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Old 07-18-2010, 11:10 AM   #1
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Improve attic ventilation?


I think my attic ventilation is inadequate because on hot summer days the second floor is 10+ degrees warmer than the first floor. An hour or so after dusk the second floor cools off, but it's never as cool as the first.

One problem I know I have is the whole house attic fan is as air tight as a sieve when not in use. The hallway where it's located is extremely warm on hot summer days. I'm making an insulated cover for it that should help.

The attic is approximately 1000 ft^2. I have a gable roof and one side of it are six roof louvers. On the same side of the house as the louvers there are four 8x16 undereave vents. According to some information I found online, I calculate my current NFA as the following:

Intake NFA = 4 * 56 in^2 = 224 in^2
Exhaust NFA = 6 * 50 in^2 = 300 in^2

For a 1000 ft^2 attic, the my total current NFA seems adequate.

Required NFA = 1000 ft^2 / 300 * 144 = 480 in^2

524 in^2 > 480 in^2

I wouldn't be surprised if my intake vents are currently partially blocked. Last fall, in preparation for adding more insulation to the attic, I installed rafter vents. While doing this I may have pushed some blown in insulation onto the inside of the vents.

The first thing I should probably do is get up on a ladder, remove the vents, and clear away any obstruction. From information I've found online, I might also need to add more vents because my intake NFA is lower than my exhaust NFA.

Ultimately, I would like to add an exhaust fan to (at least) one of the louvers. The problem here is that no one seems to make fans that can be added to a louver. I guess I could just replace one of the louvers with a powered vent, but I'd rather not get up on the roof.

Any suggestions?

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Old 07-18-2010, 11:46 AM   #2
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Improve attic ventilation?


Where are you located ?

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Old 07-18-2010, 11:49 AM   #3
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Where are you located ?
In the Kansas City area.
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Old 07-18-2010, 12:22 PM   #4
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Improve attic ventilation?


Above 30 Air Changes per Hour seems like the point of diminishing returns for attic ventilation and some recommend as little as 5 ACH.
I'd make before-and-after temp. measurements in the attic to see what you're up against. Take the average of several readings.

Your insolation is not too bad.
http://www.solarpanelsplus.com/solar-insolation-levels/
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Old 07-18-2010, 01:02 PM   #5
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Improve attic ventilation?


It seems that builders have long ago forgotten a basic physical reality: Things that are less dense rise... like hot air.

I've never met the original poster before, nor been to it's home. But I will now reach out with my incredible mind and tell you astounding details about it's house!

Ready to be amazed?

1. The stairway from the first floor to the second floor is completely open.

2. The thermostat for the home is located in an out of the way spot on the first floor.

3. In the summer, the upstairs it too hot while the downstairs can sometimes seem too cool... in the winter the same is true.

Cold air is like water. It will flow down and pool at the lowest level of the house it can. Same for hot air, but upside down - it will pool in the highest parts of the house.

While your attic might be hot, it is Unconditioned space... let it be hot. You only care about the floor of the attic where you insulate. All you really need to do is seriously insulate the attic floor, but that will only keep the heat from going down into the second floor. It does nothing for the heat already IN the house from traveling and pooling on the second floor.

A couple of strategies for you:

1. Get a programmable thermostat that has a function allowing you to run the fan alone at periodic intervals independent of temperature. Say 5 or 10 minutes of fan every hour. This will keep the hot and cold air mixing.

2. Put a door at the top or bottom of your stairway. This will keep your hot/cold pools separated into two different zones. Grand open stairways were great tools for moving air in the days before closed windows and HVAC. Now they are an architectural relic that only serves to look pretty and make us feel miserable.

3. Move your thermostat into an area that more closely resembles a living space. If there is one room downstairs that is hot when the upstairs is hot, that is where you want the thermostat.
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Old 07-18-2010, 05:35 PM   #6
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Hot air rises and cool air sinks. I can't believe I've gone several decades without realizing that. You would think that someone who takes it upon themselves to do their own house repairs and even seeks out the best way of doing so would have come across that basic principle of physics by now.

I realize there are other things I need to do and can do inside the house. My focus right now (and the purpose of my original post) is attic ventilation.
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Old 07-18-2010, 05:51 PM   #7
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Improve attic ventilation?


I think air infiltration is the worst thing here. If you go around your attic with a tube of caulk and can of spray foam, you can do a lot to keep the hot air from coming into your house. Every pipe or wire that comes thru the top plate needs to be sealed around. Every electrical box in the ceiling needs a layer of spray foam over it like a cocoon. When I replace ceiling fans I can't tell you how much air I get blow in my face from the canopy, it comes right thru all those little holes in the electrical box and into the house, dust and insects too. Any high hat needs to be replaced with an airtight IC rated model.

