Energy Use Over the Last Year In the ADU

In earlier posts, I shared how living in smaller spaces reduces per capita energy consumption, which was part of my motivation for living more compactly. Furthermore, after constructing an energy efficient ADU, Energy Trust of Oregon even gave me a $1,750 rebate for meeting Northwest ENERGY STAR standards and achieving a modeled Energy Performance Score of 35. 

Now that I've lived in the ADU for over a year, I wanted to share the actual energy bills, and compare them to the energy bills of the main house. Comparing the utility bills of the two dwellings will allow me to draw a few conclusions regarding the nuanced personal energy consumption virtues of living more compactly.

Both homes are heated by natural gas. The ADU is heated via a tankless water heater and in floor radiant hydronic heat. The main house, built in 1906 and recently weatherized, is heated through a conventional forced air heating system.

Below are the natural gas bills for the ADU and the main house, respectively. The ADU consumed 277.5 therms, and the main house consumed 930.4 therms. A "therm" is a unit of heat equal to 100,000 British thermal units.

The total therms used in the last year in the ADU

The total therms used in the last year in the main house

At only 277 therms, the ADU used only 29% as much energy as was used by the main house for heating!

That's very cool, but that's not the real story. 

The real story must include the total energy use divided by the number of occupants. The designed occupancy level is calculated by taking the number of bedrooms in the house, and adding one additional occupant. This designed occupancy level assumes that two adults are living in a 'master bedroom' together.

The ADU is a one bedroom dwelling designed for two people. And, in fact, that is exactly what happened in the ADU;  two people lived in the ADU for the full year. 

The main house is a four bedroom house, with a designed occupancy level of five people. However, in reality, the actual occupancy rates of the main house were lower than the designed occupancy level; the main house maintained an average of three occupants throughout the year. 

Interestingly, census data shows that the average national occupancy rate for homes larger than mine at 1,700 sq ft, is surprisingly low. The average occupancy rate for homes of 2,500 sq ft is actually only 2.59 occupants.

So, let's look at the therms used per capita for both the designed occupancy and the actual occupancy.

If the main house were occupied at the design occupancy rate, each ADU occupant would have used 75% as much energy as the average resident in the main house. Living in the Northwest ENERGY STAR certified ADU would have been 25% more energy efficient than living in the main house.

But, in reality, with only three occupants in the main house, the actual therms used per capita for heating and cooling in the ADU was still 138.75, but the actual therms used in the main house was 310.13. This means that by residing in the ADU, I used only 44% the amount of energy that the average occupant used in the main house. Living in the ADU has been, in reality, 56% more energy efficient than living in the main house.

These are significant data points. Here are the stories that these data points tell us:

#1) Building the new structure to Northwest ENERGY STAR standards resulted in building a very efficient building envelope and in choosing to use efficient heating systems.

#2) By living at the designed occupancy in the ADU, my partner and I each lived more energy efficiently. Building a smallish dwelling alone did not make the dwelling energy efficient. It was dwelling in a smaller footprint per capita that had the most substantial energy efficiency impact.

Said another way, if I lived alone in the ADU, and the main house was fully occupied at the design occupancy of five people, I would have actually used 50% more energy than the average resident in the main house.

#3) In smaller dwelling spaces like my 800 sq ft ADU, I was prone to live at the designed occupancy level of two. Conversely and representatively, the main house was prone not to be fully occupied (according to US Census data).

These last two points are a thinking person's fodder for a housing revolution.

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I'm also including my electrical bills from the last year for reference. Since neither the ADU nor the main house used electricity as the primary heat source (where the bulk of a home's energy is used), these data points are less relevant.

That said, indeed, the ADU was more efficient than the main house in terms of electric power consumption due to the types of appliances and light fixtures that were installed. But, one will still draw a similar conclusions that I have drawn above regarding designed and actual occupancy

Under actual occupancy rates, living in the ADU used 30% less electricity per capita than living in the main house.        Under designed occupancy rates, living in the ADU would've used 18% more electricity per capita than in the main house.

In terms of electricity, the ADU was more efficient under actual occupancy rates. But, if the main house was occupied by five people, the electricity use per capita in the main house would have actually been lower than in the ADU.

