Finishing Attic Behind Knee Wall - Building & Construction - DIY Chatroom Home Improvement Forum
Advertisement


Go Back   DIY Chatroom Home Improvement Forum > Home Improvement > Building & Construction

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

Like Tree2Likes
  • 1 Post By Marky82
  • 1 Post By spizzak
Reply
 
Thread Tools
Display Modes
Old 07-24-2017, 11:54 PM   #1
Member
 
Join Date: Jul 2017
Location: Ontario, Canada
Posts: 65
Rewards Points: 128
Default

Finishing Attic Behind Knee Wall


Hi all,

I have a knee wall on the second floor of my house with unfinished attic space behind it that I would like to finish for storage space. I thought this would be an easy project but upon reading about it, it seems like there are many pitfalls to avoid.

I have this attic running the entire length of the house (~38') and have a small, 5' tall (poorly insulated) door giving access to this space that I would like to continue to use to access the finished space. I would likely only finish about 8 feet of the 38 feet.

Currently the space behind the walls does not have a roof vent, but it looks like the space to the soffits is left open. At the top of this space, the rafters have insulation to block off the actual attic space which is vented to the outside. Ideally I'd like to insulate a small section of the roof and remove the corresponding insulation from the inside wall to create a small conditioned space for storage.

Can anyone here let me know if this is something I could do easily with my current setup or if I'm going to run into issues?

I've linked some diagrams of my current situation (in blue) and my proposed changes (in black) as well as some photos below:

http://imgur.com/a/m5OpC

Last edited by spizzak; 07-24-2017 at 11:59 PM.
spizzak is offline   Reply With Quote
Sponsored Links
Advertisement
 
Old 07-25-2017, 12:15 AM   #2
retired framer
 
Join Date: Jun 2017
Location: Fraser Valley BC Canada
Posts: 31,086
Rewards Points: 5,826
Default

Re: Finishing Attic Behind Knee Wall


Have you had any leaks in the roof when snow is on the roof?

Normally the answer would be yes with a little work to make the venting work.

The rafters over the sloped part of your roof should have been added to so there would be space for insulation and air movement.

Even if you have not had ice dams yet changing something more could be making a bad case worse.

You can do anything you want but to be fair you should understand all the pros and cons before you decide what to do.

What are you storing that needs to be in conditioned space.
Nealtw is online now   Reply With Quote
Sponsored Links
Advertisement
 
Old 07-25-2017, 08:33 AM   #3
Member
 
Join Date: Nov 2015
Location: Maine
Posts: 8,961
Rewards Points: 8,048
Default

Re: Finishing Attic Behind Knee Wall


I live in one similar to that and as an energy professional I hate it. It is being changed but difficult. Here is a link on capes to get you started.
http://www.finehomebuilding.com/2012...ttic-kneewalls

Bud
Bud9051 is offline   Reply With Quote
Sponsored Links
Advertisement
 
Old 07-25-2017, 09:36 AM   #4
Member
 
Join Date: Jul 2017
Location: Ontario, Canada
Posts: 65
Rewards Points: 128
Default

Re: Finishing Attic Behind Knee Wall


Thanks for the feedback guys. I think my plan is to insulate between the rafters with foam board insulation, and leave a small gap for airflow. I'm not sure how I'm supposed to get R49 in there (there's no way there's that much on the walls now)... any suggestions?

The second thing is for the new knee wall and the two side walls facing the existing attic space, should I put the air barrier on the outside or the inside of the insulation?

Last edited by spizzak; 07-25-2017 at 09:49 AM.
spizzak is offline   Reply With Quote
Old 07-25-2017, 10:15 AM   #5
Member
 
Join Date: Jul 2017
Location: Ontario, Canada
Posts: 65
Rewards Points: 128
Default

Re: Finishing Attic Behind Knee Wall


I forgot to reply with the fact that I haven't noticed any leaks in the winter but to be honest I haven't been looking really either. I'll have to keep an eye out for that and ice dams as well this year.
spizzak is offline   Reply With Quote
Old 07-25-2017, 10:23 AM   #6
Member
 
Join Date: Oct 2010
Posts: 5,411
Rewards Points: 526
Default

Re: Finishing Attic Behind Knee Wall


I would keep it cold space, put the storage into dust sealed containers or industrial bags. To turn it into a conditioned space, you also need soffit vents and seal from the wall plate to ridge with min 1.5" air space. Listiburek (energy efficiency build guy) has article on absolute need for the air space.
carpdad is offline   Reply With Quote
Old 07-25-2017, 10:27 AM   #7
retired framer
 
Join Date: Jun 2017
Location: Fraser Valley BC Canada
Posts: 31,086
Rewards Points: 5,826
Default

Re: Finishing Attic Behind Knee Wall


Quote:
Originally Posted by spizzak View Post
Thanks for the feedback guys. I think my plan is to insulate between the rafters with foam board insulation, and leave a small gap for airflow. I'm not sure how I'm supposed to get R49 in there (there's no way there's that much on the walls now)... any suggestions?

