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Venting Vaulted (cathedral) ceiling (retrofitting soffit vents)

Venting Vaulted (cathedral) ceiling (retrofitting soffit vents)

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  #1  
Old 04-28-13, 01:03 PM
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Venting Vaulted (cathedral) ceiling (retrofitting soffit vents)

Hi Everyone, Im new here but long time information finder on do it yourself! Thanks to all that help!

My home is a 19 year old tri level in Castle Rock CO. The roof was re shingled about 10 months ago. There are no soffit vents anywhere on the house. There are gable vents and air-hawk vents and roofer installed small sections of ridge vents over the two main parts of the house. I wish I studied up on venting before the roof went on! Also there hasn't been any ice dams or icicles in the 5 years I have lived here.

Im focusing my limited time, expertise, and patience on the cathedral 2nd level of the tri level at the moment. The living space is 30' x 15'. The back of the house faces south and that is where the kitchen is located. It definitely could be more comfortable in the summer. There is a Gable vent that is on the west side (looks like there is a small attic cavity), 2 air-hawk upper vents as well as a small strip of ridge vent that the roofer installed. As I have been reading more, I know it probably short circuits itself but right now my attention is on adding soffit vents. There is no access to this area. I did add one vent but couldn't insert a plastic baffle because of the nails coming through the deck. There are fiberglass batts installed. I inserted about 10 pieces of 1 inch pvc pipe 30" long into the 24" spcace and feels like there is a small cavity at the end of the pipe. Not sure if it opens up a little bit or if the fiberglass is compressed. Im sure pipe isnt the recommended way but it was easy. Open to suggestions on the next vent. Im basically looking to improve my ventilation without doing any heavy duty construction. Thanks so much for the help in advance!

So here are a few questions before I continue:

Do ceiling angles and roof angles differ sometimes to create an attic cavity in a cathedral ceiling?

Do I need a soffit vent between every rafter?

Do I need as many soffit vents on the north side as it is not as hot there?

Is there a better way/material to use as a baffle to get air into the attic?

Am I on the right track or way off? haha


Thanks everyone!
jay
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  #2  
Old 04-28-13, 03:06 PM
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The pipes may allow a small amount of air to flow, but will not help the bottom of the roof they bypass. The purpose of venting is to remove moisture, cool the roof in winter to avoid ice dams, and to a small extent cool the attic in the summer. The summer cooling is typically not very effective. Moisture is the most worrisome as it can promote mold and rot.

The guidelines are 1 ft² of net free vent area for every 150 ft² of attic floor. Half goes high and half goes low. Then we enter the real world and try to figure out how to do the best we can .

I'll answer your questions and then go dig up a link on venting.

Q. Do ceiling angles and roof angles differ sometimes to create an attic cavity in a cathedral ceiling?
A. Yes, a scissor truss is a good example of two different angles.
Q. Do I need a soffit vent between every rafter?
A. Yes, especially if each rafter cavity is isolated from all others. But yes also just to obtain enough vent area.
Q. Do I need as many soffit vents on the north side as it is not as hot there?
A. Venting is a summer and winter necessity and again, you need all the area you can get.
Q. Is there a better way/material to use as a baffle to get air into the attic?
A. I would need to get a better picture of the area in question. ie small attic below, slope, small attic up top, or slope all the way up to a small attic.
Q. Am I on the right track or way off?
A. You've stopped here to ask, so yes that is the right track. Your approach will change as you learn more.

http://www.fpl.fs.fed.us/documnts/pdf2002/rose02a.pdf

Bud
 
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Old 04-28-13, 10:07 PM
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Hi Bud - Thanks for responding and helping me out with all the info. I read the article and its definitely helping me unlock some pieces to the ventilation puzzle. If I don't have ice dams and icicles in the winter and don't see discoloration in the ceiling drywall, my roof must breathe somewhat right? If I can improve moisture removal and cool the attic slightly I will be stoked!

-I think it is a scissor truss system as it looks like there is some cavity at the top and warm air was coming out of the gable. If I can compute the roof angle and the ceiling angle, will I know how much space is up there?
-I will take a pic of inside the soffit for better understanding/ not sure if I can stick any kind of rigid baffle through this space. I saw someone fabricated steel ones but I don't have any kind of real tools haha. Sounds expensive - PVC Pipe is cheep! could I drill a whole bunch of holes in the pipe so there will be at least a little air leakage into the bottom of the roof?
-I really do appreciate ventilation wisdom!


So if there is about 450 square feet divided by 150 = 3 square feet of net free divided by 2 = 1.5 square feet of net free per upper and lower. 144 x 1.5 = 216 square inches needed for upper and 216 square inches for lower. I have 2 air hawk style vents and 4 ft of ridge vent plus a gable vent (I will have to measure it) but I think its 12x18.

2 airhawks = 100nfa?
4 ft ridge vent = about 16nfa per foot =64?
12x18 gable = about 80nfa?

So I think I have approximately 244nfa of exhaust. I will measure the gable and ridge vent tomorrow to be sure.

Is there a way to figure the nfa of 1 inch pvc pipe?
Is gable vent considered exhaust or is it also computed as intake?

Thanks again!!!
 
  #4  
Old 04-29-13, 04:09 AM
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This is where the real world meets the rules. They are usually looking at the attic floor when calculating sf of area and the 150 is for a typically leaky house, 300 for a tight house. When venting a vaulted ceiling I would use the area of the slope plus and flats. If you have no recessed lights and few intersecting walls then you can wiggle towards the 1/300. It creates a range of thinking and gives you a target.

