Insulating Between Floor Joists In Crawl Space - 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 Tree4Likes
  • 1 Post By Nealtw
  • 1 Post By Kory Beam
  • 1 Post By Marson
  • 1 Post By Nealtw
Reply
 
Thread Tools Search this Thread
Display Modes
Old 05-15-2018, 03:50 PM   #1
Member
 
Join Date: Nov 2014
Location: SoCal Mountains
Posts: 184
Rewards Points: 368
Default

Insulating between floor joists in crawl space


Hi all! I have to insulate between the floor joists of my tall crawl space. This crawl space is below the bedroom and bathroom.

Right now there are no vents cut in the crawl space, as the house was recently re-sided. The existing framing (blocked rectangles) for the vents are in place and all that is needed is to sawzall them open and install the vent covers for proper crawl space air flow.

The old nasty fiberglass is gone and the floor joist cavities are naked and ready for new insulation.

I have 4 bundles of Roxul R23 (2x6) batts left over from the finished basement job, and I would like to use them here, as I got a great deal on them. It is just enough to do this area.

So... in order to completely fill the joist cavity. I figured I would just buy a bundle or two of r15 roxul for 2x4 stud cavities, splice it in half - install half of the 2x4 batt in contact to the floor plywood above, then fill the rest of the stud cavity with a 2x6 roxul batt. The friction fit should hold both in place. Does that sound like it will work? Like I said... I already have these 2x6 R23 Roxul batts on hand and want to use them.

Also.... if I am cutting in the side wall vents for airflow down there, there is no point to insulating the framed crawl space walls above the foundation, right? I am in the SoCal mountains at 6K feet. 93222 - Zone 3. No AC/heating ducting. House is heated with wood stove and 2 electric Cadet wall heaters on thermostats(tiny house).

Thanks for any info/advice!

Pic of crawl space:

Last edited by Kory Beam; 05-15-2018 at 04:01 PM.
Kory Beam is offline   Reply With Quote
Sponsored Links
Advertisement
 
Old 05-15-2018, 04:01 PM   #2
retired framer
 
Join Date: Jun 2017
Location: Fraser Valley BC Canada
Posts: 38,040
Rewards Points: 690
Default

Re: Insulating between floor joists in crawl space


If the white on the concrete is an indication on how much moisture is entering the space, that venting you had before was not enough and might be the cause of the fiberglass to be ugly. More venting or power vent or dehumidifier might be the order of the day. You might consider vapour barrier on the ground run up all the way and sealed to the top of the concrete as that would not interfere with your venting.
Kory Beam likes this.
Nealtw is online now   Reply With Quote
Sponsored Links
Advertisement
 
Old 05-15-2018, 04:23 PM   #3
Member
 
Join Date: Nov 2014
Location: SoCal Mountains
Posts: 184
Rewards Points: 368
Default

Re: Insulating between floor joists in crawl space


Thanks, Nealtw.

The efflorescence on the back foundation wall has been there since I bought the place about 5 years ago. I believe it was from snow sitting against that area, and snow melt from the roof saturating the ground in that corner during winter weather. It has since been re-roofed, gutters, and a french drain to direct any water away from saturating the areas around the exterior foundation. The crawl space seems to stay dry now. Not sensing any moist smells/humidity. The original fiberglass batts were funky and torn up from rodents getting into it, so I thought it would be best to remove and replace with something fresh. The old pink stuff was original to the cabin ('73).

I have certainly considered a vapor barrier over the dirt floor and up the concrete foundation. Need to look into that more...
Nealtw likes this.

Last edited by Kory Beam; 05-15-2018 at 04:25 PM.
Kory Beam is offline   Reply With Quote
Sponsored Links
Advertisement
 
Old 05-15-2018, 04:43 PM   #4
Member
 
Join Date: Nov 2014
Location: SoCal Mountains
Posts: 184
Rewards Points: 368
Default

Re: Insulating between floor joists in crawl space


Does the insulation trick I proposed in the first post seem like a fine way to stuff the joists and get the required fill/R-value for 2x8 floor joists?

