Rim Joist Insulation Procedures
I would assume that all rim joists are not created equal, and that means that they may require different methods for insulation. Q1 – Are there any articles that help to determine what thickness of foam insulation, or R-value, to use for the rim joist for certain conditions?
For a typical rim joist in a basement where the floor joists are on the concrete foundation, I have noticed that rigid foam plus batts is recommended. And that generally 2″ of foam is used, with batts supplying the remaining required R-value. But I believe that this is for a profile with outside air on one side and inside air on the other.
What about when the rim joist is not part of the foundation structure, but part of the insulated ceiling of a garage with the entire joist space filled with insulation? My rim joist sits on the top plates of a 2×6 wall. I would think that the delta-T between the outside and inside air is not as easy to define. And I also think that the delta-T would be important in determining the R-value needed, or how thick to use for the foam insulation before the batts start.
So, Q2 – how do I determine the thickness of foam insulation that I need? Is 2″ good enough, or should I double up to 4″. I assume that once I am thicker than the 5.5” wall that adding more foam insulation is not affecting anything. But do I really need to be as thick as the wall?
There seems to be a lot of articles about insulating the rim joist area, but I haven’t come across anything with either rules of thumb or formulas to play around with. Which all means that I’m probably overthinking this, right?
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Replies
Tony. Here is an article on calculating the minimum thickness of rigid foam (https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing). Also check out the related links in the sidebar.
Where are you located? Are you asking these questions out of curiosity or because you have a project in mind?
Tony,
When the rim joist is part of an insulated ceiling assembly, the rigid foam layer is unnecessary -- as long as you pay close attention to airtightness at every stage of the construction process.
That said, installing rigid foam on the exterior or interior side of your rim joist is always a good idea, and always lowers the risk of moisture problems. For more information, see this article: Insulating rim joists.
Hi Steve,
Thank you for the link. I am in Maine, zip code 04240 (zone 6). I am asking because I have an in progress project. I have air sealed and installed 2" of Tuff-R in the rim joist area and plan to back fill with roxul batts. But before I do that, I want to be sure that I have enough foam. I don't want to do it twice, just once, so now is the time to add if I need to.
That article was for foam on the exterior, and mine is on the interior. The chart shows 2x4 and 2x6 walls, but I have no real wall thickness. [Unless the other side of the joist span 28 feet away is the wall depth :) ]
The joists are on 2x6 walls, with 2x6 walls above. So I can extrapolate that my R13ish foam is acceptable, since R11.25 is required for foam on the outside of 2x6 walls.
Hi Martin,
The 'Insulating Rim Joists' article is what I used for the initial work. It was very helpful and most everything was detailed enough to follow.
In a previous post that I did, someone mentioned that the 2" might not be enough, then I read an article that said if you use a lot of batts to back fill, you need to increase the thickness of the rigid foam. And this got me thinking again.
I have tried to get air sealing done as good as I can. There is caulking around the perimeter in every bay. (well, almost every bay, I have one that is only 2" wide, and I couldn't get the caulking gun in there.) Caulked all around the wall bottom plate and floor and plate and outside sheeting above the floor. And will air seal as the sheetrock is installed.
Most of the articles provide enough information to figure things out. This is a great site to get information, and observe the many opinions from the field.
Thank you.
Tony,
The insulated ceiling of your garage in Maine will be fine with 2 inches of rigid foam installed on the interior of your rim joist. So you can stop worrying.