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Adding sheet insulation across joists with cellulose in between joists

Adding sheet insulation across joists with cellulose in between joists


  #1  
Old 08-24-10, 05:47 PM
W
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Adding sheet insulation across joists with cellulose in between joists

Hello,

Last winter, I experienced heavy ice dams on the north-facing side of our 1930's tudor with a hip roof. To prevent this from happening again (and preventing us from parking in our driveway), I was going to: 1) Add ice melting wires on this side of the house, 2) Increase the insulation in the areas of the attic where the floor joists are exposed.

It looks like one of the previous owners had blown-in cellulose shot in between the joists. However, it is not very deep and is evidently not doing a whole lot of good. Recommended attic R-values for my area are 49 in the attic. I was going to lay sheets of unfaced fiberglass sheet or batt insulation across the joists as far into the eaves as I could go. I was browsing around, and I read that you shouldn't lay sheet insulation over blown-cellulose. I'm assuming that this means not to lay it directly over top of the cellulose - in other words, I can still lay across the joists? Also, since the cellulose does not make it to the top of the joists, will the resulting air gap between the cellulose and sheets significantly reduce R-values and/or create vapor issues (I do know to use unfaced sheets).

Thank you for any help you can provide!
Jeremy
 
  #2  
Old 08-25-10, 03:29 AM
B
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Hi Jeremy and welcome to the forum.
Ice dams are caused by too much heat and too little ventilation. The excess heat comes from three sources, solar, which on the north side shouldn't be the problem, insulation as mentioned, and air leakage. The biggest of these in your situation would be the air leakage. Before you add the extra insulation you need to air seal. I will add a couple of related links below.

Before adding batts across the joists, the cavities should be full. Leaving an air gap would reduce the effevticeness of the insulation. Adding fiberglass over cellulose should not be a problem.

The links below will talk about ventilation, but the current and future insulation should be prevented from blocking the necessary air flow from the soffits up to roof vents or a ridge vent.

With the air leakage sealed, more insulation with good ventilation, your ice dam problem should be gone. As an extra benefit, your savings on heat will pay for your efforts in just a few years.

http://www.efficiencyvermont.com/ste...ide_062507.pdf
http://www.crrel.usace.army.mil/libr.../MP02-5778.pdf

Good luck
Bud
 
  #3  
Old 08-25-10, 07:24 AM
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Bud, thanks for the information and the articles. Upon looking in my attic again, it looks like the cellulose was blown very far into the eaves, right up to the soffit. I didn't check to see if the soffit vent(s) was covered or not.

One thing I wanted to clarify in your response. You are saying that it would be okay for me to lay the unfaced batts directly on top of the blow cellulose to fill in the rest of the space between the joists? Just wanted to make sure so I can estimate the amount of materials needed.

Thanks again!
Jeremy
 
  #4  
Old 08-25-10, 07:52 AM
B
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As long as the batts are unfaced you will not be forming an unwanted air barrier or vapor barrier. What the statements you were referring to about not putting fiberglass over cellulose might have been referring to would be the, not continuing the air sealing properties of the cellulose. The cellulose does a better job of slowing air flow than the fiberglass. But fiberglass is inexpensive, relatively easy to install and has been so in 80% of the homes in America. So, despite its downfalls, air passes through it, it can work. More is better than less. Fill those cavities up to full and then lay the rest across the top. R-60 is not a bad number, but the 49 with air sealing will be fine. The nice part about attics is you can come back and add more if needed.

In the eave area, work in some baffels that will leave a minimum of a 1" gap for air flow. Calculate your net vent area and be sure you have 1 sq ft of vent for every 150 sq ft of attic floor. If you go crazy and get that attic floor well air sealed and you have a vapor barrier, the vent area drops to one sq. ft. per 300 sq ft of attic floor. Distribute that vent area half high and half low.

Bud
 
 

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