Electricity used in the last year in the ADU

Electricity used in the last year in the main house

Note the July spike in electricity use in the main house and the lack of a spike in July for the ADU.

The main house is set in direct sun, has R12 wall insulation, and has an AC unit.

The ADU is kept cool through deciduous tree shading, R33 wall insulation, and a ceiling fan. :)

Air Sealing and Insulation - The Week of Weatherization

This week has been all about air sealing and insulation.

In my previous post, I wrote about air sealing and insulating the main house--a drafty and uninsulated house, built in 1906. In the weatherization project, that house wall assembly's R-value increased from R-1 to R-12-- a huge improvement that will dramatically effect the comfort of the house, and decrease the energy bills.

This post is about air sealing and insulating the ADU. It was far easier and cheaper to air seal and insulate the new ADU construction than to weatherization retrofit the 1906 construction. In contrast to the retrofitted R-12 walls, the ADU's walls are now R-33. After much consideration, I decided to use Certainteed blown-in fiberglass insulation. This cost $2,700 for about 1400 sq ft of wall cavity (9" deep), and 900 ft of ceiling cavity (10"deep).

Blown-In Fiberglass Insulation

For those of you who are new to this, R-value is a measure of the heat resistance of a material assembly. You know how metal or glass feel cold when one side is facing cold air? They feel cold because those materials conduct heat very quickly. Conversely, Styrofoam does not conduct heat well, which is shops sometime use Styrofoam cups to contain the heat of hot liquids. The best kind of insulating building product is called closed cell spray foam, but it is 2-3 times more expensive than blown in insulation. It's also super ugly---to me, it looks like alien puke--just look at the picture of it in my main house basement ceiling and you'll see what I mean. But, it's effective as an air sealer and insulation material and it's en vogue. It's very important to air seal and insulate buildings to reduce their energy demand- and there's a number of ways to accomplish this. 

Closed cell spray foam looks weird, but it does a great job insulating and sealing

Below are three videos that show the air sealing and insulation process that we used. This was a neat process to watch in person--the third video is best one to see the insulation being blown.

Installing cardboard baffles to create a vented roof cavity


Installing netting for insulation


Blowing in insulation in walls and ceiling (aka. BIBs- blown in blanket insulation)



A video of the completed insulation a day later.

Oregon Clean Energy Works (Part II)

This week, I went through the Oregon Clean Energy Works (CEWO) program. I've wrote about the Clean Energy Works Oregon program in an earlier post, entitled Only the Worst Need Apply.

In it, I described how the program works. In a nutshell, it's a program that targets older homes in Oregon that are drafty or not well insulated and offers financial perks for homeowners to get their homes energy audited and weatherized. 

I am VERY impressed by the way that this program is funded and run. But, I wanted to reserve judgment on the program overall until I had been through it. Now that I have been through it, I want to give a strong endorsement to the program.

CEWO doles rebates for weatherization work based on how much modeled energy improvement a house undergoes in their program. Since the house had no insulation and leaked like a sieve, making major energy improvements to the house were relatively simple to achieve. The improvements were relatively cheap and resulted in substantial energy reductions. CEWO bases rebates on the level of weatherization improvements, and because my house was so leaky, I'll receive the full CEWO rebates (In the last month, CEWO has lowered the rebate amounts, so the rebates aren't as enticing as they had been.)

My monthly loan repayment for the weatherization work will be $30/month. In my case, since the energy bills are approximately $100/month, my anticipated 30% reduction will amount to $30/month. The loan repayment will be reduced by the same amount that is saved by the energy bills each month. So, essentially, for no upfront cost, my home was weatherized. After the duration of the loan repayment, the loan will be repaid, and the house tenants will benefit from the decreased energy bills forever more. The other advantage to weatherization is comfort; the house already feels less drafty.

For the energy geeks out there, the blower door test after CEWO brought the home's air sealing from 7,731 to 4,476 cubic feet per minute at 50 pascals of air pressure, a reduction in leaks of 3,288 cubic feet per minute, or from 21 ACH to 12.2 ACH at 50 pascals. This is a huge improvement in terms of air sealing. The R-value in the wall assembly went from R-2 to R-11, the roof went from R-15 to R-49, and the basement perimeter rim joist went from R-1 to R-15.

Here's a video of the blower door test: