Joists on Basement Wall Ledge
I’m trying to minimize the distance from grade to floor level to allow the smallest step out to a patio and I’m struggling to find a design for the basement wall–floor joist–main floor area with an acceptable thermal break.
This is for a single level house with full basement, CZ 6B (Colorado, 7300’).
My architect proposed using joist hangers, but with a full thickness basement wall this will create a significant thermal bridge (i.e., no significant thermal break). I expect 2” external foam on framed 2×6 walls, with basement insulation on the interior. If necessary, we could do 2” external foam on wall sections (say, top 4′) where we set the joists on a ledge so I attached drawings of both. My builder is not enthusiastic about external below-grade insulation, and apparently ants can be problematic. I expect 12” joists and 8” thick concrete walls but no engineering yet. Total joist span is ~35’ so presumably there will be a beam in the middle of the basement.
Could the design in the drawins, or some modification of it, be used to adequately support the joists and create an acceptable (if not optimal) thermal break while keeping the grade and floor level as close as possible?
My questions are:
– First, is there a better way to do this? It would be great to see or hear about alternatives.
– How much of the end of the joist has to be supported? If necessary, could we bolt a ledger board to the basement wall to increase the length of joist resting on a support?
– What would be the optimal ledge width – e.g. half (4”) of the wall, or something different, and why? I assume best to avoid needing to rip all the sills.
– Are there likely to be construction issues – is this routine or difficult? Are there details that are likely to be expensive?
– Recommendations on moisture/water barriers for this immediate area (top of concrete foundation wall)?
– Rather than use rigid foam at the ledge, would it be substantially better to set the joists and then spray a couple inches of CC foam? I’d prefer to avoid CC SF where possible. With this design, the rigid foam on the ledge (labelled as 1” foam) would be easy to cut to size and could be sealed to the concrete, hopefully avoiding condensation issues.
Thanks for any advice!
johng
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Replies
I wouldn't focus on thermal bridging, rather on total insulation.
It's more normal to put the joist on top of the basement wall (on a mudsill) with a rim joist across the ends. That way you get more space under the wall for insulation. Fill that space with polyiso insulation to the thickness of the wall and you should be good.
If you have to make the joists flush with the top of the basement wall, the way that would be done here is to use tapcons to attach joist hangers to the concrete. Then a treated sill on top of the basement wall that the first floor wall sits on.
But I'm confused. You say you have a full basement but there is insulation below the floor joists. Is the basement going to be unconditioned and vented to the outside? A basement should be either fully inside or fully outside the building envelope -- either insulated or vented to the exterior. If it's outside the building envelope then you don't want to run mechanical systems through it. So it's not much good as a basement.
John, I've set joists on interior ledges many times, for varying reasons. I have used the detail you show and think it's a good approach, except I would run the foam continuously, not between joists. Just make sure the joists have enough bearing to prevent crushing of the wood fibers. The joist specs will say what they require. If you don't have enough bearing you can bolt a metal angle ("angle iron") or wood ledger to the interior of the wall, or build a framed wall ("pony wall") under the joists.
The IRC building code says that the foundation wall needs to be at least as thick as the wall it supports, so if you're using 2x6 studs, there should be at least 5 1/2" of concrete at minimum. But I've also seen engineers allow that to be reduced.
Like your builder, I prefer placing foundation insulation on the interior when possible.
John,
As other posters have said, there are a few ways to do this.
- Detailed as you have shown with a ledge.
- Hanging the joists off a PT wood ledger let into the concrete (with or without foam behind).
- Hanging them off a pt wood ledger attached to the inside face of the concrete.
- Supporting the joists on a knee-wall that bears on the footings.
Before choosing one, or choosing between the details you have shown, find out b0th the required bearing for your I-joists, and whether they will need a rim-joist for stability. The answers may modify the sections you have drawn.
Many thanks - as others frequently note the expertise here is impressive.
DCC - the intent is to avoid a thermal bridge which would occur if the joists are hung from the top edge of the wall, or from a ledger with a full-thickness concrete wall.
Many thanks! I'll forward the drawing and your comments/concerns/recommendations to the architect and builder.
How much thermal bridging is there really going to be with an I joist? The web is pretty thin, so you're only really looking at the flanges which are maybe a couple square inches.
Putting in ledges and then thickening the wall and all the other workarounds could drive up your construction costs significantly while not really yielding that much benefit.
Just some food for thought...
My approach would be:
1. Build the 8" CMU or CIP concrete wall without the pocket.
2. Wrap the top of the CMU or concrete with a vapor barrier to prevent wicking, The wrap would be like an upside down U, going down the inside and outside.. If you live in a termite prone area consider using a product like Term Barrier.
3. For your sill plate, if you use a treated 2x8, remember your CMU block is 7 5/8" wide, and a 1x8 is only 7 1/4". If you are hanging your joists you will be better off aligning the inside edge of the 1x8 with the inside edge of the block, but that will leave you with a 3/8" ledge on the outside. If you want the sheathing to lap onto the CMU on the outside, use 2x10's for the sill and rip them to 7 5/8" to match the width of the CMU. If you are building a concrete foundation wall, try to get it built to 7 1/4" wide.
4. Attach the joist hangers to the sill plate, and hang the joists. There will be about a 1/16" to 1/8" gap between the face of the CMU and the joists.
