Thread: blow in cellulose or take down plaster and use fiberglass insulation?

1. Junior Member
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Dec 2013
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New Hampshire
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Re: blow in cellulose or take down plaster and use fiberglass insulation?

Keith, as to the math of how to get a 50% reduction of fuel by insulating walls. Von-steuben only said the walls were uninsulated to begin with not the entire house. Walls are often the last place to be insulated. If the baseline is a house with a well insulated attic, decent windows, and the major air leaks sealed up a 50% fuel drop shouldn't be that much of a surprise for blowing cellulose in uninsulated walls.

2. Senior Member Rank 2
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Tennessee
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Re: blow in cellulose or take down plaster and use fiberglass insulation?

The math still does not work. Uninsulated wall have an R-value of R-4 to R-5. This is due to having two surfaces with some value of insulation and air trapped between them. The studs add to the R-value because they are themselves good for about an R-4.4 and make up about 10% or more of the wall.

If you fill the walls with cellulose, the cellulose itself is about an R-11, but because of the studs making up 10% or more of the wall and lateral conduction of heat by the two surfaces, the actual R-value of the whole wall drops to about R-10. So the best you could hope for is a 50% reduction of the heat lost through the walls. For a total heat loss to be reduced by 50% and the walls totally responsible for that, then there would have to be absolutely zero loss through the attic/roof, windows, doors, floor and infiltration.

Adding insulation to an uninsulated ceiling is far more effective than adding insulation to an uninsulated wall. An uninsulated ceiling has an initial R-1. Adding just R-19 reduces heat loss through the ceiling by about 95%. Going from uninsulated to R-30 reduces the heat loss by about 97%. Going over R-30 is past the point of diminishing returns where the ROI (return on investment) is not justified. Bear in mind that the typical uninsulated house losses about 40% of its heat through the ceiling so adding R-19 to the attic means a savings of about 38% (.95x.4)

Please do not misunderstand me on this. I do believe that even though insulating the walls does not give as good a return as insulting an uninsulated ceiling/attic, it still is worth doing and will pay for itself on a reasonable horizon. But doing just the walls will not cut your heating bills by 50%, even if the rest of the house is well insulated, weatherstripped and energy efficient windows are installed. But when the other stuff is done, then doing the walls is a very good idea and it will save some money. I just don't like people being disappointed when the savings aren't as big as they thought.
Last edited by keith3267; 02-05-2014 at 02:59 PM.

3. Junior Member
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Feb 2014
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southeast washington
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Re: blow in cellulose or take down plaster and use fiberglass insulation?

We had cellulose blown in our 1928 bungalow walls and had a significant improvement although nowhere 50%. While looking into the costs we discovered that our local utility services provided financial rebates or credits which greatly reduced our out-of- pocket expenses. Also the insulating contractor was very helpful in pointing out potential tax credits we might have otherwise overlooked.

4. Junior Member
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Feb 2014
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Rhode Island
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Re: blow in cellulose or take down plaster and use fiberglass insulation?

Some older wiring is only sized for open air meaning it will overheat if insulated. It is a good idea to consult an electrician before blowing in insulation.

5. dj1
Senior Member Rank 2
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SoCal
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Re: blow in cellulose or take down plaster and use fiberglass insulation?

Originally Posted by durrock
Some older wiring is only sized for open air meaning it will overheat if insulated. It is a good idea to consult an electrician before blowing in insulation.
Excellent point to consider.

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