# Rockwool (Roxul?) for DIY Rim Joist Insulation?



## SPS-1 (Oct 21, 2008)

Along the rim joist, you definitely want to stop air flow. 

You can start a civil war on this forum asking if to put a vapor barrier. We need your location. But seeing that you appear to have a basement, I will guess yes you should have a vapor barrier. Vapor barrier would go on the "warm in winter" side of the insulation. But that is difficult to do properly on rim joists --- as evidenced by those silly pieces of foil that are maybe supposed to be vapor barriers.

For that application I would sooner suggest XPS foam board insulation tight fit into the cavity and then caulk around it. You could put 1.5" of XPS as mentioned above, and then put additional rockwool over top of it (really you should because XPS, EPS or sprayfoam is supposed to be covered by a fire tolerant material). 

You mentioned sprayfoam. That is a great option too. But sprayfoam (and also XPS) is a vapor barrier and if you put it towards the outside, you want it at least 1.5" thick and then no additional vapor barrier on the inside. (You don't want a vapor barrier on both the inside and outside) Thats a lot of sprayfoam if you were thinking of a couple of cans of Great Stuff --- and it makes a total mess --- mark my words.


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## wyclef (Jan 13, 2014)

Hey, thanks for getting back to me. I'm located in Upstate New York. So the thinner 1.5 XPS caulked or great stuff around the edges and then Rockwool over it because it is fire safe and then plastic sheeting vapor barrier over that? What happens if I skip the XPS and just caulk or great stuff around the edges and then vapor barrier over the Rockwool (R15?) leaving out the XPS step. Sorry if that is a dumb question I've just never done this before and am learning as I go, just trying to avoid doing this twice or more complicated than necessary. Also, not looking to start any civil wars.


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## SPS-1 (Oct 21, 2008)

If you put XPS first, then no other vapor barrier.

If you put just rockwool, then plastic vapor barrier on the inside (warm-in-winter side). But you have to make that vapor barrier a lot more air tight than that foil.


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## wyclef (Jan 13, 2014)

LOL, not sure I can get things much tighter than that foil. Whatever mess that is probably is 50 years old. I can’t wait to put that junk in my trash bin. So if this was you, would you do XPS or plastic vapor barrier? What is the benefit of one over the other? If I do the plastic vapor barrier do you even need to caulk/foam the gaps behind the rockwool or is that just good common sense to do so.


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## chandler48 (Jun 5, 2017)

Being a naildriver, my house is probably the last to get the best of everything  A year or so ago I noticed my rim joists in the basement were devoid of any insulation. That meant all I had to keep cold air out was the rim joist (wood) itself. Dumb.

I insulated it with Roxul and could not believe the difference in temperature the next winter. I highly recommend it.


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## wyclef (Jan 13, 2014)

Cool. Excited to get this project underway however I end up doing it. Just to confirm, is the R15 of this stuff what I should use for basement wood stud install? Since I'm not drywalling over it seems like the R23 would be too thick and protruding out although is the thickness of this stuff the same regardless of R value because R23 for a basement in my region would probably be more appropriate.



https://www.rockwool.com/products/comfortbatt/?selectedCat=comfortbatt%C2%AE%20downloads


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## carpdad (Oct 11, 2010)

I don't agree with rockwool. Chandler48 is in georgia so the feel may be constant but different for the coldest/windy days around the country. Rockwool is still a fiber and will let air through. Best to stop the air leak with xps foam board and spray foam the joints. The board can be cut with 1/4" gap all around and filled with spray. The gap could be larger if it helps with the fitting. Small area, so won't help much with extra r value and lower in material cost.


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## ablodneyget (Jul 15, 2018)

carpdad said:


> I don't agree with rockwool. Chandler48 is in georgia so the feel may be constant but different for the coldest/windy days around the country. Rockwool is still a fiber and will let air through. Best to stop the air leak with xps foam board and spray foam the joints. The board can be cut with 1/4" gap all around and filled with spray. The gap could be larger if it helps with the fitting. Small area, so won't help much with extra r value and lower in material cost.


I'm a big rockwool fan but I have to agree that it should not be used in this application by itself. As long as air can leak around or through the insulation, humidity will be carried to the cold band joist surface and condense. I had this happen in my crawlspace with fiberglass stuffed into the rim joists by a so-called professional. It got wet and grew mildew (intermittently) so I had to rip it out. Any time the band joist temperature approaches the interior dewpoint, there will be problems unless a vapor retarder and air barrier is installed.

A vapor retarder such as MemBrain would be ideal in conjunction with rockwool, but it would need to be carefully taped at the boundaries to keep air from leaking in.


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## mishenka (Jan 7, 2021)

Hi. I'm also fixing the same situation in the rim joists and got some great advice on this forum so far. Then found this post and just curious - which way did you end up picking?


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