Header hangers vs. jack studs: Thermal bridging?
lance_p
| Posted in Energy Efficiency and Durability on
Advanced framing articles all seem to advise the use of header hangers instead of using jack studs. The idea is, less wood and more insulation in the wall, and better the thermal performance.
Has anyone studies whether the thermal bridge of a jack stud is better or worse than a header hanger? A jack stud is very large by comparison, but a header hanger is a relatively heavy gauge piece of steel that bridges right from sheething to drywall.
Beyond the thermal bridge, could a cold spot behind the drywall cause moisture issues or condensation on the interior surface of the wall?
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Replies
Lance,
Don't forget that some headers require more than one stud for support.
Lance,
Good question. I don't know of any studies that specifically look at the question you raise. I've never heard of header hangers causing moisture problems on the interior of a wall, however.
All of that said:
(a) I remain a skeptic concerning the benefits of advanced framing. I think advanced framing is oversold.
(b) When in doubt, a continuous layer of exterior insulation solves the problems you're discussing.
For more information on advanced framing, see The Pros and Cons of Advanced Framing.
Lance,Malcolm,Martin
I wrestled, mentally, with just the question Lance asked, in addition to what is the "thermally best " and most economical header. Is it a single LVL ( Laminated Veneer Lumber) or insulated box or commercial product? In the end I found a way to " go header-less entirely! " I'm building a ranch over full basement and I went "header-less" on the main floor's eve walls by moving the header function up into the ceiling service cavity and using an LVL rim board at the ends of my 11 7/8 I-joist framed "service cavity". .To perform the "hanger function" needed above the openings below I use "blocks" of otherwise scrap LVL nailed to the inner face of the rim board, to provide support under the top flange of the I-Joists. With this "header" location, and a 1 3/4" encroachment into the service cavity I, also have less thermal bridging than even no headers. .
Jerry, I'd love to see a diagram of what you're describing, sounds neat!
Personally, I've been wondering if there's a way to use the rim board over 1st floor windows as a ledger from which to hang the joists. Yes the rim board would likely need to be an LVL or similar, but this would eliminate the need for a header all together... the rim board seems kind of redundant in an ordinary wall with a header just inches below it.
A similar approach for 2nd floor windows could use an LVL "header" above but flush with the top of the wall, and hangers could be used to hang the trusses over window spans. This gets the header out of the main insulated part of the wall.
Of course an adequate load path still needs to transfer loads to the foundation, but this would be very similar to what's already going on in a regular wall. Curious if this has been investigated before? It must have.
Lance,
Moving the headers up to the level of the rim joist is done pretty routinely. Often just with dimensional lumber, as the thickness of the header doesn't make as much difference when it is at level of the floor system. A single LVL header might have to be doubled up at some openings.
Our code requires any dimensional wood joist to bear on it's bottom surface or be supported by an approved hanger. Blocks wouldn't pass, and I'm pretty sure I joists have to bear on their bottom chord too.
Getting something similar to work at the roof level seems a lot trickier. I don't quite follow where your proposed header sits. The description "Above and flush with the top of the wall, and hangers could be used to hang the trusses" lost me I'm afraid.
Thanks for your input, Malcolm! I may discuss this 2nd floor approach with my Engineer and see what he thinks.
For the trusses, I picture them being hung from a rim board with hangers exactly like the 2nd floor joists. For a raised heel truss the outboard edge of the truss would need to be moved inboard to allow the rim board to sit flush with the exterior wall (sitting on the wall top plate). The truss would bear on its bottom surface like before, but supported by a hanger. Trusses over window openings would sit flush with the ceiling height of the other trusses sitting directly on the top plate.
This truss scenario is more just thinking out loud. It would complicate the installation of the trusses in that area (and require unique trusses in those areas), but would offer the advantage of getting the header out of the wall for someone worried about the thermal impact... Passivehouse types, maybe.