The second thing is for the new knee wall and the two side walls facing the existing attic space, should I put the air barrier on the outside or the inside of the insulation?
To make everything work right you would add to the rafters so you had enough insulation and air flow. Air barrier goes on the inside.
Nealtw is online now   Reply With Quote
Old 07-25-2017, 10:29 AM   #8
Member
 
Join Date: Jul 2017
Location: Ontario, Canada
Posts: 65
Rewards Points: 128
Default

Re: Finishing Attic Behind Knee Wall


I have no store space available now, so this space would be used as a closet. Putting things in sealed bags isn't really an option.

I don't think I'm understanding how this addition will impact things so significantly. Basically all I'm doing is building an insulated box inside a pre-existing space. There will still be a knee wall just moved out a bit further and most of the attic space is going to remain untouched. Essentially I'm just extending the slanted ceiling of half a bedroom an extra 4 feet.
spizzak is offline   Reply With Quote
Old 07-25-2017, 10:47 AM   #9
Member
 
Join Date: Oct 2010
Posts: 5,411
Rewards Points: 526
Default

Re: Finishing Attic Behind Knee Wall


Biggest challenge is build up of moisture that can grow mold and rot framing wood. Example, if you are in dessert climate, you may get away with not much venting against moisture.
carpdad is offline   Reply With Quote
Old 07-25-2017, 11:03 AM   #10
retired framer
 
Join Date: Jun 2017
Location: Fraser Valley BC Canada
Posts: 31,086
Rewards Points: 5,826
Default

Re: Finishing Attic Behind Knee Wall


Quote:
Originally Posted by spizzak View Post
I have no store space available now, so this space would be used as a closet. Putting things in sealed bags isn't really an option.

I don't think I'm understanding how this addition will impact things so significantly. Basically all I'm doing is building an insulated box inside a pre-existing space. There will still be a knee wall just moved out a bit further and most of the attic space is going to remain untouched. Essentially I'm just extending the slanted ceiling of half a bedroom an extra 4 feet.
If you are building another knee wall and allow air to be common in the attic you may get away with it.
Nealtw is online now   Reply With Quote
Old 07-25-2017, 11:40 AM   #11
Member
 
Join Date: Nov 2015
Location: Maine
Posts: 8,961
Rewards Points: 8,048
Default

Re: Finishing Attic Behind Knee Wall


Once you close the door to that new space it becomes semi-conditioned, in other words it will not be directly heated. However, the air in there will be the same as in the house. Since it is not heated the exterior surfaces may drop below the dew point and result in moist surfaces and mold. ALL closets as well as windows behind drapes suffer from this intermediate condition and the solution is, add a bit of heat. I will be conservative and say I have seen dozens of musty closets due to being isolated from the conditioned living space.

Follow the recommendations in that link to get your insulation, air sealing, and ventilation as it needs to be and consider a louvered door into that space. The louvers may not be enough but better than a solid door. Then watch out for that musty smell.

Bud
Bud9051 is offline   Reply With Quote
Old 07-25-2017, 11:44 AM   #12
Member
 
Join Date: Jul 2017
Location: Ontario, Canada
Posts: 65
Rewards Points: 128
Default

Re: Finishing Attic Behind Knee Wall


Quote:
Originally Posted by Bud9051 View Post

Follow the recommendations in that link to get your insulation, air sealing, and ventilation as it needs to be and consider a louvered door into that space. The louvers may not be enough but better than a solid door. Then watch out for that musty smell.

Bud
Makes sense- I try to keep closet doors open whenever I can for this reason. I was considering putting an air vent connecting to the adjacent room as well.
spizzak is offline   Reply With Quote
Old 07-25-2017, 04:10 PM   #13
Member
 
Join Date: Dec 2015
Location: Hamden, CT
Posts: 133
Rewards Points: 266
Default

Re: Finishing Attic Behind Knee Wall


I've been remodeling one of the upstairs bedrooms in my cape for the past year, part of which included reinsulating the entire room, so I have a fair bit of experience insulating knee walls.

