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Old 05-12-2011, 09:00 AM   #1
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Adding attic ventilation


My house is a bungalow/ranch style home. Air does not move thru it very well. Even when the AC is working (which it's currently not ) the house is still uncomfortably warm and the AC is working too hard, IMO. There is NO ventilation on the roof to siphon hot air out of the attic. The roof is probably less than 5 years old and still in good shape. The attached garage is miserable in the summer heat as well. I'd like to add some roof vents to purge the hot air out of these areas beneath the roof. What's the best method to go about doing this for someone who has never worked with roofing before? I should add that the soffet is vented (perforated) but there are no vents along the roof.


Last edited by kadetklapp; 05-12-2011 at 09:12 AM.
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Old 05-12-2011, 12:08 PM   #2
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Adding attic ventilation


Regarding the perforated soffit vents, it is very common for that product to have been installed without the necessary removal of the original wood soffit material that it is covering up, if it was a retrofit installation, so that should be checked out and verified, otherwise, those minimal perforations are now able to function at all.

As I am sure you are aware, simply put, heat rises, so since there is no remedy to allow the heat buit up in the attic space to escape, no new cooler exterior fresh is getting pulled in to replace the exhausted hot attic air, even if the soffit vent perforations are worrking.

Since you have a ranch style home, the most commonly held conclusion is that a continuous Ridge Vent would be the best option to adequately lower the attic temperature.

A well constructed Ridge Vent should contain exterior wind deflecting baffles and an interior filter, such as the Shingle Vent II, manufactured by the Air Vent Corporation.

http://www.airvent.com/professional/...ngleVent.shtml

The most optimum balance, calculated by the total Net Free Air Intake and Exhaust, would be to have 60% from the Intake Vents and the remaining 40% from the Exhaust Vents.

Ed

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Old 05-13-2011, 10:21 AM   #3
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Adding attic ventilation


Well it would appear that I already have ridge venting. Not knowing what it was, I just assumed there were no vents. But it appears that the ridges of both the house and garage are vented. My soffit is correctly vented. So why in the world do I have such horrid heat build up? There are no trees on my property, so I have no shade.
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Old 05-13-2011, 10:31 AM   #4
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Adding attic ventilation


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Originally Posted by kadetklapp View Post
Well it would appear that I already have ridge venting. Not knowing what it was, I just assumed there were no vents. But it appears that the ridges of both the house and garage are vented. My soffit is correctly vented. So why in the world do I have such horrid heat build up? There are no trees on my property, so I have no shade.
You might look into adding more insulation.
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Old 05-13-2011, 11:00 PM   #5
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Adding attic ventilation


Which part of the country do you live in? Consider adding a power vent.

Natural convection works, but is very slow.

Adding insulation will heat infiltration into the living space, but your attic will still be hot, if not hotter.
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Old 05-14-2011, 08:41 AM   #6
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Adding attic ventilation


Adding a power vent does not work with ridgevent. It will draw air from the closest point which may be the ridgevent. this would render your soffit vent useless.
go in your attic to check if the plywood on your ridge has been cut back far enough. if its only 7 inch vent & cap with blocking between the trusses getting an inch of venting on each side is tough.
A 9" or 11" vent will allow you to cut that plywood down further.
How much ridgevent do you have? only 10 ft would hardly be adequate.
If the plywood is cut 1" on each side every 3 ft is equal to 1 -75 sq inch vent 39 ft =13 vents. this is a huge amount of venting if installed properly!
If you have a ridgevent section 3 ft below the main ridge it may be that air is being drawn from this vent & not your soffits.
On gables the plywood should not be cut at the overhang as this may also provide an unwanted draw of air

Dale Chomechko
DC Roofing Inc
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Old 05-14-2011, 10:31 AM   #7
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Adding attic ventilation


The key to owning a power vent is having the on/off switch in the living space. You can turn it on/off as you wish.

But, if the ridge vent/soffit combo was doing its job by itself, the power vent wouldn't even kick on. Set the thermostat higher if you desire to.

A power vent moves air faster, as warm air rises, it will move out the hot air.

