# Expanding Foam Between Two Floor Joists Over Concrete Foundation Wall



## Howitzer (Sep 12, 2008)

I will do my best to explain my question, here goes... Cold Canadian climate!

Our floor joists run east-west. I am applying spray foam for our basement rim joists (at the ends of the floor joist on the east and west ends of our house.) This is a fairly east task, see this pic, these I understand how I can simply spray foam these areas:










My question is about insulating the south and north areas where the floor joist run parallel to the foundation wall (see pics): This one is in our unframed laundry room:









These two is from our framed rumpus room:









The outside joists run directly above the concrete wall (Joist "A" in the pic below) and I believe there is another joist outside this joist (Joist "B") and a space between them. Should I drill a hole in the inside joist "A" and insert expanding foam inside this cavity between the two joists? I believe the cavity isn't insulated but is directly below the outside walls of main floor above it. I hope I am clearly describing my situation. I was planning on drilling an inspection hole and inserting my inspection scope into the cavity (that I think exists) so I can assess what I have.









Any comments/advice/suggestions would be appreciated. I hope I am clear in what I'm asking.


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## Bud9051 (Nov 11, 2015)

Down near the bottom of this link it will show some illustrations that may relate to what you have.
When the floor joists run parallel to the exterior rim joist you will often get that annoyingly inaccessible space you see. If I assume those are 2x10 floor joists placed 16" on center then the load requirement for that last smaller space is less than the rest of the floor. Thus, IMO, the troublesome joist doesn't need to be a 2x10. If it were only 8" high the world would never know and your floor above would be fine. I'm being a little sarcastic because I have one of those spaces of my own and it has been modified so I have access.

I have never seen an engineering discussion approving a smaller joist, but logic says the smaller span should be fine with a smaller joist. If one were to feel guilty trimming it to a smaller size, they could sister a 2x8 along the open side to add strength.

What do you think?

Bud

http://buildingscience.com/documents/digests/bsd-103-understanding-basements?full_view=1


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## Howitzer (Sep 12, 2008)

Bud9051 said:


> Down near the bottom of this link it will show some illustrations that may relate to what you have.
> When the floor joists run parallel to the exterior rim joist you will often get that annoyingly inaccessible space you see. If I assume those are 2x10 floor joists placed 16" on center then the load requirement for that last smaller space is less than the rest of the floor. Thus, IMO, the troublesome joist doesn't need to be a 2x10. If it were only 8" high the world would never know and your floor above would be fine. I'm being a little sarcastic because I have one of those spaces of my own and it has been modified so I have access.
> 
> I have never seen an engineering discussion approving a smaller joist, but logic says the smaller span should be fine with a smaller joist. If one were to feel guilty trimming it to a smaller size, they could sister a 2x8 along the open side to add strength.
> ...


Thanks for replying Bud. Are you saying that joist "C" I'm talking about is maybe just a 2x8 sistered to joist "A"? I will drill an inspection hole into joist "A" so I can hopefully see what's the situation. I will let you know ASAP. Thanks again!


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## Bud9051 (Nov 11, 2015)

Your A, B, and C are not pictured well, too close. Thatis why I referenced the images in that link.

I would expect C to be a full sized floor joist which I assumed were 2x10's. Then there should be a sill plate on top of the foundation with the rim joist at the outside. Because joist spacing rarely ends up being exactly 16" on center all the way across there is often this small space left over. What I'm trying to say is it doesn't need the support of a full floor joist. 

I don't know for sure what joist A arrow is point at. Different pictures may help.

Bud


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## Howitzer (Sep 12, 2008)

Sorry for the confusion, I will try to clarify.

Joist "A" - 2 x 10 - goes directly up flush with the inside of the concrete foundation wall.
Joist "B" - 2 x ? - I am speculating it exists, I am going to drill a 1/2 hole in Joist "A" to put in my inspection scope into to see if joist "B" exists and if there is a cavity between "A" and "B". I am thinking that joist "B" also sits on top of the foundation wall, like Joist "A" does. 
Joist "C" - 2 x 10 - inside joist towards the inside of the basement. I might need to take another couple pics...

From the inside of the house to the outside, the order of joists I'm talking about is C-A-B...


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## Howitzer (Sep 12, 2008)

I drilled through Joist "A" and it appears that there is concrete on the other side. They might have installed the joist then pushed concrete up to it? see pic:


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## Gary in WA (Mar 11, 2009)

I see in the first picture they poured concrete in the joist bays to fire-proof/insulate, common around the 1950's, partially encasing the wood in concrete. Appears they did this on the running (parallel) joists as well, to fill the CMU cells for strength and against fire. Yes, foam the void and add a vapor barrier on the final layer to stop wintertime condensation from the out-going room (heated) air. Joist "B" would be your rim joists, farthest one outboard.

Gary


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## Bud9051 (Nov 11, 2015)

I agree with yours and Gary's description. Access that gap however you can and add the foam.

Bud


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## Howitzer (Sep 12, 2008)

Gary in WA said:


> I see in the first picture they poured concrete in the joist bays to fire-proof/insulate, common around the 1950's, partially encasing the wood in concrete. Appears they did this on the running (parallel) joists as well, to fill the CMU cells for strength and against fire. Yes, foam the void and add a vapor barrier on the final layer to stop wintertime condensation from the out-going room (heated) air. Joist "B" would be your rim joists, farthest one outboard.
> 
> Gary


My house was built in 1975. I can only spray foam the void between "A" and "C". Joist B is completely inaccessible unfortunately (I drilled through "A" but I didn't see any void on the other side of "A" like I was expecting).


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## Bud9051 (Nov 11, 2015)

Having been built in 75 I would expect to see a sill plate on top of the foundation and then the 2x10 and rim joust resting above that. The sill plate would also have a termite barrier under it, often flashing, but would not have been air sealed.

Your 2x10's look to be 1.5" above the concrete, not flush with the top of the foundation. What Gary described with that area being filled with concrete as an attempt to seal the house to foundation is probably what you have. Under that concrete is your traditional framing. Although it is not good to have the wood in direct contact with any concrete, moisture and termite issues, removing it now would be very difficult. The problem area you are asking about was probably filled with concrete before the deck was added as Gary said, filling the tops of any open blocks.

From a big picture point of view, add what you can and what you can't will have very little consequences in the future.

Bud


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