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-   -   Ventilation (intake) questions with partial overhangs (http://www.diychatroom.com/f9/ventilation-intake-questions-partial-overhangs-125646/)

cbrc5eric 12-05-2011 12:10 AM

Ventilation (intake) questions with partial overhangs
 
I've been doing a bunch of research on roof venting. Details, my house is a 1955 ranch with a tall attic (full sized windows on both gables). The roof is 5 years old and so far in great shape, with a ridge vent installed. I've owned the house for 2 years now. I don't see any rot in the attic when I'm poking around but in the winter when it's cold enough there will be frozen condensation on the gable attic windows which leads me to believe there is not enough ventilation.

Now, my problem is intake. I don't think there is any ventilation for the intakes. I had painstakenly attached attic baffles in each of the 50 some rafter bays only to realize I don't think there's any intake :eek: A few things to consider-

1) The house has partial overhangs both front and rear. While they do have ventilated soffit material every other panel, I think there's solid plywood behind them. I replaced my back door recently and saw the plywood while the trim was out. I'm in the middle of painting my outside trim (see pictures below) and started painting the soffits when I realized it's easier just to replace the soffits completely with all of them vented, so I'll cut open any plywood to increase the intake.

2) There obviously is no intake along the parts with no overhang, but I did research several products that can be used in this situation. I found this thread http://www.diychatroom.com/f9/no-sof...ent-how-33155/ that was very useful, looks like a DCI Smart Vent (or FasciaVent) or GAF Cobra Fascia Flow vent may be my ticket, as I DO NOT plan on cutting any holes in the roof or disrupting the existing shingles.

So my main question is- should I even bother trying to vent that area if I already have the partial overhangs? It's like all the stuff I read about ventilation with a ridge vent, but on the other hand the house has been around for almost 60 years and shows no signs of moisture damage in the roof area.




Rear
http://i262.photobucket.com/albums/i...2535770703.jpg

Front (pic is old I have trimmed the bushes, mowed the grass and am working on changing the ugly yellow trim color :laughing: )

http://i262.photobucket.com/albums/i...1634573644.jpg

jmiller 12-05-2011 06:10 AM

If ridge vent is installed with no intake it can create negative pressure in the attic that pulls conditioned air from the house (and makes for frosty gable windows in winter).

The first step might be checking the seal between the conditioned space and the attic. Are your bath fans vented out the roof, or dumped into the soffit? Does the hatch you access the attic from have tight fitting insulation? Do you have recessed lighting? Those are all an easy path for the ridge vent to pull air through.

Next step would be adding the continuous soffit vents and fascia vents where there's no soffit, which IMO would be worthwhile and DIYable.

Windows on Wash 12-05-2011 08:05 AM

jmiller is correct in that exhaust venting without supply air is a bad thing = negative pressure and air loss.

Ultimately a bad thing for efficiency but not so bad if it is keeping the roof structure dry.

Fascia flow is an option but there are probably a couple of things you can do without buying that stuff (it is expensive FYI).

Remove the vented soffits and make sure you remove the plywood or cut large holes in it.

Make sure you have the proper vent chutes to avoid the wind stripping the r-value from the insulation.

If you want to vent the non-overhang sections, I would remove the gutters, make spacers out of some sort of azek or other rot proof wood, and you can cut holes through the fascia board in between the rafter and install circular soffit vents. Shimming out the gutter will keep it from hanging across the vents and closing them off.

They come pre-screened in most cases and you will just need to make sure the interior is not blocked by insulation.

http://roofing.owenscorning.com/imag...ini-soffit.jpg

josall 12-05-2011 08:31 AM

You will need 1 sq.ft of NFA or attic ventilation for each 150 sq.ft. of attic space. Split equally between the soffit and the ridge. Depending on the brand of your ridge vent you will have about 18 sq.in. of ventilation per lineal ft. Ex: 30' ridge x 18 sq.in of Net Free Area = 540 sq.in. of ventilation or 3.75 sq.ft.

If you have 1,000 sq.ft of atic space you will need 6.6 sq.ft. of NFA of ventilation.
3.3 at ridge
3.3 at soffit

If you know the brand of your soffit you can find out what the NFA of ventilation is to calculate how much plywood you will need to cut out in the soffit.

jmiller 12-05-2011 08:44 AM

Quote:

Originally Posted by Windows on Wash (Post 786459)
jmiller is correct in that exhaust venting without supply air is a bad thing = negative pressure and air loss.

Ultimately a bad thing for efficiency but not so bad if it is keeping the roof structure dry.

The old box vents probably kept the space more dry than an over-vented ridge, which could be the only reason the decking still appears to be in good shape.

Quote:

Fascia flow is an option but there are probably a couple of things you can do without buying that stuff (it is expensive FYI).
I always figure the labor on intake repairs far outweighs the material cost, so ultimately he'll be saving no matter what if he does it himself. Luckily, the eaves with no overhang aren't very long, and the gutter can be re-used.

