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 12-22-2011, 02:57 PM   #1
Member
 
Join Date: Apr 2008
Location: NJ
Posts: 445
Share |
Default

Incremental benefit of insulating rafters in addition to ceiling joist bays?


Since I'll be adding 2x10 ceiling joists, I'm contemplating partially finishing the attic space. The attic currently has two gable vents and no soffit vents. I would like to keep this area maybe 20 degrees warmer than the outside air during the winter.

My plan is to rip out the plywood soffits and create a 1-1/2" channel up to a new ridge vent using 4 inches of xps foam insulation. I would use 2x3s as spacer blocks between the rigid foam and the roof sheathing and use a vertical piece to isolate the soffit from the joist bay. I would also close off the gable vents and leave the insulation in the joist bays for the ceiling below. I would, of course, seal with PL300 or expanding foam.

Do you think this type of installation will provide the 20 degrees I'm looking for? Would foil-faced poly-iso be a better choice than XPS?

benjamincall is offline   Reply With Quote
Old 12-22-2011, 05:19 PM   #2
Exterior Construction
 
Windows on Wash's Avatar
 
Join Date: Aug 2011
Location: VA, MD, DC
Posts: 6,207
Default

Incremental benefit of insulating rafters in addition to ceiling joist bays?


foil faced iso would be better to face to the vent space to take advantage of the radiant barrier aspect of performance.

There is no making up for R-value though. As long as you have enough and it is airtight, doesn't really matter what you use.

Windows on Wash is offline   Reply With Quote
Old 12-22-2011, 05:48 PM   #3
Member
 
Join Date: Apr 2008
Location: NJ
Posts: 445
Default

Incremental benefit of insulating rafters in addition to ceiling joist bays?


Overall, does my concept seem sound?
benjamincall is offline   Reply With Quote
Old 12-22-2011, 10:08 PM   #4
Member
 
AGWhitehouse's Avatar
 
Join Date: Jul 2011
Posts: 1,378
Default

Incremental benefit of insulating rafters in addition to ceiling joist bays?


Quote:
Originally Posted by benjamincall View Post
Overall, does my concept seem sound?
NOPE...and not because of venting or insulating...but because of fire codes. Foam cannot be left exposed within an attic space without an approved thermal and ignition barrier. Approved barriers can be sheetrock, plywood, sheet metal, or mineral fiber insulation (NOT fiberglass).

If you really want to do it, I suggest using luan instead of foam to create the air channels and fill the rafter bays with un-faced fiberglass batts held in place by tyvek sheeting stapled to the rafters.
__________________
Life's too short...so enjoy it!
AGWhitehouse is offline   Reply With Quote
Old 12-22-2011, 11:29 PM   #5
Member
 
Join Date: Apr 2008
Location: NJ
Posts: 445
Default

Incremental benefit of insulating rafters in addition to ceiling joist bays?


Quote:
Originally Posted by AGWhitehouse View Post
NOPE...and not because of venting or insulating...but because of fire codes. Foam cannot be left exposed within an attic space without an approved thermal and ignition barrier. Approved barriers can be sheetrock, plywood, sheet metal, or mineral fiber insulation (NOT fiberglass).

If you really want to do it, I suggest using luan instead of foam to create the air channels and fill the rafter bays with un-faced fiberglass batts held in place by tyvek sheeting stapled to the rafters.
I was planning on putting the xps foam in the rafter bays with 1/2" plywood affixed to the rafters.
benjamincall is offline   Reply With Quote
Old 12-23-2011, 06:57 AM   #6
Member
 
AGWhitehouse's Avatar
 
Join Date: Jul 2011
Posts: 1,378
Default

Incremental benefit of insulating rafters in addition to ceiling joist bays?


Quote:
Originally Posted by benjamincall View Post
I was planning on putting the xps foam in the rafter bays with 1/2" plywood affixed to the rafters.
Ok then. So long as the foam is sealed from exposure once the job is complete, then you'll be o.k. in the fire code realm.
__________________
Life's too short...so enjoy it!
AGWhitehouse is offline   Reply With Quote
Old 12-24-2011, 12:33 PM   #7
Member
 
Join Date: Apr 2008
Location: NJ
Posts: 445
Default

Incremental benefit of insulating rafters in addition to ceiling joist bays?


Since I'm upgrading from 2x4 to 2x10 joists, I was thinking of putting down a layer of 5-1/2" and a layer of 3-1/2" Roxul to replace the compressed R13 fiberglass that's up there right now. I'll be closing off the gable vents, but the only regular heat will be what escapes from the main floor. Do you think I'll need to place strips of poly sheeting between the joists, or will the tighter envelope and the rafter insulation keep the attic sufficiently warm to avoid a condensation problem?
benjamincall is offline   Reply With Quote
Old 12-24-2011, 03:45 PM   #8
Member
 
AGWhitehouse's Avatar
 
Join Date: Jul 2011
Posts: 1,378
Default

Incremental benefit of insulating rafters in addition to ceiling joist bays?


Quote:
Originally Posted by benjamincall View Post
Since I'm upgrading from 2x4 to 2x10 joists, I was thinking of putting down a layer of 5-1/2" and a layer of 3-1/2" Roxul to replace the compressed R13 fiberglass that's up there right now. I'll be closing off the gable vents, but the only regular heat will be what escapes from the main floor. Do you think I'll need to place strips of poly sheeting between the joists, or will the tighter envelope and the rafter insulation keep the attic sufficiently warm to avoid a condensation problem?
I'm confused...you're going to have insulation at both the ceiling and rafter planes? I wouldn't recommend that...pic one or the other...
__________________
Life's too short...so enjoy it!
AGWhitehouse is offline   Reply With Quote
Old 12-24-2011, 05:12 PM   #9
Member
 
Join Date: Apr 2008
Location: NJ
Posts: 445
Default

Incremental benefit of insulating rafters in addition to ceiling joist bays?


