Consensation on Rafter Insulation Channels

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  #1  
Old 01-27-11, 05:06 PM
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Condensation on Rafter Insulation Channels

I just discovered I have a bit of condensation building up on the bottom of the rafter insulation channels installed in my ceiling.

The moisture is between the plastic ProVents and the fiberglass insulation.

I have a knee-wall crawl space on the sides of the top floor and an attic space located above the ceiling collar beams. Between the knee-wall area and the top attic I installed ProVents to provide ventilation between the soffits connected to the knee-wall area and the ridge vent on top.

I then slid R-21 (paper-faced, paper down towards sheetrock) between the ProVent and sheetrock from attic to crawl space. I cut several small slots in the ProVent to provide ventilation between fiberglass and roof deck.

Take a look at the pics and let me know what you think. What could be causing this moisture??




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Last edited by RockRiver; 01-27-11 at 05:39 PM.
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  #2  
Old 01-27-11, 09:07 PM
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You have warm moist air coming up "chimneys" from the conditioned living space. The chimneys are formed by improperly capped/sealed stud walls and other openings. Before all that insulation went in, all openings from the conditioned space should have been closed off with expanding foam, tape, plywood, or plastic air barrier.. I used to have some pictures around to illustrate the problem and the cure. If I can find them, I'll link or post them here.
 
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Old 01-27-11, 09:18 PM
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Not exactly what I was thinking of, but probably far better: Home Sealing & Air Leakage
 
  #4  
Old 01-28-11, 06:55 AM
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tldoug has it correct. The amount of ventilation you need is affected by the amount of air leakage from the house into the attic and as stated, the best solution is to seal up all of the air leaks first. To go forward from here, you should look for the biggest leaks, seal them, then make sure the venting is working.

When knee walls are present, I frequently find plumbing vents passing unsealed through them. Look at the roof from outside to locate them and then go inside to see if they are leaking air from the basement.

Bud
 
  #5  
Old 01-28-11, 11:00 AM
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I'll pull up all the floor insulation and check for warm air leaks. I know there are two vent stacks and a bathroom vent going through the knee-wall. I cut the floor insulation around the pipes, but did not seal around them 100%. Hopefully this will help!
 
  #6  
Old 01-28-11, 11:52 AM
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It is best not to use or rely on fiberglass insulation to block air leakage. It may slow it down, but by design, air moves easily through it.

Blocking air leaks in the basement can also reduce the flow, and often they are easier to access. Attic is best though.

Bud
 
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