Is Roxul a decent/suitable insulation for the floor joists in a crawl space?
Kory Beam is offline   Reply With Quote
Old 05-15-2018, 06:21 PM   #5
Member
 
Join Date: Jan 2018
Posts: 712
Rewards Points: 1,360
Default

Re: Insulating between floor joists in crawl space


Quote:
Originally Posted by Kory Beam View Post
Does the insulation trick I proposed in the first post seem like a fine way to stuff the joists and get the required fill/R-value for 2x8 floor joists?

Is Roxul a decent/suitable insulation for the floor joists in a crawl space?

I haven't used Roxul personally, but I don't see why it wouldn't work. The main thing about floors is to get good contact of insulation to subfloor. You don't want an airspace there.



I wouldn't rely on friction fit to hold those batts in place. Over time you will have batts falling out. What I have done is pull masons string taut across the bottom of the joists, and staple the string to the bottom of each joist so that it holds the batt up. Do this on 16" centers or so, going back and forth. There are also wires you can get that wedge between the joists to hold up the batts, but I haven't used them.


The other point I would make, and people might disagree with me, is that it is actually OK to compress fiberglass in a joist cavity. Higher density insulation is a good thing, to point anyway. That's why an R 21 batt is denser than an R 19 batt, even though they both are made for a 5 1/2" cavity. The most important thing is to eliminate or minimize any voids in insulation in the stud cavity.
Kory Beam likes this.

Last edited by Marson; 05-15-2018 at 06:26 PM.
Marson is online now   Reply With Quote
Old 05-15-2018, 06:54 PM   #6
retired framer
 
Join Date: Jun 2017
Location: Fraser Valley BC Canada
Posts: 38,040
Rewards Points: 690
Default

Re: Insulating between floor joists in crawl space


I like the string idea, I have seen almost anything put up but mostly they fall down when they get heavy with moisture, letting you know you have other problems.
Kory Beam likes this.
Nealtw is online now   Reply With Quote
Old 05-18-2018, 12:03 PM   #7
Member
 
Join Date: Nov 2014
Location: SoCal Mountains
Posts: 184
Rewards Points: 368
Default

Re: Insulating between floor joists in crawl space


Out of curiosity...

Would cobbling polyiso or xps Foam board to the subfloor in the joist cavities be a bad idea? Like might cause moisture trapping issues and mold at the subfloor?

Was thinking that 1-2" foam board cobbled in those joist cavities, then sealed with can foam (like done at rim joist).... then fill rest of joist cavity with the 5.5" r23 batt insulation (roxul). Would this be a moisture trap no-no? Also wondering if that would be a problem for getting termite inspection if sometime in the future If I decide to sell the house?
Kory Beam is offline   Reply With Quote
Old 05-18-2018, 12:27 PM   #8
retired framer
 
Join Date: Jun 2017
Location: Fraser Valley BC Canada
Posts: 38,040
Rewards Points: 690
Default

Re: Insulating between floor joists in crawl space


Quote:
Originally Posted by Kory Beam View Post
Out of curiosity...

Would cobbling polyiso or xps Foam board to the subfloor in the joist cavities be a bad idea? Like might cause moisture trapping issues and mold at the subfloor?

Was thinking that 1-2" foam board cobbled in those joist cavities, then sealed with can foam (like done at rim joist).... then fill rest of joist cavity with the 5.5" r23 batt insulation (roxul). Would this be a moisture trap no-no? Also wondering if that would be a problem for getting termite inspection if sometime in the future If I decide to sell the house?
A plywood subfloor is considered the vapour barrier so having another in contact and sealed should be fine.
Not sure about termite inspections.
Nealtw is online now   Reply With Quote
The Following User Says Thank You to Nealtw For This Useful Post:
Kory Beam (05-18-2018)
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


Similar Threads
Thread Thread Starter Forum Replies Last Post
Crawl space insulation question gregsenne Insulation 11 01-16-2013 03:26 PM
Insulating crawl space Cheveldave Insulation 2 11-24-2012 11:34 PM
insulating crawl space rhader Insulation 5 11-07-2012 12:02 PM
Insulating crawl space jenoble99 Building & Construction 10 06-18-2009 12:09 PM
insulating crawl space near plate measure thrice Building & Construction 13 01-26-2009 03:11 PM




Top of Page | View New Posts