[LexHouse]

From the photo below, can anyone here ID the square inch of NFA per square foot of this soffit? The house is about 16 years old and I was told that the siding may be made by CertainTeed so perhaps this vented soffit 'was' also made by them? I write 'was' because I could not find this style on their webpage.

Thank you.

 

[Windows on Wash]

Its not much whatever it is, but you have a nice deep overhang.

What is the spacing like from inside the attic? Most times, the bigger issues with the small slots is that they clog with debris and are rendered worthless at that point.

 

[LexHouse]

Thanks for your reply, the soffits are well open to the attic and are 24" wide (this is on the rear of house, front and sides are shorter). There is a single 2x4 running parallel to the eve so the vented soffit pieces can be secured to that. These do look like small slits compared to the images of CertainTeed's latest offerings so am unsure of the NFA.:vs_worry:

 

[Bud9051]

Hi Lex,
Closest I could find was here:
Hello, and thank you for your message. There is 4.6 square inches of airflow per square foot. But they don't state in their specifications the NFA. Fortunately, someone asked and their answer was "There is 4.6 square inches of airflow per square foot."
I did a rough estimate before I searched and came up with a similar number, so that sounds good to me.

If you are following the 1/150 or 1/300 rule, there are many variables not included in that calculation. Even for a simple high/low application there is a lot of wiggle room and then try calculating venting with many heights.

If you need more help, just ask.

Bud

 

[jlhaslip]

Make certain that a decent portion of the calculated square feet of venting is in the top portion of the roof. I am not familiar with US code, but here in Canada, if I remember correctly, a third of the sq ft needs to be in the top third of the roof, but my code books are not available to confirm. Maybe it is 50% in the top third. ???

And more square feet is better than not enough.

 

[Bud9051]

@jl, The code generally shoots for 50/50 and then suggests the lower vent area be slightly larger if possible. But the reality is, almost all combinations will work and the trade-off is just another part of the design. Example, if the ceiling is well air sealed, then the range runs from 25/75 to 75/25. Either of those would provide 90% of what a 50/50 balance gives you. Make it 75/35 and you right where you want. But, if the ceiling plane is not sealed as well as it should be, then the northern climates should stay with 40% high and 60% low and use the 1/150 rule.

Bud

 

[LexHouse]

All good info, thanks. :thumbsup:

I've also read that a 50/50 is what the standards are, but then 60 intake to 40 exhaust is better, and also 66/33 where exhaust being half of intake. Once I determine the NFA in sq inches passing through my style of venting I can then calculate the required, or sufficient amount of, exhaust needed and then choose the type of exhaust options to go with my new roof. This will be in a new thread.

 

[Windows on Wash]

Err on the side of too much. Doesn't really matter if it is 50/50 or 60/40 if you ask me.

I think Joe Lstiburek mostly put that 60/40 idea out there because if ventilation is normally shorted, its on the intake side.

 

[Bud9051]

Joe's math is based upon the delta "p" across the ceiling which forces conditioned air out of the house and into the attic. Air sealing is the best defense, but even if only moderately well sealed the reduction in air leakage using the 60 low and 40 high is minimal. That delta changes because attics also have a stack effect and with more vent area towards the soffits it reduces the negative pressure a the attic floor which reduces the total the total delta across that ceiling. Must be a shorter way to explain that. Last I played with the numbers it was about 5% when there was a strong stack effect, not very often. And that is just an occasional 5% of the leakage from house to attic which should be small to start with. Long term, it would be hard to measure a benefit.

Bud

 

[Gary in WA]

Lots of info, but we need more; where are you? On the eastern coast of Maryland can be at 10' elevation or 1496' elevation with temps varying as much.http://msa.maryland.gov/msa/mdmanual/01glance/html/weather.html
Rainfall averages 40", do you have many cooling degree days...use the AC for any length of time? A breathable roof underlayment or not?

Gary

 

[LexHouse]

Hi Gary, thanks for your reply. I looked up my address and it said I was at around 50' above sea level, I'm about in the middle of MD Eastern shore. I recently bought the house but I expect to be using the central A/C this summer for sure. The new roof I expect to put on will have a synthetic undergarment, GAF Deck-Armor which is supposed to offer "Revolutionary 'Breathable' Technology" (GAF's literature)

This thread helped answer the NFA inches per SqFt of soffit my house has (thanks to Bud9051). I used the 4.6" NFA soffit intake calculations to help determine which exhaust venting options would best suit my hip style roof. I used the 4.6" NFA in another thread which is also in this Roofing/Siding forum here: http://www.diychatroom.com/f9/exhaust-choices-new-roof-details-included-386282/

Thank You

 

[Windows on Wash]

I would skip the breathable underlayment. I have been saying this for years, but I see no benefit to them and only drawbacks.

OSB is already sub 2.0 if that is what you sheathing consists. My concern with breathable synthetics is that they might allow more moisture to be pushed into the attic than out.

The roof, when installed right, is less than 1 perm as well. Nothing is coming out of that type of roof, but the sun can push moisture in over time. I would rather have my deck be completely impermeable in this case.

http://www.rci-online.org/interface/2011-12-Lstiburek,%20Karagiozis,%20Gassman.pdf

 

[LexHouse]

OK. The sheathing is plywood. Thanks for the link which I've read. To me, it basically says that the shingles work as the barrier and that a breathable underlayment doesn't add any value. I was told by GAF roofers that the synthetic underlayment provides good traction to walk on, lighter and easier to install, maybe this is the real reason it is offered?

 

[Windows on Wash]

The synthetics are superior in my opinion, I just don't think the GAF Deck Armor (one of the breathable ones) is worth a penny over standard synthetics.

 

[Gary in WA]

My question on underlayment was a "red herring" for a member that feels he HAS to answer for me- at my questions to others....without knowing why I mentioned it, even, lol. Hook, line and sinker.. You dangle a carrot in front of a donkey to move him, and sometimes it works...lol.

I used this a few years ago on this forum; http://sweets.construction.com/swts_content_files/20865/575423.pdf

Another on different brands; http://www.nyashi.com/newsletter/2006/June files/roof membranes May 06.pdf

IMHO, as you live on the coast in a maritime climate, you don't want too much venting with the added moisture it brings; eg; but colder- https://hpo.bc.ca/files/download/Report/Attic-Research-Study-Final.pdf

Check for plywood staining near the soffits, make sure the bath exhausts are vented out (preferably through gable ends), and air seal- as said already, because as wood framing moves with the seasons the drywall ceiling/wall joint stays tight. Air leaks from between the wood top plate and drywall, caulk or foam here- esp. at outside wall to help prevent ice dams; http://www.extension.umn.edu/environment/housing-technology/moisture-management/ice-dams/index.html

Clean those slots as the wet air passing through them deposits moisture around the inlets which in turn collect dirt; http://cthandiman.com/services/vinyl-siding/soffits-and-fascia/ to reduce the NFVA. Optimally would be an install (with opposite directional continuous metal vents) close to the gutter-not the house- for better/drier airflow. When close to house it brings in more moisture that is on the siding with the wind blowing across the ground and up the vertical wall, then into soffit, but you have what you have.

Gary

 

[LexHouse]

Gary, thank you for your reply. Thanks for those links. Your reply would fit in well in my thread asking about exhaust style option, which this thread was needed first as I didn't know the NFA of the soffit style my house has. http://www.diychatroom.com/f9/exhaust-choices-new-roof-details-included-386282/

Thank You