Moisture in attic - Inadequate ventilation, I assume

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Old 02-09-07, 06:41 PM
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Moisture in attic - Inadequate ventilation, I assume

This past summer I had my roof replaced. The roofer replaced the old roof caps with a ridge vent, and then cut in a number of soffit vents. Unfortunately, I think that not nearly enough soffit vents were cut. On the front of the house, directly in the middle and running half way to each end is a gable, so they cut 2 small soffit vents on each side of it, along with probably 10 in the back.

Fast forward to this winter, I went in the attic last week and noticed a ton of ice crystals formed on the underside of the sheathing. I attributed it to the severe cold we had at the time, and the sudden onset. I have approximately 6" of blown insulation and 12" of pink on top of that over 90% of the attic, other than a small portion with a floor used for storage, and that has pink where I can put it.

I thought perhaps it was a heat loss due to a cathedral ceiling, that has three walls shared with the attic and only covered in R19. I added R13 all the way around, but when I was in one corner I noticed that the moisture was really heavy, to the point where I had a couple of drips of water on the vapor barrier. I should mention that I have ice dams on about 20ft of roofline, but I had attributed that to the heavy ice storms we had gotten earlier this year and no days warm enough to melt it yet. Well, I looked closer and could see ice build up on the inside of the soffit. .

Now, I do not have dams on the back of the house, where all the soffit vents are, only on the two sections with only two vents each. I can only assume that this is the issue. Am I on the rigth track?

Also, I still have two gable vents, one powered with a thermostat that isn't kicking on, of course. Should I close those up now that there is a soffit/ridge system?
 
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Old 02-09-07, 07:06 PM
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Every roof in cold climates will get some frost, it's pretty inevitable. Cathedral ceilings are usually worse because they are closer to the sheathing which concentrates the amount of humidity in the attic. (less attic space).

You are probably right about the soffit intake. You could add additional soffit intake, but make sure the rafter bays are not blocked over the top of the exterior walls in those locations. Perhaps you realize the most common method is to install a styrofoam baffle against the roof sheathing above the exterior wall, then stuff insulation underneath it.

Ice dams most often begin due to the close proximity between your exterior wall's top plate and the roof sheathing. When insulation completely fills this area, it keeps both the wall and the roof sheathing warm, which causes ice to form (and build) on the shingles. To reduce that effect, you can place styrofoam baffles in EVERY rafter bay and place insulation below the baffle. this will allow a measure of cold air to circulate below the sheathing which, in theory, should keep your sheathing colder, which in turn should help prevent icing. In theory. If your rafters are 2x4 or 2x6, that's not much space. Heat rises. An alternative idea might be to install thick ISO foam under the baffle, then tuck a little insulation under that to insulate the top plate. Rigid ISO foam is a better air blocker and thus has a better R value per inch, compared to fiberglass.

You mentioned "vapor barrier" in your reply, and hopefully your fiberglass insulation does not have a vapor barrier facing up. This came up in a topic back in December: http://forum.doityourself.com/showthread.php?t=286880

As far as the gable vents are concerned, yes, they should definately be removed or at least blocked and sealed up from the interior. Gable vents, when used in conjunction with ridge and soffit vents, short circuit the intended flow of air from bottom to top. They can reduce the amount of air that is drawn up through the soffits which reduces the amount of ventilaton that occurs in the lower areas of the attic.
 
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Old 02-09-07, 07:14 PM
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Good catch on the vapor barrier mention. I had actually pulled out the pink insulation so I could lay down plywood to add the insulation around the cathedral walls.

As for the baffles, are they necessary? I do not have them, but the insulation stops where the soffit starts and they are not blocked.

How much moisture is normal? It's obvious only in this section, nowhere else in the house.
 
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Old 02-09-07, 07:43 PM
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The benefit of the baffles is that it allows you to insulate the top plate, which is what causes most ice dams. If your insulation does not cover the top plate (can't tell from the way you worded it?) that's not good. If it is insulated and the rafter bays are not blocked, perhaps all you need to do is add additional soffit venting.

It's hard to say how much is normal. Usually you'll get some frost on the sheathing, similar to what's on your car's windshield in the morning- just a light frost. It is usually light enough to just cause a drip here or there, which evaporates without causing any damage. If there is so MUCH frost that when it melts, you have a stream of liquid water running down the rafters or sheathing, causing wet spots on ceilings or running down exterior walls and into windows (I've seen this on a home with a cathedral ceiling) then you've got a problem... and that problem is likely due to a lack of warm-side ceiling vapor barrier, or unsealed ceiling light cans more than anything else- ventilation or insulation.
 
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Old 02-11-07, 01:29 PM
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It's definately not that bad, not at all. There were 2-3 drops of water that i saw.

My insulation does go to the top plate, but not over the soffits.
 
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