attic insulation

Old 03-21-01, 02:37 PM
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Red face

My electric bill approaches $600 dollars in mid-summer on a 2500 sq ft house. And I run my AC (2 units) at about 80 degrees. Yes, I've cleaned the grills in the air handling units and had the units cleaned and checked. So I've started an insulation/sealing project. My question:

I have heard it helps greatly to insulate under the roof in the attic and I've heard that will help very little. Which is true?

I have the baffles and I'm ready to start, but before I "waste" money I thought I'd ask for some more opinions or, hopefully, facts. By the way I have 3 fans in the attic. One roof fan and one at each end of the structure. They are set at about 90 degrees.

Any help on the insulation question or the entire project in general is appreciated.

by the way: Insulation projects stink, heh?


Old 03-21-01, 10:31 PM
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Location: Arlington, WA
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First, determine what is the minimum amount of attic insulation required where you live. Call your local bldg. dept. and they will tell you that. They will express it in "R" value. Next, find out how much insulation you already have. Poke your head in the attic and measure the depth. Once you have those two bits of info, post it in this forum and we can determine if additional insulation will be beneficial for you or not. With 2 HVAC units on a 2500 sq. ft. house, I'm assuming you live in an area where it gets pretty warm (AZ, NV, No. CA, etc,)
Old 03-22-01, 01:20 PM
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"I have heard it helps greatly to insulate under the roof in the attic and I've heard that will help very little. Which is true?"

Depends on what you mean by under the roof. The floor of the roof and ceiling of the uppermost floor needs to be insulated. The top of the attic or the underside of the roof does not need to be insulated unless you want to heat and cool the attic.

Typically you want to have R-38 or about 12" of blown or batt fiberglass insulation in the attic. R-19 would be about 6" of insulation. The larger the R value, the less transference of heat/cool.

Since you mention you already have baffles I am guessing you are planning on putting insulation on the roof rafters. That would be helpful if you plan on using the attic space as living space and heat/cool that space. If you do not want to heat/cool the attic, then a more effective place to put insulation would be the floor of the attic.

Basically you want to build a shell of insulation around the areas you want to heat/cool. If you move the insulation out to the roof rafters, the HVAC system will heat and cool all the areas till it reaches the insulation barrier.

Additionally since you have fans in the attic, I assume it is attic storage space and not living space. You might look into installing ridge vents. This will allow hot air in the attic to vent naturally without the assistence of fans. By ensuring the existing soffits are vented and installing a ridge vent, you will have constant change of air in the attic. As the hot air rises, it will draw air in the soffits and out the ridge vent. This will cool the attic space.

Remember that cool air falls and hot rises. Since you have two units, you would want the larger more efficent AC unit to service the upstairs. It will generate the air that will fall to the lower floor. The lower unit will have less work load. You would want just the opposite arrangement for your heating units.

For a longer term solution, look into broad leaf shade trees. You can plant fast growing trees that will provide shade during the hot months, and then drop their leaves to allow sun in during the colder months.
Old 03-22-01, 06:11 PM
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Well, I can't really agree that insulation projects stink.... I'm an estimator for an insulation company who has worked her way up from batting.

Insulation in your attic will definitely help to keep your heating and cooling bills down.

R30 is a good R-value to go for. The difference in efficiency between R30 and R38 isn't enough to justify it. It does, however, depend on local building codes.

"Under the roof" confuses me a little..... I'm hoping that you mean "in the attic", which is actually the "floor" of the attic.....

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