Quote:
Originally Posted by AGWhitehouse View Post
I'm confused...you're going to have insulation at both the ceiling and rafter planes? I wouldn't recommend that...pic one or the other...
I want to keep the attic at least 20 degrees warmer than the ambient air, but I don't necessarily want to have the attic at the same temperature as the rest of the house. I would also like to take advantage of the deep ceiling joists. My rafters are only 2x6, which leaves me with about 4" of insulation after I create the channels for the soffit vent.

What's the problem with an insulation sandwich? I have insulation in my ceiling and between the floor joists in the basement.
benjamincall is offline   Reply With Quote
Old 12-24-2011, 10:40 PM   #10
Registered User
 
Join Date: Mar 2009
Posts: 9,968
Default

Incremental benefit of insulating rafters in addition to ceiling joist bays?


http://www.buildingscience.com/docum...bout-diffusion

http://docs.google.com/viewer?a=v&q=...g%2520Code%252

Gary
__________________
If any ads are present in my answer above, I do not condone/support/use the product or services listed, they are there against my permission.
Gary in WA is offline   Reply With Quote
Old 12-26-2011, 11:35 AM   #11
Member
 
Join Date: Apr 2008
Location: NJ
Posts: 445
Default

Incremental benefit of insulating rafters in addition to ceiling joist bays?


So, based on what I read in Gary's articles and in R806.4 IRC, I would need to ensure that the underside of the roof deck remains at 45 degrees.

If I put R20 of impermeable insulation on the underside of the ceiling deck, R15 Roxul in the gable ends and R38 Roxul in the attic floor, will I achieve that temperature on the underside of the rigid foam attached to the roof deck? I would obviously remove all venting.
benjamincall is offline   Reply With Quote
Old 12-26-2011, 11:36 AM   #12
Member
 
AGWhitehouse's Avatar
 
Join Date: Jul 2011
Posts: 1,378
Default

Incremental benefit of insulating rafters in addition to ceiling joist bays?


Quote:
Originally Posted by benjamincall View Post
What's the problem with an insulation sandwich? I have insulation in my ceiling and between the floor joists in the basement.
The air between the ceiling and floor is tempered...you live in it. The attic is not tempered and you'll be creating a class I vapor retarder outside of the thermal envelope, which, by code, already has a class II vapor retarder in it. To avoid excessive moisture build up within the attic space you'll need to temper it and exchange it, which just adds cost and maintenance. Your money & time would be better spent adding insulation at the ceiling plane and leaving the roof un-insulated.
__________________
Life's too short...so enjoy it!
AGWhitehouse is offline   Reply With Quote
Old 12-26-2011, 11:38 AM   #13
Member
 
AGWhitehouse's Avatar
 
Join Date: Jul 2011
Posts: 1,378
Default

Incremental benefit of insulating rafters in addition to ceiling joist bays?


Quote:
Originally Posted by benjamincall View Post
So, based on what I read in Gary's articles and in R806.4 IRC, I would need to ensure that the underside of the roof deck remains at 45 degrees.

If I put R20 of impermeable insulation on the underside of the ceiling deck, R15 Roxul in the gable ends and R38 Roxul in the attic floor, will I achieve that temperature on the underside of the rigid foam attached to the roof deck? I would obviously remove all venting.
You would have to do an energy analysis considering HDD, orientation, average wind speeds, etc. Unless you do this, you really can't accurately estimate the true performance of a system.
__________________
Life's too short...so enjoy it!
AGWhitehouse is offline   Reply With Quote
Old 12-26-2011, 01:17 PM   #14
Member
 
Join Date: Apr 2008
Location: NJ
Posts: 445
Default

Incremental benefit of insulating rafters in addition to ceiling joist bays?


Quote:
Originally Posted by AGWhitehouse View Post
You would have to do an energy analysis considering HDD, orientation, average wind speeds, etc. Unless you do this, you really can't accurately estimate the true performance of a system.
So, I don't have an easy way to determine the temperature of the insulation on the underside of the ceiling deck?

I'm not trying to be a pain in the neck, I was just hoping I wouldn't have to relegate my attic to 16 temperatures each winter. I guess I could install the R38 Roxul in the joist bays without a vapor retarder and find a safe way to hook up a space heater with a thermostat to kick in if the temperature dropped below 45.

I have the following information, but I'm not sure what to do with it:

HDD 4888, 5/12 roof planes facing SW and NE, average wind speed 13.3 MPH (16 MPH in January), annual low (16F in January), percent possible sunshine 56. The house is heated using a mini split heat pump on the main floor. The temperature is maintained at about 70F. The house footprint is about 630 ft with an eight foot ceiling (5040 ft). The volume of the attic will be approximately 1015 ft.

benjamincall 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
Ceiling joist Redbeard58 Building & Construction 3 07-03-2011 08:09 PM
Insulating Attic - Peaked Roof addition over Built-Up Roof nominivans4me Insulation 1 04-16-2011 11:27 AM
Ceiling Joist Worries clayrabbit Building & Construction 11 01-06-2008 05:07 PM
Ceiling Joist Beam Construction? Stargazer Building & Construction 1 08-26-2007 10:50 PM
ceiling joist this old house Building & Construction 2 03-23-2006 09:00 AM




Top of Page | View New Posts

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