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Old 03-14-2012, 02:37 AM   #1
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ridge vent, gable, moisture in the attic


a couple years ago a new roof was installed. basic 1985, 1450sq ranch home, single level with soffit vents on every other 2x4, ridge vent installed all along the top of the roof leaving only the last couple feet on each side of the home non vented up top per ridge vent spec. one side of the home was re sided and the gable vent sealed, the other side was put off and we left the gable vent open. i now have rusty shingle nails poking through and traces of grey mold starting. very very minor but the fact they can grow means i have to find out why. after reading up it appears that a single gable left open can short circuit the ridge vent from working properly.

the home also has can lights which i've considered spray foaming, on top of that one of the bathroom vents was loose and partially venting to the attic.

tonight i fixed the bathroom vent and siliconed tar paper over the single gable vent

this summer i can sponge and spray microban the few mold spots

any advice? my other concern is what would happen if i had too much ridge vent and not enough soffit? before the new roof the home had 2 gable vents, one on each side and 3 canister vents.

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Old 03-14-2012, 06:07 AM   #2
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ridge vent, gable, moisture in the attic


Get up there and turn off the lights--you should be able to see light coming in through the soffit vents-

Removing the wood soffit and replacing with a fully vented aluminum one could be the answer.

Also check your new ridge vent--did they cut back the roof sheeting far enough to allow good air flow?

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Old 03-14-2012, 07:19 AM   #3
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ridge vent, gable, moisture in the attic


I'm having a similar issue that I posted on about a week ago:

Attic mold/venting questions

There's a picture of my ridge vent in there and GBR_in_WA noted that the sheeting wasn't cut back enough (4th picture down). Might be something to look at?

To add a question to yours, is it harmful to have every soffit bay clear and venting? (mine is like yours, every second bay is "open")
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Old 03-14-2012, 08:05 AM   #4
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ridge vent, gable, moisture in the attic


You should always shoot for 2:1 soffit (intake) to ridge (exhaust) ratio.

Air sealing the attic floor will help control condensation movement as well.
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Old 03-14-2012, 04:35 PM   #5
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ridge vent, gable, moisture in the attic


think i found my issue. i have a terrible intake to exhaust ratio. for some reason i was thinking every other was vented. after looking it's worse



last night i left the garage trap door open and stood above it to see if i could feel any flow. you can actually feel the air flowing up if you stand over this opening



is my ridgevent, can't remember the brand but it's the standard that woodfeathers sells here in the NW



and actual opening width



just wanted to bounce this all off of a few pros before i go adding soffit venting this summer. my idea for the next 90 days or so is to leave the garage trap open so cool dry garage air can flow up.

i also of course fixed the bathroom duct that was half pouring in the attic and blocked off the single gable vent
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Old 03-14-2012, 05:14 PM   #6
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ridge vent, gable, moisture in the attic


Air seal the floor and all breaches in the envelope.

Moisture should not be much of an issue off the garage as it is unconditioned space.

Your improperly vented bathroom vent was the primary causative force in this case. Leaving the scuttle vent to the garage only dumps VOC and noxious fumes into the attic and potentially into the living space. Neither are preferred.
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Old 03-14-2012, 05:33 PM   #7
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ridge vent, gable, moisture in the attic


Quote:
Originally Posted by Windows on Wash View Post
Air seal the floor and all breaches in the envelope.
what does this mean? the rock is taped and finished. do you mean spray foam the canister lights and whatnot?
Quote:

Moisture should not be much of an issue off the garage as it is unconditioned space.
are you saying it's a good idea to leave the garage hatch to the attic open until i fix the soffits in a couple months?
Quote:

Your improperly vented bathroom vent was the primary causative force in this case. Leaving the scuttle vent to the garage only dumps VOC and noxious fumes into the attic and potentially into the living space. Neither are preferred.
what is the scuttle vent to the garage?
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Old 03-14-2012, 06:15 PM   #8
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ridge vent, gable, moisture in the attic


The code is 1:300 ratio of vent area to ceiling area. When I say vent area I mean "Net Free Vent Area (NFVA)" which takes into consideration the mesh and whatnot. You can usually find this on the manufacturers website. About 15square inches per foot is average for a ridge vent. Ideally you would have 50/50 split of ventilation at the ridge and eaves. This should help you figure out how much area you need to open up!
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Old 03-14-2012, 08:04 PM   #9
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ridge vent, gable, moisture in the attic


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what does this mean? the rock is taped and finished. do you mean spray foam the canister lights and whatnot?

