Crawl Space Ceiling/floor Insulation With Roxul - 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 Tree5Likes
Reply
 
Thread Tools
Display Modes
Old 03-23-2019, 03:51 PM   #1
Member
 
Join Date: Nov 2014
Location: SoCal Mountains
Posts: 176
Rewards Points: 352
Default

Crawl space ceiling/floor insulation with Roxul


Hi all!
I need to insulate the tall crawl space at my mountain cabin. The crawl space is vented. There is no air-conditioning or heating ducting. Cabin is heated with wood stove & electric Cadet wall heaters on thermostats.

The old & ratty pink stuff has been removed. Just clean 2x8 joists and exposed plywood subfloor. 11' x 18' area to be insulated. The bedroom and bathroom are above this crawl space.

My question:
I have about 200 sqft of new R23 Roxul batt insulation that is left over from another project. Can I use this Roxul insulation to insulate the ceiling in this vented crawl space?
So... R23 Roxul fit into joist cavities, placed in contact with the naked plywood subfloor. There would be no vapor barrier (like you have on faced fiberglass insulation) between exposed subfloor & Roxul, as the Roxul batts are not faced. Just plain batts.

Would this be acceptable method of insulating?
SoCal mountains. 6000'. 93225.
No moisture issues in crawl space... always seems bone dry in there.
Appreciate advice/info!

Last edited by Kory Beam; 03-23-2019 at 04:10 PM.
Kory Beam is offline   Reply With Quote
Sponsored Links
Advertisement
 
Old 03-23-2019, 04:52 PM   #2
Exterior Construction
 
Windows on Wash's Avatar
 
Join Date: Aug 2011
Location: Washington DC Metro Area (VA, MD, DC)
Posts: 19,839
Rewards Points: 3,350
Blog Entries: 32
Default

Re: Crawl space ceiling/floor insulation with Roxul


You can. You should still use some rigid foam to back the joists and seal the seams. This will help with the thermal bridging and any condensation issues that could develop over time.



http://buildingscience.com/documents...in-crawlspaces
Kory Beam likes this.
Windows on Wash is offline   Reply With Quote
Sponsored Links
Advertisement
 
Old 03-23-2019, 05:09 PM   #3
retired framer
 
Join Date: Jun 2017
Location: Fraser Valley BC Canada
Posts: 23,047
Rewards Points: 13,982
Default

Re: Crawl space ceiling/floor insulation with Roxul


The sub floor is considered the vapour barrier and is on the warm side.

Think of a attic upside down air below insulation to allow drying.

I think the rock wool is the right product I would fill the space to the bottom of the joist. then i would apply a window screen for insects and the a wire mesh to help hold the insulation in place and keep critters out of it.

The dirt below should be covered with a 10 mill sheet poly and sealed to the foundation if you have one.
Kory Beam likes this.
__________________
Do not use flat bottom gables with scissor trusses.
Nealtw is online now   Reply With Quote
Sponsored Links
Advertisement
 
Old 03-23-2019, 05:22 PM   #4
Member
 
Join Date: Nov 2014
Location: SoCal Mountains
Posts: 176
Rewards Points: 352
Default

Re: Crawl space ceiling/floor insulation with Roxul


Nealtw, the Roxul batts are R23 for 2x6... I don't think the batts are thick enough to completely fill a 2x8 joist cavity. Is it necessary to fill to cavity to the bottom of the joists?
Kory Beam is offline   Reply With Quote
Old 03-23-2019, 05:26 PM   #5
Exterior Construction
 
Windows on Wash's Avatar
 
Join Date: Aug 2011
Location: Washington DC Metro Area (VA, MD, DC)
Posts: 19,839
Rewards Points: 3,350
Blog Entries: 32
Default

Re: Crawl space ceiling/floor insulation with Roxul


Depending on moisture conditions in the crawl, the framing ends can be condensing locations. Please see the link to reference more effective means of insulating the space.

Besides the thermal bridging of the R-7 weak points of the framing, the foam provides a more effective air barrier (less important with Roxul, but still of some benefit).



Windows on Wash is offline   Reply With Quote
Old 03-23-2019, 05:27 PM   #6
Exterior Construction
 
Windows on Wash's Avatar
 
Join Date: Aug 2011
Location: Washington DC Metro Area (VA, MD, DC)
Posts: 19,839
Rewards Points: 3,350
Blog Entries: 32
Default

Re: Crawl space ceiling/floor insulation with Roxul


Quote:
Originally Posted by Kory Beam View Post
Nealtw, the Roxul batts are R23 for 2x6... I don't think the batts are thick enough to completely fill a 2x8 joist cavity. Is it necessary to fill to cavity to the bottom of the joists?

