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darren 01-20-2006 01:25 PM

attic condensation
A few weeks i blew i some insulation into my attic.

In my roof i have two gable vents and 2 roof vents(hoise was built in the 50). I asked around and people were saying i should be fine.

I went up into the attic today to take a few after pics for myself and noticed frost on the underside of the roof. The weird part if it is only on half of the roof, the other half has very little on the nails and that is it, the bad side has it all over the sheating and nails.

I did notice a couple of spots on the frosty side that could have used a little more insualtion, other then that it all looks good.

Any suggestions what would cause this(it is only on the one side) and any suggestions on what to do to fix it.


K2eoj 01-20-2006 01:51 PM

I'll guess the the north sloping roof has the most condensation. More vents is probably the solution. There are some new scools of thought on keeping warm moist air out of the attic space but venting the attic is the old standard solution. I would also look for any obvious leaks. There can be a number of different reasons why one house would have more humidity than another.HS.

Bonus 01-22-2006 08:35 PM

You said that half the roof was frosty, is that half side to side or top to bottom?
Make sure the intake vents (soffitt) are clear and you're not relying on the gable and cap vents for all the air you need. Some theories of venting say that gable vents and cap vents should not be used together as they 'short-circuit' the airflow, and you wind up with dead air down by the eaves.

Teetorbilt 01-22-2006 09:30 PM

To abbreviate the above posts, you may have blown insulation over the soffit vents. Check for this first.

darren 01-23-2006 11:26 AM

I only have two gable and two roof vents, and no soffit vents, i think this is where my problem lays.

Before i did the insualtion i did not notice any bad water damaged so i thought i was good, maybe i jumped the gun to early.

In the spring or summer I plan on adding soffit vents. On the front of my house half the length sticks out an extra 2 or 3 feet, so i could very easyly add soffit vents to about 20 feet of my house without any attic work. Will this be effective enough or do i have to spread them all around the house.

Thanks for your help

Bonus 01-23-2006 07:24 PM

You're certainly better off with venting the entire length, otherwise you can end up with 'dead air' pockets.

Teetorbilt 01-23-2006 08:24 PM

Darren, you neglected to state where you are. The full soffit vents suggeted by Bonus could be catastrophic in a hurricane zone.

Bonus 01-23-2006 11:26 PM

Is that right Teetor, how come? I've never even visited a hurricane area. :)

CGofMP 01-24-2006 09:51 PM

The closest I have come to a hurricane is the movie twister and THAT was about tornados, but

*speculation mode 0n*

I kinda suspect having that much venting would make the roof fill like a parachute under extreme winds. That could result in the roof being updrafted to another county :-)

*speculation mode off*

Teetorbilt 01-24-2006 09:58 PM

Too much soffit ventilation in a compression zone can pressurize the roof and literally 'blow it off'.

A semi-dramatic (but in no way impossible) short course in fluid dynamics.
The wind speed is 150 MPH. It hits the side of your house as a linear or laminate flow and the molecules are tightly packed because they are all travelling together at the same speed. They hit the side of the house and are stopped but they have to go someplace because of more coming from behind, like the people that get crushed in a panic driven event. They have to make it around or over your house and they take the path of least resistance. The ones nearest the sides go around the ones nearer the center have to turn around, go back and over the roof or go through it (soffit vents). The soffit vents are in a position where the air is being compressed and trying to turn around, this leads to turbulence which causes heat and expansion. This force is applied to your roof from the inside.

Next time you watch a show on hurricanes or tornados pay special attention to HOW the roofs come off. If it's not a gable end failure, they almost always fail near the center first and them drag the rest behind.

CGofMP 01-24-2006 10:01 PM

Dr. Tetor,

That begs the question... When ya see folks prepping for hurricanes they are layin plywood overtheir windows and doors etc.

Should they also then be closing off their soffit and or other roof vents?

Bonus 01-24-2006 10:27 PM

Fascinating, Teetor, thanks for the lesson, that makes sense.

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