PDA

View Full Version : Attic vent sticky needed....



mrcaptainbob
11-16-2010, 11:14 PM
Can't find the neat info I recently read here about the formulas for attic venting. So many square feet attic should have so many square feet venting. Also, There was a quick blurb about the exhaust (ridge...) being sized smaller than the inlet (soffit...). Are there other attic venting rules that I should know about?

pushkins
11-17-2010, 08:14 AM
1:1 is the rule in other words for every 1 square foot of soffit vent you need 1 square foot of exit space. Now the area formula is 1:150 for every 150 square feet of attic space you need 1 square foot of entrance and exit air space.

Soffit intake and ridge exit is the ideal, what you don't want to do is add powered vents if you have ridge with gable intake.
Ridge vents should never be sized smaller than intake vents, if the air cannot escape then it simply won't draw it in.

Don't skimp on intake and exit venting, the better the airflow the more efficient it will work, also remember that vent size rules are subjective, just because the vent is 1'x1' doesn't make it 1 square foot, the square footage required is calculated on "actual" air space, so that mesh over it decreases the size as does any directional louvers etc... most vents will have " actual airflow" size on the box, if they don't then measure and deduct 20% for an allowance.

mrcaptainbob
11-17-2010, 11:37 PM
Dare I say it? Thanks for letting me vent about the sticky....!
Thanks for the great info, Pushkins. I will apply it to this situation.
I do have a real challenge on one part of the roof. The second floor was built ten feet less than the first level. That first level roof deck is thirty feet long, one end of which ends at a brick wall, the other ends at an exterior fire place box surround. Presently there is no soffit vent. Needless to say, there is no way of 'ridge' venting and it appears gable venting is out as well, as there are no gables. But the heat from the room is still transferred to that attic space. My plan is to remove the soffitt and facia, and using the long hose try to blow insulation into that smallish space using the access provided by the rafters through the facia end. After getting a comfortable level of insulation blown in I plan to apply the airdams from the facia ends. At this point I can install new facia and add much needed vented soffit. For exhausting the air from that space I plan to use 3" pvc at the verticle wall connecting that first level attic and slipping the pvc through the soffit of the second level overhang. The thinking is to allow the first level mini-attic to vent up to the main 2nd floor attic. Aesthetics may be controlled by placing those verticals at the extreme corners of the wall and either side of the only window on that wall. Not an ideal solution, but can think of no other way to control the yearly ice damming of this part of the roof. Any thoughts?

pushkins
11-18-2010, 08:14 AM
I'm not sure I'm following your description of your attic spaces correctly.

If my thinking is right there is another solution, you still can add a ridge vent, it doesn't need to run the full length just as long as you want, lower on the roof you can add "pop vents" this would allow the circulation of air from low to high. The holes cut for the pop vents and ridge would allow you also to add insulation much easier and much more uniformly than trying to blindly pump insulation in.

mrcaptainbob
11-18-2010, 11:26 PM
That roof deck.... consider a wedge two feet high on the high side to 4" high on the low side....30 feet long. The high side is midpoint on a two story wall, the low side is the eve. One end is on a brick wall, the other end is at a chimney enclosure. There is flashing between the two story wall and the high side to provide rain and snow protection. It being the high side is where the venting would have/need be. ( I believe). But going through the calculations based on what you provided, the 3" pvc has a an approx. area of 7 sq.in. A ten by 30 foot area is three hundred sq. ft. needing two sq' ft' of vent. 288 sq. in. / 7 sq.in. is a bit over 41. I would need 41 three inch pvc vent stacks for proper ventilation. It would look like a stupid pipe organ!! I need to find a better way.....

pushkins
11-19-2010, 08:51 AM
On the high side is there any soffit at all ? If so you could add the long strip vents up under this soffit, the strips are 2" wide and I think you can get them in 2' lengths, regardless of available length you just keep adding onto the end of the previous one. Couple this with roof pop vents placed low on the roof deck.
If you cannot use the above option then about the only way you have efficiently left is to add pop vents low on the roof and up as high as you can on the roof deck, that would mean at worst 3 pops low and 3 pops high.

mrcaptainbob
11-19-2010, 11:54 PM
The only venting possible is in the soffits. That's good for the intake. The exhaust is the issue. Since the top most part of the roof is adjacent to a wall, there's no ridge available. Using any of those pod type vents may not be a good idea, as the roof angle is shallow enough to keep a deep enough layer of snow that could easily cover them. It has happened in the past on that roof. The angle is about 27" high by about 100 inches long. This is such a challenge to get it done as close as to what's needed.
I sure appreciate the help in working this out...

pushkins
11-20-2010, 12:07 PM
100" span with 27" rise = about 3.7:12 close enough to call 4:12 for this scenario, normally that slope wouldn't be an issue with snow collection but as the wall ridges against another wall I see your problem. Can you cut through from the other part of the house attic space to open up a void between these two areas ? or is that the fireplace end ?
If it's just a veneer brick wall end you could cut out a 2 sq. foot area of brick to allow air flow between the two.
If not this end what about the exterior end or is that the exterior fireplace ?
I tell ya what, lets call things left , right etc... This is how I'm seeing it...looking at the home the roof in question is on the right side with the main home on the left and behind, the part I'm not sure about is the fireplace (exterior) on the far right end or built into the room on the left ?

One solution that may seem like a lot of work on the surface but 100% solve the issue all round would be to build a sort of cupola, it could be built up at the highest point of the roof up against the rear wall (three sided) BUT you'd have some flashing to do on the wall to cupola joints. OR you could build it 4 sided and bring it forward a foot or two down the roof this would mean only flashing it to the roof deck (much easier). The cupola could be whatever height you think will avoid snow and look cosmetically appealing (I'm assuming this is on the front of the house?) you need 2 sq. feet so that would mean a hole approx. 16"x16" perfect to fit between either 16 or 24" on centers rafters. The following link is a simple cupola you would just need to make adjustments for roof slope on the base.
http://conceptsamerica.com/images/Brooke%20Mini%20Ornamental%20Cupola%2016.jpg
You could make an even more simple one by changing the roof to a simple gable end style roof the cupola venting would actually be built into the gable kinda like a mini house. Cover the holes with soffit vents.

mrcaptainbob
11-20-2010, 10:56 PM
Yes, Pushkins, you're correct in your assessment. On the one end (east end) is the exterior brick of the main house. The other end (west end) is enclosed by the exterior fireplace surround. Except for the removing the facia (north side), there is no other access to that 'attic'. it sorely needs insulating, it sorely needs air dam and it sorely needs venting. There have been instances when snow removal was a necessity, as the bedroom window's sill on the main house (south and the 'high' side) is about eight inches from that roof deck. Considering this roof deck has walls on three sides is a real dilemma.

pushkins
11-21-2010, 07:47 AM
Okay then I'm sticking with my soffit vents and cupola solution.
Seems to be the best and most practical. Either three sided flashed to the rear wall or four sided down a little on the roof. Four sided would make installation much easier as flashing would only be needed to the roof deck.
For cosmetic reasons you could build one in whatever style you like, longer and lower or shorter and taller.

mrcaptainbob
11-21-2010, 10:10 PM
Hmm...that does seem to be a good solution. For a more 'hidden' appearance, based on your suggestion, I'm now thinking a two sided affair in the south east corner. It would be against the brick (east) end wall and the main (south) wall. Something that could rise from that first deck to the soffit and eves at the gable. On those two walls would be louvered vents, (think closet doors). A sort of two sided cupola. I'm entertaining variations as I type. Thanks, Pushkins for the lead....

pushkins
11-22-2010, 07:43 AM
Your welcome, good luck,