new attic insulation over old

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  #1  
Old 01-12-04, 01:53 PM
jgotts
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new attic insulation over old

Greetings,

i think i need to boost the insulation in my attic as i am getting pretty serious ice dams. (i know i probably need to improve the ventilation as well). The insulation in there is some kind of blown in material and it doesn't come up to the tops of the rafters. Should i blow in more, or can i lay blanket insulation (or batting on top). i've read that i can do the latter if i lay it across the rafter so i don't compress the existing blown-in insulation.

Thanks.

-- jon
 
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  #2  
Old 01-12-04, 05:03 PM
C
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If the blown does not fill the space, filling the space is the next step in insulation. You can use batts or blown to complete the insulation. The drill on laying batts at right angles to the joists covers completing the coverage of any loss through the joists themselves. It also provides for more uniform coverage since there won't be battts stacked with a slot between them.

You would want not to leave a void under the added insulation. The insulation itself will cause some compression, but that would be the case with anything that is stacked.

Hope this helps.
 
  #3  
Old 01-13-04, 01:47 PM
jgotts
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At the risk of asking a dumb question

Chris,

Thanks for the info. A few questions by way of a follow-up:

- Why is having a gap btwn the old and new a bad thing, and if there would be one do i need to increase the blown-in stuff until it's high enough to fill that gap?

- You're saying laying the batts across (right angle to) the joists is the better way because it's possible to loose heat through the joists themselves?

- i've been told by a insulation sales person that the batts may sag in the middle between the joists and that would cause some compression, but it sounds like you don't think that would be a big deal? Indeed if there's a gap between the old and new, that would be a good thing.

Thanks again.
 
  #4  
Old 01-13-04, 08:26 PM
C
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1. The dead air space is not as good as continuous insulation.

2. That is the philosophy, in part. Laying across facilitates making a continuous layer rather than tending to try to follow the buildup between the joists.

3. Sagging and compressing the insulation is not important. All insulation will tend to compress as it is layered deeper and deeper. Problems arise when the insulation is compressed beneath flooring to a point beyond what is expected from normal accumulation. This sort of compression reduces the insulation value.

Between batts and blown insulation, you will find that blown insulation is cheaper for DIY.

Hope this helps.
 
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