As for the pipes with no bug screens to reduce air flow, it would just be somewhat less than the area. But, with one or two inches of clear vent space all the way up the results are minimal, so the area of the pipes will probably be less than minimal.

Remind me on the intake and exhaust question, have to run.

Bud
 
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Old 04-29-13, 09:35 PM
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Hello!

I do have one recessed light, should I 86 it? Any recommendations? Im guessing my house isn't "tight". Its definitely high production grade construction. Im thinking my vent project is one where a little is better than nothing I'm trying to do the best I can but probably won't give me the correct numbers. I'm still feeling good about it though. Still open to suggestions too. Its a work in progress!

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Pic 1 = inside the soffit
Pic 2= some prototype baffles
Pic 3= DIY soffit baffles installed!

-So Im wondering if Gable vents are computed as exhaust and intake?

-Is there a way to compute NFA of a 1 inch pipe?

-Can I buy 2 inch pipe that is ripped lengthwise or something like that. Would make it easy. Drilling holes etc in the pipe takes me awhile. I have about 15 more to go or so for the lower part. I have to say that inserting the pipe is a very tight fit. Not sure if I could get anything bigger in there.

On a positive note, I am actually having a lot of fun with this project!
 
  #6  
Old 04-29-13, 10:48 PM
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You will hear the warning about gable vents short circuiting the air flow from soffit to ridge. But the reality is whichever direction the air flows, in or out the gable vents, it is extra air flow. Count it as high venting and try to get what you can down low.

Also, you don't have to trade off one for the other. Calculate your target net free area and shoot for half high and half low, but if you get more in either area, don't reduce the other. If you get 60% in one, still shoot for 50% in the other.

Use 90% of the cross sectional area of those pipes. The 10% reduction is just a WAG to account for resistance of the tube.

I realize you are having fun, but I feel guilty allowing you to wander down this wild path, all-be-it maybe better than nothing. Maybe someone else has tried it, but not that I've run into.

Bud
 
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Old 04-29-13, 11:52 PM
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Hi Bud,

Thanks for your posts, Not sure what the 90% cross sectional is and WAG?
Don't feel guilty, just not sure what else to do without getting in over my head or spending a ton of money. If there is another big hail storm (2 roofs last 5 years) I will go back to the drawing board knowing a little bit more about it.

In my neighborhood there aren't any soffit vents on any of the houses. You have to look far and wide to see one. Gable City

When I get to the upper part of house with an attic that is accessible I will do it by the book.

This might be cutting edge technology - that or a total waste of time.

Thanks again Bud!
jay
 
  #8  
Old 04-30-13, 06:58 AM
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The cross sectional area of a pipe is a circle, so pi times the radius squared would give you the vent area of each pipe, times my 90% WAG. The WAG is a Wild A$$ Guess.

And learning and finding this forum along with Building science corp give you a huge step up from where you were a year ago. I may have been the one chatting with you, but dozens of experts were following to keep me inline. This is an amazing web site.

Bud
 
  #9  
Old 04-30-13, 01:40 PM
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Hi Bud and Everyone

Im feeling motivated - All the good vibes of the forum and talking to you about my situation makes me want to improve, repair, and get in there and vent some attic space! Thanks again for taking the time to educate and coach a rookie.

I posted and then realized I did the math wrong:

So 3.14 x .5 squared x 10 pipes x 90% WAG () = 7.065 per vent
7.065 x 7 vents eventually = 49.455
49.455 x 2 = 98.91

I think this is right but not sure. The first time I did the math a had tons of NFA

So theoretically might be better than nothing. haha. Maybe I can get more pipe in there.

Thanks again Bud and to everyone else reading and to this forum. giving me direction, cofidence and making me feel somewhat useful.


jay :HF2:
 

Last edited by curbdog; 04-30-13 at 02:03 PM. Reason: math error
  #10  
Old 05-01-13, 03:32 PM
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Ridge vents are excellent for certain applications like cathedral ceilings, low slopes, and metal roofs. We use custom-made steel ridge vents with steel end caps for greater strength and more colour choice. We do not recommend aluminum ridge vents as they squish over time and then begin to leak. The most efficient ridge vents are the Max-Ridge vents which are used in special cathedral ceiling applications requiring a lot of ventilation. Some homeowners are sometimes surprised to see a bit of snow in their attics. Please note that all vents do take in a little powdery snow. Ridge vents are the most notorious for this because they take in snow as the wind sweeps it over the roof peak. However, despite the site of some powdery snow in the attic, this is usually harmless as it is in a small enough quantity and soon melts and evaporates.
 
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Old 05-01-13, 09:43 PM
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Thanks Roofmaster! I appreciate the info - speaking of snow, it's been doing it all day and cold. After seeing all my hard work (not really) only adding up to a sad 7.065 net free area per soffit vent, I went to the home improvement store today just to look around and see if I can utilize some other materials. And there it was - calling to me - 10 foot sections of vinyl gutters! Very flexible but sturdy and rigid. Im hoping I will be able to flex it open and slide it in the soffit creating a much bigger space for the air to get sucked in. Im excited about it! When the weather improves, I will see if I can make it happen. It's fairly cheep as well about 6 bucks per section. Come on nice weather!
jay

looks like this

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  #12  
Old 05-03-13, 10:25 PM
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Nice day today to try to install my new prototype baffles

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I made 2 1/2 foot sections and trimmed them down a bit with a utility knife.

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The one on the right was the first one I tried with just one side trimmed down. For the rest, Im going to trim down both sides to make installing easier.

I tried to get 3 sections in there but I couldn't make it happen. Next one maybe. I would say overall, it's not too bad.
 
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