Why Can't My Air Conditioning Keep My House Cool On Hot Days?
July 2012 was the hottest month on record in the US! 90-100 degree days was almost the norm. It was almost 100 degrees here in Cleveland on Saturday 7-7. (That's hot for us Midwesterners, folks in Lousiana probably think we're wusses.) On that day most of us, including me, cowered in our houses like there was a blizzard outside. I sweated just thinking about going outdoors.
Meanwhile, was your AC cowering as well? Could your air conditioner keep up with the heat? No? Why can't your AC keep up? What the heck! Why can't it do its job? Oh the humanity! You pay the company with the service contract to come check it out every year, shouldn't it keep things cool?
Don't blame the air conditioner! The root cause of your air conditioning not keeping up on hot days is probably the hot attic, plus a lack of insulation and air sealing to keep the heat out of the house.
Fair Warning: this is an advanced post, so down the rabbit hole we go!
Here are the real causes:
1. Radiant Energy (with a side of convection and conduction)
2. Temperature Stratification & Reverse Stack Effect
3. Lack of Insulation
4. Holey Ceiling with Air Leakage
Radiant Energy, Defined (Jargon Demystified!)
Radiant energy only travels in one direction and does not need matter to transfer through, like convection (gas and fluid) and conduction (solids). The sun and fire are the best examples. Radiant energy is essentially unseen light, but you feel it when you walk out of the shade into the sun.
The sun beats on your roof, which radiates that heat into the attic. This heat is what makes your second floor uncomfortable. Here are a couple pictures of roofing temperatures, 120 and 130 degrees. 140-150 happens in poorly ventilated attics.
Temperatures peak in an attic in late afternoon or evening, when the sun has been beating on them the longest. Just in time for you to come home from work and notice the house is REALLY hot. It is also seriously dangerous to go in an attic at 4-6 PM.
These extreme temperatures are caused by the sun's radiant energy. That 130 degree air is trying to get into your house! If your air conditioner is trying to hold 70 degrees, that is a 60 degree difference it is trying to make up for with only a few inches of insulation trying to stop the heat. It's like wearing a spring jacket on a 20 below day - it just doesn't cut it!
Temperature Stratification (Jargon Demystified!)
Heat rises, so the higher you go in your house, the hotter it gets. Unless you stop the heat from coming in.
If you ever had a warehouse job, you know that when you get close to the ceiling in a Wal-Mart or factory, or any other large building, it gets hot on the ceiling, often 20-30 degrees hotter than on the floor.
It should be 2-3 degrees hotter per story. Many homes experience 10-15 degrees per story. They need building science. Once applied, here is what you get:
This is the first story of my house on an interior wall. It is 86 degrees. (I don't have air conditioning. I head for the finished basement!)
This is on the second floor of my house, right below the attic hatch, which is often a hot area. At 89 degrees, it is almost 3 degrees hotter than downstairs.
Because I know a guy who will do it cheap, I have about R-65, or about 17 inches of cellulose in my attic. Not bad for an 1835 house! This means that I only have a 3 degree rise in temperature from my first to second floor.
I have pull down stairs to access my attic. I love Attic Tent pull down stair covers, they do a fabulous job stopping air leakage. I also have a 1" foam board cover over the Attic Tent. This is the largest potential hole in my ceiling, but at 89.4 degrees it is only .4 degrees hotter than the wall below it. Temperature stratification has been whipped into submission!
That's not quite the whole story, though. A well insulated and air sealed attic will reduce temperature stratification, but why?
Reverse Stack Effect (Jargon Demystified!)
First, let's start with plain old stack effect. In the winter, heat rises in your house and exits in the attic, just like a smoke stack. In the summer, the hot air from the attic pushes into the house, letting hot, muggy air in and roasting your second floor in the process. This is called reverse stack effect and is based on the Second Law of Thermodynamics which says that heat goes to cold.
Since it is easier to understand, here is what the winter stack effect looks like. Note the air escaping through recessed lights, the attic hatch, and so forth. The air to replace it comes from the basement.
