Welcome to the DoItYourself Forums!

To post questions, help other DIYers and reduce advertising (like the one on your left), join our DIY community. It's free!

Rim joist insulation issues


Bud9051's Avatar
Member

Join Date: Oct 2008
Posts: 9,764
ME

12-10-16, 06:43 AM   #1  
Rim joist insulation issues

This is a continuation from another thread to avoid drifting.
http://www.doityourself.com/forum/en...nsulation.html

Hi Tony, I thought this would be better than more back and forth on amatuer's thread.

You said: " Bud, I hope you had a good sleep and certainly don't disagree with what you're saying but it's considerably different from my basic understanding of rigid insulation in rim joists. My understanding has been that stopping moisture is the focus and that takes me to permeability.

Sticking with XPS, 1" has a perm rating of 1.1 making it a Class III permeable vapor retarder. Alternatively, 2" has a rating of .7 (not proportional) making it a more desirable Class II semi-permeable. This has been my basis for recommending 2" rigid insulation.

XPS has an R-value of 5 per inch. This means that, per the table, 1" XPS may be used in zones 5, where I live. While I get that from an R-value perspective, it seems to disregard the permeability aspect. I mean, where the point is moisture flow, why use a permeable level of XPS anywhere?

Finally, amateur, I'm sorry if I temporarily hijacked your question.

Tony P.

Although vapor permeance is part of the consideration, maintaining the inside surface of the rigid foam above the condensation temperature and air sealing around the rigid to prevent warm air from bypassing it and reaching the cold rim joist are the two more important issues. The link below does a good job at explaining the options.

Note: about halfway down under Spray foam insulates and seals it says "In mild climate zones, either open-cell spray foam or closed-cell spray foam will work; however, in climate zone 6 and colder zones, itís safer to use closed-cell spray foam." Bet you didn't know you live in a mild climate (CT). But open cell is certainly vapor permeable and is considered a low risk in zones 5 and warmer.

Amateur is listed as being in WI and they are zone 6 or 7 (cold) so he should stay with the closed foam.

Insulating Rim Joists

Bud

 
Sponsored Links
Tony P.'s Avatar
Member

Join Date: Jan 2015
Posts: 422
CT

12-11-16, 04:08 AM   #2  
Bud, thanks for the information. I'm glad to get into this so I can understand the issue. While I accept the need for >2" rigid insulation in colder regions, it still makes sense for there to be a minimum level of moisture protection and I continue to think that's reached at 2".

 
Bud9051's Avatar
Member

Join Date: Oct 2008
Posts: 9,764
ME

12-11-16, 04:42 AM   #3  
Hi Tony.
Here's an article which will help, took me awhile to find it. But it explains why vapor barriers are not necessary in many instances and locations, it is air leakage that moves the moisture. In the example, he is using unpainted drywall with a very high permeance. Add a couple layers of paint or in the application we are discussing replace the drywall with 1" of rigid insulation with a much lower permeance, and diffusion is no longer a concern.

In a rim joist, once air sealed, the concern shifts to maintaining the inside surface temp of the rigid above the dew point.

Bud
You Don't Need a Vapor Barrier (Probably)

 
Tony P.'s Avatar
Member

Join Date: Jan 2015
Posts: 422
CT

12-13-16, 04:39 AM   #4  
Bud, that all being the case, I'd like your thoughts on insulating my home purchased about 18 months ago. The home is on a slope with a basement walk-out in the back, so the basement is roughly 50% poured (front and side) and 50% framed (rear and other side).

The framed portion shows that someone had insulated with fiberglass in the past with a few signs of old mold. No insulation is on the walls now. I've been planning to add rigid insulation to the framed portion but 2" XPS over 100 liner feet is expensive and the payback's not huge.

Beyond that, the basement is not as humid as most but that's probably a function of the framing. So, to some extent, adding rigid insulation would increase humidity / moisture in the basement.

I'm interested in your thoughts on using only 1" rigid foam to create restrict air leakage given all you've described.

 
Bud9051's Avatar
Member

Join Date: Oct 2008
Posts: 9,764
ME

12-13-16, 09:16 AM   #5  
Installing a batt insulation in the framed portion of the wall is not a problem as long as you do a great job of air sealing those cavities. Modern air tight construction will even glue the drywall to the framing, a pain but it does air seal. Note, I like Roxul for the batt insulation, very dense and easy to work with. I use a long bread knife to cut it.

For the concrete and below grade the rigid provides an insulation factor and serves to prevent basement air from reaching the exterior cold concrete. The primary areas of concern for the concrete are those above grade and extending 2' below grade. Assuming you will install 2x4 framed walls, keep them tight to the rigid as the air gap provides a path for moisture vapor to be carried up to the colder exposed areas.

No vapor barrier is needed as the painted drywall will provide sufficient control and once the air leakage is eliminated there so little moisture passing through it is best to just allow it to dry to the inside. When even a tiny amount of moisture is passing through when it hits a true vapor barrier is starts to accumulate until the surface of that vb is as wet as the source of the moisture.

As for judging humidity pick up a simple meter and measure it along with the temperature. Cooler temps will register with a higher relative humidity even though the moisture level may not be an issue. Several related threads here.

In your climate region the 1" rigid should be sufficient to maintain the inside surface above the dew point along with r-15 in the cavities on the inside. But you still want to air seal those walls.

Bud

 
Tony P.'s Avatar
Member

Join Date: Jan 2015
Posts: 422
CT

12-13-16, 11:34 AM   #6  
Just one last bit of clarifying information here because you mention drywall. It's a walkout basement. To be complete, there is no drywall. The studs by the walk-out area are all exposed on the interior as is the sheathing, which is covered (on the outside) with vinyl siding, see photo.

Name:  basement wall.jpg
Views: 67
Size:  24.1 KB

My plan is to add the 1" XPS between the studs and sealed with Great Stuff, followed by R-15 fiberglass insulation


Last edited by Tony P.; 12-13-16 at 11:51 AM.
 
Bud9051's Avatar
Member

Join Date: Oct 2008
Posts: 9,764
ME

12-13-16, 03:54 PM   #7  
With the vinyl siding on the other side it is essentially an exterior wall above grade. The 1" of rigid will do no harm and will add a bit of r-value but is not necessary above grade.

The math doesn't seem to work perfectly. If that is a 2x4 stud wall and you install the 1" inside the cavity you only have 2.5" of space left and you can't get r-15 into that space. If 2x6 then you have 4.5" of remaining space and r-15 doesn't fill it.

A common way to meet the r-20 code for walls is an r-13 cavity fill and then cover the entire wall with r-5 rigid. Because the rigid would be covering the face of the studs it reduces the thermal bridging and thus qualifies for the r-20 requirement. Plus you eliminate all of the cutting and fitting. You can still picture frame the inside of each cavity with caulk if you want to ensure a perfect air seal.

Bud

 
Search this Thread