Conscious Choice: Energy Fixes for Older Homes

May 2001

Energy Fixes for Older Homes

by William Hurrle

New houses these days have a tidy look, even as they are being built. Corners are square, windows are tight, and housewrap tucks the structure in like a bedtime blanket. But if you have an older home, idiosyncrasies and gaps are the rule. You probably experience higher energy related costs than new-home owners, and you may even feel a breeze here and there. You’re not stuck with this tradeoff between character and efficiency, however. There are ways to improve energy efficiency in the older home.

Remember that there is a range of "old." Houses built before 1975 have different problems than those built after. Homes built before 1950 have other characteristics, and time mixes everything up together through additions and improvements. There are truisms: Large users can be large savers. Saving energy is detailed work. Big gains are possible, but accumulating small savings is more usual.

The first step: calculate how many "miles per gallon" your house gets. The mpg for buildings is Btu/sfDD: British thermal units per square foot of heated space per degree day. A Btu raises the temperature of one pound of water one degree Fahrenheit. A heating degree day (DD) is the difference between 65 degrees and the average outside temperature. The DD total for a given period should be on your utility bill; if it is not, ask for it.

To find your house’s Btu/sfDD you need to determine your total energy use in Btus; total square foot area of the heated parts of your home (don’t forget the basement!); and the DD total for the period in question. The typical range is 4 to 8 Btu/sfDD. An efficient home will use less than 3 Btu/sfDD.

Your utility may choose to describe energy use in therms, kilowatt-hours, or gallons. But you can translate this information into Btus using the ratios in the "mpg" example below.

Calculate the efficiency of your furnace. Some fuel heats the house; some goes up the chimney. The furnace name plate might give input Btu and output Btu. Divide the input number by the output number to find your fuel efficiency. Generally, furnaces that burn with inside air and have chimneys are 50 to 60 percent efficient. If they have a fan on the exhaust flue, their efficiency may rise to 80 percent. If they have two plastic pipes, one to supply combustion air, one for exhaust (combustion is sealed from inside air) they are about 90 percent efficient.

Boilers range from 50 to 85 percent efficiency; wood burners from negative efficiencies for open fireplaces to 75 percent for EPA-approved wood stoves. Electric heaters are 100-percent efficient in the home, but whole-system efficiencies are closer to 20 percent, and less if extended back to the coal mine, the beginning of the electricity food chain.

Improving Efficiency

There are two strategies for increasing your home’s energy efficiency. Reducing demand or improving supply. On the demand side, you can reduce your purchase of fuel with more efficient end use or increase the efficiency of the appliances that supply energy. This is relatively inexpensive, and it can be done incrementally. Success in demand-side reduction means that supply side equipment can be downsized.

We can examine this principle with calculations regarding electricity, the most expensive fuel. If a kWh of electricity is a dime, the price of a million electric Btus is $29.30 (1,000,000 Btus ÷ 3,413 = 293 kWh; 293 x .10 per kWh = $29.30). The price sounds cheap but the total cost of using electricity is at least twice that when such hidden costs as health and environmental impact and government subsidies are taken into account.

You can save energy and money with existing lighting fixtures by using compact fluorescent lights, which use a third of the energy of incandescent lamps to produce equal lumens. They also last much longer and don’t produce as much heat, which saves on air conditioning. Put constant-on TVs, computer systems, etc. on power strips, and turn them off at their electrical source.

To save further, shop prudently. Among electrical appliances, Energy Star-approved products use less energy. When replacing old units, buy Energy Star products and choose the most efficient and smallest you can live with. When a home is tightened and insulation improved, when landscaping shades the south and west, when the roof is light colored, even air conditioners can be downsized. Old air conditioners have SEERs (seasonal energy efficiency ratings) of 10 or less. New ones are 12 or better. In Japan they go to 27. Even when people buy newer products, however, they often purchase air conditioners that are oversized, resulting in short cycling and little reduction of humidity. Bigger is not necessarily better.

To reduce the fuel demand of your water heater, wrap it with an insulating blanket and insulate the first five feet of hot water pipe. Replace old shower heads with low-flow shower heads that have shut off valves. These use 1.5 gallons per minute or less yet still give good showers. Use the shut off valve when you lather up. Savings can be $0.25 per shower in energy, water, and sewer charges.

Seal Tight

In older homes, heat and cold infiltration might be 40 percent of energy use. Thus, an effective way to save energy and increase comfort is to seal holes in the building shell where air bypasses insulation. Some holes are obvious: where utilities penetrate walls; around chimneys and flues; around windows, doors, and attic scuttles; and where lights go through ceilings. Pay attention when winds blow to find them.

