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How to Save Energy

Mythbusters: Home Energy Efficiency Measures That Actually Save Energy

Have I mentioned that New Orleans is a hot and humid climate? Here, my colleague and good friend, Paul Bobbitt, installs a rigid bath fan duct in the Big Easy.
Image Credit: Rob Moody, Organic Think Inc

Still stirring from the DOE Weatherization conference, I’ll review the energy-efficiency strategies that Michael Blasnik says saves money in homes. Read my first Blasnik blog about flawed computer models that gauge energy savings. The whole idea here is that computer models are a great way to track energy-efficiency measures, but a computer is not needed for determining the upgrades that will make a significant difference in energy usage. People with the right knowledge and some diagnostic equipment can do the trick.

So what can make a big impact on home energy bills? Remember, as I said in my last blog, the more energy a home uses, the bigger the energy savings can be after upgrades. On average there is a 15% linear relationship when comparing energy usage before upgrades to energy savings, so don’t expect 50% savings from typical energy efficiency upgrades in a 1,000-sq.ft. home with $50 per month utility bills.

So here’s what Blasnik shared:

  • Adding insulation to uninsulated walls and partially insulated attics. Insulation works. Imagine that! Insulating uninsulated walls can save on average 0.2 therms per square foot per year. A therm is a unit of heat energy and equal to 100,000 BTUs and the product of burning about 100 cubic feet of natural gas. Therms are what the natural gas companies bill you for. The average cost per therm for natural gas in the US is $1.11. I’ll round that to one dollar for simplicity’s sake. In this case, 0.2 therms per square foot per year would yield a cost savings of around $400 per year for a 2,000-sq.-ft. home. The colder the region, the higher the savings for heating efficiency measures.
    • Blasnik says that adding insulation to uninsulated walls is very cost effective, but energy models underestimate the effects by 50 to 70% because the R-value of the entire wall assembly is underestimated. Continuous sheathing and air film can add R-3.5 to R-5 to a wall, which makes an exponential difference in heat transfer in an uninsulated wall. The critical caveat to this strategy is the next bullet point->
    • AIR SEALING! The rated R-value of insulation will be undermined if air infiltration is present in walls and attics. Check out Energy Star’s Thermal Bypass Checklist for places to look for energy loss. This can save 5 to 8 therms per 100 cubic feet per minute (CFM50) in reduced air leakage. What that means is that tightening up a home’s envelope by 100 units at CFM50 (measured with a blower door test) will save around $5 to $8 annually. The tighter a home, the more savings can rack up.
      • The best way to effectively locate air leaks is with a blower door creating a pressure difference. This can be very cost effective in very leaky homes that measure more than 3,000 CFM50 of leakage. There is a critical caveat here as well. If a home is tightened up and insulation is added, there could be a ticking time bomb: moisture issues throughout the home. Indoor humidity must be controlled. Air sealing is another measure that is underestimated by 50 to 70% in energy calculations due to overstated wind effects and the fact that the particular model used to crunch this ignores thermal regain in attics and foundations.
      • Additional measures for home envelopes in hot climates. If cooling equipment ductwork is located in an attic above the thermal envelope, then a radiant barrier applied to the roof can save 10 to 20% on cooling load annually. The take-away here is to keep the hot sun out of homes. Again, not rocket science, but measures such as shading and window film can be effective from 10 to 30%, depending on shading outside the home.
      • HVAC. Replacing inefficient heating systems is effective in high heating load regions, and replacing inefficient cooling systems is effective in high cooling load areas. Again, the biggest users are going to be the biggest savers. For heating, the industry baseline for replacement in natural gas units is savings of 1,000 to 1,600 therms per year. Energy savings of up to 20% can be seen by replacing a unit at 75% efficiency with one that operates at 92%. Computer models project 30% lower savings than actually seen because old furnace efficiency is underestimated at 60 to 65% instead of the more realistic 75%.
        • Air conditioner replacement is only cost effective if the home has a very high cooling load. Most cooling costs are typically below $1,000 per year even in the warmest climates. Sealing ductwork outside insulated and conditioned space could save between 6% and 20% of HVAC costs. An air conditioner tune-up could save 5 to 12% if the contractor knows what he’s doing and accurately measures the charge of the system. For cooling equipment replacement, it makes the most sense to target energy hog systems and very-high-use situations.
        • Hot water systems. Fix leaks! Also, high-efficiency washers can save 800 kWh/35 therms per year along with 11,000 gallons of water. (Average cost per kWh in the US is $0.0125.) Again, savings will depend on usage: the more loads of laundry done, the greater the potential savings. Very-low-flow (1.5-1.75 gallons per minute) showerheads can save up to 15 therms per year per shower.
        • Refrigerator replacement. A piece of equipment called a watt meter can measure the energy usage of a fridge. If a fridge is an energy hog, it will be obvious quickly, and replacing bad ones can save 600 to 900 kWh per year for typical older models and up to 2,000kWh per year for the most inefficient ones. Once again, measured energy savings are not accurate, more than likely due to testing issues.
        • Lighting. Replacing incandescents with CFLs can save 15 to 50 kWh per year. These can still be a cost-effective measure even though they save 50% less than projected due to burnout, removal, and greater use by homeowners because the bulb is more efficient. Furthermore, incandescents are like little personal space heaters, adding to the home’s heating ability in the winter. This decrease in heat must be accounted for. Motion detectors for exterior safety lighting can also be a big energy saver.
        • Phantom loads. Unplug unneeded stuff! It doesn’t help cooling to leave an air handler motor running continuously in the summer. In fact, running the fan in your HVAC system all the time in the summer could pick up hot, humid air from holes in ductwork outside the insulated space and deposit that into the conditioned areas of a home. Moving the fan switch on the thermometer to the auto setting can save 3,000 kWh per year. If you really don’t need a secondary fridge or freezer, get rid of it to save 400 to 2,000 kWh per year.

        These analytics from Blasnik and Associates are clearly enlightening and useful to the masses. As an ecologist, paying attention and observing reality makes a lot of sense to me, as does the LARGE size of the data sets that Blasnik used as a base for his conclusions. I heard someone say that green bling is not what’s necessary to move this market along; the main weapons in this war for a greener planet are knowledge, ability, caulk guns and cans of foam.

One Comment

  1. Michael Blasnik | | #1

    clarification
    Thanks for covering my talk. I'd just like to clarify something -- almost everywhere that you say "underestimate" you should be saying overestimate and vice-versa. The same thing for lower and higher. The models over-estimate the energy losses associated with uninsulated walls and attics and leaky buildings. Many audit software applications also often underestimate the efficiency of existing heating equipment (which again leads to over-estimated savings). The consistent result is that energy savings are over-estimated for pretty much every energy saving measure and the primary reasons are poor algorithms and poor assumptions.

    Also, the way to deal with IAQ when air sealing homes is to install mechanical ventilation -- perhaps as simple as a better control on an existing bath exhaust fan. I did say that air sealing is not necessarily very cost-effective in tighter homes -- in part because the air leakage reduction will be smaller but also because the need for mechanical ventilation may offset much of the savings. Air sealing is very cost-effective for older leaky homes in cold climates -- mostly in the Northeast, Midwest, and Mid-Atlantic regions.

    One more thing -- the relationship between energy usage and savings from retrofit programs in cold climates is better approximated as about 33% of the usage above 600 therms/year -- not as just a straight 15% savings. In leaky older homes that lack wall insulation, measured retrofit savings of 30%-50% are fairly common. The measured savings from many retrofit programs are strongly related to how often they treat homes that are poorly insulated and leaky.

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