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

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

Reply
 
Thread Tools Search this Thread Display Modes
Old 01-26-2012, 05:05 PM   #1
Newbie
 
Join Date: Apr 2010
Posts: 15
Share |
Default

Insulating rim joists properly


I have a new home that is uses R13 unfaced fiberglass to insulate the rim joist. Lately I have noticed that it is wet and looks like the beginnings of mold are forming. At first I thought it was water leaking in but after reading, it looks like it might just be condensation. The house is about 7 months old and has a full, unfinished, totally below grade ~1400sqft basement. I only have two vents down there but they are always open and it does not feel moist. There also is no wall staining.

It looks like I need to caulk the rim joist cavities and mud sill and use XPS with foam or caulk to form a moisture barrier. My question is how thick? Can I use 1/2 inch xps for the moisture barrier and then put the fiberglass back in there there to do the bulk of the insulating?

I'm going to ask the builder for a solution, but with this being the code here in KY I'm not expecting much.

Mike

blk94f150 is offline   Reply With Quote
Old 01-26-2012, 08:26 PM   #2
Member
 
Join Date: Dec 2011
Location: zentral Iowa
Posts: 1,179
Default

Insulating rim joists properly


I'm not sure how much to use, but I would stay away from the 1/2 material.

I start out with the outside sheathing going down over the rim joist, and I use 1" xps for the sheathing. So it is already covered with 1" on the outside. Then I take all my door and window cutouts and rip them down to the floor joist width, and cut them piece by piece to the width of the cavity. You'll have some 14 1/2's, some 14 7/16ths, 14 5/8, etc. I use my fist or a hammer to squeeze them in there very tightly. I may have enough material to get 3 layers on the inside, so I'm up to r-20. Then I put r-13 fiberglass on top of that.

I think 2 layers of 1" material on the inside will work, then fiberglass on top of that. As I said, I smash it in there very tight and I have xps on the exterior as well, so I don't caulk around the edges. No big deal to caulk around the edges if you want.

cleveman is offline   Reply With Quote
Old 01-27-2012, 07:11 AM   #3
Member
 
Join Date: Jul 2011
Posts: 803
Default

Insulating rim joists properly


I would cut them a little smaller than the opening and then use a canned spray foam around each piece. It will be more work, but will end up being like you used real spray foam. I would aim for 2 inches of XPS if possible. Either by stacking or just getting 2 inch off the bat.
mikegp is online now   Reply With Quote
Old 01-27-2012, 12:15 PM   #4
Member
 
Join Date: Dec 2008
Location: Fairbanks, AK
Posts: 1,847
Default

Insulating rim joists properly


I'd chuck the fiberglass while you have it out. It is pretty poor insulation to start with, and if it is not in an airtight cavity(putting it on the outside) it is even worse. If you want more info on this, greenbuildingadvisor.com has tons there. Your R value should exceed what you code calls for (codes are minimum) and air seal like mad, as mentioned above, or you will possibly have rot.
jklingel is offline   Reply With Quote
Old 01-27-2012, 01:19 PM   #5
Member
 
Join Date: Oct 2009
Location: Minneapolis, MN
Posts: 495
Default

Insulating rim joists properly


I am in the process of finishing of and plan to spray in. Either hire out if not too expensive or buy a froth pack (many youtube vids) and DIY. I am not that far yet.

The area directly above the cement block has "poured" in cement between the joists making it difficult to put in pieces of XPS. Spray it and do it right.

B
Beepster is offline   Reply With Quote
Old 01-27-2012, 02:55 PM   #6
Newbie
 
Join Date: Apr 2010
Posts: 15
Default

Insulating rim joists properly


Froth-Paks are too expensive for the R-value and board feet you get out of them. I guess I'll use XPS with great stuff around the perimeter. Is 2 inch Polyshield XPS? It only has an R-value of 7.7 with 2 inches. The Owens Corning Foamular boards are R-5 per inch. Would 1 inch of Foamular along with Great stuff and R13 fiberglass do the trick to keep my rim joists from getting moist? I figure that no matter what I do, I'll stick that fiberglass stuff back up there as it can't hurt as long as it's not getting wet.


Also is there any reason to caulk the joints behind the foam board before putting it in?
blk94f150 is offline   Reply With Quote
Old 01-27-2012, 03:52 PM   #7
Member
 
Join Date: Jan 2011
Location: South Jersey
Posts: 160
Default

Insulating rim joists properly


Quote:
Originally Posted by blk94f150 View Post
Froth-Paks are too expensive for the R-value and board feet you get out of them. I guess I'll use XPS with great stuff around the perimeter. Is 2 inch Polyshield XPS? It only has an R-value of 7.7 with 2 inches. The Owens Corning Foamular boards are R-5 per inch. Would 1 inch of Foamular along with Great stuff and R13 fiberglass do the trick to keep my rim joists from getting moist? I figure that no matter what I do, I'll stick that fiberglass stuff back up there as it can't hurt as long as it's not getting wet.


