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Community and Q&A

Why is our home so inefficient?

switters | Posted in Energy Efficiency and Durability on

We bought an 1,800 sq. foot house in the north Berkeley hills in November of last year. There was a small mold issue which we remediated in December and January. During that time we also did an extensive HVAC upgrade, with a 3-zone heating/cooling system, smaller/more efficient duct work, an in-line dehumidifier, and in-line air filter. (Our daughter is sensitive to mold and other airborne allergens, which is why we installed the dehumidifier and air filter.)

The house had a pre-existing, small solar array (1.7Kw). About 90% of our lights are LED, and most appliances are fairly new and energy efficient. About 60-70% of our windows are double pane; there is a very large window in the living room (about 8′ x 8′ square) that is single pane, and a few others. We have a gas oven, and electric washer and dryer. There are only 3 of us in the house, we turn the thermostat down at night and only up to 68-70 when it’s cold during the morning/evening hours, and are pretty conscious about turning lights off when we’re not in rooms.

I’ve been quite surprised to see that our energy usage is significantly higher than other similar homes around us. Here is a screenshot of the comparison from PG&E’s website:

(Note: the large spike in Jan/Feb was probably due to high amperage appliances that the mold remediation and HVAC installers used during the renovations that were happening during those months.)

I am trying to figure out where the problem lies. Some possibilities:
— Our house is poorly insulated in some areas. It’s a Cape Cod style with very steep roof and dormers. Because the roof isn’t properly vented, it isn’t possible to insulate the sloped portion of the roof without replacing it. The flat section of the second story ceiling is insulated, but with only about 60% of the thickness that is recommended. When we did the HVAC upgrades I investigated upgrading this, but there is knob & tube wiring up there and it would have to be redone at a cost of $10+k before more insulation is added, and I wasn’t ready to spend that given everything else we were spending. The basement/floor is insulated and air-sealed (foam was sprayed into all holes and gaps). The walls are not insulated, but my understanding is this doesn’t make a huge difference.
— The house is not weather-sealed
— We have a chest freezer in our laundry room
— The dehumidifier runs a lot in order to maintain our target RH of 48%. We do this to make mold growth as unlikely as possible.

Any thoughts? Our electricity bill has ranged from $150-$250, which seems really high, especially given the (albeit small) solar array and other efficiency upgrades. Thanks.

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  1. davidmeiland | | #1

    I'm curious about the three-zone HVAC: is this gas, electric, or heat pump, why the three zones, etc. Also curious if perhaps you are using mostly electric and others are using more gas, thus the PG&E data looks like you are using more. It sounds like an older, mostly-uninsulated (or under-insulated) house that will need to be upgraded wherever possible to get the usage down. And, what about ventilating instead of dehumidifying?

  2. switters | | #2

    Thanks, David.

    We went with 3 zones with the idea that it would save energy. The upstairs is one zone, my home office is another, and the kitchen/living room is a third. We don't spend any time upstairs during the day, so that zone is kept cool in winter or warm in summer. If we heat/cool the bedrooms at night, we keep the system off or reduced in my office and kitchen/living room.

    One thing I forgot to mention is that we do spend a lot of time in the house. I work from home and so does my wife.

    We definitely use more electricity than gas. Average gas bill per month is about $50-$90, depending on season.

    We have a condensing gas furnace and AC outdoor condenser. It's a top-of-the-line Carrier system with a Carrier Infinity programmable thermostat. Our dehumidifier does both dehumidification and ventilation.

    The original house was built in 1927 (only about 1,200 sq. feet) and was not insulated. The previous owners did add a bit of insulation to the flat part of the attic/roof (which is only a small part of it, given how steep the roofline is). They also did 600 sq. ft addition in 2000 which is now the kitchen and living room. That part is completely insulated (walls, floor, and ceiling).

    I agree that insulating the roof/ceiling is probably the most important thing we can do. The upstairs rooms get very hot/cold. But it would cost us at least $20k for a new roof and another $10k to rewire the attic. Was just hoping there might be another explanation or thing we could do, but that's probably just me in denial!

  3. GBA Editor
    Martin Holladay | | #3

    This is just a guess, but operating a whole-house dehumidifier can use a lot of energy. If you feel you have to operate the equipment because of special medical issues, it is what it is. Sealing air leaks in your thermal envelope will lower the cost of operating your dehumidifier.

  4. switters | | #4

    Thanks Martin. We did already seal the leaks as part of the extensive HVAC/energy upgrades.