Not all air bypasses are obvious. For example, a dropped soffit or a stud cavity open to the attic can let outside air into your house. Usually the largest bypasses are in sill box joints, the structure right on top of the foundation masonry, and the ceiling/attic. To find them, you may want to hire a blower door technician. A blower door is a variable speed, whole-house fan that fits in an air-tight jig placed in a doorway. It depressurizes the building, gets a number for the tightness of the shell, and finds air leaks. Caulk and foam is cheap.

Duct joints leak. If ducts run through unconditioned spaces (attics, crawl spaces, garages) they must be insulated and sealed — RCD #6 mastic is best; duct tape will dry out and fall off. If rooms distant from the furnace are cold, seal the duct runs. Also, be sure the return air path is adequate and open. Good returns are the secret to efficient forced air system function.

Air leaks in older homes may let in so much cold outside air that humidity is low and humidifiers are added. Newer homes leak less and are under ventilated. Either situation may result in condensation on windows — a sign of excess humidity that may also condense inside walls. The target in winter is 25 to 30 percent relative humidity to prevent condensation, and thus mold, in walls.

In older homes it is often necessary to decide where the air barrier and vapor retarder/thermal boundary will be. In a one-and-a-half story home, do you insulate the floor and knee wall or the roof? In a home with a crawl space, do you insulate the floor or the wall?

Blow dense pack cellulose insulation into walls. Fiberglass batts are less dense, don’t fill odd shaped spaces well, and don’t block air movement. Blow cellulose into attics. Fiberglass is not dense enough to stop conductive loops of very cold air that degrade its R-value. Batts gap around truss webs. Wall insulation open to moving air needs backing to prevent air scrubbing. Fiber insulation doesn’t stop the flow of air. Seal holes first.

Ventilate Right

Sealing tight, under-ventilated homes in which furnaces, hearth appliances, and water heaters burn indoor air can result in dangerous backdrafting. A clothes dryer blows out 150 cubic feet of air per minute (cfm). If only 75 cfm comes through the walls, the other 75 cfm has to come from somewhere, most likely the water heater exhaust vent. When the water heater fires, rising combustion gases can’t overcome the downflow of air pulled in by the clothes dryer so they spill into the house. Home performance contractors (blower door guys) can do the pressure diagnostics and prescribe fixes that deal with sealing and ventilating, taking into account health and building durability — this treats the whole house as a system. HERS (home energy rating system) technicians offer the high end of this service.

Inefficient older homes can leak 0.5 air changes per hour (ACH). New houses average 0.28 ACH; high performance homes, 0.1 ACH or less. Ventilation should supply 7.5 cfm per person plus 1 cfm per 100 square feet.

Systemic Problems

Residential construction is done by a score of trades. None see the house as a whole system. Roof color impacts cooling load, yet roofers don’t talk with heating, ventilating, and air conditioning (HVAC) guys. Insulators don’t do air sealing, nor will framers, electricians, or plumbers, nor will the HVAC contractor. Rascals plague the industry. Insulators fluff blown-in product with air in order to use less of it. It is piled high, at least around the scuttle, but its density is low and it settles. Furnace men say a heat exchanger is cracked, terrorizing homeowners into buying a new furnace, which is then oversized. Window salesmen present new product as the solution to high bills.

Construction is undermined by the low-bid drive to the bottom. CATNAP Construction (cheapest available technology narrowly avoiding prosecution) rules. A "code" building is the least efficient that can legally be built. All builders say they build efficient homes but few if any can tell you how many Btu/sfDD they’ll use. Some will answer they can’t tell because owner operation varies. This is true, but the shell-appliance package has a number just like a car has a mpg number. A lead-footed driver won’t achieve its potential; ditto a lead-footed householder. If the thermostat is pushed to 78 degrees and never set back at night, if lights are left on, if storm windows left off, if an old fridge grinds away in the basement to chill beer — you get the idea.

In older homes, air sealing and adding insulation to walls, sill boxes, and ceilings can improve heating energy use by 20 percent. But replacing a 50 percent efficient furnace with a 92 percenter won’t remove 42 percent from the bill. In cold climates heating is about 58 percent of energy use in inefficient homes; 36 percent in efficient ones. In newer homes health and durability issues tend to dominate. Air leaking through and around ceiling can lights condenses on roof sheathing. Mold grows, wood rots. Ditto air leaks in sill box corners. Air leaks from attached garages allow carbon monoxide (CO) into negatively pressurized basements. Then the furnace fan goes on, distributing CO into the house.