Someone on this forum showed pictures of a smoke test that was performed, and with the ridge vent/soffit combo, there was hardly any air movement.
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Old 05-14-2011, 05:46 PM   #8
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Adding attic ventilation


The reason for your smoke test showing less is because all your air is going to one power vent
Yes a power vent will move more air but a ridgevent is continuous throughout the house
If you used only one power fan on a large house it may not only create dead zones but as I have seen several times, air flowing through one place makes on area of the roof cooler and can buckle the shingles. It almost looks like moles tunneling underneath them.
I am not saying its a bad idea, just that all the areas of the house need consideration

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DC Roofing Inc
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Old 05-14-2011, 06:57 PM   #9
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Adding attic ventilation


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Originally Posted by Dale Chomechko View Post
The reason for your smoke test showing less is because all your air is going to one power vent
Yes a power vent will move more air but a ridgevent is continuous throughout the house
If you used only one power fan on a large house it may not only create dead zones but as I have seen several times, air flowing through one place makes on area of the roof cooler and can buckle the shingles. It almost looks like moles tunneling underneath them.
I am not saying its a bad idea, just that all the areas of the house need consideration

Dale Chomechko
DC Roofing Inc
The person who posted the pics from the smoke experiment was from a house that did not have a power vent.

WRT "one power vent on a large house," that issue can be addressed via two lower rated power vents evenly spaced on the roof. As you may well know, power vents are rated by how much air they can move in one minute.
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Old 05-15-2011, 02:17 PM   #10
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Adding attic ventilation


The smoke test was not done on a housem but a cardboard mock-up instead just for illustration purposes.

All Ridge Vents DO NOT operate as efficiently as the other.

Roofers/Carpenters sometimes neglect to either cut out the slot line or cut the slot too thin, not to the manufacturers specifications and also, sometimes, even if the slot is cut out properly, the felt paper or the shingles cover it up.

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Old 05-15-2011, 05:19 PM   #11
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Adding attic ventilation


This video is of a smoke test on a cardboard scale model mockup which illustrates why the idea of a "ventilation short circuit" is a myth.
http://www.roofingcontractorreview.c...cuit-Myth.html

This video is of a real house/attic illustrating the less than stellar performance of ridge and soffit vents.
http://www.roofingcontractorreview.c...moke-Test.html

This video is of the same attic/house after installing a power roof fan, which illustrates the power fans effectiveness in removing hot air from the attic. Watch closely and you will see that the hot air is drawn from the main attic space (the path of least resistance) and not the nearby ridge vent.
http://www.roofingcontractorreview.c...tic-space.html

Can anybody point out any similar tests or scientific studies that do not agree with the above?
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Old 05-15-2011, 09:50 PM   #12
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Adding attic ventilation


Clearly, real test data trumps theories any day.
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Old 05-16-2011, 06:51 AM   #13
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Adding attic ventilation


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Originally Posted by kadetklapp View Post
Well it would appear that I already have ridge venting. Not knowing what it was, I just assumed there were no vents. But it appears that the ridges of both the house and garage are vented. My soffit is correctly vented. So why in the world do I have such horrid heat build up? There are no trees on my property, so I have no shade.
You partially answered your question. Roof color, orientation, outside temperature, location, etc all play into the equation. No matter how well the attic space is vented, it's always going to be hot up there on a sunny day.

Power vents can be a solution, but before installing one, it's important to make sure there is adequate inlet and/or the insulation plane is air sealed. If the inlet is inadequate, the attic will become depressurized and conditioned air can be sucked from the living space - not a desirable result.

Power vents have a limited life and if installed correctly, should not be audible from the living space. So, an annual check should be made to make sure they're still operating. We find many during re-roofs that have been dead for years. Also, you're increasing your electricity consumption added to the cost of the fan and subsequent replacement. I'm guessing the average power vent lasts about 6 years. No hard data to back that up - just a feel from encountering dead units.

Solar powered units, while free to run cost considerably more and quit working at sunset. The embodied heat stored in the framing/roofing still needs to be exhausted after the sun stops increasing the gain, so they may not be a good choice for primary venting.

If your living envelope is insulated properly, the main reason to vent the attic is to expel moisture. High attic temperatures may reduce the life of asphalt shingles, but the question is "does it reduce the life of the shingles enough to justify buying and installing 3 power vents over the life of the roof plus the power to run them"? Probably not. If you have "by the book" venting in place, I'd spend my money on air sealing and added insulation.

edit: I'd not be too concerned about the "short circuit' effect theoretically created by mixing types of exhaust vents. IMHO, you can't "over vent" an attic. Even if there is some "short circuiting", moisture and heat are still exiting.


Last edited by seeyou; 05-16-2011 at 06:57 AM.
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