Plastic vapor barrier needed? new attic insualtion

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  #1  
Old 11-07-15, 05:58 AM
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Plastic vapor barrier needed? new attic insualtion

Live in the northern part of IL - VERY cold
I am redoing my attic insulation
Currently there is almost nothing up there but what I do have is a shredded type of fiberglass insulation only about 2-3 thick

I am about to have fiberglass insulation blown in & I am doing all the sealing prep work myself before hand.

Since I am moving the old insulation to do crack sealing with spray foam this would be the time to add a 6mil plastic vapor barrier

Is the plastic vapor barrier under the blown in insulation necessary?

Thanks
 
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  #2  
Old 11-07-15, 07:07 AM
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Since the application of a plastic vapour barrier from the top side is impossible to seal correctly, you are better of focusing on the air sealing as you are. The next best thing you can do to achieve an acceptable vapour barrier is to apply two coats of oil paint (or primer) on the interior walls and ceiling.
 
  #3  
Old 11-07-15, 07:11 AM
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Desirable in very cold climates and may be required by code. Installing one at this point is difficult, either just covering the bottom of each cavity or running the plastic up and over every rafter. If you do an excellent job of air sealing, including the top of every wall, then the need for a vapor barrier drops dramatically. With good air sealing, you can shift the function of the vapor barrier to the paint below. All paints will slow the vapor flow, but there are some actually recommended for this purpose.

An other consideration is, how humid you keep your home in the winter. If you crank up a humidifier then you need more protection to keep that moisture out of the attic. Along with that, you should also review your attic ventilation, soffits, ridge, gable, or other.

Note, can foam is easy, but I like a good caulking for most air dealing. And, where sealing vertical air passages you should use a fire rated material.

Link below is on air sealing.
http://www.efficiencyvermont.com/ste...ide_062507.pdf

Bud
 
  #4  
Old 11-07-15, 07:29 AM
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Bud - The insulation seems to be fairly easily moved & I have to move in order to do some new electrical & spray foam sealing

My plan was to put down strips of 6 mil vapor barrier 20ft x 6 ft - overlapping, stapling around each joist, & taping then adding the insulation over it

But before I went through all the trouble wanted to make sure that it was the "correct" method - I was reading many conflicting thoughts about vapor barriers
 
  #5  
Old 11-07-15, 11:51 AM
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What you are proposing will work, just keep it tight as any gaps encourage air circulation which increases thermal conduction (heat loss).

For the record, the thinking in regards to vapor barriers has moved away from them with only the far north and deep south still being advised. It is now the air sealing that is recognized as the primary concern, both for moisture and energy efficiency.

How is the ventilation up there? Be sure to add baffles with tails that keep the insulation away from the bottom of the roof and keep the incoming air from blowing on the end of the insulation. The tail that blocks wind washing is especially important with blown fiberglass because it is so light, there have been cases where the first few feet of insulation has been blown away from the exterior walls.

Food for thought, but cellulose is another consideration for blown in insulation. Code is a minimum so don't hesitate to go above that amount.

Bud
 
  #6  
Old 11-08-15, 05:41 AM
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Finished installing half the baffle vents already, other half going in today.

I have several fan looking vents at the gable tops but I am not sure what they are called because I didn't put them in. I am going to be installing some new light conduit before all the crack sealing. Then the vapor barrier, with your suggestion extremely tight to the joists.

Finally the fiberglass blown in insulation - not a fan of the cellous because of chemicals it is treated with & possible mold issues if it gets wet. I know they say it's really not an issue but makes me uneasy. My goal is to blow in an R44-49 with that much insulation I want to make sure my prep is completely right because I NEVER want to be doing this all again any time soon! You have been a tremendous help thanks!
 
  #7  
Old 11-08-15, 09:11 AM
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Take pictures of everything you would want some future buyer to know about. Once buried no one can tell how much air sealing you did. Going above the code minimum for your area (R-38) is good.

The jury is still out on a lot of the new methods and materials. I'm old and old school and fiberglass has been used in attics for generations and the new fiberglass is better than the old.

"I have several fan looking vents at the gable tops " if those are actual exhaust fans you should no longer need them. With reasonable ventilation and a super job on air sealing and insulation there should be no moisture or ice dam problems.

Bud
 
  #8  
Old 11-12-15, 12:57 PM
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I would not put a VB in a ceiling. It needs to be able to let the moisture out of the house.
 
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