Insulating An Attic Framed With 2x6 - Insulation - DIY Chatroom Home Improvement Forum

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


Thread Tools Display Modes
Old 09-14-2012, 09:43 AM   #1
Join Date: Sep 2012
Posts: 5
Rewards Points: 10

Insulating an attic framed with 2x6

Hey, I have a question about how to best insulate my attic. It's a 1.5 story wartime house, the upper floor comprised of two bedrooms. Noticing it can get really hot in there, while working at least in the warmer months - stiflingly so at times. I'd like to keep it cooler.

I've attached a diagram to illustrate my thoughts.

It's framed with 2x6 rafters and ties, then has 2x4 knee walls. Currently, the space behind the knee walls is insulated with cellulose; the knee walls, rafters and collar ties have old rockwool insulation, then more cellulose blown in within the space below the peak. I'm planning on ripping all this out and re-insulating; admittedly, this will only be for one side of the house, as the board is still up in the other bedroom.

Looking at the diagram, my thought was maybe to go with either 1" or 1-1/2" blue ridgid foam first, leaving around a 1" air baffle between the foam and exterior sheathing to allow air to pass between the peak and the space behind the knee walls. Below the foam, I would place batts of 3-1/2" fibreglass insulation, followed directly by drywall.

How does this sound? You can see in the diagram, there are passive vents installed on either end of the house for the peak space and both sides of the house for the space behind the knee walls. Do I even need baffles? Would vapor barrier be a good idea?

Cost/space ARE a factor. The rooms are small; I can furr out the rafters if needed but prefer not to.

Edit - house is located just outside of Toronto, summers can be hot and winters can be cold.

All suggestions/information appreciated, thanks!
Attached Thumbnails
Insulating an attic framed with 2x6-atticinsulation.jpg  


Last edited by dustycreek; 09-14-2012 at 09:54 AM.
dustycreek is offline   Reply With Quote
Old 09-14-2012, 11:59 AM   #2
Join Date: Sep 2012
Posts: 5
Rewards Points: 10

I'll add, alternately I'm now considering instead going with 1" (or whatever is common) foam baffles and then as much fibreglass as I can fit in.. guessing R20 is probably too much unless I peel some off.


dustycreek is offline   Reply With Quote
Old 09-15-2012, 04:56 AM   #3
Exterior Construction
Windows on Wash's Avatar
Join Date: Aug 2011
Location: Washington DC Metro Area (VA, MD, DC)
Posts: 16,312
Rewards Points: 5,558
Blog Entries: 5

It is pretty well thought out, however, I would plan on covering the rafters with rigid foam prior to drywalling. They will still be super cold in this application and in that climate, you will have some condensation issues.

Rip down some 1.5" foam board and apply those strips to the underside of the roof deck to create continuous vent channels. Fill the rest of the rafter bays with rigid foam cut to fit the width (do not use fiberglass). Seal all the seams with sealant/foam for air seal. Cover the rafters with a rigid foam to eliminate the thermal bridging and cold rafters. Cover with drywall.

This should give you the best assembly and still maintain the venting.

You will not need a vapor retarder if you use the right foam board and how much you cover the rafters with will determine your final R-Value.

I would not put less than an R-38 in that assembly.

Air seal and kneewall floor. Blow in additional insulation. Install joist blockers. Foam seal the kneewall and back with rigid board as well.

Air seal the top attic floor. Blow in R-50.

Windows on Wash is offline   Reply With Quote
The Following User Says Thank You to Windows on Wash For This Useful Post:
Gary in WA (09-15-2012)
Old 09-15-2012, 10:15 PM   #4
Registered User
Join Date: Mar 2009
Posts: 11,730
Rewards Points: 526

Well said! ADA the drywall to seal the knee wall cavities on room side (esp. around electrical boxes) and caulk the wall/floor joint (required in U.S. Codes now), weather-strip the attic access opening;

If any ads are present below my answer or words underlined/colored, I do not condone/support/use the product or services listed/linked to, they are there without my consent.
17,000 dryer fires a year, when did you last clean the inside of the dryer near motor or the exhaust ducting?
Gary in WA is offline   Reply With Quote
Old 09-16-2012, 10:15 AM   #5
Join Date: Sep 2012
Posts: 5
Rewards Points: 10

Thanks for the replies guys, and the detailed write up Windows On Wash. I'm going to cost everything out and decide what to do.
dustycreek is offline   Reply With Quote

Thread Tools
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
Insulating your attic Bob Mariani How To Guides 5 07-07-2013 08:49 PM
finishing insulating attic Roundeye Insulation 12 01-12-2012 09:10 PM
insulating attic walls maverick1096 Building & Construction 5 07-25-2010 01:38 PM
Insulating an attic for finsih ddave23 Building & Construction 10 10-05-2009 12:24 PM
Insulating a finished attic properly? kokanee123 Roofing/Siding 13 09-18-2009 09:48 PM

Top of Page | View New Posts


Search Engine Friendly URLs by vBSEO 3.6.1