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Old 04-05-2010, 12:26 PM   #1
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Proper Balanced Ventilation?


When re-roofing this weekend I pulled out the old power vent and installed cobra rigid vent 3. I want to make sure I install soffit vents in a way that will provide proper balance to the ventilation system.

There is 44' of open ridge vent (actually cut out from the roof, it's not cut all the way to the edge, despite being installed end-to-end). This yields 5.5SF of free ventilation at the ridge, so I am aiming to have at least that much at the soffits, ignoring the 4SF or so that are provided by the gable vents.

My existing soffit vents are woefully inadequate, only providing less than 2F of ventilated space. They will be replaced, and my question is as to how to do that.

About 1/4 of the home has cathedral ceilings, and they are insulated in a way that does not provide for ventilation (the bats pretty much fill the space, and the soffit vents are only between certain rafters, not all of them).

The ridge-line runs east-west, and the cathedral ceiling is on the Southeast quarter of the house. My plan was to provide evenly spaced soffit vents on the north eve, and large vents on the Southwest eve, roughly balancing the ventilation between north and south, even thought the south would be poorly ventilated on the eastern side.

Is this a good plan?

In the meantime I have removed one 8'x10" piece of Soffit from the north side of the home, in the center of the span to provide for temporary ventilation. Is this OK for a couple of weeks?

Last question: the 24-year old three tab feltless roof did not show any signs of blistering, etc with the old power vent arrangement, will my GAF timberline roof with 15# felt be more or less susceptible to heat damage? (Color is about the same, maybe a shade lighter)

Thanks to everyone here who provided advice going into this project. I learned a lot and had confidence guiding the installers through the process.

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Old 04-05-2010, 02:07 PM   #2
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Proper Balanced Ventilation?


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About 1/4 of the home has cathedral ceilings, and they are insulated in a way that does not provide for ventilation (the bats pretty much fill the space, and the soffit vents are only between certain rafters, not all of them).
Start by adding continuous soffit venting to the cathedral section. They might draw enough to overcome the insulation issue. On the other hand, they might not, but without being vented eave to ridge, you will have trouble here.
As a guideline, I use 1 square foot of venting per 300 sq ft of attic, with one third of the venting being placed in the top third of the roof.
You may need to cross strap the roof under the sheathing the next time you have to re-shingle to provide better ventilation in the cathedral section.

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Old 04-05-2010, 02:19 PM   #3
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Proper Balanced Ventilation?


Ok, will do... should I try to beat down the batts with a very long stick?
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Old 04-05-2010, 04:03 PM   #4
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Proper Balanced Ventilation?


I have a similar situation where the cathedraled edges of my second floor (about 2-3 ft) was filled in with blown in insulation. The soffit vents were boarded over. There is a ridge vent in the peak and a power gable vent. My question: Is it worth it to open up that section of ceiling, remove the blown in, open up the soffit vents place the rigid spacers then fiberglass craft face before replacing the ceilings. Or is the power gable vent adaquate. I had roofing estimates and the roof replaced last fall but could not get two roofing comapnies to agree on what is best. I realized that all that insulation wicks heat from the heated space and contributes to ice jams but I am very diligent about raking the snow off the edges to prevent ice buildup.
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Old 04-05-2010, 04:07 PM   #5
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Proper Balanced Ventilation?


With blown in you might be able to suck out the insulation with a strong shop vac and a long rigd tube (i'm thinking PVC). This is not professional advice, but it might keep your ceiling in place?
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Old 04-05-2010, 11:35 PM   #6
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Proper Balanced Ventilation?


I considered vacuuming but that still leaves me with now way to install the new fiberglass and rigid soffit spacer plus, you should see the length of nails the roofers used. I thought they were overkill but they insisted they were necessary to comply with the manufacturere's wind rating.
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Old 04-06-2010, 04:29 AM   #7
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Proper Balanced Ventilation?


Try this. Remove the blown insulation with a shop vac using 2" pvc as an extension. Then cut some 1" poly-isocyanurate (R-7 per inch) about 3" wider than the rafter spacing. Cut 3/4" on one surface to allow you to bend it over doubling the depth at each side. This gives you this 3/4" piece as a spacer which is placed along the sheathing giving you protection from the blown insulation (which you will place back) as well as maintaining the air space needed for ventilation. I hope I explained that okay.
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Old 04-21-2010, 02:42 PM   #8
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Proper Balanced Ventilation?


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Originally Posted by Bob Mariani View Post
Try this. Remove the blown insulation with a shop vac using 2" pvc as an extension. Then cut some 1" poly-isocyanurate (R-7 per inch) about 3" wider than the rafter spacing. Cut 3/4" on one surface to allow you to bend it over doubling the depth at each side. This gives you this 3/4" piece as a spacer which is placed along the sheathing giving you protection from the blown insulation (which you will place back) as well as maintaining the air space needed for ventilation. I hope I explained that okay.
Echase... We recently introduced a new intake ventilation solution called Cobra FasciaFlow intake vent. It combines the form of a fascia board with the function of an intake vent. It has a true/full 9 sq. in. of NFA which matches up well when using on the front/back of the property along with the Cobra 3 at the ridge (18 sq. in. of NFA). If you're interested, check out our website for more information http://www.gaf.com/Roofing/Residenti...aFlow-Vent.asp

Thanks and best of luck with your project!
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Old 04-21-2010, 04:44 PM   #9
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Proper Balanced Ventilation?


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Originally Posted by GAF Materials View Post
Echase... We recently introduced a new intake ventilation solution called Cobra FasciaFlow intake vent. It combines the form of a fascia board with the function of an intake vent. It has a true/full 9 sq. in. of NFA which matches up well when using on the front/back of the property along with the Cobra 3 at the ridge (18 sq. in. of NFA). If you're interested, check out our website for more information http://www.gaf.com/Roofing/Residenti...aFlow-Vent.asp

Thanks and best of luck with your project!
Two comments on this idea.
1) OP may have the same issue of improper insulation covering this area.
2) installation by a Diyer is somewhat troublesome and more work than needed as a retrofit.

But a great solution for short eaves where there is no room for soffit vents.

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