# What type of insulation for basement rim joist?



## Gary in WA (Mar 11, 2009)

http://www.rd.com/57548/article57548.html
Be safe, Gary


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## diy4life (Oct 31, 2008)

Secutenudu,

If you have enough air circulation for your furnace and hot water heater, then you shouldn't be worried about moisture build up. Moisture problem only becomes serious if your basement can't breath fresh air. For my basement project I used faced and most of the time, one of my basement window is slightly left open for outside air. 

Just curious..How old is your house and what type of house is this?

Diy4Life


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## Scuba_Dave (Jan 16, 2009)

GBAR has it, rigid foam is the best way to seal the rim joist area
Key word is seal
Any time the warmer air of the basement can get to the cold rim joist it can & will condense
Moisture causes mold
I used fiberglass for now & will be going back to use rigid foam, then possibly fiberglass over that
I don't have the time right now to cut & fit the rigid block

If you have a dry basement then you may never have a problem with moisture
But even a basement that appears dry can have water vapor infiltrate
I have a humidstat in the basement that I use to monitor moisture
I also have a dehumidifier that has a digital readout
We have a high water table here - near a stream


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## secutanudu (Mar 15, 2009)

Thanks for the replies. I want to do this on my house and my parents' house. 

Mine is a 50's brick cape with a block foundation.

Theirs is a 1970's colonial with a block foundation, brick/wood shake siding.


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## ccarlisle (Jul 2, 2008)

Although it depends on other things, we find that the "best" is *sprayed-on foam* since it acts as insulation plus a moisture/vapour barrier; only thing is it's not cost effective on a small scale. Next best is the rigid extruded polystyrene where the foam around each piece acts (as well) as a vapour barrier. That system is the most cost effective for a DIYer...

But we're in a cold zone...

Using faced or unfaced batts isn't a matter of which one is more convenient or easy-to-apply. It almost sounds like you think it is. The facing is a vapour barrier and is specified for where it's needed, not for practicality. It depends too on what zone you are in, it's not an "I-have- a-bigger-engine-than-you" thing, and no-one else's choice of insulation or vapour barier has one bit of relevance unless other conditions are the same.

Either way, you should wear a mask at installation of fibreglass...but we're not talking asbestos fibres here! A moisture problem remains a moisture problem regardless of whether or not you have faced, unfaced, or any other type of insulation and moisture problems are to be dealt with independently of which insulation method you chose. Moisture impacts the choice but conditions have to be seen as a whole, not individually.

What you are most worried about at the rim joist (between insulation and air infiltration) is _air infiltration_; at least it is in cold zones, where I trust you are located. It is typically an under-insulated and leaky area and is well-known for letting air - warm or cold - through it. It is really hard to completely air-seal the whole area unless you use spray-on foam. 

A 50's brick cape -are you in New England? FYI: If you have a block foundation, there are inherent problems with moisture there too, some may be larger than the problems related to the rim joist.


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## secutanudu (Mar 15, 2009)

My parents and I both live in New York, though I am upstate, they are down, so yes we are in a cold zone.

Thanks for all the info - it seems like rigid foam is the way to go...


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## diy4life (Oct 31, 2008)

*Good thinking*



secutanudu said:


> My parents and I both live in New York, though I am upstate, they are down, so yes we are in a cold zone.
> 
> Thanks for all the info - it seems like rigid foam is the way to go...



Good information by Carisle but I would like to add my two cents. Fiberglass batts by themselves wether faced or unfaced won't help to stop air infilteration or moisture problem, but it will dampen or slow the air down. However, air will eventually get inside. Almost all the time, you need a barrier to go along with the fiberglass batts - caluks, plastic sheets, rigid or spray foams, etc. As for as moisture problem goes, moisture needs to be moved from one place to another by air circulation. One time I had less than quater of an inch of rain water all over my basement floor that came from under my basement door. Even though we cleaned and mopped the floor as much as we can, the whole basement smelled and felt dampness and it wouldn't go away until I decided to leave one of my basement window open for a day or two. At the end of the day, my whole basement was dried up and clean.


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## stadry (Jun 20, 2009)

insulation doesn't keep anything cold OR hot,,, it ONLY slows the transfer of,,,,,,,,,,,,,, heat ALWAYS runs to cold, btw.


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## secutanudu (Mar 15, 2009)

How is air infiltration a problem around a rim joist, when there is brick or other siding on the other side? I just wouldn't have expected that.

I take it rigid foam surrounded by caulk acts as both an air and humidity barrier?


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## Scuba_Dave (Jan 16, 2009)

Brick is porous - as is cement - water vapor will pass thru
Fiberglass paper is a vapor retarder, it will not stop water vapor
And heat will slowly work its way thru, doesn't stop it 100% 

And with cold weather its best to use rigid to totally seal the cold from the rim joist "out"
It's a task that is going to be a PIA for me...probably anyone
Which is why I used fiberglass as a Temp measure


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## secutanudu (Mar 15, 2009)

But the wood itself...is not porous, right? Can water vapor/air move through wood? Or is it actually going above and below the joist?

I will probably (eventually) use rigid foam topped with fiberglass. I assume that fiberglass is way better than nothing, right? Is there any danger to having exposed fiberglass insulation up around the top of the basement wall, assuming it remains untouched after installation?

PS. Can a moderator please fix my typo in the thread title?


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## Gary in WA (Mar 11, 2009)

But the wood itself...is not porous, right? ------ No, wood is porous. 

Can water vapor/air move through wood? ------- vapor can.

Or is it actually going above and below the joist? ------ Yes, above and below. Anyplace numerous pieces are used, especially solid wood members (expand and contract with the seasons. Engineered wood to a much lesser degree. 

I assume that fiberglass is way better than nothing, right? -------- Yes, barely. Better to caulk all joints you can reach, then rolled glass batts.

s there any danger to having exposed fiberglass insulation up around the top of the basement wall, assuming it remains untouched after installation?
---- only if you are looking up at it from below during vibrations or air movement into it.

Be safe, Gary


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## secutanudu (Mar 15, 2009)

Clear answers, thanks Gary.


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## High Gear (Nov 30, 2009)

Yippy Ki Yaa my first post! 

Hi guys I've been lurking in the shadows the past few months and hey I finally joined.

Anyways I've done the rigid foam rim joist upgrade. I have I joists so they all get notches.

I used 2" thick fomular150 and doubled them up so should be an R-20

I used a table saw and was done sawing in a couple/three hours ( notches too).

I cut them about a 1/4" smaller with the hope that I would have much/any trimming for the crowded ones. 

This worked out well, very minor trimming on just a few.

I used window (low expansion) foam instead of caulk , but would likely use caulk next time as its far less messy. 

My question in this is would ceiling panels be an adequate fire stop or do you need to directly cover them with drywall ect ??


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## ccarlisle (Jul 2, 2008)

No need to cover the rim joist area insulation with drywall, according to the newer revisions of the IRC...


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## High Gear (Nov 30, 2009)

Thanks CC, I was a little worried about that .


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