# Rim Joist Air Seal and Insulation Options



## Dynasty (Sep 11, 2019)

I am seeking advice on air sealing and insulating my rim joist area of my home (full basement in Ohio). Home was built in 2019. I noticed some areas of the rim joist were allowing daylight in (trades punched holes for HVAC, plumbing, etc). I caulked the top and bottom of the rim joist ares and spray can foamed what I could. Batt insulation was laid in the spaces in a sloppy manner which lead me to investigate and improve the area.

At this point I find I have 3 options:

1.) Use 2" rigid foam board against rim joist, seal with spray can foam, lay batt insulation. Cost = approx $150 but will take hours to completed cutting each foam board to fit and there will be some areas I am unable to complete due to obstacles in the way.

2.) DIY spray foam kit from one of the big box stores. Cost = approx $350 and should be much faster than option 1, but I have no experience with this and have read some of the kits are low quality and are a headache to work with

3.) hire an insulation contractor to professionally spray foam. Cost = approx $650 (per quote 2.5" thick)

I started the rigid foam board process and it is going to take 15- 20 hours for me to complete. I am considering the DIY or pro spray foam options, but my main question is: will insulating the rim joist area SIGNIFICANTLY increase my home's effeciency and lower energy bills?


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## Nealtw (Jun 22, 2017)

Seal it as best you can do a better job with the FG and the cut a piece of drywall just a bit small and set it in there and caulk it in place.
What is wrong outside that light could get in.


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## SPS-1 (Oct 21, 2008)

Option 1B) --- 2" rigid insulation against the rim joist and seal with acrylic caulk. Makes far less mess than sprayfoam in a can.


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## Nealtw (Jun 22, 2017)

SPS-1 said:


> Option 1B) --- 2" rigid insulation against the rim joist and seal with acrylic caulk. Makes far less mess than sprayfoam in a can.


If the basement is unfinshed you have exposed foam, still need the drywall.


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## SPS-1 (Oct 21, 2008)

Nealtw said:


> If the basement is unfinshed you have exposed foam, still need the drywall.


Or you can cover the foam with a layer of rockwool.


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## Nealtw (Jun 22, 2017)

SPS-1 said:


> Or you can cover the foam with a layer of rockwool.


You sure? Do the make denser product like a board


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## Dave Sal (Dec 20, 2012)

My crawlspace temperature used to be just above freezing during winter, resulting in cold hardwood floors in the living / dining / kitchen above. I was worried about freezing water lines, as they run through the crawlspace. I insulated the rim joist with rigid foam and sealed with caulk, and also sealed up the two manual vents to the outside. I have a remote sensor for a thermometer down there and now the temps stay an average of 63 degrees year round. 
This is one DIY job that I think is definitely worthwhile. It took me about 8 hours total to complete the job. I split it up in two, four hour work days. Like the OP mentioned, it is tedious work cutting the foam boards to the proper size.


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## SPS-1 (Oct 21, 2008)

Nealtw said:


> You sure? Do the make denser product like a board


I have seen a couple of builders in my area that sprayfoam the rim joist and then cover with batt rockwool insulation. I have that in my new place. The foam can be toxic if it burns, so needs a cover that provides XX minutes of fire resistance. Rockwool is fire resistant (it actually is basalt rock). Rockwool is available in board also (but not as common), but I have not seen it used for this purpose. Might give a neater appearance.


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## Bud9051 (Nov 11, 2015)

There are methods for cutting the rigid foam that can make it easier. I have a small circular saw that turns at a lower speed. With a plywood blade it cuts nice straight lines. I alsi use my saber saw. 2" is not easy to cut with a utility knife. 

What is below the rim, any insulation on the foundation walls?

Bud


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## Dynasty (Sep 11, 2019)

Bud9051 said:


> There are methods for cutting the rigid foam that can make it easier. I have a small circular saw that turns at a lower speed. With a plywood blade it cuts nice straight lines. I alsi use my saber saw. 2" is not easy to cut with a utility knife.
> 
> What is below the rim, any insulation on the foundation walls?
> 
> Bud


Currently, the walls have no insulation. That will change in the future once the basement is finished.


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## nobbyv (May 30, 2018)

In my experience, the Dow spray foam kits are pretty DIY-friendly and reliable. I used the 650 sq ft one (bought off Amazon for $800) to do my rim joists. Took about an hour (~1800 sq ft basement). Note that building code may require you to cover this with a fire-resistent covering (drywall or intumescent paint) depending on how thick you go (believe the 2018 IRC says thicker than 2" needs a fire-resistant coating).


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