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Will starting insulation to the interior of the stud bay help keep the mudsill dry?

Tyler Pagel | Posted in General Questions on

I’m looking for feedback on an interior insulation retrofit for the above grade framed portion of basement walls on a 100 year old home in Zone 6. It’s yet another basement insulation question, but among the other usual questions that I have, I’m most interested in whether my plan will help to keep my mud sill and framing dry.

I’ve responded below with attachments which should illustrate the following text in a much quicker way! Consider the following text additional info.

The above grade portion of the framed walls vary from 3-4.5ft due to a sloped lot and varying foundation wall height (if this helps inform cost/benefit analysis for insulation levels). The wall assembly is framed with true 2×4 studs and is as follows:

Cedar Clapboard> Tar paper> true 1×8 shiplap board sheathing> Inconsistent UFFI foam in some stud bays from mud sill to joists (does not extend to cover rim joist, so pretty ineffective)> 1/2 of the basement painted lath and plaster and another 1/2 basement painted 1/4in High Density Fiberboard over lath with plaster removed, this covers from the top of the mud sill and stops at the joist bays leaving the rim joist exposed and uninsulated


In the future I plan to insulate the foundation walls from the interior with at least 2 inches of rigid foam and finish it with 2×4 framing and cavity insulation. However, I need to address bulk water and efflorescence issues first. Not to mention what to do with the section of exposed chimney brick that extends down to the footing.


Since this retrofit will be conducted from the interior, done by me, I hope to leave the current lathe and plaster and UFFI mostly alone. I’d like to install interior rigid foam and then batt insulation towards the interior from there, basically a flash and batt but moved to a different position in the assembly. I will have 6in to 9.5in of cavity to work with which includes 4-5in (depth varying due to plaster vs hardboard sections of basement wall covering) to the foundation wall edge and then an additional 5.5in where 2in of rigid foam, and 3.5in of framing, would be in the future.


I am thinking of cap nailing 2in of rigid foam at R10 over the current wall making sure to properly seal between the sheets and around I’d place 3.5in of Roxul batts at R15 horizontally in place behind the future interior partition wall framing. That assembly would be ~R25 with no thermal bridging and ~40% R value from rigid foam to avoid condensation issues, and most importantly a vast improvement over what I currently have. Let me know if more insulation is warranted or would be cost effective in this space. I could continue to fill the future interior basement framing cavity, but would then need to adjust my amount of rigid foam as I understand the building science guidelines from reading this site.


The mudsill is covered by an angled cement detail (I assume for aesthetic reasons since the mudsill is already bolted to the foundation). The top of the foundation walls is rather rough and, when I removed a section of this cement curb detail for exploration, found wood shims leveling the sill with a roughly 1/8-1/4 gap beneath. There is no capillary break other than that air gap between shims after removing 100 years of dirt/dust and debris.


I know that there is currently, and will likely be more moisture present in the foundation walls once insulated. The foundation walls exhibit efflorescence and minor surface spalling beneath bubbled drylok in places. There is no sump pit and no perimeter footing drain.  I will attempt to insert some sort of capillary break beneath the mudsill in the future, and luckily there is no observable rot.


Now that I would be removing the blast of warm air that currently escapes my house and dries this framing, would my plan be fine from a moisture perspective? I assume that leaving the mudsill open to the 4 inch cavity above (rather than sealing directly with foam if I had remove the lathe and plaster) and my relatively vapor transmissive exterior assembly will provide enough drying to the exterior that it should continue to not be an issue.


What to do at the detail where the wall meets the floor above?


I haven’t seen anything that addresses what to do when connecting a new insulation retrofit to a poorly insulated space, maybe it just isn’t that important? The floor deck above is diagonal planking that extends to the exterior sheathing, it is covered in tar paper and hardwood flooring above. Would it make sense to cut and cobble the same rigid foam between the joists against the floor planking at the top of the exterior wall stud bay/rim joist? Basically I’d be extending the vertical plane of rigid foam outward horizontally to the sheathing. My intention is that this rigid foam might serve both as a minimal amount of insulation to the floor above at the perimeter and also help reduce airflow into the wall assembly above therefore reducing winter inward air draw due to the stack effect. I am not sure if this detail would raise any fire blocking concerns, or even accomplish much. If anyone has suggestions, I’m open to them.


In summary:

Will my interior rigid foam plan as described work? Is more insulation warranted?

Does placing the insulation toward the inside and leaving the stud bay dry the mud sill and framing once insulated?

What should I do where the new interior rigid foam reaches the floor above?


Thanks in advance if anyone has any advice to offer, I’ve tried to give as many details as possible but let me know if I’ve left something out.

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  1. Expert Member
    Peter Yost | | #1

    Hi Tyler -

    Most of us in the building industry are better with images than words; you are painting a picture of your situation/problem/question in an awful lot of words. Do your best to hand-draw your question, scan it, and attach to your question. It's simply too much to ask of your words alone.


  2. Tyler Pagel | | #2

    Thanks for the feedback Peter, I realize now that I hid my question behind quite a wall of words. I've attached a diagram of what I propose to do and two annotated photos.

    My three questions remain:
    Will my interior rigid foam plan as described work? Is more insulation warranted?

    Does placing the insulation toward the inside and leaving the stud bay dry the mud sill and framing once insulated?

    What should I do where the new interior rigid foam reaches the floor above?

    Thanks again to anyone willing to give me advice.

    (Edited, an issue with the attachments originally)

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