Attic Insulation: Do I Need It?


Old 07-28-05, 10:43 AM
Thread Starter
Join Date: Jul 2005
Location: USA
Posts: 160
Attic Insulation: Do I Need It?

The reason I ask is because I live in south Florida. Obviously, heat loss isn't the issue. And since the concern here is keeping cool, and cool air falls, what benefit does attic insulation offer?
The house is 20 years old, with existing insulation that appears to be about two inches, maybe three, of powdery dust. I can't imagine an R value of more than 4 as it is now.
What got me to thinking of all this is the pending installation of a new central AC system. I figure if I'd dropping $5K, I ought to do my best to keep the cool air where it belongs.
So, what do you think? Add some insulation up there? And if so, am I better served with fiberglass blankets, or the blown in type? Any estimates, also, on the cost to have about 1,300sf of the stuff installed for me? Thank you!
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Old 07-28-05, 12:21 PM
Join Date: Apr 2005
Location: WA
Posts: 117
While heat in a room will flow up (convection), he also travels by conduction from hot to cold.

In this case, you've got two spaces (outside, unconditioned, and hot), and inside, conditioned, and cold. What separates those two spaces is your walls and roof.

If your house was just a box, with no attic, insulation would help your ceiling no more than it would help in your walls. Its all based on surface area and temperature difference. Insulation does nothing but help retard the conduction of heat.

Now, if your house has an attic, it is likely significantly hotter than the outside air temperature. This makes the ceiling insulation much more useful, as the temperature difference is greater.

You can pretty much always benefit from insulation, just don't go overboard. Use your zipcode to find the ideal amount (R-value) of insulation for your area.
Old 07-30-05, 05:53 AM
Thread Starter
Join Date: Jul 2005
Location: USA
Posts: 160
Thanks for the reply. And while I think I understand how heat travels, so to speak, I still must wonder if the three inches of original, 20 year old cellulose that the builder used is adequate for my location and climate. The R value for my zip code is 38, according to the government standards. But that is ridiculous. In six years of living here, my heat has never been turned on. Heck, I don't even know if the unit works.
I contacted a local insulation contractor, and they will come and blow in six inches of fiberglass on top of the existing insulation. The six inches translates to an R factor of 19. And they'll do the job for just under $400. Not a terrible sum to spend but, again, I'm trying to determine just how much of an effect this will have on my cooling bill and new AC system. I'm certainly willing to have the job done if it will pay for itself over time, so to speak.
Old 07-30-05, 09:24 AM
Join Date: Mar 2005
Location: USA
Posts: 6,126
Attic Insulation: Do I Need It?

Attic insulation is needed.

You are right that you do not care about losing heat through the ceiling to the attic. What you should be concerned with is gaining heat through the ceiling into the house. You certainly don't want some of that hot 120 to 140 degree heat in the attic warming your house do you?

The insulation keeps the ceiling cooler in the summer, preventing it from warming the air in the house. You are right that heat rises, but fans circulate the air, mixing the warm air (due to the ceiling heat loss) with the cool air from the AC. This means your AC will run more.

If the ceiling joists or truss cords are not covered with insulation and are exposed to the hot attic, they will warm up and transfer the heat through. Wood is not a wonderful insulator, but it certainly is better the the steel studs you may find. It is interesting that the tables for insulation ignore this fact that is included in the ASHRAE literature. - You are lucky you have wood.

I think more is known today than you builder (not an expert) knew 20 years ago. Also, the energy costs versus the cost to insulate are now much different (energy has gone up more than insulation cost).

Do not overinsulate, but it sounds like now is the time to upgrade. I imagine you are getting an efficient AC, so why not match the insulation. In addition to cost, you will appreciate the comfort.

Old 07-31-05, 04:09 PM
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Join Date: May 2005
Location: Tucson
Posts: 176
heat travels both ways. Remember how hot the doors of older ovens used to get? and how Mom hated to bake in the summer because it made the whole kitchen warmer? Well, you've got one king-size oven over your head in the summer, and heat wants to go where there's less heat if it can get there. With only 3" of insulation you aren't presenting that oven with much of a challenge, and your entire ceiling becomes a giant radiator.

that R-38 is there for a reason, and it's the minimum you should have.

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