Having the living area of the house sealed off from the attic is the main issue. After that, a layer of good insulation will be enough. I do a lot of electrical work in attics, it will be 130 degrees up there yet I pull up a measly piece of R-19 insulation and the metal electrical box will be cool to the touch, most of the heat is going UP, not down into the house. Having good ventilation might help your home a bit, but not THAT much. It will help your roof quite a bit, tho. You also need some ventilation to keep the moisture at bay.
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Old 07-18-2010, 05:51 PM   #8
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suggestions to what? you seem to have more than required venting,your gonna fix the intake vents you screwed up,your not going on the roof,your gonna make an insulated cover for the leaky fan one day...i think your doing fine
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Old 07-18-2010, 07:42 PM   #9
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Originally Posted by Proby View Post
I think air infiltration is the worst thing here. If you go around your attic with a tube of caulk and can of spray foam, you can do a lot to keep the hot air from coming into your house. Every pipe or wire that comes thru the top plate needs to be sealed around. Every electrical box in the ceiling needs a layer of spray foam over it like a cocoon. When I replace ceiling fans I can't tell you how much air I get blow in my face from the canopy, it comes right thru all those little holes in the electrical box and into the house, dust and insects too. Any high hat needs to be replaced with an airtight IC rated model.
I spent many weekends last fall doing this very thing. Every crevice and hole I could find got filled with caulk or foam. I even replaced all my old recessed lighting fixtures with new ICAT fixtures. I did all of this in preparation of adding insulation, but I still haven't gotten around to it.

Looks like getting the extra insulation in needs to be my primary goal.
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Old 07-18-2010, 08:04 PM   #10
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"On the same side of the house as the louvers there are four 8x16 undereave vents." ------- So all intake are on one side of a gable house? What intake is on the other side?

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Old 07-18-2010, 09:07 PM   #11
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"On the same side of the house as the louvers there are four 8x16 undereave vents." ------- So all intake are on one side of a gable house? What intake is on the other side?

Be safe, Gary
Yes, all on one side. No intake on the other side, at least none that I can see on the undereave.
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Old 07-18-2010, 09:27 PM   #12
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Improve attic ventilation?


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all on one side. No intake on the other side, at least none that I can see on the undereave.
So only a part of your attic sees moving air.
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Old 07-18-2010, 11:40 PM   #13
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And you do know when figuring the NFVA to divide by 300 WITH a vapor barrier installed between ceiling/insulation, otherwise use 150....... And that is with plastic, not a vapor retarder paper on the insulation: http://www.airvent.com/homeowner/pro...it-specs.shtml

http://www.energysavers.gov/your_hom.../mytopic=11810

http://www.advancedenergy.org/buildi...%20Studies.pdf

Be safe, Gary
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Old 07-19-2010, 08:35 AM   #14
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Improve attic ventilation?


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Originally Posted by Proby View Post
I think air infiltration is the worst thing here. If you go around your attic with a tube of caulk and can of spray foam, you can do a lot to keep the hot air from coming into your house. Every pipe or wire that comes thru the top plate needs to be sealed around. Every electrical box in the ceiling needs a layer of spray foam over it like a cocoon. When I replace ceiling fans I can't tell you how much air I get blow in my face from the canopy, it comes right thru all those little holes in the electrical box and into the house, dust and insects too. Any high hat needs to be replaced with an airtight IC rated model.

Having the living area of the house sealed off from the attic is the main issue. After that, a layer of good insulation will be enough. I do a lot of electrical work in attics, it will be 130 degrees up there yet I pull up a measly piece of R-19 insulation and the metal electrical box will be cool to the touch, most of the heat is going UP, not down into the house. Having good ventilation might help your home a bit, but not THAT much. It will help your roof quite a bit, tho. You also need some ventilation to keep the moisture at bay.
So there is no consequence to taking a can of Great Stuff and just burying an electrical box (Light receptacle) from inside the attic. Besides if you do have to do work on that can or change it the pain of removing the foam.
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Old 07-19-2010, 09:02 AM   #15
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So there is no consequence to taking a can of Great Stuff and just burying an electrical box (Light receptacle) from inside the attic. Besides if you do have to do work on that can or change it the pain of removing the foam.
No, I did not say to spray foam a can (high hat), I said to buy air tight IC rated high hats.

As for an electrical box, if you need to work on it, you do it from inside the house.

If you ever need to remove that box or change around the electric in some way, the spray foam will be the least of your problems.

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