5. At the exterior, even if you need a no step entrance, do not bring a concrete walk or drive directly to a door threshold. Keep the subgrade about 12" down from the top of the block sloped away from the house. Fill against the house with about 4" of drainable #57 stone. Use crusher run as a subbase below the concrete that is a few feet out from the house. Then frame a pt "deck" between the door threshold and the concrete walk or drive on top of the #57 stone. This allow the grade to stay below the CMU, and the grade can be pitched away from the house, but you can still get a no step entrance.
CL,
Why not bring the concrete walkway right up to the threshold? It's the standard detail for houses, multi-family dwellings, and commercial buildings.
In many jurisdictions, the AHJ for residential requires the grade or concrete walk/drive be 2" or more below the lowest wood framing member so that moisture will not get trapped against wood that is part of the structural system. This precludes bringing the concrete all the way to the threshold.
When you see this detail in residential or multifamily, it is usually slab on grade construction.
There are two factors in commercial construction that differ. First, commercial usually does not have wood that can rot if moisture is trapped as an integral part of the structure. Second, many commercial buildings are concrete slab on grade, or if there is a level below the entrance grade, that portion of the structure is CIP (Cast in Place) concrete.
I guess I wasn't clear about the main question:
How can I minimize the vertical distance between the floor level and outside ground (grade) level _with a thermal break? This is CZ6, north side of the house, and the ground will likely be frozen from November to March.
Malcolm - if it was just a walkway to the threshold, I’d do as you suggest. But the area of concern is much larger – about 100 linear feet
The question is what construction/detail can economically provide for a thermal break in this situation? If the joists are hung from a full-thickness stem wall, the only insulation is the sill board or air space, subfloor, and flooring – i.e., not much. See attached detail - the bold red arrow is the "thermal pathway".
Michael and Malcolm got it - the issue isn't bridging though the web or joist, it's conduction thru the concrete, which will be at ground temperature, and thru the subfloor and flooring.
With a ledge in the stem wall there’s space for foam and thus a thermal break (see original diagrams), whether the joists are supported only by the concrete ledge or are supported entirely or partially by an interior basement stud wall or a ledger bolted to the concrete wall. The key is that with any of these alternatives there's foam, not concrete, under the subfloor where it’s inside (interior to) the first floor stud wall. Ideally (for insulation) the top of the stem wall will be narrower than the 2x6 sill plate, but that's a question for the engineer. And as Michael noted, I really want a space between the concrete and the ends of the joists to facilitate installing the rigid foam insulation.
Depending on the final design, I’m still wondering whether this is a situation where it’s worth using CCSF, and whether there are any special concerns about moisture barriers.
Thanks again for the responses – it’s really helpful to sort out the issues.
In order to avoid that gap the inner edge of the wall studs has to overlap the outer edge of the joists. One way would be simply to make the wall thicker.
Another way is to have the joists sit on top of the foundation, and have the foundation end one joist thickness below grade. You've then traded one problem for another, how to detail a rim joist that's below grade. Frankly I'm not sure that's so much worse than having the bottom of the wall exactly at grade. In either case I think I would excavate down maybe 18" below the bottom of the joists and 8" out pour concrete to make a good barrier against pests.
You can avoid the thermal break by insulating the exterior side of the concrete or CMU wall. Waterproof it with self healing material and apply insulation.
Does your exterior finish need to bear on the foundation (ie brick) or is it entirely supported by the framing above the sill (ie siding)?
CL - We plan to support the finish on the framing. Insulation on the exterior concrete wall will require resolving a bunch of other issues - i.e., possible but I'd prefer another solution. If we end up with exterior foundation insulation I hope this would only be 2 or 3 feet below the top of the wall, rather than full depth. IRC wants 15/19 for CZ6 and that's a lot of foam. We're planning a walkout basement on the S side, along with a sliding glass door opening to a deck that will need to be on posts and attached to the wall on the S - this would pose the same issues as the N side, plus attaching the deck ledger. That makes the external foam less attractive.
Simpson makes an ICF ledger connector that lets a ledger hang through foam. You can see if you builder can pour the foundation with this connector+foam on the inside and hang your joists from that.
Still sounds more complicated than rigid on the outside. If you don't want foam, go with mineral wool.
P.S. You can clear span 35' with I-joists and floor trusses. For me, structure without interior support is always better even if it adds extra material cost as it saves on labour and allows more flexibility in the space.
Some interesting ideas to discuss with the builder. I sure wish I had some actual experience with these things.
DCC - I hadn't even considered the idea of an external treatment that would permit a wood sill at grade. A economical way to do that is, in my mind, the preferred alternative. Something else to look into! This wouldn't require any additional excavation so it may be practical, and it's a well-drained, dry area. We'll have an 8' wide covered porch on the N side, and at least 2' eaves elsewhere so I don't expect moisture issue at the foundation.
Akos - Thanks for the pointer to the Simpson connectors. I'll look into those. I wonder how hard, and how much expertise would be required to embed foam and the connector into the top of the wall pour. From GBA posts my impression was that ComfortBoard was quite attractive to mice, but perhaps less so to ants. That would be a good alternative to foam.
John,
Consider using this detail or a variant on it.
- You form the ledge before the pour using a let-in pt ledger and foam, attached to the inside of the concrete forms, with threaded-rod or anchor bolts already in place to embed in the concrete and secure the ledger.
- You can either frame it as shown, or drop the sill-plate (set the ledger 1 1/2" higher) for less thermal bridging.
- You can then use conventional joist hangers, and the attachment of the SIPs should be simple.
Malcolm -
Very nice! The extra effort to set the foam and ledger seems like a minor inconvenience compared the alternatives. Great idea!!