Proper Cantilever Insulation? - Insulation - DIY Chatroom Home Improvement Forum
Advertisement


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

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

Like Tree2Likes
Reply
 
Thread Tools Search this Thread
Display Modes
Old 08-02-2020, 03:05 PM   #1
Member
 
Join Date: Mar 2017
Location: PA
Posts: 92
Rewards Points: 184
Default

Proper cantilever insulation?





Current insulation is just fiberglass. Is the above picture the proper way to insulate cantilevers?


I know spray foam would probably be better, but it is not an option.
mac1990 is offline   Reply With Quote
Sponsored Links
Advertisement
 
Old 08-02-2020, 03:16 PM   #2
retired framer
 
Join Date: Jun 2017
Location: Fraser Valley BC Canada
Posts: 38,140
Rewards Points: 890
Default

Re: Proper cantilever insulation?


Almost on the far left you want a piece up against the rim up in the gap.

And inside the house above the block wall
__________________
Do not use flat bottom gables with scissor trusses.
https://www.fema.gov/media-library-d...6/chapter7.pdf
figure 7-14
Nealtw is online now   Reply With Quote
Sponsored Links
Advertisement
 
Old 08-02-2020, 03:35 PM   #3
Member
 
Join Date: Mar 2017
Location: PA
Posts: 92
Rewards Points: 184
Default

Re: Proper cantilever insulation?


Quote:
Originally Posted by Nealtw View Post
Almost on the far left you want a piece up against the rim up in the gap.

And inside the house above the block wall

So, put some foam board vertically on the header/rim joist, opposite of the siding?

and I already have foam horizontal above the cinder block wall inside the house?
mac1990 is offline   Reply With Quote
Sponsored Links
Advertisement
 
Old 08-02-2020, 03:55 PM   #4
retired framer
 
Join Date: Jun 2017
Location: Fraser Valley BC Canada
Posts: 38,140
Rewards Points: 890
Default

Re: Proper cantilever insulation?


Quote:
Originally Posted by mac1990 View Post
So, put some foam board vertically on the header/rim joist, opposite of the siding?

and I already have foam horizontal above the cinder block wall inside the house?
Yes that would do it.
mac1990 likes this.
__________________
Do not use flat bottom gables with scissor trusses.
https://www.fema.gov/media-library-d...6/chapter7.pdf
figure 7-14
Nealtw is online now   Reply With Quote
Old 08-07-2020, 06:31 AM   #5
Member
 
Join Date: Oct 2010
Posts: 6,148
Rewards Points: 2,000
Default

Re: Proper cantilever insulation?


Just thought it up and is a wild guess, but hopefully, based on some past experience.
Air seal is most important. 1.5" xps board against the rim, AND same board on the floor side of the cantilever. Seal the joints with spray foam. Sides also. Then push in fiberglass insulation. This part does not have to be neat or pretty. Stuff the space and you won't be hurting any r value.


Better would be use the foam boards, 3" is available, to fill the space. Because it is cantilever and the floor is exposed to weather, more r value is better to keep that part of the floor warmer. If possible, I'd go for r50 but going past r60 gives you nothing for the money.
carpdad is offline   Reply With Quote
Old 08-07-2020, 06:53 AM   #6
Member
 
Join Date: Oct 2008
Location: Ontario
Posts: 6,290
Rewards Points: 11,494
Default

Re: Proper cantilever insulation?


I would use XPS. EPS is not going to be a proper vapour barrier. XPS is, and will have higher R value.
Cut the XPS to a push-fit and then caulk (just cheap acrylic exterior caulk) the edges (if you really want to go the extra mile) rather than spray foam. Spray foam in the can will make a big mess, especially in an inverted application.


.

Last edited by SPS-1; 08-07-2020 at 06:55 AM.
SPS-1 is offline   Reply With Quote
Old 08-07-2020, 07:34 AM   #7
Member
 
Join Date: Nov 2015
Location: Maine
Posts: 10,737
Rewards Points: 276
Default

Re: Proper cantilever insulation?


Hi Mac, you didn't say what your concerns are, heat loss to the outside or cold floors above the cantilever?

The advice so far is addressing mostly heat loss to the outside, however packing that space full of good insulation will do little to help warm the floors above.

If the basement space exposed to those joist cavities is heated then the other option is to insulate the bottom of the cantilever and leave a warm air gap above. I couldn't find the article that specifically discusses this approach but the article below does a little.
https://www.buildingscience.com/docu...in-crawlspaces

Air sealing is always required but the floor above will have trouble staying warm without a source of heat. Warm air in the house is quickly pushed up by the cold air from that wall and or window,

Bud
__________________
I volunteer my help and opinions, but you are responsible for what YOU choose to do with that information.
Bud9051 is offline   Reply With Quote
Old 08-07-2020, 07:48 AM   #8
retired framer
 
Join Date: Jun 2017
Location: Fraser Valley BC Canada
Posts: 38,140
Rewards Points: 890
Default

Re: Proper cantilever insulation?


