Why is rimjoist insulation any different than wall cavity insulation?
BrunoF
| Posted in General Questions on
There are many articles (many of which conflict with each other) about rim joist insulation details but why not just insulate that area with the same materials as the wall cavity?
in my case, I will have a sealed and insulated crawlspace with my sheathing detailed to be the air barrier. Seems like I can just use batt insulation like the rest of the house and avoid all the risks of foam.
What am I missing here?
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There are two separate but related requirements for an insulated assembly -- air sealing and vapor sealing. The reason they are separate is that most of the commonly used construction materials that are airtight are not vaportight.
The problem with rim joists is getting them vaportight. If you don't, water vapor from the interior gets in and condenses and causes problems. In a wall assembly the most common insulation is faced batting, the facing provides the vapor protection, and it's cut so that you can easily do a reasonable job of getting it adequately sealed. The areas behind rim joists are irregularly shaped and just not well-suited to attaching facing.
The other issue is drying. The facing is never a perfect barrier, some vapor gets past it, and the assumption is that the wall is going to be able to dry to the exterior through the sheathing and cladding. In the rim joist you have a thicker wall, the joist itself adds to the thickness, which makes it harder for the assembly to dry to the exterior.
It all stems from the observation that rim joists tend to have more issues with condensation.
Are those issues more prominent in cold climates? The reason I ask is that here in the mixed humid climate of NC, no one really uses faced insulation and walls are meant to dry in and out.
Absolutely.
Going from memory, but I think special treatment of rim joists is only required in climate zones 4 and above.
BrunoF,
https://www.greenbuildingadvisor.com/article/insulating-rim-joists
Malcom, thank you. That article part of the source of my confusion as it conflicts with the US Energy Star Website’s guidance.
several of their documents recommend using unfaced, batt insulation at the rim joist as shown here: https://www.energystar.gov/saveathome/seal_insulate/basement_crawlspace
One thing i noticed in Marin’s article is that all the illustrations show an unsealed, uninsulated crawlspace and maybe that is why there is differing guidance?
In my case, the CMU foundation walls will be sealed and insulated, the floor and piers sealed and conditioned air pumped in. I have also used three beads of congega HF under my mudsill gasket, caulked the rim joist to sill plate interface before joists were set and have taped and sealed OSB for the sheathing.
In thinking through my setup, it seems like my crawlspace is essentially the same as my living space which I why I am questioning the closed cell foam route. Additionally, I have concrete porches that meet the house with impermeable flashing between them and the sheathing…it seems that those areas would only have to way to dry which is in, but if I seal that path with closed cell foam wouldn’t it create other issues?
BrunoF,
Given what you describe, and where you are, I don't think the foam based approach is necessary, or really beneficial.