I like your idea too, but would hesitate suggesting it on customers house without ever having seen it in the field. Interesting though.

Windows on Wash 12-05-2011 09:10 AM

Quote:

Originally Posted by jmiller (Post 786492)

I like your idea too, but would hesitate suggesting it on customers house without ever having seen it in the field. Interesting though.

We have used it previously with good success. Customer had some custom trim and other things and wanted to be as surgical as possible. Every other rafter bay and some 4" pipe to hook it up, you should have felt the amount of air it was pulling once we opened it up.

shazapple 12-05-2011 10:30 AM

Those circular vents don't really flow much air. The 4" diameter vent has 2.43 sq. inches of NFA, which means you would need 4 of them per foot to balance your ridge vent. You are better off going with a continuous soffit http://roofing.owenscorning.com/home...us-soffit.aspx

Windows on Wash 12-05-2011 12:50 PM

Quote:

Originally Posted by shazapple (Post 786533)
Those circular vents don't really flow much air. The 4" diameter vent has 2.43 sq. inches of NFA, which means you would need 4 of them per foot to balance your ridge vent. You are better off going with a continuous soffit http://roofing.owenscorning.com/home...us-soffit.aspx

Where to you come up with 2.43 square inches? Are you assuming that the 4" vent pictured is only +80% obstructed?

The original poster does not have an overhang in those locations and near complete removal of the fascia board is probably not an option.

The purpose of the circular vents was in those locations without overhangs and an unobtrusive way to get some intake air at those locations.

Perhaps those vents weren't the best example as they are a bit obstructed. Here is a better example. In a 4" capacity, I would estimate them to be less than 40% obstructed which would provide roughly 7.5 square inches of ventilation per vent.

http://www.homedepot.com/catalog/pro...c7b3d7_300.jpg

I would estimate that the footprint of the home is less than 800 square feet necessitating 5.3 square feet of attic ventilation. If he just did the soffits in those circular vents, he would need a bunch of them (roughly 50) but he has a soffit overhang on roughly 60% of the roof. These vents are likely unnecessary for the total ventilation requirement but will give him some added convective movement of air in those areas without intake air.

jmiller 12-05-2011 02:49 PM

The tough Q: how does one provide intake ventilation where the porch roof covers the eave.

Windows on Wash 12-05-2011 03:40 PM

Quote:

Originally Posted by jmiller (Post 786685)
The tough Q: how does one provide intake ventilation where the porch roof covers the eave.

Drop the porch soffit, access previous overhand, make sure soffits are completely open (no need to install soffit as you can just cut it opening and cover it up with some metal window screen) re-install new porch soffit (hidden vent is fine for beaded board look or non vented will not really obstruct that much air flow).

If the framing covers everything up, you can just cut a hole in the roof at the cross gable and put in some vented soffit on the porch.

If the storm door is closed, it is most likely not an air tight enclosure and the convection in the attic should drive the make up air into the vents.

jmiller 12-05-2011 03:44 PM

Thanks, BUT, I'll pretend I didn't think of it.

shazapple 12-05-2011 03:58 PM

Quote:

Originally Posted by Windows on Wash (Post 786617)
Where to you come up with 2.43 square inches? Are you assuming that the 4" vent pictured is only +80% obstructed?

It is from the manufacturers website
http://roofing.owenscorning.com/home...ni-soffit.aspx

The other one you showed is 4 square inches of NFA
http://www.homedepot.com/h_d1/N-5yc1...atalogId=10053

Windows on Wash 12-05-2011 04:25 PM

Thanks for that link.

I am not sure how they come up with that number. I guess I was figuring on the vent portion be 4" with a flange that extends beyond that.

4" @ 60% clear, yielding about 2.4 inches x 3.14 = 7.5 sq/inches.

Go figure though. Must be much more obstructed than that.

If the homeowner wants, take a 4" hole saw and just put some window screen over the hole. It won't be 4" clear but it will give you more than 4 sq/inches.

cbrc5eric 12-05-2011 06:44 PM

Quote:

Originally Posted by jmiller (Post 786392)
If ridge vent is installed with no intake it can create negative pressure in the attic that pulls conditioned air from the house (and makes for frosty gable windows in winter).

The first step might be checking the seal between the conditioned space and the attic. Are your bath fans vented out the roof, or dumped into the soffit? Does the hatch you access the attic from have tight fitting insulation? Do you have recessed lighting? Those are all an easy path for the ridge vent to pull air through.

Next step would be adding the continuous soffit vents and fascia vents where there's no soffit, which IMO would be worthwhile and DIYable.

Yup! I've done the first extensively, crawled around the attic, sealed up electrical and vent stack penetrations. No recessed lights, and I've sealed everything as best as I could. One big offender was right over the stairs, it was completely open! Check out the dirt in the insulation:

http://i262.photobucket.com/albums/i...ket_31316_.jpg

http://i262.photobucket.com/albums/i...ket_31317_.jpg


Quote:

Originally Posted by Windows on Wash (Post 786459)
jmiller is correct in that exhaust venting without supply air is a bad thing = negative pressure and air loss.

