Insulating Rim Joist (90 yr old house, no sill)


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Old 02-15-21, 12:47 PM
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Insulating Rim Joist (90 yr old house, no sill)

Hello

We recently bought a 90 year old house in Missouri. The basement is unfinished. Poured concrete. There are sills with bolts in the corners, but not around the rest of the foundation wall, as is typical. The joists sit right on top of the concrete. The rim joist/box plate is not insulated. In some cases there is a gap, enough for a pinky say, between the rim joist and foundation wall.

I'd like to insulate this area. We will not finish the basement, but it is used for storage and has some fitness equipment. So people will be down there from time to time for extended periods.

I have done a lot of reading on this topic. Here is what I have learned, and where I need help.

1) Fiberglass batt used to be the go to here but due to moisture issues and wood rot is no longer recommended
2) Spray foam seems to be what most people recommend given the issues created by the joists sitting on the concrete i.e. moisture rotting structural wood over time
3) The poly foam board plus can of great stuff foam DIY option crops up a lot. Some risks here of not getting a good seal and could end up looking messy. I'm not a great DIY'er. I know my limits and then hire a local well recommended 'handyman' to help me.
4) Rock wool sounds interesting but people are very divided on it's application and I'll still need to seal it with drywall for fire blocking - or so I've read.

I'm trying to accomplish two things.

1) Seal between the concrete foundation and rim joist for air gap and critter control. Want to keep pests out as much as reasonably possible.

2) Insulate. I don't mind if it isn't the greatest R-value. As long as I've done something to help out because right now (written during cold snap here in mid-Feb '21) those areas of the floor above are noticeably cold.

Any and all advice is welcome.

 
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Old 02-15-21, 04:03 PM
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1. You need to solve the water problem first. No matter what insulation you use the moisture/high humidity will still be there damaging the wood.

2. Research your situation carefully. Spray foam doesn't magically fix moisture issues. In some cases it's vapor barrier properties can make things worse by trapping moisture inside your wood.

3. Yes, spray can foam can be messy if you are not careful.

4. Rock wool suffers from many of the same issues as other materials especially if you've got an ongoing water problem.

1. Sealing gaps that are allowing daylight, drafts and insects in can be a big help. How to seal depends on how big they are but spray can foam is one of the more common methods as you can stick the tube in a crack and pull the trigger.

2. Your mentioning of a moisture problem, wood rot and pests has me thinking that insulation should be a far second to the big problem which is the water/moisture. Until you get the moisture solved or under control then insulation could cause problems.
 
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Old 02-15-21, 04:16 PM
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Fiberglass batt used to be the go to here but due to moisture issues and wood rot is no longer recommended
To reinforce what PD has stated, insulation, moisture, wood rot are all separate issues, not related but share the same basic issue!

Assuming the wood rot is due to moisture/water that has to be fixed, you have gaps, got to get them sealed.

If it were me, I be looking at 2-3" expanded foam board, cut to size and glued and caulked to seal the foundation. More time consuming but would provide a solid seal.

After that any insulation would work, they all do the same thing!
 
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Old 02-15-21, 06:28 PM
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Thank you for the replies. We do not have any moisture issues in the basement.
 
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Old 02-15-21, 10:15 PM
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but due to moisture issues and wood rot
We were going with your description!
 
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Old 02-16-21, 06:26 AM
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I'm assuming you are referring to this item?

1) Fiberglass batt used to be the go to here but due to moisture issues and wood rot is no longer recommended

This was a broader statement. As in, fiberglass batt used to be what was applied to rim joists, but it is no longer recommended due to causing moisture issues.

I hope this clarifies my post and I welcome all replies.
 
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Old 02-16-21, 03:28 PM
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but it is no longer recommended due to causing moisture issues.
Not trying to nit pick, insulation, of any type, has absolutely nothing to do with causing any kind of moisture issues.

I suspect what your stating is that it's not good to use WITH moisture (nobody will argue that point). Solutions to remedy moisture/gap issues have been provided!
 
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Old 02-16-21, 04:19 PM
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https://www.retrofoamofmichigan.com/...-want-to-avoid

2. Flash and Batt Moisture Builds Up On the Foam and in the Fiberglass

If an air seal isn’t created, then condensation will form as warm air meets cold air in the wall cavity. This condensation will lead to the growth of mildew and mold. It can also lead to wood rot as the fiberglass will retain that moisture.
Solution: Ensure the insulation installed in your wall cavities creates an air barrier and doesn’t promote mold or mildew growth.
 
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Old 02-16-21, 11:59 PM
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Ensure the insulation installed in your wall cavities creates an air barrier and doesn’t promote mold or mildew growth.
Yes, that is what we are stating, the spray foam or foam boards are what you use to air seal the rim joist, no different than what you do in an attic. Seal the air gaps, thus no moisture, no water, then the insulation stays dry!

Insulation itself is not an effective material to seal openings/gaps!
 
 

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