# Frost and Mold in Rim Joists



## JoeSims (Feb 2, 2019)

Hello,

Went down stairs today to caulk around a penetration through one of the rim joists. When I went to pull back the batt insulation I noticed a lot of frost. The north east has been in a cold blast. It was -5 a few days here. After looking in a few other joist mays it seems that its wide spread. 

I also found a few spots where the OSB changed color and is black... Not sure if its mold or just discoloration. Possible to tell from these photos?

Here are a few pics of whats going on. Would love to get some opinions on what I need to do.


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## Nealtw (Jun 22, 2017)

You have been trying to seal air from coming in and that is not the problem for the most part. Loose fitting bats will allow warm moist air from the house to be drawn to the cold on the outside wall, then that moisture condenses there. 
I would paint all the stained areas with copper, End cut treatment for treated wood. That will kill what ever might be growing there. 
Get everything dried out fit foam board in there and seal the edges with spraw foam or caulk,


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## Bud9051 (Nov 11, 2015)

Fiberglass is the source of the problem it just doesn't stop the air. 
leave all joist bays open so they will dry. To be polite I refer to the dark area as mildew but it is probably the early stages of mold. Once dry a paint brush and a 10% mix of bleach will eliminate the stain and any mold if present.

Then Neal's suggestion of some rigid insulation is good, many threads here about that approach. If you can find Thermax it has a heavy foil facing that can allow it to remain exposed. Other rigid foam products may need a thermal covering, like drywall. Some have used Roxul after the rigid foam as the thermal barrier, local code officials are the final authority.


Bud


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## JoeSims (Feb 2, 2019)

Thanks guys. Few other questions... 

How thick of rigid insulation do I want?
What do i do between the band joist and the parallel floor joist? I hardly have enough room to get my hand in between them. I can pull out the fiberglass but its going to be hard getting foam board and caulking around it.


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## Bud9051 (Nov 11, 2015)

Your climate zone (or nearest big city) will help us judge how thick is thick enough. Codes vary but if you plan on adding Roxul over the Rigid you should exceed any code.

The climate zone tells us how much R-value is needed in combination with additional air permeable insulation to ensure the first layer any air reaches will be warm enough to not form condensation. A little long winded but I suspect 2" would be max needed. 1" might get you by.

In addition, while you are filling in those end pieces caulk or foam every seam you can see including sill plate to foundation. I have done many Infrared inspections and the rim areas are always leaky. Plus, that cold air is contributing to the condensation issue.

Now, for that narrow joist parallel to the rim. I have one (had, so to speak) and just took my reciprocating saw and modified it. A quick check upstairs and it really wasn't holding any weight so instead of being a 2x10 it became a 2x8. With the angle on the bottom it was very easy to work. Judgement call.

Bud


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## JoeSims (Feb 2, 2019)

Bud9051 said:


> Your climate zone (or nearest big city) will help us judge how thick is thick enough. Codes vary but if you plan on adding Roxul over the Rigid you should exceed any code.
> 
> The climate zone tells us how much R-value is needed in combination with additional air permeable insulation to ensure the first layer any air reaches will be warm enough to not form condensation. A little long winded but I suspect 2" would be max needed. 1" might get you by.
> 
> ...


I dont think I will be able to cut my joist as its an I-Joist. Ill have to figure out a different way of getting in there. 

On the parallel side since its an I-Joist they used as the outer rim... If I use 2" rigid foam board do I just notch around the top and bottom of the I-Joist? Or do I do two 1" pieces? You can see what I am talking about in the 3rd pic. 

Should I flatten out the spray foam the builder used between everything or just go over it? 

Do I need to do anything between the concrete foundation and the sill plate? The builder did not install any spray foam there. 

The house is about 5 years old. I am in climate zone 5 if that helps at all. 

Thanks for all the advice!


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## Bud9051 (Nov 11, 2015)

My bad, wasn't paying attention to the pictures.

Is your last picture showing the rim board on the opposite end of the house where the joist bay is full width? Given the potential damage from moisture I think you need to come up with a way to insulate and protect that difficult area. Since cutting a joist is violating the holy grail of engineered materials think of it as simply removing that joist. If you cut out whatever you needed to gain access and then cut to fit a 2x? the height of the web and sistered it to that joist it would return to near its original strength or at least the strength of the sister. Since that area of the floor isn't experiencing a lot of load (TBD) I don't see an issue.

What is the depth of that lip top and bottom of the joist?
Since you will be adding the rigid and sealing in in place you don't need to worry about the can foam the builder used. trim or flatten as necessary.


More discussion if you think you want to go this way.

A couple of pictures of that tight spot might help.

Bud


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