Mounting Solar Panels Over Passive Attic Vents
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Will panels installed over passive low profile attic vents have an effect on attic ventalation?
In particular I am looking at a full hip roof that is all cut-up, most workable roof planes are small and have multiple passive attic dormer style vents. The roofing is concrete tile and the vents are shaped to blend in with the tile.
There will be 4-5" between the tile/vents and the panels, so no loss in overall vent area.
I have been researching attic vents, and what info I have found, is mostly unclear and often contradictory. If anyone has info or can point me in the direction of some, it is greatly appreciated.
In particular I am looking at a full hip roof that is all cut-up, most workable roof planes are small and have multiple passive attic dormer style vents. The roofing is concrete tile and the vents are shaped to blend in with the tile.
There will be 4-5" between the tile/vents and the panels, so no loss in overall vent area.
I have been researching attic vents, and what info I have found, is mostly unclear and often contradictory. If anyone has info or can point me in the direction of some, it is greatly appreciated.
Comments
Yes, they can.
Beyond that it gets pretty complicated. There's many types of roof vents, with different outlets. Some are directional and some are "all around". The first issue is whether or not the existing vents are functioning as they should. In a surprisingly large number of cases the answer is "no", which means putting a panel above them doesn't change anything.
As a rule-of-thumb, if the panel is the same distance above the vent top as the vent top is above the roof air flow will not be seriously affected.
In some cases the panel heating up will create more draft across the vents (chimney effect).
Sounds like you've got just about the worst roofing scenario possible for installing PV's. Concrete tiles, blended vents, multiple facets ... ARGH!
From some testing of solar panels and roofing for fire ratings... Link from poster:
Flammability Testing of Standard Roofing Products in the Presence of Stand-off Mounted PV Modules
It appears that ~5" is optimum for a "chimney effect under a solar panel.
Over 5", the panel had much less effect on flame spread.
Below 2", the panel did not affect flame spread (no "useful" chimney effect).
So, basically, that is how I would look at the attic vent issues... Using flame spread as an allegory for general air circulation issues. I think California gives their maximum RE Rebates if the panels are something like 5-6 inches, or more, higher than the roofing (more air gap, better air flow, better cooling, higher Vmp voltages).
How much air gap are you looking at?
-Bill
Bill;
So he's sort of at that critical will it/won't it point!
It depends... Maybe, Maybe not...
-Bill
Isn't it nice of us to reinforce the confusion?
I found that placing solar panels on my uninsulated roof really cooled down my attic by a whole bunch... Probably more than the roof vents would by themselves.
-Bill
Sadly, I think this is the most correct answer for this situation.
One of our local AHJs has banned the positioning of PV modules over roof vents. Don't know why, I don't ask them much anymore. They pretty much have beat me into submission.
Ventilation is created by air convection, heating of the air in that attic. The purpose of the vents is not to cool the attic but dehumidify the air. Block soffit vents for one winter in Seattle and be prepared to scrap the roof due to mold & rot.
From a fluid flow perspective less heating in the roof will draw less air flow, this could cause issues whether the panels are covering the vents or not. Installing panels over roof vents is not going to restrict airflow at all, air flow is caused by a differential pressure. Air enters the attic, heats up, expands, pressure increase, hot air leaves the roof. Maybe in certain wind conditions pressure variations could cause reversal of flow or increase flow. But in all no effect other then less heating of the attic and less airflow.
The OP is in Boise. They don't have the severe moisture problems like Seattle - or Vancouver, B.C.
The process is the same, whether you're removing moisture or just heat.
Even in dry climes, venting attic spaces is required to vent moisture that permeates the vapor barrier, assuming there is one. In the winter, warm heated air mixes with cold attic air and condenses. Venting is critical.
I personally would not worry about covering roof vents with PV. If the roof is properly designed, it will have vents on both aspects of a roof, while PV will only be on one. Now if the inspector says you can't do it that is another matter, but one can cite the UBC chapter and verse regarding attic vents, and in my older edition of the UBC the ratio of venting to attic area was prescribed, as was the relationship between intake and exhaust vents. If you have covered the code, I don't imagine that it is going to effect much.
Tony
I have come across a manufacturer of vents that has actually taken this into consideration and has literature on the subject. Basically says its ok to mount panels 4-6" above the top of the vents.
I have info and can post a link to the manufacturers site if private links are ok.
Not a problem. Links that answer questions, commercial or not, are always welcome.
-Bill
After trying to contact a few different manufacturers of low profile passive attic vents, O'HAGIN'S INC., is the only one to reply so far.
Here is a link to a PDF doc. found on their website that specifically addresses thier vents used in conjunction with photovoltaic panels.
http://www.ohaginvent.com/pdfs/PhotovoltaicPanel.pdf
And here is their website.
http://www.ohaginvent.com/default.asp
I don't think it's allowable to place a panel (or anything) over a plumbing vent, but a simple attic vent should be OK.
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As long as the vent isn't actually blocked, no worries. Plumbing vents' main purpose is to let air in so the drains work right. You'd be surprised how little venting that takes.