I have 2x6 rafters in my house as well. You will never R49 on the sloped sections, unless you want to give up A LOT of space, so forget about that. Do the best with what you have.

I would forgo building a smaller knee wall in the back and instead insulate the roof deck so the entire knee wall space becomes conditioned. Go with site built ventilation baffles that run from your soffits all the way to above the insulation in your ceiling. You have 5.5 inches of free space. Code says you need a 1 inch air gap for venting but, as carpdad mentioned, 1.5" is better. Start by tearing down the drywall on that side of the room, cut 1.5" strips of wood to use as spacers, nail/screw them to the sides of the rafters then get 2 inch XPS rigid foam board and cut it 1/4 to 1/2" less than the space between rafters. I used XPS because it has a higher r-value in cold weather compared to polyiso which doesn't work as well in cold climates. Put the foam board against the 1.5" spacer blocks and use nails/screws with plastic caps to mechanically hold in the foam board in place. Next get great stuff windows and door canned foam (buy a gun so you can use the pro version, it's much easier to control) and spray it in the gaps between the rafter and foam board. When two pieces of foam board meet, use construction adhesive safe for foam to glue the ends together then use 3M 8067-All Weather Flashing tape to tape the seams (get the 3" version).

After you do all the rafter bays you will now have air traveling from your soffit vents to the ceiling without coming into contact with the conditioned space. At this point you have 2" of space left in the rafter bays. You could use fiberglass or rock wool but it's difficult to keep those air tight. I would go with another layer of 2" foam board. You can glue it directly to the first layer and use the canned foam on the sides. Make sure to stagger any seams so those from the first layer aren't on top of the seams on the second layer. It's good practice in case a seam fails. Lastly I would cover all the rafter bays with sheets of 1" XPS foam board to reduce thermal bridging. Mark where the studs are and use the nails/screws with plastic caps to fasten it directly to the studs. Finish by covering the foam board with an approved ignition barrier such as 1/2" drywall.

If you're leaving the other section of the knee wall unconditioned (i.e. leaving it alone), build a wall between the two so the outside air from the other knee wall doesn't travel to the side you'll be using as a closet.

This is called the cut and cobble method or poor man's spray foam. There are numerous articles and youtube videos. It's a pain, and very time consuming, but it is one of the better retrofit options.

Good luck with whatever you decide to do!


Here are some pics from my room reno:







Nealtw likes this.
Marky82 is offline   Reply With Quote
Old 07-25-2017, 04:27 PM   #14
Member
 
Join Date: Jul 2017
Location: Ontario, Canada
Posts: 65
Rewards Points: 128
Default

Re: Finishing Attic Behind Knee Wall


That is super helpful, thanks! It seems like I have all the info I need to finish this now
Marky82 likes this.
spizzak is offline   Reply With Quote
Old 11-07-2017, 06:29 PM   #15
Member
 
Join Date: Jul 2017
Location: Ontario, Canada
Posts: 65
Rewards Points: 128
Default

Re: Finishing Attic Behind Knee Wall


I'm reviving this thread because I'm only just getting around to doing this project. I had decided to use Marky82's recommended cut & cobble method to insulate the ceiling of the closet, but discovered an issue.

The ceiling of the room adjacent to the would be closet (marked with a '?' in this pic) is insulated without any air gap (pic). I haven't noticed any issues in the winter with ice dams but regardless this looks like it's incorrect to me. My roof deck is planks rather than plywood, so perhaps that allows for more venting?

I don't want to tear down the existing drywall in the bedroom, so I can't really change the way that existing section is insulated, and so even if I put air gaps in the new section, they will still be blocked at the existing section (circled part in this pic).

My question is for the foam insulation that I plan on installing below it should I still put the airgaps even though they will be blocked or should I just follow the existing section without air gaps?
spizzak is offline   Reply With Quote
Sponsored Links
Advertisement
 
Reply


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
Issue Insulating Knee Wall prone to Ice Dams joen270 Insulation 7 02-24-2015 06:28 PM
Insulating your attic Bob Mariani How To Guides 5 07-07-2013 09:49 PM
Question: BNC cables through attic (studs to wall, or ceiling drywall?) eatmyturf Electrical 4 04-05-2013 11:25 PM
Knee wall w/ soffit vents no ridge vent -- seal soffit vents? tblaz Roofing/Siding 1 04-24-2011 05:12 PM
Drywall in attic with knee wall bparise Drywall & Plaster 9 11-19-2009 06:35 PM




Top of Page | View New Posts