building passive solar home. want to know efficiency of continuous ridge vent versus cupola vent. Prefer cupola re:aesthetics.
Â Just in general, a continuous ridge can provide a better ventilation ratio (1-to-150 to 1-to-300 recommended) In other words, 1sq. ft of open ventilating area for each 150 sq. ft (min, at 1-to-150 ratio) of attic space measured at its floor level. Besides, it would provide a better distribution ofÂ the hot air because it can escape from either part of the roof. With a cupola, the hot air would have to reach the cupola first. That said, with a large enough cupola you can still reach the recommended ratio.Â Please note that vented attic design in itself is not mandatory and is rather a design option.Â You do have sealed attic design available to you as a code-recognized option.Building Science has a great article by Joseph Lstiburek on the subject of attic ventilation. If you haven't read it yet, I strongly recommend it:
It does not review a cupola as an option but has plenty of general info on attic ventilation you may find useful in making an informed decision.
Thanks for the info.Â I have witnessed the capability of a sealed attic performing well.Â My design incorporates a (white) metal roof and HDM (concrete filled block).Â The house will be in climate zone 2/3 and I will be taking advantage of agressive p.m. whole house venting.Â Due to the HDM design my greatest concern is cooling in summer.Â I am hoping the HDM will allow the house to retain the early a.m. temp through the afternoon until the exterior temp dropsÂ to a manageble level when I will begn venting.Â I had planned on utilizing a hip roof but now am considering a hybrid hip/gable (gable at each end of apex of roof) continuing w/hip to eave.Â I believe this combined with a sufficiently large cupola will allow for sufficient venting.Â I will be incorporating a whole house fan for assist.Â Do you foresee any problems?
Not recounting problems, just having a few comments. I am not here to dissuade you from building your house just the way you like it. Especially considering that you've put so much thought into it already.
A large cupola may really look quite out of place on an average size house, visually large as if the house structure would not be able to support it. If that happens, it will always be the first thing that catches someone's eye when they first see the house. Especially considering that some of the ridge length will be taken away by the part hips on each side. The ratio between the ridge length and the cupola width is what makes this works visually.
Speaking of "aggressive PM venting" - are you close to the sea and trying to make use of predictable prevailing winds? You may want to investigate a possibility of constructing the cupola with louvers that you can control (possibly even automatically, like through reading the direction of a vane)Â - in a strongÂ wind you would definitely want the windward louvers closed and the opposing ones open.Â
Cupolas and heavy rain (especially with wind) don't really go together well. If you're in predominantly dry area, then it's not a problem. Otherwise you'd have toÂ figure out the way to mitigate the water that the wind will push through the louvers. You can make them overlap more but then the airflow would get restricted - a kind of a delicate balance here.
Should I assume that you were talking about an attic fan rather than a whole house fan (since we're talking about cupola)? Attic fans should blow outwards, thus reducing the pressure inside the attic and actually lowering the effectiveness of a cupola located above the ridge. I've never installedÂ one but there are cupola designs that incorporate a large fan that blows through the cupola chimney up and out through the louvers. That may be a better way to use both a fan and a cupola for hot air evacuation.
An insect netting should be incorporated into the cupola design (else wasps will appreciate the easy landing approach) thus further reducing its effectiveness for moving air.
If you have your mind set on a cupola appearance, would you consider making it into a (large and attractive but sealed) skylight rather than a part of the hot air evacuation system? Just a though...
Best of luck with your dream house!
Homeowner - thanks for taking the time and providing the information.Â One thing I did not make clear in my post is that the hybrid gable/hip (if that is the right terminology) will not shorten the roof line as much as a pure hip roof design.Â The gables would incorporate gable venting which should assist the cupola in total attic venting.Â The gables at the apex and each end of the roof would include gable vents then the hip roof would continue below them to the eaves.Â I am thinking the end gable vents combined with a properly scaled cupola will provide sufficient ventilation.Â The total (east/west) orientation of the house will be 50 ft. long.Â I will be incorporating a porch (total perimeter) around the house which will add another 20 ft to the lengthÂ and width of the footprint.Â Â
I have studied prevailing wind information that shows average 7-10 mph from SE but that doesn't preclude rainstorms that can be extreme as you point out.Â I have read some literature (taken w/a grain of salt -Â from vendors) that profess design that will prevent precip intake through the cupola and provide sufficient venting.Â I do not live w/in 100 miles of the coast but will certainly investigate the possibility of remote orÂ "sensed" control of cupola vents.
The whole house fan I am describing will vent the house from the highest interior ceiling into the attic.Â I am not technically proficient (or motivated) to "run the nubmers" on the dynamics of ventilation but am hopeful the end gable vents combined w/the cupola will evacuate sufficient air.Â
I will certainly incorporate your idea of insect netting - low cost to prevent possible future headaches.Â
The house will sit on 25 acres in a rural location so I dont have to worry too much about burglars & thieves & villians (oh my!) and will be able to open any/all doors and windows at night.Â
I really appreciate your thoughtfulness in responding.
Â Adam, best of luck with building your dream home! I can already picture you tending a vineyard with an impressive roof line or a beautiful house in the background :)
Yes, agree with the cupola at least for style purposes. It works not just for ventilation but style too. With the availability of highly resistant materials, you don't need to worry about early troubles. You can also get recommendation from a roofing contractor like
Hmm.. This is pretty much a matter of personal preference. Both types of vents help provide air circulation in the attic and allow for excessive attic air to escape through the sides on the top of the roof. The choice basically depends on the structure of the home and the climate of the area where the home was built.