Lance,
I've got it now. If the trusses have raised heels then why not?
I should have mentioned I'm using the same details on my house. I don't have updated details with the floor, but here's a snap of the roof header detail.
Lance,
The roof trusses aren't complicated even with the integrated header. The trusses can bear on the inner half of the top plate with the header on the outer half when needed. It's all taken into account when the trusses are designed.
Regarding your floor framing why not go one step further and remove the rim? Around here 11 7/8 rim is $1.80/ft and the LVL is $4/ft. By notching the studs and setting a ledger (or face mounting with screws) you can eliminate the rim joist and pick up some $. It reduces the thermal bridging quite a bit also.
Search for Swedish platform framing.
User 692etc,
I can see two complications.
Trusses typically need 3" of bearing, so maintaining a uniform span where there are no headers might not work with a 2"x4" wall.
Raised heel trusses tall enough for an LVL header usually need continuous sheathing up to the top chord for stability (and to prevent wind-washing of the roof insulation). If the trusses are inset from the exterior face of the wall this becomes difficult where there is no header.
Lance,
Here is an oblique view hope it helps.
.
Malcolm I think you misunderstood me. If you read my post it says when needed. The rest of the trusses would be traditional bearing on the full 3.5".
Lance please consider Swedish platform framing. For cost alone it is worth it on my project. Another benefit is 4" cavity between the notched ledger (2x6 wall 2x12 ledger) and sheathing that we're insulating to r15 with batts. I noticed your other post about insulating the rim.
User 992etc,
"The trusses can bear on the inner half of the top plate with the header on the outer half when needed."
This is what caused my confusion. It implies that the trusses are all the same length and "bear" on the inner wall when no header is present. I agree: having two different spans for the trusses is no big deal.
Edit: One other area that has me confused. Are you Jerry, or another poster?
Jerry,
Those I joists are just to create a service cavity aren't they, and will be fastened to the trusses above? If so i doubt it matters how they are fastened at the perimeter. That is a pretty expensive way to create a space for ducts though.
No worries about using hangers and ceiling irregularities. Joist and truss hangers are too thin to affect it.
I'm glad we are on the same page Malcolm. My name is Brandon.
Jerry, I-joists are not made to be top-bearing trusses. The glue used to connect the web to the flanges is extremely strong, but not designed for the high localized stresses of a top-bearing truss. It's easy to get proper joist hangers, either face-mounted or top-mounted, why not use them? Is saving nickels but causing a safety issue and voiding the warranty worth it?
Edit to add: I'm a fan of using flush headers when appropriate. It's less necessary when using continuous exterior insulation or double stud walls. Every different truss specification raises the risk of something going wrong and causing confusion on the construction site, but when necessary they are easy enough to hang from properly sized hangers.
Cheers Brandon.
Malcolm,
With the blocks nailed to the inner face of the LVL rim board/header I'm creating a " top chord bearing" situation for the I joist, which I must admit I've never seen as "approved" for I joist. Top chord bearing is definitely available on floor trusses, I'll see if I can get details of it's use and limitations with I joist from ? If I had to use hangers I'd need to notch the plate below the hanger or accept an uneven floor & ceiling. Without question the blocks will work and support the very light loads of a ceiling and attic floor with insulation above. But are they code legal??
Edit: A code legal way of combining the header s and rim board is to make the inner surface of the rim flush with the inner surface of the wall between two top plates and use "face mount" hangers for all the ceiling I joists. This means the outer surface of my air barrier will take a 2 1/4" step inward as it transitions from being the sheathing on the outer surface of the wall to the LVL @ mid wall and a 1 3/4" step outward @ the second top plate. That second top plate is the top 1 1/2" of the "composite" rim board and it's top surface is aligned with the top of the I joist. NO BLOCKS! The inter wall mineral wool will need to be thicker outside the LVL and it saves a bunch of cut OSB pieces but costs hangers, not a significant cost shift.
