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Old 02-22-2012, 07:17 PM   #1
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In desperate need of advice...


on the best way to insulate my "to be" finished attic. We just found mold on the roof sheathing behind the poorly installed fiberglass in the rafter bays that was installed without ventilation. I'm going to have the mold treated and a ridge vent and soffit vent installed, but after countless hours of reading conflicting research on the web I'm still unclear on the best method to re-insulate.

Here are the details:

The home is in the northeast (NY)
The rafters are 7 1/2" deep and are spaced 21 o.c. (with a 19" cavity)
roof is a 9/12 pitch
Heating is baseboard radiator
1x6 collar ties form a 64" flat ceiling
I can't afford to spray the whole thing with closed cell foam
I want to finish the room with drywall once it is properly insulated

I would like to fur out the rafters and install baffles the length of the rafter bay from soffit to ridge and then install fiberglass with a continuous 6 mil poly. I read on buildingscience.com a spec for installing the baffles with foam to create a continuous air tight channel for the vent, but then I am unclear if it is still ok to installl a vapor barrier on top of the fiberglass?? Wouldn't I be creating a double vapor barrier potentially trapping moisture between the air tight baffles and the poly?

I will be grateful for any advice/opinions

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Old 02-22-2012, 07:42 PM   #2
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What do you mean by "1x6 collar ties form a 64" flat ceiling?" Are you saying that once this space is finished, the ceiling will be only 64" high? If so, it will be useless as living space.

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Old 02-22-2012, 08:08 PM   #3
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it sounds like you didnt have good air flow with packed insulation, adding baffles fluffy insulation then vapor barrier should be the way to go.
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Old 02-22-2012, 08:23 PM   #4
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Quote:
Originally Posted by md2lgyk View Post
What do you mean by "1x6 collar ties form a 64" flat ceiling?" Are you saying that once this space is finished, the ceiling will be only 64" high? If so, it will be useless as living space.
There is plenty of headspace. I meant the flat part of the ceiling spans 64" wide. The sloped part of the ceiling is about 10'8"... The height from floor to ceiling is 8'2"
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Old 02-22-2012, 08:30 PM   #5
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Quote:
Originally Posted by princelake View Post
it sounds like you didnt have good air flow with packed insulation, adding baffles fluffy insulation then vapor barrier should be the way to go.
That is true and I'm hoping adding the soffit and ridge vent will remedy that. My dilemma is whether I will have problems if I use a vapor barrier on the interior surface of the insulation if I also use the sealed baffle system pictured at the end of this article: http://www.buildingscience.com/docum...n-roof-venting
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Old 02-22-2012, 10:42 PM   #6
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i looked over that article theres no real talk about vapour barrier but lots about getting a good seal. it looks like from that i would say dont use it but what other way do you get a proper seal.
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Old 02-23-2012, 07:10 AM   #7
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From what I can deduce, both the sealed baffle system and the interior vapor barrier would have to be absolutely perfect otherwise any migrated water could get trapped in between. Do you know of a vapor barrier that works like tyvek or could I use tyvek?
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Old 02-26-2012, 03:38 PM   #8
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You could use Certainteed's "Membrain" as it is adjustable to moisture content. OR, just asphalt paper-faced (Class 2- vapor retarder) insulation, also variable with moisture;http://www.buildingscience.com/docum...vapor-barriers , rather than poly (Class 1- true vapor barrier).

Code in Zones 5-8, and 4C accept a Class 2 in walls/ceilings: http://publicecodes.citation.com/ico...001_par003.htm

With the ADA for drywall, you will be stopping most of the diffusion through the ceiling: http://www.buildingscience.com/docum...wall-approach/

That is mentioned here, under "Approach"- notice the last sentence: http://www.buildingscience.com/docum...on?full_view=1

The diffused moisture is far less than small openings around electrical boxes let in over a heating season- Fig. #4; http://www.buildingscience.com/docum...researchreport

Be sure to air seal the attic AND the crawlspace floor above to deter the "stack effect". Keep the air/thermal barrier intact throughout, especially at the knee-walls. More info if interested.....

If still worried, just increase the foam board thickness at the vent to reduce the risk of cavity temperature dew points- similar to a wall: http://www.greenbuildingadvisor.com/...ally-necessary
Good BSC links at the bottom of article....

Gary
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Old 02-27-2012, 12:38 PM   #9
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Thanks so much! I'll get to reading

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