# ridge vent, gable, moisture in the attic



## racebum (Mar 8, 2010)

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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## oh'mike (Sep 18, 2009)

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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## burnt03 (Sep 20, 2009)

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

http://www.diychatroom.com/f9/attic-mold-venting-questions-135944/

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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## Windows on Wash (Aug 30, 2011)

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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## racebum (Mar 8, 2010)

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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## Windows on Wash (Aug 30, 2011)

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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## racebum (Mar 8, 2010)

Windows on Wash said:


> 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?


> 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?


> 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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## shazapple (Jun 30, 2011)

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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## Windows on Wash (Aug 30, 2011)

racebum said:


> 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.
> 
> ...


See above.


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## racebum (Mar 8, 2010)

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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## racebum (Mar 8, 2010)

shazapple said:


> 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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## Windows on Wash (Aug 30, 2011)

racebum said:


> 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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## racebum (Mar 8, 2010)

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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## Windows on Wash (Aug 30, 2011)

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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## seeyou (Dec 12, 2008)

Windows on Wash said:


> 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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## seeyou (Dec 12, 2008)

racebum said:


> 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.


1st, air seal the ceiling.

2nd, as WOW advised, do some measuring and calculating and verify that you actually need more intake vent. If you do, use a reciprocating saw and remove the blocks you want to vent. Cut new blocks with vent holes and install the screen and paint them on the ground. Install them by nailing through the rafters from the other side.


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## Windows on Wash (Aug 30, 2011)

seeyou said:


> 1st, air seal the ceiling.
> 
> 2nd, as WOW advised, do some measuring and calculating and verify that you actually need more intake vent. If you do, use a reciprocating saw and remove the blocks you want to vent. Cut new blocks with vent holes and install the screen and paint them on the ground. Install them by nailing through the rafters from the other side.


Good idea right there. :thumbsup:


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## racebum (Mar 8, 2010)

spent an hour or so studying roof ventilation. correct me if i'm wrong but, 

1. the ridge vent acts as a suction device pulling air up, if the soffits are unable to supply enough air it draws from the easiest source. my loose bathroom vent being the #1 reason i have a few mold blotches, even without the fan on negative pressure was pulling it up. 

2. adding soffit venting will reduce the negative pressure in the attic and make it less likely to draw on the multiple cracks and crevices in the ceiling that could pump conditioned air into the attic. it will also lower my heat bill since i won't be sucking heat up the top

3. use bleach or some type of mold killing product to scrub spots, then fog the attic with microban. was considering using my auto paint gun with a fine tip and a lot of air pressure to shoot a couple gallons of microban on the osb. at the moment the only traces of mold are on the osb or upper joists.

and of course seal every place i possibly can along the ceiling. was thinking of buying some pink 1/2" foam insulation and duck taping together boxes to slip over the canister lights


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## seeyou (Dec 12, 2008)

racebum said:


> spent an hour or so studying roof ventilation. correct me if i'm wrong but,
> 
> 1. the ridge vent acts as a suction device pulling air up, if the soffits are unable to supply enough air it draws from the easiest source. my loose bathroom vent being the #1 reason i have a few mold blotches, even without the fan on negative pressure was pulling it up.
> 
> ...



The purple quote is kind of, but not exactly, true. Tinkering with the venting before air sealing is treating the symptom instead of the disease. The conditioned air is mostly being pushed rather than pulled. If you have a forced air heating system, the house interior will be somewhat pressurized when it's running, forcing moisture laden conditioned air into the attic. Opening and closing interior doors causes localized pressurization. Weather related pressure drops can cause conditioned air to migrate. Think of the sum of the openings through the ceiling as a partialy open window. Each hole around a wire you seal closes that open window a little.

The blue quote is a good idea. Check to see if the cans are IC (insulation contact) rated. As cheap as can lights are, it might be worthwhile to swap them for ICs if they're not and add some insulation to your attic after you air seal.


