Hello all,

I've been doing research on the best way to insulate/retrofit my rim joists, a 1931 stucco home. My floor joists sit on top of block foundation walls. The spaces between the joists have been filled with sloped/pitched concrete.

Now, I understand 2" of 3lb foam is common, as well as polyiso or xps foamed in place. However, I've also been looking at the research from building science regarding embedded beams, in which they state that "However, static thermal simulations indicate that there might be significant risk of wintertime condensation within the cavities at typical interior humidity conditions, based on the beam pocket surface temperatures relative to indoor dewpoints."

I have not seen my specific case discussed anywhere, as roughly half of the beam is "embedded" in the sloped concrete. In addition, it may be possible to remove all of the sloped portions, but then I am not sure of how they are attached to the rim and would 2" 3lb foam provide similar structural effect (if that is the purpose of the concrete)

http://www.buildingscience.com/documents/confpapers/cp-1201-masonry-wall-insulation-interior-embedded-beam-simulations

Posted By ibilisi on 23 Feb 2014 10:34 PM
Hello all,

I've been doing research on the best way to insulate/retrofit my rim joists, a 1931 stucco home. My floor joists sit on top of block foundation walls. The spaces between the joists have been filled with sloped/pitched concrete.

Now, I understand 2" of 3lb foam is common, as well as polyiso or xps foamed in place. However, I've also been looking at the research from building science regarding embedded beams, in which they state that "However, static thermal simulations indicate that there might be significant risk of wintertime condensation within the cavities at typical interior humidity conditions, based on the beam pocket surface temperatures relative to indoor dewpoints."

I have not seen my specific case discussed anywhere, as roughly half of the beam is "embedded" in the sloped concrete. In addition, it may be possible to remove all of the sloped portions, but then I am not sure of how they are attached to the rim and would 2" 3lb foam provide similar structural effect (if that is the purpose of the concrete)

http://www.buildingscience.com/documents/confpapers/cp-1201-masonry-wall-insulation-interior-embedded-beam-simulations

Use of 3lb foam is actually pretty rare in this application, but 2lb foam is common.  The most common application for 3lb foam is sprayed-on insulating roofing.

Climate matters in this decision, as does the amount of ventilation space behind the stucco and the rim joist/sheathing, and what type of weather-resistant layering (asphalted paper, rosin paper, or whatever) is between the stucco & sheathing/rim-joist.  Where is this house located, and what is the stack up?

I think you may have your numbers reversed. Here in Minnesota we regularly remove the sloped concrete and spray foam 2# to at least three inches, but more often with reckless abandon filling the cavity. From experience, the alternatives are unacceptable.

Sorry, I did have my numbers in the wrong order. 3" of 2lb is what I meant to write.

This is 1930 stucco standard on wire with what seems to be tar paper over sheathing. I don't see a gap other than standard. In certain areas there are wax rolled newspaper batts on the outside of the sheathing. But not everywhere, very odd. I'm zone 6, Minneapolis.

Glad to hear from badger re:experience as you are in my climate zone and sounds like you've dealt with these before. They are odd ramps of concrete but they do pop out. Looks like tar paper is placed on top of the wall block and then this concrete was placed on top of the tar paper, sloped.

Right now I'll sketch down the 2lb foam and "remove sloped concrete". But I'm open to other ideas as well.

Thanks all

We often have our clients remove concrete and we review critical mechanical components like outdoor faucets, before we spray 2# foam. I did this in my own 1921 farm house. It is one of the most important areas to insulate to limit unwanted air infiltration. This can be a very considerable percentage of an older homes' total heat load.