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Old 07-18-2009, 09:11 PM   #1
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Attic ventilation


I've been reading up on this subject as I want to install baffles to prevent open up better airflow from the soffits. Everywhere I read mentions soffit vents and how to install them, but arn't soffits full of holes to act as venting already? Or are these normally covered in plywood or something? I never noticed when I went in the attic for a different job.

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Old 07-18-2009, 09:32 PM   #2
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Attic ventilation


A soffit is only the area that is covered under the rafter area to the house
It can be solid or vented
A soffit vent can be a strip vent, vinyl sheets w/venting holes, or even round/square vents






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Old 07-18-2009, 09:46 PM   #3
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Attic ventilation


Hmm never seen soffits like that, guess that might be on older houses? The ones I see everywhere including my house are like this: http://img.archiexpo.com/images_ae/p...ffit-11976.jpg So if I have those do they also act as vents?
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Old 07-19-2009, 06:58 AM   #4
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Attic ventilation


You have perforated soffit material. It is pretty much the norm in new construction as well as most homes where eaves trough and fascia, etc have been up graded. Vents are normally installed in plywood older style soffits.
It is not uncommon to find this in place covering the old plywood reducing the ventilation benefit or to find a retrofit where soffit material was replaced but there were no baffles added to help the air flow up between the rafters to the rooftop. Baffles are recommended in at least every third rafter space. Do not remove the insulation covering the top plate of your outer wall
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Old 07-19-2009, 05:00 PM   #5
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Attic ventilation


I just had a better look at it looks like they indeed just added the peforated soffit to plywood. I can't really see it from inside the house (ceiling is opened up so did not need to go in attic) but I put my arm in with a hammer and felt it hit plywood instead of metal/plastic.

What is the best way to go about venting these with no access from inside? Here is a picture of how it is setup.

I'm thinking of removing the soffits from outside then just making holes with a drill and jig saw then readd the soffits, is this a good way of doing it?
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Old 07-19-2009, 05:32 PM   #6
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Attic ventilation


Sounds good, though I would use my 5-1/2" recip. saw, cutting a 4" continuous slot in the ply, 1" away from the fascia board. The farther away from the house, the better. Wind can bring snow and rain into the attic if holes are closer to the house, as it follows the vertical building up. Add the lower pressures in the attic to help suck it in. Don't forget to re-nail the ply, if needed. Notice the balanced or slightly more soffit vents per ridge vents here: http://books.google.com/books?id=Z8a...#PRA1-PA616,M1
Be safe, G
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Old 07-19-2009, 06:03 PM   #7
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Attic ventilation


IRC 2000 has a rule for ventilation openings that even roofers were surprised to learn about.
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Old 07-19-2009, 09:30 PM   #8
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Attic ventilation


What would that be, Yoyizit? And what is the code number? Be safe, G
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Old 07-19-2009, 09:39 PM   #9
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Quote:
Originally Posted by GBAR in WA View Post
What would that be, Yoyizit? And what is the code number? Be safe, G
This has the right code number but the text is not quite the same as my book. The vapor barrier has to have a certain permeability, in perms.

"R806.2 Minimum Area. The total net free ventilating area shall not be less than 1 to 150 of the area
of the space ventilated except that the total area is permitted to be reduced to 1 to 300,
provided at least 50 percent and not more than 80 percent of the required ventilating area is
provided by ventilators located at least 3 feet above eave or cornice vents with the
balance of the required ventilation provided by eave or cornice vents. As an alternative,
the net free cross ventilation are may be reduced to 1 to 300 when a vapor barrier is
installed on the warm side of the ceiling. "

Here's a better one
http://www.chinoaz.net/dev_services/...entilation.pdf

Last edited by Yoyizit; 07-19-2009 at 09:41 PM.
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Old 07-21-2009, 12:58 PM   #10
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The 1/300 rule was mentioned in the article on page 604. Page 615 talked about a balance or close to balance for the vent locations. Maybe I should have mentioned to read the whole article, even backing up some pages. This from older research. Thank you for bringing that up.