    The dehumidifier is on a 115v circuit and draws 5 Amps. Doesn't seem like it would be too much of an energy hog?

    I guess our only options at this point would be spending the $$$$ on roof/insulation, and possibly upgrading our solar to a higher Kw system.

  5. Jon_Lawrence | | #5

    Chris - Do you notice your heater running a lot? If not, then it may be your dehumidifier as Martin pointed out. I would buy a Kill A Watt and connect it to your dehumidifier for 24 hours to see how much that is adding to your load. Next, I would do the same for the freezer and then check the on and idle draws of your other plug loads.

    That 8' x 8' single pane window is a huge energy loss. That is basically the same r value as uninsulated 8' x 8' plywood wall. I bet that window would show white hot on an infrared camera on a cold winter day.

    Depending on what the Kill A Watt shows, you might be better of buying a new picture window and addressing the source of the excess humidity before dropping $30k on the roof and attic.

  6. davidmeiland | | #6

    If your heat is gas, and your electric use is the issue, then you have to look at the individual loads as suggested above. Assuming your use is the blue line on the PG&E graph, it looks like you're normally in the range of 20 KwH per day. If the dehu is 5A 120V then it's using 600 watts, and if it runs 12 hours per day it's using roughly 7 KwH by itself, roughly a third of your total. I'm confused by your comment that your dehumidifier does both dehumidification and ventilation, does that means it's connected to your HVAC and the blower motor is running as part of the process?

    I've gotten a lot of mileage out of a simple TED 1001 electricity monitor at my place. After a short while I learned to recognize what was on at any given time, just by looking at the wattage on the display. $25 on eBay.

  7. GBA Editor
    Martin Holladay | | #7

    Good catch about the ventilation reference.

    If your equipment is introducing outdoor air as part of a "ventilation" feature, you should definitely verify how many cfm of outdoor air is being introduced to your house. If this feature hasn't been properly commissioned, you might be overventilating -- and it's extremely expensive to overventilate if you are also operating a whole-house dehumidifier.

    Any time you are dehumidifying, you want to reduce ventilation to the absolute minimum level you can tolerate.

  8. switters | | #8

    David, I have the Ultra-Aire 105H. Outdoor air is ducted to the unit via a 6-inch round duct, and it is connected to the HVAC system. With the controller I can decide how much fresh air vs. recirculated air I want. For example, I can set it to 10 min of fresh air ventilation and 50 minutes of recirculated air.

    I ordered a Kill A Watt as recommended by Jonathan and will start testing loads. Your calculation for the dehumidifier is illuminating. That will be the first thing I'll test.

  9. Andrew_C | | #9

    Can you clarify how your basement is sealed and insulated? It sound like the floor and walls are insulated, and that penetrations in the rim have been sealed with spray foam? The sealing leads to my real question: have you had an energy audit done that included leakage testing, and was this done before or after sealing the basement rim?

    Also, there's more than one Berkley; can you specify which climate zone you're in?

  10. davidmeiland | | #10

    Not sure how widespread this is, but where I live you can borrow a Kill-a-Watt from the library. You will find it useful, but it is really helpful to have some way to look at 240-volt loads, for which you need a TED or similar device.

  11. switters | | #11

    Thanks again, everyone. Martin, I'm not sure how to figure out how many CFM of outdoor air is being introduced. Do you know where I would look? I checked the dehumidifier spec sheet and it says this:

    257 CFM @ 0.0" WG
    206 CFM @ 0.2" WG
    146 CFM @ 0.4" WG

    Does that tell you anything?

  12. GBA Editor
    Martin Holladay | | #12

    The best way to measure your ventilation air flow is to hire a home energy rater -- someone equipped with tools to measure airflow.

    If you want to make your own measurements, start by reading these two articles:

    Is Your Ventilation System Working?

    Simple Methods for Measuring Air Flow

    The simplest method would be to use the garbage bag method. Start with the garbage bag fully inflated -- something that can be achieved with a swooping motion through the air -- and have someone time the deflation of the garbage bag when it is held around the outdoor air intake of your ventilation system when the fan is running. Perform the test 5 times and take an average reading.

  13. GBA Editor
    Patrick Mccombe | | #13

    One more thing nobody has mentioned. Modern high-end furnaces have blowers that will automatically adjust their speed to make up for resistance (static losses) within the ductwork. If your new ducts (you said they're smaller) are overly constricting the airflow, the furnace blower will use a lot more electricity. I'm told that return air systems are often especially screwed up.