Also is there any reason to caulk the joints behind the foam board before putting it in?
For my crawlspace I caulked all the joints in my rim joists just to be extra careful and stop that air flow. I then placed 2" XPS board over it and sealed it with great stuff foam. I used Roxul vs. fiberglass but same difference for the most part. Good luck!
cbaur88 is offline   Reply With Quote
Old 01-27-2012, 07:50 PM   #8
Registered User
 
Join Date: Mar 2009
Posts: 9,968
Default

Insulating rim joists properly


I agree! Caulk the wood joints to stop incoming air, foam the 4 edges of the foamboard to stop interior air from condensing on the wood rim: http://www.buildingscience.com/docum...ce-insulation/

If under the IRC, the foamboard can be left exposed: http://publicecodes.citation.com/ico...016_par015.htm

Of course, always check with your local AHJ.
I was surprised to read that as little as a 1/32" gap can degrade your cavity insulation. Plus, water vapor molecules are much smaller than air molecules, we can't win...LOL.

Gary
__________________
If any ads are present in my answer above, I do not condone/support/use the product or services listed, they are there against my permission.
Gary in WA is offline   Reply With Quote
Old 01-27-2012, 11:08 PM   #9
Member
 
Join Date: Dec 2008
Location: Fairbanks, AK
Posts: 1,847
Default

Insulating rim joists properly


Quote:
Originally Posted by GBR in WA View Post
I was surprised to read that as little as a 1/32" gap can degrade your cavity insulation.
Gary
I think it was on the ORNL site where I read that a 1" sqr hole in sheet rock will bleed as much moisture as will diffuse through the rest of the sheet. Hence, air seal.
jklingel is offline   Reply With Quote
Old 01-27-2012, 11:20 PM   #10
Newbie
 
Join Date: Apr 2010
Posts: 15
Default

Insulating rim joists properly


Can anyone comment on 1 inch vs 2 inch XPS? Will 1 inch be adequate to keep the condensation away? Obviously 2 will double the R-value, but I'm going to put the R13 glass insulation back in there as well anyway.
blk94f150 is offline   Reply With Quote
Old 01-28-2012, 05:55 AM   #11
Exterior Construction
 
Windows on Wash's Avatar
 
Join Date: Aug 2011
Location: VA, MD, DC
Posts: 5,962
Default

Insulating rim joists properly


Quote:
Originally Posted by jklingel View Post
I think it was on the ORNL site where I read that a 1" sqr hole in sheet rock will bleed as much moisture as will diffuse through the rest of the sheet. Hence, air seal.
More than that.
Attached Thumbnails
Insulating rim joists properly-moisture-migration-airflow-vs.-diffusion.jpg  
Windows on Wash is offline   Reply With Quote
The Following User Says Thank You to Windows on Wash For This Useful Post:
cleveman (01-28-2012)
Old 01-28-2012, 11:27 AM   #12
Member
 
Join Date: Dec 2008
Location: Fairbanks, AK
Posts: 1,847
Default

Insulating rim joists properly


Windows: Chit. That is pretty dramatic. Where did you get that pic (I'd like to site it if I even use the photo). Thanks. john
jklingel is offline   Reply With Quote
Old 01-28-2012, 11:06 PM   #13
Registered User
 
Join Date: Mar 2009
Posts: 9,968
Default

Insulating rim joists properly


Quote:
Originally Posted by blk94f150 View Post
Can anyone comment on 1 inch vs 2 inch XPS? Will 1 inch be adequate to keep the condensation away? Obviously 2 will double the R-value, but I'm going to put the R13 glass insulation back in there as well anyway.
---- depends on your location; http://www.buildingfoundation.umn.ed...timum-main.htm Cruise the links given...

The pic. is from: http://www.energysavers.gov/your_hom.../mytopic=11760

I was referring to air movement degrading cavity/rigid insulation.

Gary
__________________
If any ads are present in my answer above, I do not condone/support/use the product or services listed, they are there against my permission.
Gary in WA is offline   Reply With Quote
Old 01-28-2012, 11:28 PM   #14
Member
 
Join Date: Dec 2008
Location: Fairbanks, AK
Posts: 1,847
Default

Insulating rim joists properly


Quote:
Originally Posted by GBR in WA View Post
GAry: Thanks for the link.

jklingel is offline   Reply With Quote
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
OSB Stronger than Plywood? Weathermaker01 Flooring 15 12-22-2012 09:06 AM
Garage ceiling joists and beam question Truth Building & Construction 8 01-05-2012 07:52 PM
Sistering Joists and Adding a Beam Mikeman Building & Construction 6 09-30-2011 08:11 PM
Replace or sister floor joists WillK Building & Construction 6 03-09-2011 08:55 PM
Strengthening Joists Before Basement Finish braverichard Building & Construction 16 06-20-2010 07:28 PM




Top of Page | View New Posts

Copyright © 2003-2014 Escalate Media. All Rights Reserved.