Try Renewables

After demand is reduced and supply equipment made as efficient as possible, then add renewable energy. EPA-approved wood stoves are 75 percent efficient and only pollute six grams of particulates per hour of burn. If you buy a face cord of oak for $75, it is about the same price as natural gas. However, the wood cutter will spend the $75 locally, increasing community cash flow by at least $150. A utility will immediately export 85 percent of your money out of the local economy.

You can also look into adopting solar power. Solar hot air systems are like windows without a hole-in-the-wall penalty. They are particularly welcome in homes with hydronic heat because they move air. You may want to make a small investment here; winter days are short and often cloudy. Solar hot water systems sized to give 100 percent of winter needs will produce industrial quantities of hot water in summer. Better to size for summer gain, use solar for almost all hot water from May through September, and boost with conventional heating the rest of the time for a total of 60 percent of the load. (Solar hardware should face within 20 degrees of south; no shade.) Installations that are aesthetic might ding performance, but they’ll win more acceptance from neighbors, and maybe create a neighborhood trend.

Currently, solar electricity (photovoltaic) is expensive to install, however, smaller systems connected to the power grid (no batteries) can supplement your power needs and sometimes exceed them. When you generate more electricity than you pull from the grid — especially during summer afternoons when central generation can’t keep up with demand — it makes your meter go backward as it sends your excess power back into the grid. Many utilities have buy-back programs that credit your utility account.

You can’t get more efficient than that!

MPG Example

Energy Conversion to Btus:

• Natural gas: 100,000 Btus per therm or ccf (hundred cubic feet)

• Electricity: 3,413 Btus per kilowatt hour (kWh)

• Propane or LP: 91,500 Btus per gallon

• Fuel oil #2: 138,000 Btus per gallon

• Wood: 20-23 million Btus per cord (4 ft x 4 ft x 8 ft) of dry hardwood

• Coal: 26 million Btus per ton

Degree day example: If the high temperature for the day is 30 degrees and the low is 16 degrees, the average is 23 degrees [(30 + 16) ÷ 2]. The difference is 42 (65 - 23). This is called a 42 degree day.

To determine your home’s Btu/sfDD, multiply square footage of heated space by the degree day total from your utility bill, then divide the result into the total Btus used for the same period. Btus should be adjusted for furnace efficiency.

For example, an 1,800 square-foot home with a heated basement might use 600 kWh of electricity at 90 percent efficiency (about 10 percent of electricity lights the home, the rest ends up as heat) and 450 therms of natural gas at 50 percent efficiency to power through a 1,200 degree day month (40 degree days x 30 days in the month). For this example we’ll assume the basement is also 1,800 square feet for a total of 3,600 square feet (basements are generally not included in the stated square footage of a house). Calculated using the conversions listed above:

(600 [kWh] x 3,413 [Btu/kWh] x .9 [efficiency])
+ (450 [therms] x 100,000 [Btu/therm] x .5 [efficiency])
÷ (3,600 sf x 1,200 DD)
= 5.6 Btu/sfDD.

The typical range is 4 to 8 Btu/sfDD. An efficient home will use less than 3 Btu/sfDD.

Resources

No Regrets Remodeling: Creating a Comfortable Healthy Home that Saves Energy, by the editors of Home Energy magazine, 222 pages, 1997, $19.95.

Residential Energy: Cost Savings and Comfort for Existing Buildings, John Krigger, 2nd edition, 280 pages, 1996, $35.00.

Web sites: www.buildingscience.com, www.healthhouse.org.

Shelter Supply has a catalog with ideas and products that are helpful, 800-762-8399.

Energy Federation Inc., has catalogs for compact fluorescent lights and fixtures, weatherization and ventilation products, 800-876-0660.

Efficient appliances

Windows

Green building

Solar: www.mrea.org, www.aaasolar.com

Chicago/Illinois resources

Illinois Department of Commerce and Community Affairs (the home of the state energy and weatherization bureaus). Energy rating, John Morely, 217-785-2007; renewable energy, Rex Buhrmeister, 217-557-1925.

A list of HERS technicians in northern Illinois, Brenda Teaster, 312-413-4959.

The Center for Neighborhood Technology has a Community Energy Corp., 773-486-7600.

There are low-income and other subsidies/rewards for eligible householders who improve their energy use or install renewable energy hardware.

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