0 Helpful?

Spray foam inside rim joist?

I know this is quite commonly done but am I creating a durability issue in my unique situation? My rim joist and main floor joist ends sit upon a 2x6 interior basement wall. The outer 2x8 wall of the PWF basement is filled with MW bats and has "Thermoply" on it's inner face and it's top is 1/2 way up the rim joist. (Lateral soil loads on the PWF wall are transferred to the floor system through the rim joist) The "Thermoply" serves as primary air barrier in the basement and keeps warm air from the PT Plywood exterior basement sheathing. Unfortunately "Thermoply" is very low permeability which leads to my concern about spray foam on the inside of the rim joist as the lower half of the rim joist will have no ability to dry either way. The upper half of the rim joist can still serve as the air barrier without the "Thermoply" giving drying ability to the outside on the upper half of the rim joist. through the mineral wool and vapor open exterior there. To restate my question: Can i, without undue risk of moisture related problems, use closed cell spray foam on the inside of my rim joist if the lower 1/2 of the opposite side is covered by a vapor barrier?

Asked by Jerry Liebler
Posted May 28, 2014 4:19 PM ET
Edited May 29, 2014 9:23 AM ET


8 Answers

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Jerry... You have posted assembly details that are your design dozens of times. They are your design. You need to be the ultimate one to know your design and risk it performing properly for you.

From what i have read posted your assemblies are not simple nor are they efficient to build as to labor hours nor are they assemblies most of us in the business build.

Have you a building permit? Are you building? What are your start and finish dates?

Simple plans are the way to go IMO. Your assemblies are the opposite.

But... But I digress.

Take a WUFI course... Build your ideas and assess thereafter... Get a job with Joe L? That would be a kick.

Happy birthday

Answered by aj builder, Upstate NY Zone 6a
Posted May 28, 2014 4:36 PM ET
Edited May 28, 2014 4:40 PM ET.


Most of my posts have been "exploratory" as my plans have evolved. My objectives have been and are a well sealed well insulated house with a basement at modest cost. You have not seen my plans and I take serious exception to your assertion that my assemblies are not efficient to build as to labor content. There are, no doubt some unusual details and material choices but my design will be easy to build and quite cost effective in both materials and labor. It is unlikely that anyone has built exactly as I plan but the elements are all routinely built (wood stud walls roof trusses, metal roof, ,plumbing, HVAC etc), just not combined as I have. I choose to add the complexity of double stud walls, insulated with mineral wool, a complexity and added cost but OTH I choose a simple gable roof with deep energy heal trusses rather than the complex multi dormer leak prone mess most builders choose. PWF basements are ubiquitous in some areas. Building a double walled basement as I plan will actually be far less costly than any other r 26+ basement. I will build when I find the "right" spot. I will get the permits and comply fully with all codes. I will be in the completed house 9 months after beginning excavation.

Answered by Jerry Liebler
Posted May 28, 2014 7:26 PM ET


I don't recommend your approach. If I understand correctly, you want to install mineral wool insulation to insulate your below-grade walls, which are part of a permanent wood foundation (PWF). You want to install Thermo-Ply (foil-faced cardboard) on the interior side of your basement walls, with the hopes that the Thermo-Ply will act as an effective air barrier to keep interior moisture from contacting the cold pressure-treated plywood that makes up the exterior surface of your foundation wall.

In short, I don't think this plan will work.

First of all, your mineral wool batts are air-permeable.

Second, the Thermo-Ply will be unable to effectively isolate the air inside your framed wall from the air inside your basement.

Moreover, the air that is between the studs will be interior air (with its moisture load). It won't be magic dehydrated air.

The only way to keep the moisture that is contained in your warm interior air from contacting the cold plywood in your below-grade wall is with closed-cell spray polyurethane foam or rigid foam. If you choose to install closed-cell spray foam, it needs to be installed not only on the rim joist, as you suggest, but on the interior side of your pressure-treated plywood foundation.

For more information on this issue, see How to Insulate a Basement Wall.

Answered by Martin Holladay
Posted May 29, 2014 9:33 AM ET


Exploring for years. I do not believe one other builder would find the assemblies you post ...buildable cost effectively.

Answered by aj builder, Upstate NY Zone 6a
Posted May 29, 2014 11:48 AM ET


The Southern Forest Products Association publishes a "PWF Design and Construction Guide" which gives clear instructions on how to use air permeable insulation in a PWf (on page 14).. They include provision of a 2" in-insulated gap at the bottom, above the lower plate and installation of 4 mil poly on the inner face.
I have substituted Thermoply for the poly. Thermoply is advertised as a superior air and moisture barrier and I believe it is easier to install, but if it's truly unsuitable I'll use poly as the design guide recommends.
Please answer my question.

Answered by Jerry Liebler
Posted May 29, 2014 1:44 PM ET


If you want to use mineral wool insulation, it's your choice.

I'm not sure what you mean by "Please answer my question." If you provided a sketch, your question might be clearer. Since you are proposing a basement insulation method that is likely to develop moisture problems, I'm not sure that I am the best person to answer you question about whether your decision to use Thermo-Ply is a good idea.

Answered by Martin Holladay
Posted May 29, 2014 2:28 PM ET


The question I asked is :Can i, without undue risk of moisture related problems, use closed cell spray foam on the inside of my rim joist if the lower 1/2 of the opposite side is covered by a vapor barrier while the upper half is vapor open to the exterior.

Answered by Jerry Liebler
Posted May 29, 2014 2:49 PM ET



Answered by Martin Holladay
Posted May 29, 2014 2:55 PM ET

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