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Old 06-10-2010, 12:20 AM   #1
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intake venting between rafters


I had a ridge vent installed and now I would like to add more wire mesh intake vents that go in between the rafters. Mostly, there is 2x4 blocking between the rafters. Every 4th is a vent.

Can I take out the blocking and replace with screen? If I did that, would I have to stabilize in between the rafters? Or, could I cut a large area out of the 2x4's for a screen?

Any suggestions appreciated on how I can increase my intake venting!

thx
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Old 06-10-2010, 05:48 AM   #2
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intake venting between rafters


Can I take out the blocking and replace with screen?

Yes. But there are better options available. Here's one:

http://www.cor-a-vent.com/s400.cfm

If I did that, would I have to stabilize in between the rafters?

No, the sheathing will keep them from twisting. The blocking was to close the hole.
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Old 06-10-2010, 10:58 AM   #3
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intake venting between rafters


thx for the clarification and the link - seeyou!

rob
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Old 06-10-2010, 08:48 PM   #4
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intake venting between rafters


The solid blocking is required to prevent rotation of the rafters and ensure shear-flow of the roof deck as per building codes, but you may "V" notch the blocks for air venting. (Or drill them which greatly limits the net free venting area).

Be safe, Gary
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Clothes taking longer to dry?
Clean the dryer screen in HOT water if using fabric softener sheets.
They leave a residue that impedes air-flow, costing you money.
Clean the ducting in the last six months? 17,000 dryer fires annually!
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Old 06-10-2010, 09:20 PM   #5
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intake venting between rafters


Quote:
Originally Posted by GBR in WA;454306[B
]The solid blocking is required to prevent rotation of the rafters and ensure shear-flow of the roof deck as per building codes,[/B] but you may "V" notch the blocks for air venting. (Or drill them which greatly limits the net free venting area).

Be safe, Gary

Thanks - I should have researched that better. The open cornice structures I'm familiar with locally have no structural blocking at the wall plane, but rather a 1x frieze that was notched slightly at the bottom of the rafters with 1x blocking attached to it. It's common to see two layers of sheathing outside the wall plane - Beaded tongue & groove notched into the rafters overlaid with what ever the primary sheathing is.
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Old 06-11-2010, 12:51 AM   #6
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intake venting between rafters


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Originally Posted by GBR in WA View Post
The solid blocking is required to prevent rotation of the rafters and ensure shear-flow of the roof deck as per building codes, but you may "V" notch the blocks for air venting. (Or drill them which greatly limits the net free venting area).

Be safe, Gary
Hi Gary - By shear-flow, do you mean down-ward pressure on the roof that could cause it to flex? With the V-notch idea, do you mean an upside down "V" that goes up about 2/3 or so into the blocking? And maybe put in 2 or 3 over the length of the block?

thx
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Old 06-11-2010, 06:24 AM   #7
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intake venting between rafters


Quote:
Originally Posted by GBR in WA View Post
The solid blocking is required to prevent rotation of the rafters and ensure shear-flow of the roof deck as per building codes, but you may "V" notch the blocks for air venting. (Or drill them which greatly limits the net free venting area).

Be safe, Gary

I've gone back and looked at numerous open cornice details. In some cases, there is structural 2x blocking between the rafter tails. But it is in plane with the wall sheathing and when it exists, it is typically of the same size as the rafter material. This leaves a "breathing' space above it due to the roof slope.

What I was referring to and I believe the OP was referring to was the trim piece that is typically installed to fill the hole between the rafters and would be outside of the blocking.

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Originally Posted by gwald View Post
Hi Gary - By shear-flow, do you mean down-ward pressure on the roof that could cause it to flex? With the V-notch idea, do you mean an upside down "V" that goes up about 2/3 or so into the blocking? And maybe put in 2 or 3 over the length of the block?

thx
Another option is to install round louver vents through the "blocking". But this will be pretty labor intensive if you've got a trim piece in front of 2x blocking.

A little more description or a pic would help. From what I can ascertain, there are at least 3 scenarios that could exist: 2x blocking that is also the trim, 1x trim with no blocking, and 1x trim over 2x blocking.
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Old 06-11-2010, 10:54 AM   #8
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intake venting between rafters


Cu - My rafters are 2x4 with 2x4 blocking in between. There is gutter on the edge of the rafters that acts as a fascia. There is nothing else. It's as you described:
In some cases, there is structural 2x blocking between the rafter tails. But it is in plane with the wall sheathing and when it exists, it is typically of the same size as the rafter material. This leaves a "breathing' space above it due to the roof slope.

I'm not sure what you mean by "breathing" space, unless you remove the blocking, etc.


My goal is to get as much intake ventilation as possible. I don't mind if it's hard, because I think it will be worth it.

I appreciate your ideas.

thx

Last edited by gwald; 06-11-2010 at 10:57 AM.
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Old 06-11-2010, 11:13 AM   #9
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intake venting between rafters


Check these out:
http://ventmastersstore.com/shop/index.php?cPath=23
Will require a lot of boring and don't offer a great deal of free air.

My concern is that having only 2x4 rafters, the insulation will be packed up against the blocking.

Are the rafters 2x4 or just the tails that are exposed outside?