A solid sheet of plywood nailed down to the joists with glue is pretty much air sealed.
__________________
Do not use flat bottom gables with scissor trusses.
https://www.fema.gov/media-library-d...6/chapter7.pdf
figure 7-14
Nealtw is online now   Reply With Quote
Old 08-07-2020, 08:17 AM   #9
Member
 
Join Date: Nov 2015
Location: Maine
Posts: 10,737
Rewards Points: 276
Default

Re: Proper cantilever insulation?


More reading:
https://www.greenbuildingadvisor.com...or-cold-floors

Bud
__________________
I volunteer my help and opinions, but you are responsible for what YOU choose to do with that information.
Bud9051 is offline   Reply With Quote
Old 08-07-2020, 08:18 AM   #10
Member
 
Join Date: Mar 2017
Location: PA
Posts: 92
Rewards Points: 184
Default

Re: Proper cantilever insulation?


Quote:
Originally Posted by SPS-1 View Post
I would use XPS.
.
I choose EPS because it does not lose R value over time and will dry out if it manages to get wet. It is also a better value per R.

The EPS I purchased has a foil face, should be fine.




Quote:
Originally Posted by Bud9051 View Post
Hi Mac, you didn't say what your concerns are, heat loss to the outside or cold floors above the cantilever?
Something I did not even think about. The floors above the cantilever are cold in the winter.

All the areas above the cantilever are heated, but only half the area below the cantilever is heated. Half the cantilever is off an unheated garage, while the other half is in a heated finished basement.

So do the following when the cantilever is connected to a heated area:
mac1990 is offline   Reply With Quote
Old 08-07-2020, 08:34 AM   #11
Member
 
Join Date: Nov 2015
Location: Maine
Posts: 10,737
Rewards Points: 276
Default

Re: Proper cantilever insulation?


If we use the example of the ceiling above a cold garage and below bedroom space above and there is no heated basement to warm that air gap, it is still beneficial to create the gap to allow it to share some of the heat from above. As long as there is sufficient space for a lot of insulation and the rim and other leak areas are well sealed I believe it a better approach. Cold floors are hard to warm up so any help they can get is good.

So, I would consider leaving the air gap for the entire cantilever, just blocking it where the joists are exposed to the unheated space.

Bud
__________________
I volunteer my help and opinions, but you are responsible for what YOU choose to do with that information.
Bud9051 is offline   Reply With Quote
Old 08-07-2020, 11:57 AM   #12
retired framer
 
Join Date: Jun 2017
Location: Fraser Valley BC Canada
Posts: 38,140
Rewards Points: 890
Default

Re: Proper cantilever insulation?


Quote:
Originally Posted by mac1990 View Post
I choose EPS because it does not lose R value over time and will dry out if it manages to get wet. It is also a better value per R.

The EPS I purchased has a foil face, should be fine.






Something I did not even think about. The floors above the cantilever are cold in the winter.

All the areas above the cantilever are heated, but only half the area below the cantilever is heated. Half the cantilever is off an unheated garage, while the other half is in a heated finished basement.

So do the following when the cantilever is connected to a heated area:
Solid blocking over the lower wall is code
__________________
Do not use flat bottom gables with scissor trusses.
https://www.fema.gov/media-library-d...6/chapter7.pdf
figure 7-14
Nealtw is online now   Reply With Quote
Old 08-08-2020, 08:53 AM   #13
Member
 
Join Date: Mar 2017
Location: PA
Posts: 92
Rewards Points: 184
Default

Re: Proper cantilever insulation?


Quote:
Originally Posted by Nealtw View Post
Solid blocking over the lower wall is code

what?
mac1990 is offline   Reply With Quote
Old 08-08-2020, 09:01 AM   #14
retired framer
 
Join Date: Jun 2017
Location: Fraser Valley BC Canada
Posts: 38,140
Rewards Points: 890
Default

Re: Proper cantilever insulation?


In yellow, no air thru that area.
Attached Thumbnails
Proper cantilever insulation?-mac-8.png  
__________________
Do not use flat bottom gables with scissor trusses.
https://www.fema.gov/media-library-d...6/chapter7.pdf
figure 7-14
Nealtw is online now   Reply With Quote
Old 08-08-2020, 09:14 AM   #15
Member
 
Join Date: Mar 2017
Location: PA
Posts: 92
Rewards Points: 184
Default

Re: Proper cantilever insulation?


Quote:
Originally Posted by Nealtw View Post
In yellow, no air thru that area.

I removed that because @Bud9051 stated the following:
"If the basement space exposed to those joist cavities is heated then the other option is to insulate the bottom of the cantilever and leave a warm air gap above."

I thought the air gap wanted the warm air from the heated basement area.


I dont know if I am even going to mess with the air gaps. How much of an air gap is needed? It may mess up the thickness of insulation I was going to buy.

Last edited by mac1990; 08-08-2020 at 09:18 AM.
mac1990 is offline   Reply With Quote
Sponsored Links
Advertisement
 
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





Top of Page | View New Posts