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Roof/soffit vents for a (theoretically) fully insulated low-slope roof?

Roof/soffit vents for a (theoretically) fully insulated low-slope roof?

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Old 07-01-17, 07:05 AM
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Roof/soffit vents for a (theoretically) fully insulated low-slope roof?

Hi, I was wondering if I needed to add soffit vents to my low-slope roof that I'm in the process of insulating and a adding a sheet rock ceiling to? There are 2x6 joists at 24" CL, I'm planning on adding R-19 - 6.25"deep x 23"wide insulation batts that should fill the entire space.

For some reason I'm thinking that vents and air flow are more important in true "attic spaces" where there are large unheated areas above the insulation and condensation could be a larger problem when the hot and cold mix. While if the entire space is insulated there might not be the same issues?

Thanks
 
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Old 07-01-17, 07:20 AM
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Ventilation

I suggest ventilated soffit, baffles where the rafters meet the outside wall to allow free air circulation from the soffit to the attic, and ridge vents to allow the attic air to escape.
 
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Old 07-01-17, 04:01 PM
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I agree with Wirepuller you need the ventilation. You will never fit the code required amount in there but that is between you and the inspector. You cannot install fiberglass insulation directly against the bottom of the roof.

The concern is not the size of the attic space but the cold bottom of the roof and that is where the condensation will form.

While you are up there seal every possible leak from below. Air leaks are the major source of moisture.

Bud
 
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Old 07-01-17, 04:38 PM
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I probably should have started with a picture because I'm not sure what I described is coming across accurately.


I'm open to suggestions, but unless I use foam board as a top layer idk how I'm keeping the insulation from touching the roof... The entire space is only 2x6s deep. There is a soffit that over hangs the "bottom" wall by 5". I'm not sure I'm keeping the 1x2s that are 12" CL and originally holding up drop ceiling-*like* tiles.

And lolz, codes, inspectors, is this doityourself.com or hirecontractorsandtakeoutpermits.com? x] Haha just kidding. I do want to do it properly, which is why I'm asking.
 
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Old 07-01-17, 06:28 PM
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The 2009 IECC (International Energy Conservation Codes) show R-38 as minimum in a ceiling. The 2012 or 2015 newer codes may have increased that but it depends upon what your local authority is following.

Although we are volunteers here we have an obligation to mention permits and codes but what you choose to do is your choice. I could run through many stories where the lack of permits and inspections has come back in a big way and at a big expense. I also started contracting in Brick so I do know NJ isn't the easiest place to slide by on that issue.

But, an existing low slope roof is almost impossible to bring up to code without major remodeling. To get that air gap for ventilation with 2x6 ceiling joists you will baffles under the roof out until there is a sufficient height for the insulation to not close off the vent space.

Here is a picture where they are using several layers of rigid in place of your R-19, optional but expensive. The rest of the illustration seems to match what you need.
https://www.google.com/search?q=pict...4ZVM9Dini1JeM:

Bud
 
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Old 07-03-17, 05:45 PM
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Thank you for the advice. Just to clarify, like you touched on, this was a repair of an existing part of the house. The roof was leaking so we replaced it, and now I'm just looking to get it insulated and buttoned up as best as possible.

Yeah, R-38 would be nice but like you said, there's no room in the existing 2x6s...I'm even going to lose the R-value if I have to compress the 6.25" deep r-19 to make room for venting

Thoughts on using these type attic vents (Owens Corning Raft-R-Mate Attic Vent 22-1/2 in. x 4 ft.-70RM - The Home Depot) between every rafter, all the way up the low-slope roof (about 12') with the fiberglass insulation (slightly compressed) beneath it, until it hits the higher, normal pitched roof over the room next to it (that does have a gable vent)? It might not be an ideal amount of venting space, but at least there'll be some and keep the insulation from direct contact with the roof.

Thanks
 
  #7  
Old 07-03-17, 07:32 PM
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That vent should be fine. When possible I like this flap on this one (or others) that protects the end of the insulation from incoming air.
Amerimax Home Products 41 in. x 22 in. Accuvent Vinyl Attic Airway and Soffit Vent in Black-ACCUVENT - The Home Depot

Once again, air seal anywhere you can as air leakage has been named the primary source of moisture in attics.

The thinking that some is better than nothing is more true than you might expect. It turns out that our guidance for net free area came about back in 1942 with no known research or testing. Fortunately it has done little harm but how much any particular home needs is an unanswered question. I've seen many with no ventilation that have done fine for many years and others with lots of ventilation and still they have mold. Ventilation is a more complex problem not well suited for a one size fits all solution. I think yours will do just fine.

Bud
 
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