See the picture below. If you have an air/insulation barrier over the insulation, it is ideal to leave an air gap under the subfloor to allow for some warming of the floor.
Windows on Wash is offline   Reply With Quote
Old 03-23-2019, 05:30 PM   #7
Exterior Construction
 
Windows on Wash's Avatar
 
Join Date: Aug 2011
Location: Washington DC Metro Area (VA, MD, DC)
Posts: 19,839
Rewards Points: 3,350
Blog Entries: 32
Default

Re: Crawl space ceiling/floor insulation with Roxul


Depending on the glue used in the plywood, it will be a Class 2 or Class 3 vapor retarder and will allow for drying. Depending on the foam and details on the seams, there will be next to zero moisture coming from the earth and into framing.
Windows on Wash is offline   Reply With Quote
Old 03-23-2019, 06:37 PM   #8
Member
 
Join Date: Nov 2014
Location: SoCal Mountains
Posts: 176
Rewards Points: 352
Default

Re: Crawl space ceiling/floor insulation with Roxul


If I am only going to be insulating with rock wool batts in the joist cavities (no rigid foam encapsulation of joist assembly), I should make sure that the insulation batts should be in full contact with the subfloor, right? Since these are R23 batts for 2x6, they would not completely fill a 2x8 joist cavity.
Kory Beam is offline   Reply With Quote
Old 03-23-2019, 06:44 PM   #9
Exterior Construction
 
Windows on Wash's Avatar
 
Join Date: Aug 2011
Location: Washington DC Metro Area (VA, MD, DC)
Posts: 19,839
Rewards Points: 3,350
Blog Entries: 32
Default

Re: Crawl space ceiling/floor insulation with Roxul


If they are tight fitting on the joist spacing, I would still like them hang down to be flush with the joist bottoms.



After that, I would cover the bottoms with some sort of wrap or plastic to hold the insulation in and create and air barrier.
Kory Beam likes this.
Windows on Wash is offline   Reply With Quote
Old 03-23-2019, 08:53 PM   #10
retired framer
 
Join Date: Jun 2017
Location: Fraser Valley BC Canada
Posts: 23,047
Rewards Points: 13,982
Default

Re: Crawl space ceiling/floor insulation with Roxul


Quote:
Originally Posted by Windows on Wash View Post
If they are tight fitting on the joist spacing, I would still like them hang down to be flush with the joist bottoms.



After that, I would cover the bottoms with some sort of wrap or plastic to hold the insulation in and create and air barrier.
Would you cover the insulation in that attic with plastic or some sort of air barrier?
__________________
Do not use flat bottom gables with scissor trusses.
Nealtw is online now   Reply With Quote
Old 03-23-2019, 09:10 PM   #11
Exterior Construction
 
Windows on Wash's Avatar
 
Join Date: Aug 2011
Location: Washington DC Metro Area (VA, MD, DC)
Posts: 19,839
Rewards Points: 3,350
Blog Entries: 32
Default

Re: Crawl space ceiling/floor insulation with Roxul


Depends on the application.
Windows on Wash is offline   Reply With Quote
Old 03-23-2019, 09:16 PM   #12
retired framer
 
Join Date: Jun 2017
Location: Fraser Valley BC Canada
Posts: 23,047
Rewards Points: 13,982
Default

Re: Crawl space ceiling/floor insulation with Roxul


Quote:
Originally Posted by Windows on Wash View Post
Depends on the application.

Mobil homes have fabric so it can breath,
__________________
Do not use flat bottom gables with scissor trusses.
Nealtw is online now   Reply With Quote
Old 03-23-2019, 09:17 PM   #13
Exterior Construction
 
Windows on Wash's Avatar
 
Join Date: Aug 2011
Location: Washington DC Metro Area (VA, MD, DC)
Posts: 19,839
Rewards Points: 3,350
Blog Entries: 32
Default

Re: Crawl space ceiling/floor insulation with Roxul


Homes and insulation don't need to breath and you don't need to vent through the bottom of the insulation in a crawl space. The vast majority of moisture is coming via connection to the exterior climate and soil at that point.


Attic doesn't have the same challenges.
Windows on Wash is offline   Reply With Quote
Old 03-23-2019, 09:20 PM   #14
retired framer
 
Join Date: Jun 2017
Location: Fraser Valley BC Canada
Posts: 23,047
Rewards Points: 13,982
Default

Re: Crawl space ceiling/floor insulation with Roxul


Quote:
Originally Posted by Windows on Wash View Post
Homes and insulation don't need to breath and you don't need to vent through the bottom of the insulation in a crawl space. The vast majority of moisture is coming via connection to the exterior climate and soil at that point.


Attic doesn't have the same challenges.
That is why you put the poly on the ground.
__________________
Do not use flat bottom gables with scissor trusses.
Nealtw is online now   Reply With Quote
Old 03-23-2019, 09:25 PM   #15
Exterior Construction
 
Windows on Wash's Avatar
 
Join Date: Aug 2011
Location: Washington DC Metro Area (VA, MD, DC)
Posts: 19,839
Rewards Points: 3,350
Blog Entries: 32
Default

Re: Crawl space ceiling/floor insulation with Roxul


I can forward your concerns to the Building Science corporation if you like.



And we put poly on the ground as well..when we are conditioning a crawl in conjunction with sealing the rims and insulating the stem walls.



I fail to see the point in it with a vented crawl. The relative humidity in the air will usually be consistent with what is in the air at that point.
Windows on Wash 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
Sealing off a Crawl Space arblargan Insulation 12 09-11-2014 10:41 AM
Water in crawl space ratherbefishing Building & Construction 2 07-19-2014 09:10 AM
Bad smell in crawl space. John_W General DIY Discussions 34 06-03-2014 12:57 PM
Odor from Crawl Space G Ollis General DIY Discussions 68 11-02-2013 01:31 AM
Vapor barrier issue between crawl space and conditioned basement gregsenne Insulation 3 10-11-2012 11:53 PM




Top of Page | View New Posts