Google Air Sealing and you will come up with countless threads and recommendations.

Creating a rigid enclosure around the recessed light fixture and sealing to the floor is part of it as is top plates, wiring runs, plumbing vents, flues, etc, etc.


are you saying it's a good idea to leave the garage hatch to the attic open until i fix the soffits in a couple months?

No and that is what I was calling the scuttle vent (the garage stairs).

what is the scuttle vent to the garage?
See above.
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Old 03-14-2012, 08:48 PM   #10
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ridge vent, gable, moisture in the attic


figured that's what you were getting at, my only thought is that sealing off the garage hatch still leaves me really sort on intake, i've been slammed with work lately and realistically can't get at this till july

as you said the bathroom was probably most of it, it's just if i can feel pressure on the garage hatch, sealing it will have the ridge drawing from the path of least resistance.
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Old 03-14-2012, 09:17 PM   #11
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ridge vent, gable, moisture in the attic


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The code is 1:300 ratio of vent area to ceiling area. When I say vent area I mean "Net Free Vent Area (NFVA)" which takes into consideration the mesh and whatnot. You can usually find this on the manufacturers website. About 15square inches per foot is average for a ridge vent. Ideally you would have 50/50 split of ventilation at the ridge and eaves. This should help you figure out how much area you need to open up!
15sq/in would be a 1x15" vent on the eve.

hmm... i have roughly 2x20" vents which equals 40sq/in so ballparking would mean i would want a vent like i have on every other eve. does mean i need to add a few more but nothing spectacular.

i'm going to tap a couple brites in the trusses and tape some paper to the osb to see what the moisture does after fixing the bathroom and sealing the gable. this summer i'll have a bit more time to do floor sealing and calculate how much more soffit venting i'll need.
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Old 03-14-2012, 10:16 PM   #12
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ridge vent, gable, moisture in the attic


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figured that's what you were getting at, my only thought is that sealing off the garage hatch still leaves me really sort on intake, i've been slammed with work lately and realistically can't get at this till july

as you said the bathroom was probably most of it, it's just if i can feel pressure on the garage hatch, sealing it will have the ridge drawing from the path of least resistance.
In order for the venting to be proper, it needs to be pulling from the ridge and picking up any moisture that has diffused or otherwise moved through near the eave. That is why soffit venting needs to be unobstructed and is ideally placed at even intervals at the eave.

I will disagree, respectfully, with shazapple. The ratio should not be 1:1 on intake to exhaust because people will invariably install more exhaust than intake. It is easier to get to and easier to do. Doing this always drive stack pressures and actually negates the effectiveness of the ventilation.

Go heavier on the intake and your attic will love you for it.

That all being said, you might have enough as is once you have fixed what was a source of bulk vapor and air seal. Once you do that, you usually lessen the necessity of ventilation for moisture control.
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Old 03-15-2012, 12:46 AM   #13
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ridge vent, gable, moisture in the attic


i will do all of that as soon as i can. the sealing around canister lights and what have you. after using HVAC tape on the bathroom exhaust vents and getting the gable blocked off it feels noticeably drier up there already and it's only been 24hrs.

the only head scratcher i have with the soffit venting is how to. the soffits are nailed inside the wall so i can't get at the sinkers holding the 2x4. i could use a 2.5" hole saw and burn a bunch of 2.5" holes along the soffits. it's just if i do that, how to get the screen behind them, it's a bit too tight to crawl up to with a staple gun.

so, in your professional advice, what would you prioritize if you were me?

1. sealing the attic floor
2. adding soffit venting
3. using microban on the few mold spots
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Old 03-15-2012, 12:53 AM   #14
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ridge vent, gable, moisture in the attic


Spray the mold to start with to just kill it from the outset.

Air seal the floor next.

If the ventilation was really problematic, the house would have a huge plume of mold in it at this point.

As far as the soffit go, you can get units that are pre-screened and you just hole saw to open them up and drop them in.
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Old 03-15-2012, 06:07 AM   #15
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ridge vent, gable, moisture in the attic


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As far as the soffit go, you can get units that are pre-screened and you just hole saw to open them up and drop them in.
He's got open cornice with 2x4 rafters. The venting is through the 2x4 blocking at the wall plate. Not much off the shelf to fit that.

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