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## racebum (Mar 8, 2010)

was working on this earlier today. the single gable end vent i had was turned into an intake by my ridge. you can literally see the water/mold path follow the airflow from the gable. 

i'm going to try and find a cheap humidity meter tomorrow to leave up there and borrow one of the industrial dehumidifiers from my partner {used for flood jobs} 

80% of the mold up there is within a few feet of that gable vent. there was a little around the bathroom vent but nothing like around the gable area.


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## racebum (Mar 8, 2010)

doing the math on ridge to soffit venting. earlier today i found these at lowes, claim a maximum 27sq/in for flow {brown grates} along with basic 3" hole vents

the problem i have is that i forgot how to figure sq/in. i plan on using a 3" hole saw to put holes in the soffits then silicone these screens over say...4 of them. 

a 3" circle has a 1.5" radius and a 9.424 circumference along with a total area of 7.068 but i totally forgot how to transform this into sq/in to get an idea of how much i'll need to get that ridge vent working efficiently 

also worth noting he humidity is down to 40% ish and holding


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## seeyou (Dec 12, 2008)

racebum said:


> doing the math on ridge to soffit venting. earlier today i found these at lowes, claim a maximum 27sq/in for flow {brown grates} along with basic 3" hole vents
> 
> the problem i have is that i forgot how to figure sq/in. i plan on using a 3" hole saw to put holes in the soffits then silicone these screens over say...4 of them.
> 
> ...


Do you mean convert to SQ FT? Divide by 144. Each 3" rd hole will give you 7/144=.05 sq ft. Gonna be doing a lot of drilling.


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## racebum (Mar 8, 2010)

well, i already have a good chunk of soffit venting, just need to add more. from what i gather i need to be at 1:1 intake/exhaust and being 2:1 favoring the soffit isn't a bad idea when possible

just realized area is sq/in since it's the total area of the circle. my ridge vent flows 32sq/in every 2 feet which would mean 5 of those 3" holes would be 35 sq/in


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## House Engineer (Feb 23, 2012)

If your gable end vent is located close to one of the roof box vents, that may be short-circuiting the ventilation at that end of the attic. I would make sure I had enough square inches of net ventilation area of roof box vents, and then seal the second gable end vent.

And I agree with Mr. W.O.W, about the benefit of providing more than ample low venting.


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## racebum (Mar 8, 2010)

House Engineer said:


> If your gable end vent is located close to one of the roof box vents, that may be short-circuiting the ventilation at that end of the attic. I would make sure I had enough square inches of net ventilation area of roof box vents, and then seal the second gable end vent.
> 
> And I agree with Mr. W.O.W, about the benefit of providing more than ample low venting.


what>?

roof box vents? like for the bathroom? those were sealed. the moisture problem was obvious when i looked in the daylight. the gable vent became an intake for the ridge vent and drew in a lot of moisture. humidity dropped rapidly the second i sealed that off. 

just curious but did you read the thread? you keep referring to "roof box vents" what i have is










last question for someone who will know but when ridge vent is quoted at 16sq/in/ft in flow is that on each side of it or total? if it's total my intake calc's will be a lot easier to arrive at


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## seeyou (Dec 12, 2008)

It's total.


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## roofguy (Feb 5, 2012)

*roof venting*

check out my site on condensation 
<Company link removed> :thumbsup:


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## racebum (Mar 8, 2010)

roofguy said:


> check out my site on condensation
> <Company link removed> :thumbsup:


more or less what i've been lead to believe so far. 

seal the attic deck. spray foam the can lights or build enclosures around them. hvac tape the bathroom vents, spray foam the wire holes. equalize soffit and ridge vent sq/in favoring more on the soffit. 

i'm just happy i caught it before it really started to mold the wood. the few grey spots i have more or less melt the second you spray them with microban

turning my bathroom vent and gable into intakes were ugh...............i have no idea how that was ever overlooked in the first place


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## Gary in WA (Mar 11, 2009)

If the soffit vents are substantially more than the exhaust vents, the air could move from one soffit vent to another close by rather than to the ridge vent. This is why there are limits on the percentage system. The soffit air feeds the ridge vent. The new “I” codes reflect this by limiting soffit at 50-60% maximum, more than exhaust: http://publicecodes.citation.com/icod/irc/2012/icod_irc_2012_8_sec006_par001.htm

