Does More Wall Insulation Reduce Cooling Cost?

by Erik North on June 10, 2013

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Hey, hey…It’s warm enough so that I can get back to publishing those air conditioning articles from last summer’s backlog.

Does More Wall Insulation Reducing Cooling Costs?

A little but way less than you think. That’s the short answer. The long answer will take…a bit longer.

Adding insulation reduces heat flow. That’s what insulation does. But does insulation highly impact the heat flowing into your house during the summer and is it the most effective way to reduce your cooling bill?

Adding wall insulation to save on cooling seems to make sense, since slowing heat movement and reducing the need to cool is our goal. In northern climates, if you suggested that insulation wasn’t the best way to retain heat and reduce your heating bill, we would laugh right in your face and maybe throw a coffee at you. And with good cause. So why does what works in the north not work quite the same in the south?

The difference is in how heat moves and the climates. In the north, solid surface heat loss is the largest component of your heating bill. In cooling climates that need air conditioning, a well insulated house is definitely easier to cool. But the solid surface radiating heat in from the wall is a smaller portion of the air conditioner’s cooling load compared to attic radiant heat and sun streaming through windows.

How Heat Moves … Briefly
I always seem to need to quickly summarize this. Heat moves by radiation, conduction, and convection. Radiation is that big ball of thermonuclear plasma in the sky. The sun’s warmth on your skin is solar radiation. Conduction is heat movement between any two materials, moving from hot to cold. Remember that time you grabbed a hot frying pan handle? The heat transfer (and cause of the burn) is conduction.

The last way heat moves is convection or air transported heat movement. If you opened a window in the dead of winter, that’d be air based heat movement.

The Climate … Also Different
In Georgia, air conditioning attempts to maintain building temps around 70 F when the hottest day of the year may be 110 F. In Maine, our heating systems maintain 70 F temps in the face of -10 F temps on the coldest nights. Conductive heat loss (heat energy moving across a wall) in driven by temperature differences and heating climates have much higher average temperature differences. Meaning: if you live in the North, insulation is more important.

The one caveat here is attic insulation in the South. The summer sun traverses at a very high angle through the sky, beating down on house roofs. Attic spaces can get dramatically hotter at these times, approaching 140 – 150 F. High levels of attic insulation become important; wall and slab insulation … not so much for cooling purposes.

So Where Are You Gaining Heat?
After all this hubbub, where are you gaining all this AC straining heat from? Well, externally you have solar heat gain, air infiltration and heat transmission across surfaces. Inside you have radiant heat off equipment (computer monitors, stoves, heating equipment, televisions), heat from light bulbs and from people and pets. *Whew* And we’re not even touching sensible vs. latent heat, time delays with thermal storage, solar angles, surface opacity, etc. etc. There are a lot of factors.

That’s Complicated, Let’s Make It Simple
The sun is the huge difference between heating and cooling a house. In the north, there’s no comparable massive heat source radiating inside the house. Heat transfers through the solid surfaces and escapes by air leaks so we insulate and air seal.

The sun is an overwhelmingly stronger factor. In northern heating climates, extra solar heat helps lighten the load. In southern cooling climates, the sun’s radiation pours through the windows massively adding to the work the air conditioner must do.

What are the solutions? Minimize sunlight whenever possible. Eliminate south facing windows, add shades, awnings, buy/build house with wide eave overhangs, plant shade trees, etc.

Adding insulation definitely improves air conditioning bills. However building walls are not generally as problematic as attic spaces or windows, both of which allow in more heat.

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{ 3 comments… read them below or add one }

Eric June 10, 2013 at 11:07 pm

Do you have any experience w/ solar screens to reduce solar heat gain? I was considering putting them on my south windows in the summer, since we don’t have any nice overhangs, trees, etc. Seems like a fairly inexpensive experiment, and looks nicer than hanging towels over the windows. :)

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Erik North June 10, 2013 at 11:35 pm

Thanks for writing. Like I mentioned in the article, anything which reduces solar heat gain helps. And something that is inexpensive that also helps…that’s a win/win.

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Rick December 7, 2013 at 12:11 pm

That’s a great write up. I live in Charleston, SC and I’m in the process of getting quotes to add insulation to my attic. I started thinking “hot air rises and cold air sinks” so how does the attic insulation help my air conditioning situation.
You explained it perfectly.
Thanks

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