M alcolm & Michael,
I agree that top mounting I joists is"risky", that includes "screwing" through their top chords onto the trusses above. Per my edit of my last post I have a way to be fully code compliant using face mount headers, I'm busy editing my "Sketchup" model. Malcolm, regarding the cost of a ceiling service cavity framed with I joist vs hung from the bottom of roof trusses. Yes the I joist add cost, is it offset by installing the ceiling air barrier from above with nails or below with lifts and screws, at least partially. Then the cost to "make it air tight",, from below or above, again from above involves significant labor savings. I joists, LVL & hangers material cost <$1/sq ft. Well worth it IMHO to have high performance and hidden HVAC
Edit: I'm adding an attachment that is an attempt to illustrate the "plan".
Jerry,
Check you elevations if you are thinking of adding an 11 7/8 service cavity above your ceiling then raised heel trusses. The house will look very top heavy under most circumstances unless you do some Frank Lloyd Wright overhangs. You are doing a PWF, correct, why not bring ducts up through a chase to a cassette or whatever you're doing, why the large cavity?
Also putting roof trusses on a flat deck can cause headaches. In theory they are supposed to be perfectly flat on the bottom chord but reality says otherwise. By restricting the truss to a flat surface you will end up fighting a couple that are down on one end and an an inch off your floor on the other, although maybe you will stick frame the roof which would make more sense.
Back to the original question I have used header hangers and Ive seen them iced up prior to drywall in the winter and you can certainly find them with a thermal camera, that said Ive never torn apart an old assembly using them so I cant comment on long term effects other than I wont put them in a structure unless I have to for the same worries the OP has.
I missed the fact that Jerry is using TJIs for a service cavity. In that case I would be much less concerned about attaching them through the top. In fact, to save a decent amount of money, I've seen TJIs cut into short lengths and screwed through an air barrier at the bottom of trusses, with an additional layer of strapping below the TJIs, in order to create a dropped ceiling.
11" I joists are commonly used as the floor systems for houses because of their long spans - and they typically represent a fair proportion of the framing budget. It just seems odd to use them in a situation where their structural attributes are largely redundant. You are essentially hanging a complete floor system from your trusses to hide a couple of ducts.
T Carlson,
I've revised my details and will have the "second top plate", that joins wall segments. The I joist hangers will be positioned vertically so that the top of the I joist is the top of the "second top plate". The top of the segments will be the plywood "tie" plate that ties the inner and outer 2x4 walls together.. Directly under the upper tie plate is a 2x4 top plate on the outer wall, but on the eve side inner walls directly under the tie plate on the inner surface is the header/ rim board. The inner wall's outer surface is the air barrier OSB. the space between the OSB and rim board/header is filled with foam except under joints in the OSB where a 2" length of LVL serves as a block. As wall segments are "erected" they will have top plates added flush with the inner face of the wall. The ceiling air barrier cover the top of the I joists and the inner 4" of the double stud walls. If it is desirable I could add a bearing 1x4 under the truss ends to "uncouple" the truss s bottom chords from the attic floor
Edit: The elevations definitely shout "somthing is different" there is, obviously more distance between window tops and the eves . The combined effects of 9' ceilings,1' thick ceilings and 1' raised heel trusses. My attitude is, that is a " status symbol" because I can't "fix it" so I must "feature it"!
Jerry,
You can fix it, porch roofs, awnings, transoms to ceiling level, false gables, even cladding choices and whatever else. For all the trouble your going through to design this thing over the last how many years you gotta make it look good.
Be sure you are aware of the different types of rimboard and their limitations, basic LP (what I use) OSB 1 1/8" rim carries less then 500 lbs per foot with a 4' clearspan max. LP does have a rimboard brochure with their tabulated values, and they do offer a few higher load upgraded versions but their gonna be nonstock when you can get LVL off the shelf and throw 1 py into the floor or in your case service cavity system and be done with no lead time.