Go Back   DIY Chatroom Home Improvement Forum > Home Improvement > Insulation

CLICK HERE AND JOIN OUR COMMUNITY TODAY...IT'S FREE!

Reply
 
Thread Tools Search this Thread Display Modes
Old 08-23-2012, 07:55 AM   #1
Member
 
Join Date: Jul 2011
Posts: 39
Share |
Default

6mil plastic under ridge vent?


I am insulating and sealing a brand new cathedral ceiling. After installing R-38C I plan to staple/tape up 6 mil plastic sheeting for vapor barrier.

My question: Do I use a continuous sheet right up and under the ridge vent and over to the other side? Or do I stop the plastic short of the ridge vent on each side?

I'm guessing I am going to go right under and over.

Thanks!

Jeff

aquasport17 is offline   Reply With Quote
Old 08-23-2012, 08:15 AM   #2
Exterior Construction
 
Windows on Wash's Avatar
 
Join Date: Aug 2011
Location: VA, MD, DC
Posts: 5,511
Default

6mil plastic under ridge vent?


Where is the home?

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

Windows on Wash is offline   Reply With Quote
Old 08-23-2012, 08:25 AM   #3
Member
 
Join Date: Jul 2011
Posts: 39
Default

6mil plastic under ridge vent?


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
aquasport17 is offline   Reply With Quote
Old 08-23-2012, 10:34 AM   #4
Exterior Construction
 
Windows on Wash's Avatar
 
Join Date: Aug 2011
Location: VA, MD, DC
Posts: 5,511
Default

6mil plastic under ridge vent?


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.
Windows on Wash is offline   Reply With Quote
Old 08-23-2012, 10:50 AM   #5
Member
 
Join Date: Jul 2011
Posts: 39
Default

6mil plastic under ridge vent?


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

Last edited by aquasport17; 08-23-2012 at 11:08 AM.
aquasport17 is offline   Reply With Quote
Old 08-23-2012, 02:01 PM   #6
Exterior Construction
 
Windows on Wash's Avatar
 
Join Date: Aug 2011
Location: VA, MD, DC
Posts: 5,511
Default

6mil plastic under ridge vent?


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.
Windows on Wash is offline   Reply With Quote
Old 08-23-2012, 02:24 PM   #7
Member
 
Join Date: Jul 2011
Posts: 39
Default

6mil plastic under ridge vent?


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.
aquasport17 is offline   Reply With Quote
Old 08-23-2012, 02:28 PM   #8
Exterior Construction
 
Windows on Wash's Avatar
 
Join Date: Aug 2011
Location: VA, MD, DC
Posts: 5,511
Default

6mil plastic under ridge vent?


It is much more than the R-5 increase when you figure the thermal bridging of the rafter framing.
Windows on Wash is offline   Reply With Quote
Old 08-23-2012, 02:36 PM   #9
Member
 
Join Date: Jul 2011
Posts: 39
Default

6mil plastic under ridge vent?


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.
aquasport17 is offline   Reply With Quote
Old 08-23-2012, 05:32 PM   #10
Exterior Construction
 
Windows on Wash's Avatar
 
Join Date: Aug 2011
Location: VA, MD, DC
Posts: 5,511
Default

6mil plastic under ridge vent?


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.
Windows on Wash is offline   Reply With Quote
Old 08-24-2012, 08:18 AM   #11
Member
 
Join Date: Jul 2011
Posts: 39
Default

6mil plastic under ridge vent?


Ok, I am convinced.

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

J

aquasport17 is offline   Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
snow country ridge vent on 2:12 pitch roof fxtm Roofing/Siding 4 11-01-2011 06:26 PM
Ridge Vent with roof vents and Soffit vents apquery Roofing/Siding 8 08-30-2011 12:18 PM
ridge vent ugabulldog Roofing/Siding 4 09-28-2010 08:59 AM
new roof, ridge vent was in est, but not installed... Zel1 Roofing/Siding 29 09-01-2010 04:30 PM
ridge vent ?'s Zel1 Roofing/Siding 6 05-19-2009 08:53 PM




Top of Page | View New Posts

Copyright © 2003-2014 Escalate Media. All Rights Reserved.