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Old 12-06-2012, 05:50 PM   #16
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Too much soffit venting?


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No, the whole idea is to keep the underside of the roof deck (Including the roof deck at the eaves) close to the outdoor temperature. did you read my post on ice damming? Most of the guys posting on here don't seem to understand what is going on, so it is easy to be misled.
Yes I read your post and it agrees with my existing understanding of ice damming which is why I paid for air sealing and insulation already.

What I'm struggling with is whether there is an opportunity on a localized basis to further help the roof avoid taking on heat by actually applying foam to the decking. Not a fully enclosed attic but just to help a certain area that leads to a valley stay really cold so I don't get melt down into the valley.

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Old 12-06-2012, 08:47 PM   #17
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Too much soffit venting?


You absolutely do not want to do that as all it will accomplish is to cook your roofing system. If you have insulation in your attic floor,that is where you want to stop hot air/moisture loss. Your roof deck should stay ventilated. to stop moisture migration, you should have a vapor barrier on the warm side, usually, drywall ceiling, vapor retarder, insulation. You want the dew point to fall in the middle of the insulation

You want outdoor air to flow up through the soffit, along the underside of the deck and out the ridge vent.
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Old 12-06-2012, 10:14 PM   #18
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Too much soffit venting?


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You absolutely do not want to do that as all it will accomplish is to cook your roofing system. If you have insulation in your attic floor,that is where you want to stop hot air/moisture loss. Your roof deck should stay ventilated. to stop moisture migration, you should have a vapor barrier on the warm side, usually, drywall ceiling, vapor retarder, insulation. You want the dew point to fall in the middle of the insulation

You want outdoor air to flow up through the soffit, along the underside of the deck and out the ridge vent.
Sounds good. I have already seen some improvement with the air sealing and insulation, so fingers crossed for when the snow falls. Last winter I couldn't get the attic below 42/43 even when outdoor was 15. This year it seems to be holding about 10-15 above, so when temps went to 20, the attic got down to 35. In warmer temps it gets closer (tonight it is 46 outside and the attic is 50.5) presumably because the closer you get to indoor temps the easier it is to be the same. Part of the issue also is that I have a furnace and ductwork in the attic, in a separate drywalled room but still it creates heat. But the better insulation now means it doesn't run as much, and with less heat coming up thru the can lights etc, it is easier for the roof exhausts to get rid of the heat that makes it out of the furnace room and into the attic. I know - bad design - but the builder did it that way before I bought the house, and I ain't ripping out my zone 2 HVAC.
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Old 12-07-2012, 06:06 AM   #19
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Too much soffit venting?


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Originally Posted by jagans View Post
You absolutely do not want to do that as all it will accomplish is to cook your roofing system. If you have insulation in your attic floor,that is where you want to stop hot air/moisture loss. Your roof deck should stay ventilated. to stop moisture migration, you should have a vapor barrier on the warm side, usually, drywall ceiling, vapor retarder, insulation. You want the dew point to fall in the middle of the insulation

You want outdoor air to flow up through the soffit, along the underside of the deck and out the ridge vent.
+1

As jagans has said, your envelope and insulation layer should be the attic floor and should be continuous and unbroken. If the air sealing contractor did their job effectively, the improvement over the original construction should be significant.

Depending on where you are located, the painted drywall is the vapor retarder level and there is no additional barrier required.

Air movement carries the bulk of the moisture so if you got that buttoned up you are way ahead of the curve.

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Sounds good. I have already seen some improvement with the air sealing and insulation, so fingers crossed for when the snow falls. Last winter I couldn't get the attic below 42/43 even when outdoor was 15. This year it seems to be holding about 10-15 above, so when temps went to 20, the attic got down to 35. In warmer temps it gets closer (tonight it is 46 outside and the attic is 50.5) presumably because the closer you get to indoor temps the easier it is to be the same. Part of the issue also is that I have a furnace and ductwork in the attic, in a separate drywalled room but still it creates heat. But the better insulation now means it doesn't run as much, and with less heat coming up thru the can lights etc, it is easier for the roof exhausts to get rid of the heat that makes it out of the furnace room and into the attic. I know - bad design - but the builder did it that way before I bought the house, and I ain't ripping out my zone 2 HVAC.
HVAC in the attic is one of the biggest issues in today's construction. Hopefully the air sealing crew addressed all the ductwork for air tightness and insulation value.
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Old 12-07-2012, 05:15 PM   #20
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Too much soffit venting?


You are losing heat from that attic unit somewhere, Is the plenum chamber insulated?
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Old 12-07-2012, 09:27 PM   #21
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Too much soffit venting?


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You are losing heat from that attic unit somewhere, Is the plenum chamber insulated?
The furnace/AC unit is basically boxed into its own little drywalled room in the attic, with a sprinkler head for fire suppression, etc. The little room is insulated with fiberglass batts and tyvek on the outside. There is a sliding door to close it off, although I must leave it cracked open a bit so the furnace can intake its combustion air. The ductwork in the attic is in good shape, and is covered in R-6 sleeve insulation.

I probably would have asked if there was an alternative but I bought the house after it was already mostly built, and I'm not changing it now.
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Old 12-08-2012, 09:54 PM   #22
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Too much soffit venting?


Why dont you bring combustion air in from the outdoors through PVC Piping, and properly seal that door? Heated air is of higher pressure and is forcing its way out into that attic. The vapor retarder should have been on the inside of the wall on the warm side. Under the drywall.
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Old 12-08-2012, 10:09 PM   #23
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Too much soffit venting?


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Why dont you bring combustion air in from the outdoors through PVC Piping, and properly seal that door? Heated air is of higher pressure and is forcing its way out into that attic. The vapor retarder should have been on the inside of the wall on the warm side. Under the drywall.
The tyvek was just something that the air sealing guys added to help seal around the fiberglass batts on the walls of that space. I'm not sure how much water vapor there is coming from that room - it is not an inhabited space. It is just a box around the HVAC unit - standup height for someone to be in there if they need to change the filter or work on the unit.

I've thought about adding another intake to the room but that requires more $ and another hole in the roof. Also the intake needs to be 100 square inches as my furnace is 110,000 BTU. How you gonna do that with a PVC pipe?
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Old 12-08-2012, 10:17 PM   #24
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Too much soffit venting?


Can you bring fresh air in through a duct that runs to the soffit?

The differential temperatures you stated meant that you are getting quite a bit of heat into that attic. Its coming from somewhere.
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Old 12-08-2012, 10:27 PM   #25
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Too much soffit venting?


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Can you bring fresh air in through a duct that runs to the soffit?

The differential temperatures you stated meant that you are getting quite a bit of heat into that attic. Its coming from somewhere.
I thought 10-15 degrees was somewhat normal?

Before the air sealing I ran a test last year and left the heater off all night. The attic temp got about 3 degrees cooler that night than it had the night before. Last year it NEVER got below 42 with the heater, as I believe there was significant 68 degree air leaking up through the cans and attic floor. After the air sealing, the temp made it down to 35 on a cold night with the heater still active.

Your idea of a duct is possible I guess.
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Old 12-08-2012, 10:55 PM   #26
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Too much soffit venting?


8" duct is 50.27". Make sure the screening/louvers are subtracted for only NFVA.

Where are you located?

Gary
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Old 12-08-2012, 10:58 PM   #27
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Too much soffit venting?


I am in Chicago

So I'd need to run 2 parallel sections of 8" duct?

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