Be safe, G
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Old 07-21-2009, 10:52 PM   #11
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Attic ventilation


Quote:
Originally Posted by GBAR in WA View Post
Sounds good, though I would use my 5-1/2" recip. saw, cutting a 4" continuous slot in the ply, 1" away from the fascia board. The farther away from the house, the better. Wind can bring snow and rain into the attic if holes are closer to the house, as it follows the vertical building up. Add the lower pressures in the attic to help suck it in. Don't forget to re-nail the ply, if needed. Notice the balanced or slightly more soffit vents per ridge vents here: http://books.google.com/books?id=Z8a...#PRA1-PA616,M1
Be safe, G
Is this what you mean? (remove perforated fascia, cut plywood, then reinstall the it back)

Think I will make this a project of mine as I have mold in my attic and I am starting to wonder if pour ventilation was the cause. It looks like whatever the dampness problem was is gone, but to be safe maybe I should fix the venting issue.

I'll also buy and install baffles as I do not have any and the insulation is in the way in most spots. Would putting baffles at every 3 joists be sufficient?

Also should I avoid venting where windows are, so I'm not just circulating air from inside into the attic? (or vise versa)
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Attic ventilation-soffit-venting.jpg  

Last edited by Red Squirrel; 07-21-2009 at 10:54 PM.
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Old 07-22-2009, 12:52 AM   #12
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Attic ventilation


I would remove fascia and then remove ALL plywood soffit. It needs all it can get!! and EVERY rafter space baffled!! and top it off with ridge vent!!
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Old 07-22-2009, 05:59 AM   #13
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Attic ventilation


You can over vent at the top with no gain so either use a ridge vent or box type vents but not both. Check the squaure inch rating of your current rooftop vents. There is a correct ratio and it should not be exceeded or it will negatively effect the flow of air

I think Ontario new home code is 1 baffle per 3 spaces. That is what the insulation contractors are recommending in KW area. If your home has had insulation added you may still be able to fish the baffles above it into the joist space. In my 4/12 pitch roof I did this with a 6' piece of pvc pipe to hold the insulation down slightly, then let it rise up to hold the baffle for stapling.

Be careful to leave as much insulation fully overlapping the top plate as you can otherwise an exposed top plate could radiate cold air down to the corners of your ceiling, causing condensation and mold.

Save your receipts all this counts toward the 15% Federal home improvement tax credit for expense after the first $1000
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Old 07-22-2009, 04:02 PM   #14
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Yes, the dampness will come back if you don't ventilate. As per article: find the NFVA of your existing perforated or replace with continuous, double angle venting strips. Balance that to the ridge vent, don't mix other exhaust types. (Close off gable or turbine vents) Cut the ply close to the fascia (to prevent the positive air pressure near the house), lining the cuts up with the width you need divided by the total NFVA vents required, half for soffit, half for ridge. Continuous is 9 NFVA per foot, about 2-3/4" wide plus flanges. If your material is rated the same, your slot needs to be only as wide. If rating is different, say 4, the you need ply cut enough for 2 of your existing channels wide, with ply cut accordingly. Don't cut so much ply to make the siding soffit sag where it shouldn't. I would baffle every bay. Here is another site to figure it out: http://www.airvent.com/homeowner/pro...it-specs.shtml Be safe, G
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Old 07-22-2009, 08:51 PM   #15
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Attic ventilation


Red Squirrel- -Are you stating that the homebuilder placed perforated vinyl soffit over the plywood soffit, without venting the plywood soffit at all? I would not doubt it, as the vinyl was probably used for an cosmetic reason, rather than a practical venting reason. I think I would go for removing the perforated vinyl soffit, cutting venting into the plywood, and re-installing the vinyl. It is a lot of work, but will be worth it for you. Good Luck, David

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