Ultimately a bad thing for efficiency but not so bad if it is keeping the roof structure dry.

Fascia flow is an option but there are probably a couple of things you can do without buying that stuff (it is expensive FYI).

Remove the vented soffits and make sure you remove the plywood or cut large holes in it.

Make sure you have the proper vent chutes to avoid the wind stripping the r-value from the insulation.

If you want to vent the non-overhang sections, I would remove the gutters, make spacers out of some sort of azek or other rot proof wood, and you can cut holes through the fascia board in between the rafter and install circular soffit vents. Shimming out the gutter will keep it from hanging across the vents and closing them off.

They come pre-screened in most cases and you will just need to make sure the interior is not blocked by insulation.

I installed the attic baffles on all the baffle bays. 16" OC construction, there was 50 of them. It was a pain crawling around the attic!

Yup, I'm replacing the soffit anyway to change the color, so while I'm in there I'll cut out what's necessary.

Still considering all options for the non-overhang part... I'll only need about 35 feet total as the house isn't that big. I saw there's FasciaVent from GAF too which comes in a roll, but so far Fascia Flow looks most appealing for my application.


Quote:

Originally Posted by josall (Post 786479)
You will need 1 sq.ft of NFA or attic ventilation for each 150 sq.ft. of attic space. Split equally between the soffit and the ridge. Depending on the brand of your ridge vent you will have about 18 sq.in. of ventilation per lineal ft. Ex: 30' ridge x 18 sq.in of Net Free Area = 540 sq.in. of ventilation or 3.75 sq.ft.

If you have 1,000 sq.ft of atic space you will need 6.6 sq.ft. of NFA of ventilation.
3.3 at ridge
3.3 at soffit

If you know the brand of your soffit you can find out what the NFA of ventilation is to calculate how much plywood you will need to cut out in the soffit.

It is about 1000 sq ft. I just plan on getting the aluminum stuff from Home Depot, I saw it the other day but since it came in 12 ft sections it wouldn't fit in my vehicle :lol: I'll have to go back with my trailer.


Quote:

Originally Posted by jmiller (Post 786492)
The old box vents probably kept the space more dry than an over-vented ridge, which could be the only reason the decking still appears to be in good shape.


I always figure the labor on intake repairs far outweighs the material cost, so ultimately he'll be saving no matter what if he does it himself. Luckily, the eaves with no overhang aren't very long, and the gutter can be re-used.

I like your idea too, but would hesitate suggesting it on customers house without ever having seen it in the field. Interesting though.

Good points. I will be DIYing. :thumbsup: Fascia Flow I'll look more into, from what I read so far it comes as a pallet and they're $100+ each one? Yikes... hope I'm wrong.

Quote:

Originally Posted by jmiller (Post 786685)
The tough Q: how does one provide intake ventilation where the porch roof covers the eave.

That's actually another one of my projects is I plan on tearing out the closed in part of the covered entry way. It's too small to be useful (can't put chairs in there) and the siding looks like such an obvious add on, where the rest of the house is brick. :laughing: I plan on leaving the peak but losing most of the area with the siding and the windows, and making it open with some columns and a new front door :yes:.

But for now, the sides of the enclosed part there are also soffit vents there. Who knows they're probably also covered with plywood...

cbrc5eric 12-05-2011 07:01 PM

Quote:

Originally Posted by Windows on Wash (Post 786617)
Where to you come up with 2.43 square inches? Are you assuming that the 4" vent pictured is only +80% obstructed?

The original poster does not have an overhang in those locations and near complete removal of the fascia board is probably not an option.

I can get creative :laughing: I'm pretty handy.

Quote:

The purpose of the circular vents was in those locations without overhangs and an unobtrusive way to get some intake air at those locations.

Perhaps those vents weren't the best example as they are a bit obstructed. Here is a better example. In a 4" capacity, I would estimate them to be less than 40% obstructed which would provide roughly 7.5 square inches of ventilation per vent.


I would estimate that the footprint of the home is less than 800 square feet necessitating 5.3 square feet of attic ventilation. If he just did the soffits in those circular vents, he would need a bunch of them (roughly 50) but he has a soffit overhang on roughly 60% of the roof. These vents are likely unnecessary for the total ventilation requirement but will give him some added convective movement of air in those areas without intake air.
The house is actually 1000 sq ft. I gotta take some more measurements to see exactly how much area I have under the overhang areas. Perhaps I'll climb up on roof too to see what kind of ridge vent it is as well...

I'm wondering who the hell put in the ridge vent without adding the right intake vents. I'm VERY curious to see what's hiding underneath the current soffit. Just hope I don't wind up destroying the gutters pulling them loose to take out the existing soffits :lol:


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