  14. Dana1 | | #14

    115V @ 5Amps is 575 watts, or 0.575 kwh for every hour it's running. If the dehumidifier is running even 25% of the time, it would be use ~100 kwh per month, a double-digit percentage of your energy bill.

    In a Berkeley CA climate the outdoor dew point averages are low enough to keep the interior under 48% MOST of the time via ventilation alone. The summertime dew point average from June-August runs about 50F, which at 75F is a 50% relative humidity, not much of a latent load even in summer, even at substantial ventilation rates. The rest of the year the latent load against a 48% RH set point would be negative- high ventilation rates would keep the indoor air under 48% RH. Martin's comments about overventilation being costly when mechanical dehumidification is need is true, for areas with outdoor dew point averages north of 55F That's an issue for eastern half of the US, but not so much for anything west of the continental divide.

    A graph of historical dew point averages for nearby Oakland can be found near the bottom of this page:

  15. GBA Editor
    Martin Holladay | | #15

    In light of Dana's observation, one experiment would be to just unplug the dehumidifier for two months. See what that change does to your electricity bill -- and determine whether your family even needs the dehumidifier.

  16. charlie_sullivan | | #16

    I'm suspicious that the HVAC fan is a bigger energy hog than the dehumidifier. Especially, as Patrick mentioned, with the new smaller ducts. If the humidity floats at 55% without the dehumidifier, it shouldn't need to run all that much to bring the humidity down to 48%, if the infiltration and outdoor air supply are reasonably small. But the Kill-A-Watt meter for 24 hours will give you more information than our speculation.

    In the winter, the dehumidifier's heat contribution will help reduce your gas bill, so it's not such a bad thing to run it, as long as you don't try to bring the humidity down below where it operates efficiently.

  17. Expert Member
    Dana Dorsett | | #17

    The wintertime outdoor dew points in the Bay Area average about 50F. At 70F a volume of 50F dew point air has an RH of 50%, still the high end of the human healthy & comfortable range. It's practically in the "Goldilocks Zone" year round.

    The amount of heat you'd get out of dehumidifying the incoming air to 30% (the low end of the human-healthy & comfortable range) would be pretty limited, and probably not very efficient, since most dehumidifiers are dialed in for bringing ~70% RH air down to ~50% efficiently, not 50% down to 30%. There is a lot more available heat (both latent & sensible) in a 20%RH difference going from 70%-->50% starting with warm humid air than there is from 50%-->30% pulling from cooler dry-ish air.

  18. charlie_sullivan | | #18

    Dana, I'm not suggesting taking it down to 30% RH. I think the 48% that Chris is using as a setpoint is a reasonable set point given a desire for margin for a safety margin vs. heath concerns with a sensitive individual in the house. But given that there are sources of moisture in the house, I think it's more likely to float at perhaps 55% humidity than 50%. So with the set point of 48%, there's 7% of dehumidification work to do. I'm not suggesting that that can provide all the heating needed. All I'm suggesting is that the increment of heat he gets from that comes at a pretty high effective COP given the terrific L/kWh rating of his top-of-the-line dehumidifier. So there's no need to feel like that electricity consumption is a waste of energy, in the winter.

  19. Expert Member
    Dana Dorsett | | #19

    It should be pointed out that even 55% RH isn't a mold hazard or a comfort problem, and is only unhealthy for folks with dust-mite allergies. Most of us are pretty comfortable and healthy at 60% RH, which is still below the mold hazard threshold.

    Keeping the dehumidistat at 48% in winter is fine too, again, with only a miniscule latent load from the ventilation air.

  20. jeffwatson | | #20

    I agree with Patrick & Charlie regarding suspicions with the A/C fan/blower.

    In my house, I observed a huge spike in my electric bill the first summer that my new furnace had been installed. Condenser/compressor had not been changed.

    I later found out that the (cooling) blower speed was set to the max, which the user manual indicated static pressure had to be less than some pretty low pressure for it to achieve the stated energy specs. Reduced the fan speed by taking it down 2 taps & the huge spike hasn't returned since, even after more A/C usage.

    I also installed a thicker air filter to reduce pressure.

    So if you've got high pressure in your ductwork & a fan that is pushing way too hard, there is a definite energy penalty. I think they make some low-energy fan motors now but I don't think my furnace has one. After coming out of the thought that the blower fan doesn't use that much electricity, looking for a low-energy fan motor isn't a bad idea to me anymore for the next furnace.

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