All that heat from your 'heat soaked' attic and roof is leaching into your living space by heating up your ceiling needlessly, sending that AC on when it does not need to be. If your attic is properly vented and your insulation is modern and does not get as heat soaked as the old style insulation, you are already saving about as much as you can. This article is for those who want to understand the how and why of attic heat soak and how it effects your energy bill.
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 whatever it is that is 'full up' with heat will take a lot longer to cool off from ambient cooler temps, because the properties of the insulation are such that heat cannot easily escape.
Modern fiberglass insulation, for example, 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. While little research on the subject is available to the public (probably because the consensus is considered settled: that fiberglass insulation is better than the old kinds and should be used), the basic concept is that 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 properly. 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 have 30%- or more- unneeded cooling costs.
'But wait,' you say in your head, 'doesn't having ventilation in the attic just remove the heat pillow I need for the winter?' The answer is 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 structure from rot!
An attic that has less than one square foot of venting per three hundred 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 non-foldable ventilation (NFVA) area of the vents.
So how do you 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.
It gets up to or over 140 degrees during the summer in an attic and as hot as 170 on the roof itself. In basic non- literal terms, that's about a ton of hot air just above you.
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 going 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. Happily, SMUD of California eliminated tier charging Jan 1, 2017. But if you do get charged extra in your second tier, you could easily be spending another $50/mo on wasted energy.
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 sq.ft. 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, 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 twice as fast, and you're looking at $15k easily, which is new car territory.
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).
Last, let's make sure we know first what a 'poorly ventilated' house really is.
What makes a house poorly ventilated?
Two things: the ventilation of the house itself, and the ventilation and insulation of the attic.
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. If this is not done daily, and your house fan does not bring in outside air consider your house improperly ventilated by definition. Yes, it's work turning on those fans, getting off the couch and pushing a couple buttons.
Now for the attic: During the daytime, the attic heats far faster than the rest of the house due mostly to solar radiation on the shingles. So what can a homeowner do to avoid this most basic cause of house heat? The answer is not an easy one: Light colored shingles will help but will still get heat soaked. Metal roofs will reflect a lot of solar radiation. Having a tree block the light, or to a lesser extent, install solar panels. Solar panels will help but seldom do they cover the entire roof and if they're too small the shingles will still become heat soaked by their neighboring sunny shingles. And since both trees and solar panels can cost thousands, why not start with the basics and make sure your attic is properly ventilated? The cost will almost certainly be lower, and without the risk of tree limbs falling on your neighbors car during some windy night!