Crawlspace rim joist insulation in mixed humid climate?
I keep going around in circles on this one because so many folks in the building industry here in central North Carolina have different opinions on how to insulate the rim joist in a sealed crawl space. It is very difficult to separate the “that’s how I have always done it” methods from the science based best practices methods.
Doing research online does not seem to add much clarity as I see the same range of suggestions and they are often at odds with each other.
I see online at the US Energy Star page that several of their documents recommend using unfaced, batt insulation at the rim joist as shown here: Basement & Crawlspace Air Sealing and Insulating Project | ENERGY STAR
Martin Holiday’s article here from Fine Homebuilding says to never use batt insulation; but maybe his article is for vented, unconditioned crawlspaces? Insulating Rim Joists – Fine Homebuilding
So, what should I do to minimize condensation risk on my new construction home with regard to my rimjoists, in my sealed crawlspace? My walls are sheathed with OSB that has been taped and sealed at the bottom where it covers the bottom plate. Ideally I want the rim joist to dry in and out just like the rest of my walls. It is very important for it to be able to dry in where the concrete porches are since that area has imperiable flashing on the exterior to separate the concrete from the house.
Thanks!
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Replies
Navigating the varied advice on insulating rim joists, especially in a climate like central North Carolina, can indeed be challenging. The key is to balance moisture management, insulation effectiveness, and the unique aspects of your home's construction and sealed crawlspace. Given your research and the differing opinions you've encountered, let's break down the best approach based on building science principles.
Best Practices for Rim Joist Insulation in Sealed Crawlspaces
1. Use of Unfaced Batt Insulation:
The recommendation from Energy Star to use unfaced batt insulation at the rim joist can be applicable in certain conditions but is not universally the best practice. The main issue with batt insulation in this location is its tendency to absorb moisture and not effectively block air leakage, which can lead to energy inefficiency and potential moisture problems.
2. Avoidance of Batt Insulation According to Martin Holladay:
Martin Holladay's advice to avoid batt insulation in rim joists is largely based on moisture management and air sealing effectiveness. In sealed, unconditioned crawlspaces or basements, preventing air infiltration and controlling moisture are paramount to prevent mold growth and wood rot.
3. Best Approach for New Construction with Sealed Crawlspace:
Given your sealed crawlspace and the need for the rim joist to dry inward, especially beneath concrete porches with impermeable flashing, consider the following:
Rigid Foam Insulation or Spray Foam: Both materials provide excellent air sealing and insulation. Closed-cell spray foam offers the added benefit of moisture management by acting as a vapor barrier. If using rigid foam boards (e.g., XPS or polyiso), ensure they are cut to fit snugly between joists and sealed at the edges with spray foam or caulk to prevent air leakage.
Flashing and Moisture Control: Since you mentioned impermeable flashing on the exterior, ensure there's a clear drainage path for any water that might infiltrate the flashing area. This helps prevent moisture accumulation at the rim joist.
Allowing for Drying: Ideally, the insulation method chosen should not trap moisture within the rim joist assembly. Closed-cell spray foam, while acting as a vapor barrier, effectively blocks moisture from entering the assembly from the outside but should be used judiciously to not entrap moisture that may originate from the interior.
4. Conclusion:
For your new construction home with a sealed crawlspace in central North Carolina, focusing on air sealing and insulation that also allows for appropriate moisture management is key. Closed-cell spray foam insulation at the rim joist area is generally considered a best practice in your scenario, given its ability to provide both air sealing and moisture control. Ensure any exterior flashing is properly installed to facilitate drainage and that the overall wall assembly is designed to allow for inward drying where necessary.
Given the variability in construction practices and local climate considerations, consulting with a local building science professional can provide tailored advice ensuring your specific concerns, especially regarding moisture management and insulation effectiveness, are adequately addressed.