Rim joist insulation question

[Trying2bFrugal]

I am starting to work on basement framing and reading people suggesting to add insulation on rim joists than the
unfaced fiberglass batt thar builder put to pass codes.

I pulled one batt and can feel the cold air coming inside house,  very little though.

Is cutting a 2" insulation foam board with plastic facing house and use foam to fill gaps then use existing fiber glass batt (this old man youtube video) a good idea?

Will the small air leak is bad or would I see air circulation good for the moisure to dry up?

The reason for foam board is,  the wood used on external rim joist itself is going to transfer heat and foam board can add more resistance to it. While it make sense theoritically. I have no structural knowledge to judge.


What you guys think about it?

Edit: house built in Aug 2022,, SE Michigan. There is a 3" flexible hose running from outside to inside and open. Builder said it has to be open all times near furnace and said it is new standard code. I forgot to mention this, this is where I got the thought of sealing rim joists or not as the intentional air in is going to be there all times.

[Shuchong]

I think this depends a lot upon climate, but where I live, there are concerns about over-insulating rim joists/using inappropriate materials and causing condensation to build up in the colder months, leading to mold and rot.  This is one of those areas where I'd talk to local guys and figure out what code is in your area. 

[Paper Chaser]

The basement is presumably conditioned space, so walls should be insulated as much as possible. No need to leave it uninsulated for ventilation as your HVAC will take keep the basement conditioned like the rest of the house, controlling humidity levels.

Rigid foam is perfect for this application as it insulates while also cutting down on air movement/drafts.

[ChpBstrd]

I think this depends a lot upon climate, but where I live, there are concerns about over-insulating rim joists/using inappropriate materials and causing condensation to build up in the colder months, leading to mold and rot.  This is one of those areas where I'd talk to local guys and figure out what code is in your area.

This was my thought too. Maybe leave an air gap between the rim joist and the rigid foam insulation. Sounds like it is already able to breathe.

The problem with asking a local repair guy is they may have never encountered this specific issue. Or they may have insulated rim joists and feel they did a good job, but 20 years later the thing rots out and they aren't there to ever see or learn from it. If this project is in an accessible space, maybe go ahead and install the foam blocks, and then a few months later pull them out and give the joist a scan with a moisture meter and look for mold.

Replacing rotted rim joists and sill plates can be tricky. Here's how I know:
https://forum.mrmoneymustache.com/do-it-yourself-forum!/your-nightmare-project-my-piece-of-cake/msg2803615/#msg2803615

[lthenderson]

I've never lived in a house with insulated rim joists and live in the Midwest where we get plenty of cold weather in the winter. I think a lot of that is due that most basements aren't finished to the point where there is someone living there for hours at a time. If I had a bedroom where I spent a large amount of time in, I might consider putting in rigid insulation and foam just to maintain comfort.

[sonofsven]

I am starting to work on basement framing and reading people suggesting to add insulation on rim joists than the
unfaced fiberglass batt thar builder put to pass codes.

I pulled one batt and can feel the cold air coming inside house,  very little though.

Is cutting a 2" insulation foam board with plastic facing house and use foam to fill gaps then use existing fiber glass batt (this old man youtube video) a good idea?

Will the small air leak is bad or would I see air circulation good for the moisure to dry up?

The reason for foam board is,  the wood used on external rim joist itself is going to transfer heat and foam board can add more resistance to it. While it make sense theoritically. I have no structural knowledge to judge.


What you guys think about it?

Edit: house built in Aug 2022,, SE Michigan. There is a 3" flexible hose running from outside to inside and open. Builder said it has to be open all times near furnace and said it is new standard code. I forgot to mention this, this is where I got the thought of sealing rim joists or not as the intentional air in is going to be there all times.

Yes, that'll work. Go around the gaps between rim and floor and sill plate with a small bead of spray foam  first to seal up the air leaks first.
Are you putting a finished ceiling up against the floor joists? Are you framing walls inside the concrete walls as well?

[Sibley]

This is also a question of how old your house is. In old houses that don't have wall insulation insulating the rim joists is an important part of air sealing. Tops and bottoms of the exterior walls, attic floor, around doors/windows and other holes in the wall.

[Midwest_Handlebar]

I used closed cell spray foam on the rim joists when we finished our basement a couple years ago:

https://www.awarehousefull.com/dupont-low-gwp-froth-pak-650-spray-foam-insulation-kit-15-hose/?sku=DOW%2012031907&gclid=Cj0KCQiAzoeuBhDqARIsAMdH14HN8bVDUQ8Ve111WWGOcQPCJOa1e_rCM4ha_MPLYM_EeTVNEfahISEaAv_fEALw_wcB

I opted to go with the spray foam because of the air sealing properties and ease of application. Additionally, I used R-6 rigid foam on the walls and R-13 faced bat insulation in the stud bays. Between this and the spray foam, it improved the energy efficiency of the house pretty dramatically.

[nereo]

Insulate rim joists with either closed-cell spray foam or rigid foam sealed around the edges with spray foam. This will serve as both an air barrier and prevent condensation issues

See: https://www.greenbuildingadvisor.com/article/insulating-rim-joists

[bacchi]

And get a foam gun. I threw out so many half used cans of foam before I found out about them.

[nereo]

And get a foam gun. I threw out so many half used cans of foam before I found out about them.

+1. Foam guns are awesome. No more waisted, half full cans. Cheaper “contractor size” cans, and far greater control of flow and bead size.

[Trying2bFrugal]

And get a foam gun. I threw out so many half used cans of foam before I found out about them.

+1. Foam guns are awesome. No more waisted, half full cans. Cheaper “contractor size” cans, and far greater control of flow and bead size.

Can you link? I will check it out.

[nereo]

And get a foam gun. I threw out so many half used cans of foam before I found out about them.

+1. Foam guns are awesome. No more waisted, half full cans. Cheaper “contractor size” cans, and far greater control of flow and bead size.

Can you link? I will check it out.

Here’s the name brand. I’ve wound up with two, one off brand Amazon variety which works about as well as the slightly more expensive one

Make sure you have a can of foam gun cleaner on hand ($6) for when you want to toss out a used can or switch. It’s basically acetone with a propellant.

The savings on foam is so much that it’s paid for the foam gun several times over in just a few years.

https://www.greatstuff.dupont.com/products/pro-14-dispensing-gun.html?src=gs_na_ppc_gun_google_100323_19554387317&gclid=EAIaIQobChMIvoPG7YGkhAMV2FFHAR1rOgEDEAAYASAAEgKHYvD_BwE