Reverse stack effect is a little trickier. While it is true that heat rises, the stronger law in play during the summer is the Second Law of Thermodynamics which says that heat goes to cold. What ends up happening is that the hot attic air (120-140 degrees) pushes into the 70 degree air conditioned house through air leaks like recessed lights, bathroom fans, attic hatches, and so forth. Because air is coming in, more air has to go out to make way for it. The cold air typically exits through your basement, like so:
So that's the theory behind your uncomfortable attic, let's dive into the practical part - how you actually fix it! There are 2 parts to the solution:
1. More insulation. (Shocking to hear from an insulation contractor, right?)
Most attics in older homes only have 2-3" of insulation like this:
This is about R-6 to R-9. The Department of Energy recommends R-49 up to R-60 for our climate zone in Northeast Ohio. This is not enough to keep the heat out of the 2nd floor!
Even if you have a newer home, until 2008 Ohio code was only R-30 (now it is R-38 here), like this:
This addition from about 2004 has R-30 fiberglass batts. Note the fact that you can see wood studs. Those only have an R-value of 6 or 8, depending on whether they are 2x6 or 2x8. They are thermal bridges, allowing a lot of heat to pass through them and not the insulation. Blown insulation does a better job here. Plus, note the bathroom fan in the foreground and the recessed light in the background (the metal hump). These are both holes in the ceiling. Which brings us to:
2. Holey Ceiling with Air Leakage
Thermal Bypass (More Jargon Demystified!)
A hole through your attic ceiling that leaks air from inside your house to the outside.
A thermal bypass can be a recessed light, plumbing stack (or poo pipe as one of my customers called it), ceiling fan, light fixture, chimney, attic hatch, open wall cavity, or any number of other things.
Air leakage is bad. It lets cold air inside in the winter and hot air inside in the summer. It causes drafts in the winter and muggy spots in the summer. Plus it allows a ton of moisture to transfer into your attic, which can cause mold. There really isn't an upside. It's just plain bad.
Put your hand by your attic hatch on a hot day, is it hot? Then heat is leaking in.
Regardless of whether your house was built in 1835 or 2012, you have air leakage in a holey ceiling.
Here are a few actual examples of air leakage.
This attic hatch actually had 3 hatches over a stairwell, the top had no proper cover, the middle that you can see below also had no cover, then there was a door at the bottom. This house had major comfort problems on the second floor, even though it had R-19 in the attic, which is better than the typical older home I look at.
This large hole around a plumbing stack (the black pipe) went all the way to the basement. It is essentially a chimney for air to move through. Note the dark fiberglass insulation, a surefire sign of air leakage. Yuck. And very typical.
This is a recessed light not rated for insulation contact (non-IC). These leak like sieves. Note the blue light coming through 3 holes in the white can. In fact, when the blower door was turned on in this house it sucked black dirt from the attic in and stained the ceiling. Non-IC lights are usually older and found in 1960-1980 homes, but they are still made and (gasp) installed today because they are $5-10 cheaper than a good one. Your new recessed lights stink too.
This is looking straight down the hole next to a chimney in a 1950s house. This hole went all the way to the basement. A chimney outside the chimney!
Conclusion: Stop Blaming Your Air Conditioner For Your Hot House!
It's just physics. Heat rises is one part - temperature stratification. The second part is reverse stack effect, or heat also goes to cold, which is what happens when hot attic air and radiant energy push into your living space through air leaks and thin insulation.
The solution isn't to call your AC guy and complain.
The solution is to air seal your attic and add more insulation.
As proof, I showed you a 3 degree difference on my 1835 house between the first and second floors due to a well insulated and air sealed attic. If my house can be fixed, yours can too.
Don't take my word for it, though, check out the testimonials from Jane D, Jason K, Susan S, and Lisa T at the bottom of my testimonials page. I constantly have customers tell me how much more comfortable their house is now.
Uncomfortable houses in the summer is a fixable problem. Stop suffering and call a good home performance guy! (Otherwise known as an insulation contractor.) You'll also save on your air conditioning bills and you may earn up to $1250 in gas company rebates!
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