls completely.

At the end of the Autodesk video Amory mentions the 10xE principles of integrative design, and I want to share those here:

  1. Define shared and aggressive goals.
  2. Collaborate across disciplines.
  3. Design non-linearly.
  4. Reward desired outcomes
  5. Define the end-use.
  6. Seek systemic causes and ultimate purposes.
  7. Optimize over time and space.
  8. Establish baseline parametric values.
  9. Establish the minimum energy or resource theoretically required, then identify and minimize constraints to achieving that minimum in practice.
  10. Start with a clean sheet.
  11. Use measured data and explicit analysis, not assumptions and rules.
  12. Start downstream.
  13. Seek radical simplicity.
  14. Tunnel through the cost barrier.
  15. Wring multiple benefits from single expenditures.
  16. Meet minimized peak demand; optimize over integrated demand
  17. Include feedback in the design.

In Amory’s lecture he talks about using integrative desing in building design for heating and cooling, in auto design for using less fuel, and in factory design for pumping fluid. Stay tuned for a bit of a deeper dive into these topics in the future, including how the integrative design principles lead to radically different approaches in each of these categories.

Passive Heating do-it-now

Ok, maybe you don’t have any big renovations planned for your home, but you still want to make your living space more heat energy efficient. Let’s go back to the second goal in passive heating:  Seal up your building so the heat doesn’t escape. Here are some simple things you can do to seal up your home and keep it warm without burning so much gas this winter.


The thing about warm air is that it can escape through really tiny holes and cracks, so we want to do our best to fill them all in. Start by checking your windows, where the frame of the window comes in contact with the wooden sill. Is it sealed? If not, use caulk all the way around to fill in and block any potential leaks. Now look at the junction between the glass and the window frame and do the same. You can find clear caulk especially made for windows for this project.

Window Plastic

To add an extra layer of sealant, (or if you live in an apartment and can’t get permission to caulk your windows,) go with the old standby of window plastic. Wipe down the sill well, and make sure it is dry before putting down the double sided tape to help ensure a good seal.

Weather Stripping

Doors are the other prime leak location. Especially older wooden doors whose wood has begun to weather and warp. You can help stop up those possible leaks by putting weather stripping on the edges of the door. A draft guard along the bottom edge works well to block leaks too. Make sure you measure your door and the gaps between the door and the jam to ensure you get the appropriate size weather stripping and draft guard. You want the weather stripping to be slightly thicker than the gap it is filling to get a good seal. So there you have it, three simple ways to make your house better at passive heating. The great news is, these three things can also help keep your house cool during the summer as well. And we’ll have more on that coming up.

This post contains affiliate links.

Are you looking for an introduction to passive design? You can find it here.

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