Hanging floor joists: Hanging or on top of foundation wall?
airfix
| Posted in General Questions on
My engineering is showing that hangers are used to mount the floor joists from the foundation wall. The hangers drop the floor joists below the top of the concrete foundation. I wasn’t expecting this. Instead I was expecting to see the floor joists sitting on top of the foundation wall.
What are the pros/cons of doing it one way or the other when it comes to installing exterior foam on the above grade walls, installing interior foam on the basement below grade walls and then air sealing and insulating the sole plate?
Steve
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Replies
Is your grade high? Often if you have a no-step threshold the floor joists will be dropped like that to prevent the ground from touching the wood rim joist.
We are on a steep lot, with a 22% natural grade. So the grade is high at the back of the house. However the foundation wall should be at least 6" above grade.
Our rear patio elevation is at the same height as our interior floor so that's probably why they did it that way. Does this method pose any problems for installing foam or sealing the rim joist sole plate?
Steve
Yes, the details will be different because you have no band joist to seal and insulate. Regardless of what you do, you will probably be unable to get any meaningful insulation between the top corner of the foundation wall and the interior. With dropped joists, the subfloor generally stops on or next to the corner of the foundation. At most, you might be able to get 3/4" of insulation between the top of the foundation and the underside of the finished flooring.
You will have more difficulty insulation the interior of the foundation, because the insulation will have to wrap around all of the joists and hangers, and also because the hangers themselves will act as thermal short circuits.
You could ask your engineer to design standoffs that allow you to hang a ledger inside the foundation, with the inside face of the ledger as far from the wall as the thickness of interior foundation insulation you wish to use. Then, the joists can hang from the ledger using standard hangers. Depending on the design of the standoffs, there should be little thermal bridging, and you may even be able to get some insulation between the ledger and the foundation wall.
Peter,
Thanks for the reply. If I understand correctly:-
I'm planning a 1.5" interior foam on the inside of my basement. So the ledger would mount directly to standoffs that are mounted to the foundation wall. The standoffs would be small to allow the 1.5" thick foam to be flush to the inside of the ledger. I assume the joist would still mount from a hanger off the ledger therefore the top of the joist would be flush with the top of the ledger which would be flush with the top of the foundation wall?
Would the sub-floor come to the outside of the ledger essentially stopping at the inside of the standoff? Then are you proposing I fill the spaces between the standoffs and ledger with insulation?
In this scenario my rigid foam would come up to the bottom of the ledger. I'd still have to figure out how to air seal the foam to the ledger to the foundation? Air sealing will be a nightmare, I guess closed cell spray foam seems the most logical choice for sealing around the joists and ledger?
Do you have anywhere I can see a diagram or picture to make sure I'm visualizing what you are describing correctly?
Steve
Steve,
You could also reduce the width of your foundation wall at the top to accommodate insulation and a ledger?