Ever a good idea to install a powered roof vent?
Mike_LCBI
| Posted in Energy Efficiency and Durability on
I know these powered roof vents get a bad rap, but is it ever a good idea to install them?
Such as if the attic floor (ceiling below) has been air sealed. This way the powered roof vent should not be pulling conditioned air from the house, rather it’s pulling mainly from the soffits or other areas of easy entry.
Also assuming, the power roof vent is temperature controlled and electric efficient.
Zone 4 represented
Thanks
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Replies
I would say no. The money would be better spent addressing problems the roof vent was meant to cure.
First off a roof vent is going to depressurize the attic. Any leaks from the interior are going to be exaterigated (sp). So more air is not leaking from the house. Also you need to understand that radiant heat tranfer is greater than convection (hot air).
The roof is giving off more of its heat via radiant heat which more will pass through the air pourus fiberglass insualtion. This is what will reach the interior.
The hot air in the aittic will rise and fow out with proper static vents. It will take a large amount of air flow to keep the attic close to the exterior temp, That air flow will have a negative effect on insulation and will pull air from the house.
Spend the money on air sealing and adding cellulose insulation.
Robert is right. Even before considering the cost of the electricity required to run these units, attic fans often increase rather than decrease your energy bill (because they tend to pull conditioned air out of your home). Remember, no ceiling is airtight. Even after you have sealed your ceiling, all you have done is reduced, not eliminated, air leaks.
Add on the fact that you have to pay for the electricity to run these things, and you really end up losing.
I would have to respectfully disagree. I believe there are times that a power vent makes sense. For instance a hip roof, or a complex roof with ridges at multiple levels.
They get a bad rap because they are almost always installed incorrectly.
I do agree that static vents would work fine if your roof design allows for them and you should always follow some basic rules of attic ventilation. I believe Robert is correct about the air sealing and having the proper amount of insulation, but we don't know what you have. Your house may already be at the optimal level. You should still access you ventilation needs and make sure it is appropriate.
1. Size the ventilation requirements properly, Do the math, always have more intake than exhaust. This prevents the negative pressure in the attic. Air will come from the path of least resistance, you need to check everything in your attic and make sure your least resistance is the open soffit vents.
(While your in the attic make sure the bath fans/ fart fans hoses exit the roof properly and are not just thrown over in the soffit area.)
2. Never mix exhaust ventilation types. You should not have static vents, ridge vents with a power vent. This is an incredibly common problem.
3. Throw the thermostat that comes in the box away and get a humidistat/ thermostat. Set the controls appropriately for your area so this thing runs only when is needed.
4. If your house is so large that 2 power fans are required, they should be wired together so they come on at the same time. The manufacturer will let you know how many can be tied to the same thermostat.
5. Don't cover exhaust vents with plastic or turn them off in the winter time.
You should always install exhaust vents at the highest point on the roof but not be able to see the vent top sticking up from the front of the house. Set the vents where the top of the vent is level with the ridge.
Ronnie
3rd generation roofing contractor
Ronnie,
And what problem is this powered attic ventilator trying to correct?
If the ceiling is sealed and the attic insulation meets code, then attic temperatures are irrelevant -- unless, of course, you have ducts in your unconditioned attic.
The only problem I am trying to solve is proper attic ventilation. There are roofs that you cannot achieve this with static or ridge vents. I don't agree that attic temperatures are irrelevant.
We only use solar powered attic fans for reaching code required ventilation for the attic. We never use hard wired attic fans for ventilation because of the energy penalty. Solar powered fans work great mostly for aesthetic purposes and to minimize dormer vents or even worse those unsightly and noisy whirlybird vents. We mount the solar powered fan high on the ridge and the panel on the South or West facing portion of the roof to maximize the run time. We typically use ohagin vents for the lower ventilation mounted about 14" from the the floor of the ceiling so we can insulate directly over the top plates of the exterior walls at the eaves. This also allows easy installation of any attic insulation baffles. Our clients are always happy when we are done with their new roof because it looks cleaner with less penetrations and large venting.
http://www.hardingconstruction.biz
@Martin your comment comes across as pretentious and condescending. Of course attic tempuratures are relevant; that heat is radiated through the attic materials. My ceiling is significantly warmer on a sunny day, and the insulation meets code. Building code is only the allowable minimum. Building to code is not a cure-all for insulation problems.
