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Old 03-17-2014, 12:00 PM   #1
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Rafter vents needed if no edge vents?


Hi, I've talked to contractors and others and got conflicting opinions on whether I need vents or not and what thickness insulation to use.

My roof has 1) no edge/soffit vents and 2) valleys. I do have a ridge vent across the top. The house was built in 1948 and has 2x6 rafters. I have no plans to make these bigger (2x8, 2x10 or 2x12). The 2nd floor is occupied (i.e., not an attic) and will have sheetrock. Above the ceiling is a 2ft attic space. There are 3 1'x1' gable vents, 2 are visible in the picture. There is no possibility of adding edge/soffit vents in the valley, which makes up the entire front half of my house.

From my understanding, foam/plastic baffle vents (like ProVent or DuraVent) are intended for allowing air to flow between an edge vent and a vent in the attic (ridge, gable, etc).

Since I don't have edge vents, is a baffle necessary? Some argue it is always needed, to keep some, even if limited, airflow between the insulation and plywood under the shingles to keep the moisture down. Others argue without edge vents, it's not needed since there are no edge vents.

Since the baffle is 1" or more, this would affect the insulation I use. If I used a baffle and put 6 1/4" R-19 insulation in, the insulation would be compressed as 1) the thickness of the rafters is only 5 3/4" (I think this compression alone would reduce R-19 to R-17 or R-18) and 2) the baffle is at least an inch thick (further reducing the R value... to what, I don't know).

If I used a baffle and 3 1/2" R-13 insulation, there would be no compression and I'd get the full R-13 value. Actually, in this case, I wouldn't need the baffle, which would save a little cash.

If I used no baffle and 5 1/2" R-21, which is twice as expensive as the R-19 since it only comes in bats, I'd get the full R-21 value. I estimated for the size of my house that using a roll of R-13 or R-19 to be about $400-$450 and using batted R-21 to be double that.

I'm leaning towards the last one, with the value of having R-21 vs R-13 (or maybe 14 or 15) outweighing the need for airflow between my insulation and plywood.

Thoughts?
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Rafter vents needed if no edge vents?-roof.jpg   Rafter vents needed if no edge vents?-rafters.jpg  

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