Regardless, you are under your own State Energy Code of balanced system with ½ intake, ½ exhaust to use the 1/300 rule. If adding more soffit venting percentage past 50%, be sure to meet the 1/300 rule first. “*Ventilation in attics*
“The attic ventilation requirement is one square foot of net
free vent area per 150 square feet of ceiling. This is often
abbreviated as 1/150. If half the vents are placed low, at the
eaves, and half the vents are placed high, at the ridge or
gable end, the vent to ceiling ratio may be reduced to 1/300.” “Flat ceiling insulation” – just above Fig. 1: http://www.cbs.state.or.us/external/bcd/programs/energy/energy_publications/ResPub_all.pdf

To clarify: meet your 50/50 rule for 1/300 first; then add additional soffit venting to get your greater soffit percentage.

10-15 % (or 55-58%) more soffit than exhaust is recommended here, pp. 615: http://books.google.com/books?id=Z8...page&q=attic airflow with gable vents&f=false

2:1 is here: http://www.inspectapedia.com/interiors/atticcond10.htm
BSC recommends 60/40 best: http://www.buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting
From the picture, it appears your ridge vent doesn’t have baffles, which are much better by giving you negative pressure (suction): http://oikos.com/esb/30/atticvent.html

http://www.buildsite.com/dbderived/airvent/derived_files/derived20936.pdf

The circular vent Net Free Ventilation Area (NFVA) is minus the screening and louvers: (1.735 NFVA) https://ventmastersstore.com/shop/3-round-screened-vent-black-p-128.html

May need to re-figure your NFVA: http://www.airvent.com/homeowner/products/intakeSoffit-specs.shtml

Gary


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## racebum (Mar 8, 2010)

i don't have the cool baffled ridge vent but boy do i ever relate to



> The ratio of soffit intake to roof outlet should be at least 2:1 to avoid unnecessary these heat losses from the building. A serious error is a roof outlet vent net free area that exceeds the air inlets at lower roof edges or eaves. When this occurs in a climate where building heating is needed during part of the year, warm air leaking into the attic or roof space and exiting at the ridge vent (or other vents high on the roof) creates a convection air current that draws excessive heat out of the building during the heating season, leading to unnecessarily high heating costs.


as it sits now i have somewhere close to .6sq/in of intake for every 1 sq/in of exhaust. your links and others have made me realize what seems so obvious now, the ridge IS venting and is pulling air from the easiest source. it's why the gable end turned into an intake when it was open and why i need to seal my attic deck and add more intake. i added about 50sq/in of intake today and will continue to do so until i get to at least 1.1sq/in of intake for every 1sq/in of exhaust. i'm not sure i can achieve 2:1 but i think 1.5-1 could be in the cards 

thanks again for the links i will read through them all

oh, for my IC rated can lights, is spray foaming around the loose edge between it and the sheetrock okay or does each canister require a box or foam baffle to be placed over it?


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## Gary in WA (Mar 11, 2009)

Seal the joint to drywall at the lights, IC need no covers: http://www.finehomebuilding.com/PDF/Free/021105092.pdf

http://www.bpi.org/Web Download/Ins...Install_Certif_Scheme_Handbook_V1.3_DRAFT.pdf

http://www.google.com/url?sa=t&sour...sg=AFQjCNHwd56o0AxLi8-V03E5cMUmwWATQw&cad=rja

http://www.google.com/url?sa=t&sour...sg=AFQjCNEV_sFpJPB8DwLjAd-t6PJoO842EQ&cad=rja

That should hold you for the next few nights....

Gary


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## Bob Mariani (Dec 1, 2008)

Windows on Wash said:


> 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.


where did this info come from. My understanding was to have a balanced vented area. Basic physics... one CFM in will means one CFM out. so if you are allowing twice as much in, where does it go?


