It’s perhaps that time of month again to look at that power bill. It’s perhaps summer, and your AC has been going, and going. It would sure be nice to not have to run that air conditioner for so long every day, but it’s hot and that’s all there is to it.
Or is it?
When looking at power use, it’s important to know just how huge of a factor your cooling is putting into that bill, since it is often the biggest power user in the summer when temps go up. Having more vents and better insulation can have a startling impact on savings when implemented in a house that is ventilation deficient.
There are a couple things that can drive your AC bill up, and they both pertain to how well your house can expel heat that has built up during the day in the attic. For those people having problems getting rid of the heat in their house well after sunset, they may need not look further than up to find the problem.
For those who face the problem night after night with the house struggling to get as cool as the outside at night, consider doing two things, especially since power utilities widely charge more for peak hours when you need it the most:
1: See what kind of insulation you have, being safe getting up there (it’s a job that calls for high mobility).
2: Calculate how much net free venting area your attic vents have. You need 1 square feet per 150 square feet of attic space of net-free ventilation area (NFVA accounts for the screen/mesh of the vent).
Attics underneath a sunny roof will get hot in the summer. There’s no avoiding that. But ensuring your house is well equipped to vent that air out as quickly as possible after the sun goes down can and will save you money on cooling. A house with adequate venting but wrong insulation is also not optimized for cooling, and vice versa.
All that heat from your ‘heat soaked’ attic and roof are leaching into your living space by heating up your ceiling needlessly, sending that AC on when it does not need to be. If you have ‘blown in cellulose’ or vermiculite or other type of insulation that relies on sheer mass, your attic is going to be heat soaked in the hot summer.
So, what is heat soak, first and foremost?
Heat soak is a term we use that’s regularly used and reserved for car turbochargers. It basically means ‘saturated with heat’ meaning what is ‘full up’ with heat will take a lot longer to cool off from ambient cooler temps. This happens in attics because of the insulation used: because the properties of the insulation are such that heat cannot easily escape.
For a good insulation take modern fiberglass insulation. Fiberglass, for example, with little mass and other properties, has very little heat soak as opposed to the older vermiculite and rock wool insulation that is good for winter conditions but terrible for summer cooling bills. The consensus: fiberglass insulation is better than the old kinds and should be used. Conductive heat transfer is one reason, another is the flammability of cellulose insulation (basically just ground up paper) and others. Fiberglass will simply melt, while other types may burn like no tomorrow, which is a prime reason to upgrade to fiberglass now.
Back to the basic concept of heat soak. The more mass a material has, the more it can soak up heat.
Your old insulation is like a heat sponge and will continue to warm your house well into the night if the heat has no way to escape the attic quickly. You’ll be running that AC overtime, which will drive your energy bill into the next rate tier (where- in California at least- the cost per kilowatt nearly doubles) AND your AC unit will wear out faster, meaning a multi-thousand-dollar bill that could have been staved off for years possible if only our attic was better at heat management.
How much more is your AC on than it needs to be? We believe an improperly ventilated and insulated house can easily have 30% – or more- unneeded cooling costs, depending on the severity of the problem.
‘But wait,’ you say in your head, ‘doesn’t having ventilation in the attic just remove the heat pillow I need for the winter?’ It seems to many no, since your attic insulation is above the ceiling, and your heater is made to blow into your home, not the attic. If your insulation is sufficient, the attic temperature is fine to be near ambient outside temperature, which is actually desirable so as to prevent condensation and roof rot. Another concern about having a hot attic in the winter is the warm air creating an ice dam as described here, which leads to roof leaks and, if untreated, the destruction of the house’s habitability from rot!
An attic that has less than one square foot of venting per 150 square feet of attic is considered poorly ventilated. However, some professionals insist on that number being much greater, especially for desert homes and places known to not get a breeze. Also, important to remember: due to the designs of vents which include grating, it is not enough to simply measure the dimensions of the openings of your vents. You need the actual net free ventilation (NFVA) area of the vents.
Collect the information that will calculate how much more you will save. Either through empirical gathering of data from your house as you upgrade your insulation and venting, or by calculating the percentage of time your AC is on needlessly during the morning and night hours. Look at your bill and ask yourself what you think, given your house size, the vents you have, and the insulation system you have, what you should be running the AC for. What is your goal? Is it attainable? Ask a specialist in your area what is realistic.
In short: The more mass a material has, the more it can soak up heat. Fiberglass insulation has less mass and some unique properties that make it ideal for insulation.
This second method is a bit more guesswork as we need to find out the number of hours of AC you use needlessly in the summer. Start with this knowledge: your AC need not be on past sundown and before several hours of morning sunlight. So, let’s say from the period of 9pm to noon the next day (a span of fifteen hours) it runs for five hours (this is a poorly insulated and ventilated house built in say, the 1950s, of which there are millions of in the U.S.).
Looking at your bill from a month of heavy AC use, determine the amount by looking at your cost per kilowatt hour and your A/C power usage (typically 3.5kw).
For a 15 cents per kilowatt cost (national average is 12 cents as of 2017, in 2001 it was 7.1 cents), let’s say your AC runs five hours unnecessarily every day with a typical 3.5kw power usage. You’re paying 52.5 cents an hour to run your AC.
5 hours extra daily is 155 hours every 31-day month, so you’re spending $81.35 extra every month in the summer, plus you may already go over into the second-tier rate of pricing. This second-tier rate of pricing varies state by state, but in the southern and valley heat of California the tier is variable and notorious.
Without tiered charging, without taking into account roof rot from poor ventilation, and not taking in AC repairs, and with the extra 5 hours of daily AC use for 4 months of the year, that’s $325.40 of money gone every year, or in the ballpark for the cost of new insulation materials ($505.37 for a 900 square foot home, here’s a handy link to help you calculate your home‘s new installation cost). Over a 30-year period (which is the typical mortgage period) with energy hovering around the given cost, you’re looking at spending $9,672 extra, or enough for a very reliable used car from a few years ago. Add in a new AC from being on and wearing out more quickly and you’re looking at $15k easily, which is new car territory. Let’s say instead of 5 hours extra per day, your attic has better insulation and venting but not quite enough: even one extra hour of AC per day could run you a couple grand over time – not including the extra wear and tear on your AC!
Having shade on the house itself is the single best way to keep your house from heating in the first place. You can also reduce the albedo of your shingles by changing to a lighter color (which slows their rate of being heat soaked) though your mileage may vary with the efficacy of that.
Talking about the house itself: just opening the windows and with a couple fans in the morning and evening can be enough to help bring down AC costs by a significant margin. Having a whole house fan that exhausts attic air out can certainly help with ventilation while taking only a fraction of the wattage of AC, though you’re still relying on energy. Wind powered ‘whirly bird wind turbines’ also provide venting, but you can’t always count on a steady breeze, so it’s best to have optimum non-powered venting to being with. You’ll be saving lots of money and hassle down the road.