I think I would look at the Cor-a-vent product closer. You could remove the blocking one at a time, install the Cor-a-vent and re-install the blocking a little further out if you were able to remove it un-damaged. Replace with new if not. Then, while the bay is open, air chutes could be installed to make sure theer's a clear shot for the vent air.

Beat's crawling on your belly like a reptile in insulation in a hot attic.

Quote:
Originally Posted by gwald View Post
.

I'm not sure what you mean by "breathing" space, unless you remove the blocking, etc.

In your case, the structural and cosmetic aspects of the board between the rafters has been taken care of in one piece. Locally, I typically see a 2x4 block between 2x6 rafters with a cosmetic 1x actually closing off the bay.
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Old 06-11-2010, 12:13 PM   #10
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intake venting between rafters


The rafters extend all the way from the ridge to the top plate and extend out about 2 feet, which makes up the eaves. That's it, rafters and blocking in between.

I'm not crazy about another 2x4 sitting out there. Maybe the answer will be a lot of those round vents.

thx
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Old 06-11-2010, 09:34 PM   #11
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intake venting between rafters


Ideally, you would want at least ---- 9 sq.in. of soffit venting per lineal foot: http://www.airvent.com/homeowner/pro...it-specs.shtml

To achieve this in the button round vents would require 8 of the C-25’s (2-1/2” hole) with a sliver of wood between each in every 22-1/2” bay.


The notching was for 2x6 rafters and larger where you had room and a closed soffit. To install the Coravent would lose your shear in the roof which hardly anyone pays attention to anyway....



Be safe, Gary
__________________
Clothes taking longer to dry?
Clean the dryer screen in HOT water if using fabric softener sheets.
They leave a residue that impedes air-flow, costing you money.
Clean the ducting in the last six months? 17,000 dryer fires annually!
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Old 06-11-2010, 11:49 PM   #12
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intake venting between rafters


GBR - What do you think about cutting a 2.5" tall rectangle with a jigsaw and then nail a mesh screen over it. Seems like ventilation would be better and cheaper since you don't have to buy so many of those C-25s. Probably easier as well.

thx,
rob
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Old 06-12-2010, 04:54 AM   #13
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intake venting between rafters


Quote:
Originally Posted by gwald View Post
GBR - What do you think about cutting a 2.5" tall rectangle with a jigsaw and then nail a mesh screen over it. Seems like ventilation would be better and cheaper since you don't have to buy so many of those C-25s. Probably easier as well.

thx,
rob
What type of insulation do you have and how much? Since the rafters are only 2x4, it's likely the insulation is packed against the blocking unless it's woefully inadequate.

This: http://www.cor-a-vent.com/in-vent.cfm

or similar might be a more logical route to try. You don't give your location, so I don't know if you're in snow country or not. I don't have any experience with this type product, but you're going to be blocked by insulation if your attic has any in it at all.

Another answer would be to add a series of static vents just above the insulation.

Quote:
Originally Posted by GBR in WA View Post
Ideally, you would want at least ---- 9 sq.in. of soffit venting per lineal foot: http://www.airvent.com/homeowner/pro...it-specs.shtml
I'm curious how you ascertained that from the info given. We haven't been told the attic size or whether it's a hip or gable roof. We'd need that info to determine how many vents of a certain size per bay, wouldn't we?
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Old 06-12-2010, 01:27 PM   #14
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intake venting between rafters


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Originally Posted by seeyou View Post
What type of insulation do you have and how much?
This is SF Bay area, so winter is not a problem. Very temperate here.
I have the intake problem on 2 different attics, and the big one is blown in and works well! I've cleared away insulation from the vents, plus I could put baffles in. It's the small attic that I'm focusing on because it has NO exhaust vents! NONE. So not only is it a bizillion degrees in there on a hot day, but the roof intersects the wall half way up to 2 of the rooms and it will radiate heat from the attic, through the wall all night on a hot day. The small attic doesn't actually have a ridge vent (the large one does), but will soon have something like it. I'm going to install a vent, just like a ridge vent, but for wall/roof intersection. Since I'm putting in the new exhaust, I want to beef up the intake too. This is the one:
Flash FilterVent



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The roof is in really good shape, and I need to do a retrofit type of product.
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Old 06-12-2010, 03:16 PM   #15
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intake venting between rafters


"I'm curious how you ascertained that from the info given. We haven't been told the attic size or whether it's a hip or gable roof. We'd need that info to determine how many vents of a certain size per bay, wouldn't we?" ---- Based on the article stated, 18 NFVA per foot exhaust and 18 NFVA for intake (soffits) divided by 2= one side--9 NFVA 18 at the top, whether ridge or hip, 9 at the bottom, each side for a balanced system. The first chart explains 9 for "continuous", other chart 18 for ridge, based on 150 min.NFVA by code and better vent manufacturer recommendations as brought out in the article.

Leave a portion of the blocks at the ends for strength, use 1/4" hardware cloth for screen.

Be safe, Gary
__________________
Clothes taking longer to dry?
Clean the dryer screen in HOT water if using fabric softener sheets.
They leave a residue that impedes air-flow, costing you money.
Clean the ducting in the last six months? 17,000 dryer fires annually!
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