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Old 06-24-2008, 05:05 PM   #1
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Knee wall Ventilation advice needed


Our 1947-ish cape cod style home had the upstairs finished by previous owner. It gets really hot up ther in the summer, and I suspect the superheated space behind the knee walls has something to do with it. Also I've observed moisture in this space. However, just found out the bathroom was venting into this space, which may have been the cause for the moisture.
These spaces do not have free airflow to the ridge because they are packed with insulation.
So for ventilaiton behind the finished and insulated knee walls I'm having THE EDGE installed on the roof line to create intake venting. However, I am having problems figuring out how to get adequate venting to the ridge for outtake, should have thought this through a bit more. Would you add side vents out both gable sides high up on the knee wall space to act as an outtake vent? Or perhaps feed 3/4 pvc tubing between the insulation and the roof in order to create space for air circulation up to the ridge (every 16")? Thoughts?
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Old 06-24-2008, 05:32 PM   #2
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Knee wall Ventilation advice needed


I never heard of using tubing, pvc or any other, i assume that would work.
Fiberglas baffles should be used.
Creating airflow beetween each rafter/truss is the ideal way to go, but if need be i'm sure doing every other would work, it will surely work better than it is now.
Using gable vents as exhaust, should be the last resort (no other option).

Trapped heat can and will create some moisture/dampness in areas like that.

Creating the airflow to the ridge is a must, but having proper exhaust vents up there is equally important, no matter if you have pot vents or ridge vent, make sure theres enough of it.
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Old 06-24-2008, 11:33 PM   #3
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Knee wall Ventilation advice needed


I'm assuming you don't have adequate access to remove what insulation you need to to install baffles in the rafter bays and re-install the insulation?
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Old 06-24-2008, 11:46 PM   #4
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Knee wall Ventilation advice needed


Is the insulation sealed in by drywall?

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Old 06-25-2008, 05:42 AM   #5
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Knee wall Ventilation advice needed


This may be where the roof is headed. Other than having a tin roof, it was the same as you described. http://www.rooferscoffeeshop.com/sho...6&file=979&s=0 The rafter ends rotted off on both ends. We closed it back off with ridge-vent and it's still standing, but when it gets re-roofed, rafters and sheathing will need replacement.

When we opened the top for venting, we used 16' boards to knock out the insulation to salvage the roof somewhat. Depending on access, you could do the same and install foam channels and insulation. Doubt it though.
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Old 06-25-2008, 12:29 PM   #6
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Knee wall Ventilation advice needed


The insulation is sealed in by plaster in the finished area. the unfinished attic has a combination of Bats with Cellulose over the top. The angled portion of he roof is over 6' so insulation baffles won't fit unless I double them up, which I could do. I'm hesitant to pull out the current insulation as I'm unsure if I can easily get more in. Am I causing more harm than good if I have intake vent without proper outtake?
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Old 06-25-2008, 05:36 PM   #7
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Knee wall Ventilation advice needed


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Originally Posted by N8E View Post
The insulation is sealed in by plaster in the finished area. the unfinished attic has a combination of Bats with Cellulose over the top. The angled portion of he roof is over 6' so insulation baffles won't fit unless I double them up, which I could do. I'm hesitant to pull out the current insulation as I'm unsure if I can easily get more in. Am I causing more harm than good if I have intake vent without proper outtake?
I don't think so, per se. But I can see trouble on the horizon. I don't understand much about the venting. Ed can explain much more than I. Without being able to state all the causes and effects, I can only say that the pics I showed are NOT atypical.

"perhaps feed 3/4 pvc tubing between the insulation and the roof in order to create space for air circulation up to the ridge (every 16")? Thoughts?" Doubt that will accomplish anything.

How far, time-wise, are you away from a new roof? Could pull enough wood to vent the area.
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Old 06-26-2008, 05:57 AM   #8
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Knee wall Ventilation advice needed


Your definately going to increase the chances of moisture/dampness to occurr in that space if you have intake with out exhaust.
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Old 06-26-2008, 09:56 AM   #9
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Knee wall Ventilation advice needed


There was a woman on this forum who had the same situation as you and She and her elderly and sickly mother had to move out of the home due to mold infestation.

Do it right is the only suggestion if you want to avoid the possibility of dampness and condensation turning into a mold problem.

Even if that means removing the roof and some of the decking to gain access into the rafter bays.

You Must have 100 % continuous intake ventilation.

You Must have a channel for air to flow over the top side of the insulation of at least 1 1/2" to 2" of free clear space for proper flowage.

And, you Must have 100% continuous Ridge ventilation to allow each individual rafter bay to exhaust out the heat and humidity.

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Old 06-28-2008, 11:23 PM   #10
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Knee wall Ventilation advice needed


definitely feed the pvc tubing thru the rafter bays to achieve the ventilation,I would suggest 1 1/2-2" pipe though,I`ve suggested this to others here in the past
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