Go Back   DIY Chatroom Home Improvement Forum > Home Improvement > Roofing/Siding


Thread Tools Search this Thread Display Modes
Old 09-27-2010, 12:30 AM   #1
Join Date: Aug 2010
Location: Montreal Quebec, Canada
Posts: 13
Share |

Insulating vaulted ceiling

Have a vaulted ceiling, rafters are 12" engineered I beams. I can get 2" polyurethane foam boards at a good price. I was going to place the foam boards between the rafters leaving 1 1/4 " space between board and roof sheating, under the foam R35 fiberglass insulation, then vapour barrier then drywall.
Would this be a good way of insulating the ceiling?
Do I make the foam board layer air tight, or do I allow the insulation to breath?
My concern is the fiberglass between two non breathable layers....foam and vapour barrier.

Attached Thumbnails
Insulating vaulted ceiling-vaulted_insulation.jpg  

dodge1930 is offline   Reply With Quote
Old 09-27-2010, 07:31 AM   #2
Join Date: Dec 2007
Posts: 63

Insulating vaulted ceiling

Does this cathedral roof have continuous ventilation flow through the 1/4 inch gap? IE: soffit inlet and a vented ridge cap. Its sometimes recommemded to make the gap 2-3 inches. The 1/4 inch could be too restrictive. Is it asphalt shingles with tarpaper underlayment? Depending on your climate, you may want to omit the vapor retarder below for a more permeable air barrier instead.

rodeo is offline   Reply With Quote
Old 09-27-2010, 08:30 AM   #3
jlhaslip's Avatar
Join Date: Dec 2009
Location: Canadian Rockies
Posts: 1,280

Insulating vaulted ceiling

R35 might be too much batt insulation for that situation. The batt stuff needs room to fluff up.
Where are you located?
Originally Posted by Bud Cline View Post
Go ahead and apply for a variance, those guys at City Hall can use a good laugh.
jlhaslip is offline   Reply With Quote
Old 09-27-2010, 09:46 AM   #4
Join Date: Aug 2010
Location: Montreal Quebec, Canada
Posts: 13

Insulating vaulted ceiling

I'm located in the eastern township of quebec, canada

the air space is 1 1/4 (one and a quarter).
The 2 " of polyurethane (not polystryrene) I understand gives me a R value of around R15 (The foam is foiled faced on both sides.

I was trying to acheive the upper R40's in the ceiling.

The roof has a continuous ridge vent with venting along the soffits.

The roof in covered with metal over 15/32 OSB.

I could drop the fiberglass down to an R30 9.5 inches.

Plus I will have strapping attached to the bottom of the I beam for the drywall, that could give me an extra 3/4 - 1 inch of space for the fiberglass.
dodge1930 is offline   Reply With Quote
Old 09-27-2010, 01:02 PM   #5
Registered User
Join Date: Mar 2009
Posts: 9,968

Insulating vaulted ceiling

You don't want air gaps at the batt insulation, behind or in front of to give convective loops. http://www.buildingscience.com/docum...archterm=insul
Especially in the channels created by the strapping.


Need the v.b.: http://www.buildingscience.com/docum...rchterm=attic+


I would use foam or a sill sealer as a thermal break to the rafter bottoms from the heated room. http://www.coloradoenergy.org/procor...f/r-values.htm

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 09-27-2010, 03:03 PM   #6
Join Date: Aug 2010
Location: Montreal Quebec, Canada
Posts: 13

Insulating vaulted ceiling

So if I as per my drawing. leave the 1 and 1/4 inch air gap above the foam board to vent.
install the foam board making it air tight.
install fiberglass
cover with vapour barrier
install strapping
install drywall onto strapping.

Foam board R -15
R30 fiberglass
total r = 45

Would this work?

dodge1930 is offline   Reply With Quote

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
Crown moulding with a vaulted ceiling? cstrat Carpentry 34 06-17-2011 06:51 PM
HELP! lighting question for vaulted ceiling pauls General DIY Discussions 5 07-03-2010 04:53 PM
Rafter Ties on Vaulted Ceiling timsgills Building & Construction 2 01-07-2010 03:02 PM
Ceiling condensation icemand Building & Construction 1 12-15-2009 05:16 AM
Vaulted Ceiling Insulation Problem casey_wa Roofing/Siding 13 05-08-2006 09:55 AM

Top of Page | View New Posts

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