# 6mil plastic under ridge vent?



## Windows on Wash (Aug 30, 2011)

Where is the home?

What type of insulation was used to get to R-38.


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## aquasport17 (Jul 12, 2011)

It's in Southern Vermont. Owens Corning R-38C High Density fiberglass batts are going into 2x12 rafters (I understand the code is R49 but that's not my question). I'm simply wondering if I should seal the ridge vent by running the plastic up, under and down the other side, or only run it up TO the vent.

Note that there is about a 3 foot crawl space under the ridge, so the cathedral doesn't go right to the top.

I am also considering rigid foam over the 2x12 rafters instead of plastic sheeting to get me a little more R. Depends on budget. 

Jeff


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## Windows on Wash (Aug 30, 2011)

Skip the vapor barrier and do the foam.

R-39 batts will not give you a decent amount of performance.

If you have more room, it would be nice to see some loose fill over the rafters as well but if it is tight, you can do anything with that.


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## aquasport17 (Jul 12, 2011)

Rigid foam may be out of the budget. This is a part time getaway house - but I still want to do it right, and do it within budget. I could add 1" R5 foam possibly. 

My question still remains. Do I run whatever vapor barrier I use all the way up and under the ridge to seal off the ridge vent from the living space, or not?

Maybe I will use the rigid foam up to the attic area and over the flat part of the cathedral, leaving the higher section above the ceiling with R38C in the rafters, and R30 in the small attic area? The top looks like this: /-\ with the small space above the cathedral about four feet high to the ridge.

J


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## Windows on Wash (Aug 30, 2011)

Yes.

The vapor control layer would be continuous on the warm side of the drywall.

How is the roof vented?

I can guarantee that with 1" foam (taped, foamed, and sealed joints) you wouldn't have any vapor issues though.


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## aquasport17 (Jul 12, 2011)

It's traditionally vented with a ridge vent and soffit vent. I plan to use 1" baffles between the R-38C and the roof deck. 

Yes I agree the 1" rigid foam is the way to go but it's 10 times the cost of plastic for only an R5 increase. 

Plus, I think I would need furring strips to secure my tongue and groove to the rigid foam - adding more time and expense. 

I'm undecided.


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## Windows on Wash (Aug 30, 2011)

It is much more than the R-5 increase when you figure the thermal bridging of the rafter framing.


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## aquasport17 (Jul 12, 2011)

I have heard a lot about thermal bridging. Hard to imagine the cold coming through a half inch of plywood and 11-1/4 inches of rafter! I guess. 

And then where does it stop? Might as well do the walls too? Those have the Roxul R23 Batts.


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## Windows on Wash (Aug 30, 2011)

Wood = R-1 per inch.

2 x 12 = R-11.25

Figure your framing is minimum 20% of your assembly (usually runs higher to 30%).

At that point, your R-38 assembly is working like R-25.

At closer to the 30%, R-22.

Condensation is also and issue on those cold rafter as well.


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## aquasport17 (Jul 12, 2011)

Ok, I am convinced. 

Can I nail the pine tongue and groove directly over the rigid foam?

J


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