Staggered framing
Salibonz
| Posted in General Questions on
I am looking to build a somewhat passive home while keeping my cost low. Can I frame a 2 story home with staggered 2×4 studs 24″ o.c. also using floor trusses and roof trusses. Also using zip r6. I am hoping to reduce thermal bridging and still be cost effective. My worry is that 2×4 24″ o.c. is not acceptable by code for a 2 story home, but in staggered framing there would be a stud every 12 inches. Is this a viable option? Thank you in advance.
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Staggered studs and zip R6 is an expensive combo. If going staggered, why not just widen your bottom and top plate and skip the Zip R6?
I believe you can frame with 2x4 24" on center for a 2 story home if the loads are aligned.
Reason for zip r 6 is to reduce thermal bridging at the top and bottom plates as well as king and jack studs. I figured the zip r is expensive but I would still save on labor costs if I had to add rockwool on the exterior as well as furring strips.
I am trying to keep cost down by utilizing staggered 2×4 rather than standard 2×6 16o.c and achieving a thermal break. Upon googling information I saw that 24 inch o.c is not an option for a 2 story home with 2×4 framing, but can't find anything on staggered 2×4
Yes, I can't find anything either... My builder said it was ok. I'm not sure that inspection is going to go well. We have 2x4 double stud walls 24" OC currently being framed. 2nd floor is under way now.
What I mean is rather than zip R6, do a 2x12 base plate instead of 2x8 or whatever you were planning. The plates would have thermal bridging, but the assembly would be easier and achieve similar or better results. Or just go with a double stud wall.
Basically, you are likely introducing two complexities to the framer that may be new to them rather than just 1. Both of them will require a little extra thinking about every detail that is second nature for standard walls. It's not a ton, but it's something.
So your architect designed a double stud 2×4 wall 24 oc? So one wall must be the bearing wall. I guess if engineered correctly it can be done. Thank you for your input
Sort of, it was drawn up as 2x6, I said to do 2x4 16 OC instead of 2x6. Miscommunication and now we are here. Builder says it is fine, I'm skeptical. The outer wall is technically load bearing. But they are tied together at the top plate with plywood. It makes for a very stiff wall before it's braced.
If “Advance framing” is not prescribed in your local code you will need the have an engineer draw and stamp your plans making it approved.
“Advance framing” puts the studs 24 inches on center but it also puts the second floors joists directly over the first floors studs and under the second floors studs and the roofs truss sits on top of the studs so there is a clear load path to the foundation.
If you build a stager stud wall you would not be spending money on Zip+R or any foam board insulation. Your choice is to make the wall thicker and fill it with cheap fluffy insulation or you build a single stud wall and cover it with expensive foam board but a little of both cost way to much making it a bad idea.
Before you decide you want a “passive house” consider what are YOUR goals. I know passive house has made some changes but last I looked it was about an absolute number of BTU per square foot regardless of the climate or the cost of construction and paying lots of money to get a certificate.
Consider a “pretty good house” instead of passive lots of info about it if you go looking.
Walta
Walta, I agree, a pretty good house is the way to go. I was just that with staggered studs you would still have thermal bridging and labor costs to put a blanket on the outside would be far to much. My logic...framing with 2×4 is cheaper than 2×6 but I would be using more lumber but...I would create a thermal break. And to increase that thermal break...using zip r. Same labor, no extra work, just a pricier product.
My biggest concern is framing with 2×4 24 o.c. is acceptable ( even though it would be staggered every 12" o.c.) on a 2 story home.
Or should I just forego the whole idea and keep things simple.
You have much less thermal bridging with staggered studs though, even with the top and bottom plates. That's why double stud walls perform pretty well. Remember that even with staggered studs, you have much wider top and bottom plates, which means less thermal bridging (compared to a "regular" wall), because the wood of those top/bottom plates has some insulating value (I allow about R1 per inch). You also avoid thermal bridging for the studs themselves, which (for an 8x16 foot wall, as an example) is about a 62% reduction in thermal bridging compared to a regular wall (about 128 feet of stud face for a regular wall compared with about 48 feet of stud face for the top and bottom plates and two corners in a staggered stud wall). For longer walls, the efficiency gain will be greater.
I agree that if cost savings is your goal, using Zip-R is contradictory. I'd go with a thicker staggered or double stud wall and skip using Zip here.
Note also that doubling up the top plate can help with truss alignment issues.
Bill
Hey bill, appreciate the help, so going with a, say 2×6 staggered wall and eliminating zip r, this would be a cheaper wall assembly, and still be efficient.
It's likely to be cheaper, and can be made to be as efficient, or even more so. Basically the thicker you make the wall, the more R value can fit, and the more efficient it is. You could save some money making the wall a little thicker and using cheaper-per-unit-R-value fiberglass over mineral wool too.
Bill
Ok bill I appreciate that and am going to look into that
Note that only one of the two walls has to be 2x6, the other can be 2x4.
It's generally easier to think of one wall -- the 2x6 one -- as the bearing wall and the other as a non-structural accessory.
Salibonz,
What climate zone are you in, and what R-value are you aiming for with the walls?
I'm in climate zone 6a. I am trying to reduce thermal bridging......affordably....while still optimizing my r value. I'm content with an
R 24 if I have low thermal bridging
.