Jeremy,
The intent of my comment was to be reassuring; I'm sorry it came off as condescending.
You are correct: Building to code is not a cure-all for insulation problems. It's important for builders to meet code minimum levels for insulation, but it's also important that (a) your ceiling be airtight, and (b) the insulation be carefully installed.
It's legal in most areas of the U.S. for builders to install ducts in vented unconditioned attics. Even though that's legal, I strongly recommend that GBA readers keep ducts out of unconditioned attics. If you follow this advice, and if the insulation on your attic floor meets or exceeds minimum code requirements, and if your ceiling is close to airtight (as it should be), then the temperature in your attic is irrelevant to occupant comfort and energy bills.
You can safely stop worrying about the temperature of your attic if you follow this advice.
Here's another way to say what I'm saying: if you want to reduce the rate of heat flow through your code-minimum insulation layer (R-49 in zones 4 and colder zones, or R-38 in Zones 2 and 3) -- a rate which is already tiny -- then the most cost-effective approach is to install more cellulose on your attic floor, not to install a powered attic ventilator.
There is at least a 40 year history of studying and carefully measuring the energy use, shingle life, and cooling effects of attic ventilation, much of which is compiled here:
http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1496-05.pdf
The upshot is that most of the time it will increase total energy use (AC + attic fan). The exceptions are when the fan is fairly modest cfm, self-powered (solar), at modest insulation levels:
http://www.fsec.ucf.edu/en/publications/html/FSEC-GP-171-00/
"Although attic ventilation has been shown to reduce attic air temperatures and cooling loads the only examination of powered attic ventilators has shown the electricity consumption of the ventilator fans to be greater than the savings in air conditioning energy (Burch et al., 1979). In recent years, however, photovoltaic ventilator fans have become available which have no parasitic consumption of line electricity. These tend to be expensive, but are easier to install since no wiring is required."
and
"Comparing periods with similar weather conditions, the test revealed that the PV vent fans have the potential to reduce measured peak summer attic air temperatures by over 20oF. However, the impact over the cooling season is fairly modest with well insulated attics. Measured space cooling reduction was approximately 6% - worth about 460 kWh annually at the test home."
Note: The "well insulated" attic in this case was R19 blown fiberglass plus radiant barrier, not current code minimums. Current IRC code min attic insulation in most of FL is now twice that (R38) and in zone 4 it's 2.5x that (R49).
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm
Ronnie's comment number 3 has a start at describing what it would take to make a high performance attic ventilation system. As is often the case, the Europeans are ahead of us and have actually figured it out. Ventotech has commercialized a concept developed with Chalmers University of Technology in Gothenburg, which controls a fan and a damper based not just on relative humidity, but based on dew point to ensure that outside air is brought it only when it will actually reduce the humidity in the attic. The fan is a high-efficiency ECM unit. http://www.ventotech.com/index_en.html
Testing shows that it it effective in keeping the attic conditions susbtantially further away from mold potential
http://www.ventotech.se/docs/FinalCopenhagen2008Color.pdf
I would not recommend powered attic ventilation with anything less than that system. But there is not a North American version available, as far as I can see. When there is, I want one for my shed.
Charlie,
Remember, in Sweden, ceilings are airtight. In the U.S., very few ceilings are airtight.
While an attic fan may be sometimes useful in a Swedish attic, it will usually cause more problems than it solves in a U.S. attic.
In Sweden something like 2/3 of all new homes are built in factories, and the building efficiency codes are performance based, field verified post installation, with penalties for both the designers & builders if they screw it up. The financial incentives for more sophisticated ventilation controls are clearer in Sweden than in North America.
A common misconception in the US is that attic ventilation is about attic cooling, which is at best a secondary function, the primary function being purging moisture. In the humid southeast in air conditioned houses attic ventilation often brings more moisture into the attic than it purges, since the outdoor dew points can dwell above the indoor ceiling temps for weeks or even months at a time.
Martin, I agree. The smart dew point based control and the efficient fan are only part of what makes that system work--excellent air tightness is essential. I implied that it would be a good thing if that product was available here, but the number of attics where it would be helpful is quite limited, and ready availability might cause more harm than good.
I still want one for my shed.