Humidity in Attic
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zbwmy said:
i get into attics every week and i can say without hesitation that ridge vents are almost useless. they are hot as hell in the summer. if i had a roofer out id have him cover it up and install the unpowered "turbine" type vents
you may need two however as they wont move as much as a powered type
then again they cost nothing to operate and the motor cant fail! you also really need to move the bath vent outlets to (preferably just through) the roofline under an open cap
you may have a ventilation problem, but the moisture is the heart of it
your attic shouldnt be close to 80% rh even with a poor ridge vent
Your problem is very familiar to me. Let me explain:
Two and a half years ago, we had a new roof put on after a hail storm. The original sheet metal ridge vent was replaced with a plain Cobra vent, and we thought all was well.
In early December, I went into the attic for some Christmas stuff, and found the sheathing dripping wet (never happened before for 20+ years). Called the contractor, who maintained he'd used the same vent for years with no problems. By spring, we had a serious mold problem, and called him back. He took a look and said it was moisture from a separated bathroom duct he found, which he 'fixed' (for more $$). Had Servpro out to get rid of the mold (more $$$, insurance wouldn't cover). Thought all was well.
Next December, more wet sheathing. Called another contractor, who checked the bathroom ducts (2) and found them in horrible condition (crushed and full of holes). Had them replaced and hard-piped to soffits, and closed the soffit vents for 4-feet around to prevent back-drafts (more $$, insurance covered 'moisture damage', and mold was minimal because we got on it early). Put a temperature and humidity sensor in the attic (which later indicated summer attic temperatures as high as 150-160F), and thought all was well.
And this December? Yup. Wet again.
Did some digging online, and found some recent research that says:
o Ridge vents do not work because warm air rises, as is widely believed;
o Ridge vents work because of wind;
o Ridge vents without side baffles do not work, because the wind goes right through them;
o Ridge vents with side baffles work because the wind is forced over them, making a vacuum at the vent openings, and pulling the air out.
This is why our original baffled ridge vents worked, and the baffle-less Cobra vents do not.
We recently replaced the Cobra vent (yet more $$) with a baffled ridge vent, which is slowly drying the attic (and contents) from 30-40% moisture meter readings to 20-25% readings. Am hoping to get the moisture down to 16%, which is when mold doesn't grow.
My question is: what winter moisture and summer temperature levels should one expect for a well-ventilated attic.
I hope some of this is helpful to you all.
Two and a half years ago, we had a new roof put on after a hail storm. The original sheet metal ridge vent was replaced with a plain Cobra vent, and we thought all was well.
In early December, I went into the attic for some Christmas stuff, and found the sheathing dripping wet (never happened before for 20+ years). Called the contractor, who maintained he'd used the same vent for years with no problems. By spring, we had a serious mold problem, and called him back. He took a look and said it was moisture from a separated bathroom duct he found, which he 'fixed' (for more $$). Had Servpro out to get rid of the mold (more $$$, insurance wouldn't cover). Thought all was well.
Next December, more wet sheathing. Called another contractor, who checked the bathroom ducts (2) and found them in horrible condition (crushed and full of holes). Had them replaced and hard-piped to soffits, and closed the soffit vents for 4-feet around to prevent back-drafts (more $$, insurance covered 'moisture damage', and mold was minimal because we got on it early). Put a temperature and humidity sensor in the attic (which later indicated summer attic temperatures as high as 150-160F), and thought all was well.
And this December? Yup. Wet again.
Did some digging online, and found some recent research that says:
o Ridge vents do not work because warm air rises, as is widely believed;
o Ridge vents work because of wind;
o Ridge vents without side baffles do not work, because the wind goes right through them;
o Ridge vents with side baffles work because the wind is forced over them, making a vacuum at the vent openings, and pulling the air out.
This is why our original baffled ridge vents worked, and the baffle-less Cobra vents do not.
We recently replaced the Cobra vent (yet more $$) with a baffled ridge vent, which is slowly drying the attic (and contents) from 30-40% moisture meter readings to 20-25% readings. Am hoping to get the moisture down to 16%, which is when mold doesn't grow.
My question is: what winter moisture and summer temperature levels should one expect for a well-ventilated attic.
I hope some of this is helpful to you all.
Humidity in Attic Part 2
Just a followup to my previous post:
In about a month after replacing the cobra vent, the attic RH went from a more or less steady 85% to between 60 and 68% (depending on ambient temperature, wind, and RH), the sheathing moisture went from 30-40% (lower on south side) to 16 to 22%. No new mold can be seen.
I think I have the problem licked.
Just a followup to my previous post:
In about a month after replacing the cobra vent, the attic RH went from a more or less steady 85% to between 60 and 68% (depending on ambient temperature, wind, and RH), the sheathing moisture went from 30-40% (lower on south side) to 16 to 22%. No new mold can be seen.
I think I have the problem licked.
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Interesting post.
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Hi,
Found this old thread while diagnosing a similar issue in my own home. I have GAF cobra vent on my roof that was replaced 4 years ago. Recently had really bad condensation on the underlayment after a cold and snowy spell. Was your cobra vent like a nylon string type thing. Almost like a Brillo pad or dish scrubber? I’ve had two different people in and both questioned that saying that it was very thick and didn’t seem to let much air in.
Thanks!
Jeff
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