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## racebum (Mar 8, 2010)

i even get this one. having more intake than exhaust is not only recommended via some of the links above. it prevents negative pressure in the attic from too much exhaust which would suck from the easiest source...that can be the house. other unsealed areas, you name it. from what i gather is the idea is to have enough soffit that the ridge exhaust can easily pull on the soffit. being right at 1:1 gives you no wiggle room if anything gets blocked. my own home is going to come in at 1.4:1 which should be open, breezy and prevent high humidity conditions up there. 

i've been adding 50-100sq in of intake per day in my free time and i literally have watched the humidity meter drop a few % each day


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## Windows on Wash (Aug 30, 2011)

Joe Lstiburek

He typically aims for a 60/40 split on intake vs. exhaust.

He always preaches to aim high on the intake side. Intake air is more often overlooked or obstructed, creating an imbalance the other way. If there is too much exhaust, you just drive stack pressure and air loss from the envelope.

Intake air is also a bit more difficult to get right in most cases vs. just about anyone can easily add in additional ridge vent or box vents to make up for lack of exhaust air.

Most of the homes that I go into with persistent venting issue are lacking intake volume or have obstructed some aspect of it.


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## Bob Mariani (Dec 1, 2008)

okay I can accept that one. Reason is more of a practical one due to the vast lack of attention to details of the construction industry in general. However you can also go overboard. If you pressurize the attic space you drive heated air in the summer into the conditioned space below. The ideal is one to one! And completely sealing the attic floor is far more critical than attic venting in most climate zones.


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## Gary in WA (Mar 11, 2009)

In post 29, I showed our (U.S.) codes are allowing for 50-60% soffit over exhaust, any more than that and you create the possibility of air in one soffit---out the soffit right next to it. No reason to go up if the resistance is less down below. Here is the idea: 10-15 % (or 55-58%) more soffit than exhaust is recommended here, pp. 615: http://books.google.com/books?id=Z8a...0vents&f=false

Many won't achieve those figures as WoW said, much easier than crawling at the soffits- especially for us old guys....There is not much research on the percentages ratio with test results, I'm still looking...

Gary


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## Windows on Wash (Aug 30, 2011)

Bob Mariani said:


> okay I can accept that one. Reason is more of a practical one due to the vast lack of attention to details of the construction industry in general. However you can also go overboard. If you pressurize the attic space you drive heated air in the summer into the conditioned space below. The ideal is one to one! And completely sealing the attic floor is far more critical than attic venting in most climate zones.


Convection will probably always steer a negative pressure even the the presence of a slightly overbalanced intake:exhaust ratio.

Stack pressures during what the the more problematic venting seasons (winter) are already in effect in the home and pushing warm, moist air up into the attic.

In those cases I would always prefer a slightly overbalanced intake vs. exhaust if I am going to err on one side or the other.

Air sealing is first and foremost and that is why I mention it in nearly every discussion about ventilation. It will nearly always mitigate or completely eliminate the needs for modification.


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## racebum (Mar 8, 2010)

well, it's been a little over a week since i did everything that was mentioned. couple cans of spray foam and hvac tape did wonders sealing the attic floor. had a couple really big home to attic vents, one being that bathroom vent, second being from the kitchen. the cutout for the exhaust vent had a good 3/8" all around going directly to the attic. that's on a...what...8" pipe so that's a good amount of air. anyway. with the gable closed down and sealing done along with a whopping 98 more 2.5" holes in the soffits {hole sawed 6-7 -2.5" holes in each soffit i chose} the air feels about the same up there as it does outside and get this...the mold is dying on its own. some areas that had grey splotches starting are either gone totally or shrinking. i microban'd the worst areas but the minor areas appear to be dying on their own with the food source cut off

for reference a 2.5" hole is 5sq/in


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## Bob Mariani (Dec 1, 2008)

Glad you took the time to read all the confusing opinions on how to fix this issue. But you did it and it is working. Good job!


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## seeyou (Dec 12, 2008)

racebum said:


> for reference a 2.5" hole is 5sq/in


Yeah - rounded up. But the resulting NET area is reduced significantly when an insect screen or louvers are added.

Glad your solution is working.


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