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Old 05-07-2015, 04:28 PM   #1
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Attic ventilation problem


A couple of years ago, I had an attic contractor insulate the attic with blown insulation and replaced a broken attic fan. Back in January of this year, we had a new water heater installed and found out that it was backdrafing (may have been a pre-existing condition). After a long process of elimination, we completely disconnected the attic fan, eliminating the backdraft. I was told by a contractor that I need an additional 10 mushroom vents. My house has no soffit vents. Will this fix the issue? Would the installation of the new water heater cause the backdraft?
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Old 05-07-2015, 06:01 PM   #2
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Where is the water heater located? In the attic or inside the conditioned space?

If it inside the conditioned space, that indicates (along with the removal of the fan and opting for passive ventilation) that there is too much communication between the conditioned space and the attic.

How are you planning on supplying additional intake air? More exhaust without intake will not necessarily solve the back-drafting issue.

Granted, the lack of mechanical pull will, in all likelihood, solve the issue but too much exhaust without intake, can create a similar pressure issue in the home.
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Old 05-07-2015, 06:44 PM   #3
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No, backdraft is caused mainly by powered attic exhaust ventilation, you need more supply vents; http://www.nachi.org/attic-ventilati...s-part1-25.htm

http://www.energyvanguard.com/blog-b...Are-a-Bad-Idea

Where are you located?

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Old 05-07-2015, 07:00 PM   #4
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10 mushroom vents!!
Why?
A ridge vent would vent the whole roof, less work and cost less to install.
No form of soffit vents then any form of roof venting is not going to be much good.
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Old 05-07-2015, 07:20 PM   #5
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I live in Chicago.

I only have about 3 mushroom vents and an unpowered attic fan. Water heater is in he basement. I'm out of options here.

So, a ridge vent would be best??
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Old 05-07-2015, 07:30 PM   #6
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Or, what is my best option for intake vents in this case? Soffit vents aren't an option.
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Old 05-07-2015, 10:27 PM   #7
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Quote:
Originally Posted by Coldcity80
Or, what is my best option for intake vents in this case? Soffit vents aren't an option.
Depends on the style, layout, and overhang situation.

What do you have?
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Old 05-07-2015, 10:32 PM   #8
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It's a raised ranch, mid-50's, no overhang. Just a couple of mushroom vents
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Old 05-07-2015, 10:34 PM   #9
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Quote:
Originally Posted by Windows on Wash View Post
Quote:
Originally Posted by Coldcity80
Or, what is my best option for intake vents in this case? Soffit vents aren't an option.
Depends on the style, layout, and overhang situation.

What do you have?
It's a raised ranch, mid-50's, no overhang. Just a couple of mushroom vents
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Old 05-08-2015, 04:24 AM   #10
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Any chance you could post a picture? I'm not a huge fan of mushroom vents down low at the eaves because I never like popping a bunch of holes in the roof on the low side.

There are a handful of options to allow for some eave venting.

There are some vented facia board designs and strip vents for the roof deck that all work. Those are just a couple of options.

Last edited by Windows on Wash; 05-08-2015 at 04:26 AM.
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Old 05-08-2015, 07:28 AM   #11
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I'd rather not add 10 new possible leak locations to my roof. A ridge vent should be a much better option.
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Old 05-08-2015, 08:09 PM   #12
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A ridge vent without intake doesn't solve the issue in total though.
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Old 05-08-2015, 08:20 PM   #13
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That's the issue here:

I HAVE NO INTAKE VENTS!



F'n old Chicago homes
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Old 05-08-2015, 08:30 PM   #14
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Quote:
Originally Posted by Windows on Wash View Post
A ridge vent without intake doesn't solve the issue in total though.

No and it can make it worse. If the ridge vents are starved for air from lack of intake it can cause negative static pressure within the house making the back drafting problem worse.

The best scenario is balanced soffit (or other the means of low roof intake) to ridge vent. By balanced I mean do the math. Vents have a NFVA rating that tells you how much ventilation per foot you are actually getting.

NFVA of First Soffit + NFVA other side soffit = NFVA of ridge.

This is minimum. If you can get to 60 / 40 split with more intake, some say better yet
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Old 05-09-2015, 06:42 AM   #15
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Regarding your intake vents, IMO use under shingle vents. The vents should be placed as low as possible on the roof.

Pending how deep your blow-in insulation is in relation to the vent it may require attic baffles to keep the air flow open.

With this vent system in place all other vents in the roof should be capped. And no fan.
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