A weekly personal finance newsletter to help you manage your money.

Friday, September 14, 20181 like

Home energy efficiency 101

The Interest: issue #4

If you punch a hole in the side of your fridge, it will become less energy efficient. (What other personal finance newsletter is dishing out that kind of insight?). We can actually calculate how much less efficient. There’s about 0.5kg of air in an empty fridge and we’ll take a total guess that the hole would result in a full air replacement every minute. The task, then, is to cool 0.5kg of air from 70º to 35º 1,440 times per day. Doing it once uses about 0.002 kWh. Doing it 1,440 times per day for 30 days uses 86 kWh which at $0.10 per kWh would cost $8.64 per month. [Sidenote: the main cost of running a fridge is cooling the food, not the air — let those leftovers cool before sticking them in.]

By contrast, a closed fridge that’s never opened will heat up not through air exchange, rather by heat moving through the materials of the fridge. That’s called conduction (& radiation and convection, it seems, but I’m not the one to take us down that rabbit hole of science). The speed at which the temperature inside reaches the temperature outside will depend on how fast the materials in between conduct heat. Or in home energy building materials lingo: their r-value.

These same 2 principles (air exchange and heat conduction) dictate how efficiently your home is heated and cooled, and understanding them puts you well on your way to passing home energy efficiency 101. Minimize air exchange by determining the boundries of your “thermal envelope” and air seal them to the extent possible. Just like your fridge, cutting holes in your thermal envelope (for sockets, light fixtures, duct work, and poorly sealed windows and doors) will increase the amount of air replacements per hour in your home, meaning you’ve got to heat or cool more air.

You can’t live in a giant plastic bag, of course — you do need air ventilation for good old oxygen and other air quality measures, but unless the air sealing is taken to an extreme, it’s not likely to be something you’d have to consider. New construction homes purposely built to be extremely air tight use mechanical ventilators. There are even fancy ones that can limit the thermal impact of exchanging for fresh air.

After the house is air sealed, then it’s a matter of wrapping the surface area of the thermal envelope with high r-value materials, to the extent possible. Insulation, of course, is engineered to do just that. The r-value of a house can be computed by a simple weighted average of the surface area. If 50% of a wall is window with an r-value of 3 and 50% an insulated wall with r-value of 14, then that wall, overall, is r-value 8.5.

For the fridge, just sticking duct tape on the hole would do a lot of good. Assuming duct tape is air tight (of course it is, right? it’s duct tape!), it eliminates the air exchange and the only thermal impact is the fact that 5% of the fridge’s interior surface area is now very low r-value, effecting the weighted average proportionately.

When I was first trying to learn home energy efficiency basics, it was hard for me to understand the concept of the thermal envelope before I pictured the fridge analogy. Thinking about it that way , it’s obvious that air leakage is bad and, if air sealed, insulation slows down temperature equilibrium with the outside.

There are several other cans of worms to open, like window quality, roof strategies (ventilated or not, etc.), maximizing solar gain through positioning the house and its windows, the impact of appliances, and so on. Lots of fodder for home energy efficiency 201.

I’ll close with a note on lightbulbs.

LED lightbulbs are a total no brainer. Just 10 years ago they cost as much as $40 each, but, now they’re almost as cheap as “regular” bulbs, only use 9W to generate as much light as old 60W bulbs do, and they last much much longer. The ROI isn’t even close. 3 hours of use per day per bulb would cost $1.08 for the year with an LED bulb vs. $7.20 for the old bulbs. LED bulbs have a 10 year life span compared to 0.9 years for old bulbs (we’ll round up to 1 year). If you’ve got 30 bulbs in your house, we’re talking $3 * $30 = $90 investment and $324 in total electricity cost for all 10 years for a total of $414. Buying old bulbs would run you $1 * 30 * 10 = $300 and would cost $2,160 in electricity over 10 years for a total of $2,460. Plus, mother nature, you know? If you haven’t already, definitely switch to LED. If you’ve got CFL, those are close enough (14W for 60W equivalent light), so you can wait for those to burn out before switching.


P.S. those new iPhones! We’re switching to Xfinity Mobile (today). It’s only available for customers of Comcast Internet, but if so, it’s only $12/GB per line, all in. Or, $45/line for unlimited. We pay $138 right now for 2 lines on AT&T and this should drop our bill to ~$60, or worst case $90, and when the time comes to get kids their own phone (when is that time, btw??), it’s just $12/mo. They also have a promotion running through 9/30 for $300 in prepaid gift card per iPhone you purchase from them for new service. Good stuff. There’s reason to doubt Comcast’s ability to impress me with their mobile service, but the price is worth trying. I’ll report back.

A weekly personal finance newsletter to help you manage your money.