I think you're over-emphasizing thermal bridging. What you should be looking at instead is maximizing whole-wall r-value vs cost. Accepting some thermal bridging may be the way to do that.
I'm in climate zone 6a. I am trying to reduce thermal bridging......affordably....while still optimizing my r value. I'm content with an
R 24 if I have low thermal bridging. I assume that r value is better when thermal bridging is reduced, thus makinthe wall unit work better.
.
I'm in climate zone 6a. I am trying to reduce thermal bridging......affordably....while still optimizing my r value. I'm content with an
R 24 if I have low thermal bridging. I assume that r value is better when thermal bridging is reduced, thus makinthe wall unit work better
There is a formula for calculating the effective r-value of an assembly built out of different materials with different r-values. If you have materials with r-values R1, R2, R3, etc, and they're in percentages P1, P2, P3, etc, you calculte:
P1/R1+P2/R2+P3/R3 ... and so on. The take one over that.
So let's say you have a wall with 2x10 top and bottom plates, staggered walls with one side 2x6 and one side 2x4, and the whole thing filled with blown cellulose. Framing wood is about R1 per inch so the 2x10 is R9.25, the 2x6's are R5.5 and the 2x4's are R3.5. But the 2x6's have 3.75 inches of insulation behind them, which is R4 per inch, so that section is R20.5. Similarly the 2x4's have 5.75 inches of insulation behind them so that section is R26.5.
The full insulation sections are R37, 9.25 inches at R4 per inch.
It's customary to assume that a wall is 15% framing. The top and bottom plates are 1.5 inches each, so 3" out of a 96" wall, or 3%. Let's assume the 2x4's and 2x6's each are the same, or 6% each. So the calculation is :
0.85/37+0.03/9.25+0.06/20.5+0.06/26.5
Representing the full insulation, top and bottom plates, 2x6 section and 2x4 section, respectively.
That comes to 0.031. One divided by that gives 31.8, which is the effective R-value of the wall.
Now, costs vary a lot by region, but I suspect you'll find that's the cheapest way of getting that effective R-value. If you feel that's not enough, probably the cheapest way to increase it would be to make the wall thicker.
Just for reference, if both walls were 2x4 the effective R would be 32.5. If both walls were 2x6 effective R would be 31.2.
If you did your original proposal, double 2x4, offset, 2x8 plates and R6 continuous you'd get 31.0.
Why the emphasis on thermal bridging? The goal should be energy efficiency and "comfy-ness", which are both related to the OVERALL R value of the wall. Thermal bridging just cuts down on the overall R value somewhat from that of an "ideal" wall which would be only the insulating material and no thermal bridges. The only way to eliminate thermal bridges is the all-external insulated "perfect wall", which brings new challenges and isn't cheap -- and will STILL have a few thermal bridges.
You should be trying to get a good overall R value for your entire wall ASSEMBLY. If you want to cut down on BTU loss, you can do either or both of reducing thermal bridging or upping the entire R value by making the wall thicker. Thicker walls have reduced thermal bridging too, in a way, because the wider top and bottom plates (and studs, for a regular wall) have increased R value due to more wood depth.
If you want to get the best performing wall at low cost, some variation of a double or staggered stud wall is probably the way to do it. If you have access to cheap reclaimed polyiso, then adding exterior rigid foam would be another option. If you can only use all-new materials, then my preference would be to use a wider double stud wall with the interior and exterior studs staggered, and batts in the interior and exterior bays. Using 2x4s on both sides, this will get you R30 if using high density fiberglass batts, or regular mineral wool batts. With 2x6s on one side, that ups the total R value to about R38. That's pretty good! You've still cut down overal energy losses in relation to a regular studwall, the thermal bridges are there but loosing less BTU due to the greater wood depth (and associated higher R value for the framing), and you have something easy and relatively cheap to build.
Remember that WOOD studs are NOT "R-0", they do have some insulating value. I think you're overly concerned with thermal bridges here.
Bill
I just built a 2000 sqft 2 story house with double stud walls 24 oc with 2x,6 framing with a 1 inch gap between the bottom and top plates creating a thermal break and 12 inch total cavity filled with dense pack cellulose. If you put a 2 inch gap between your double wall you will get an extra r 7 (assuming r 3.7 per inch with dense pack cellulose) creating a thermal break from the plates with the same r value as the foam without the labor or material cost.
That must have been a hefty price tag...doubling 2×6 framing plus labor. Although you do get a better wall and I like it, but it does come with a hefty price tag. Did you find that it was worth the extra cost. You mind me asking how much it added to your budget
That must have been a hefty price tag...doubling 2×6 framing plus labor. Although you do get a better wall and I like it, but it does come with a hefty price tag. Did you find that it was worth the extra cost. You mind me asking how much it added to your budget
I milled my own lumber from trees cut down to clear the build site and built the house myself so those costs aren't applicable to my situation but the concept is still relatable to your situation If you change the 2x6's to 2x4's. Adding 2 inches in between walls is no extra labor, eliminates your thermal bridging at the wall plates, gives you a similar r value as the Ridgid foam and eliminates the material cost for the zip R sheathing. Also to correct my original post, only my exterior load bearing wall is 2x6. The interior non load bearing wall is 2x4 24 OC
A tip on thermal bridging. Since your house is 2 stories you may well be able to eliminate many, if not ALL the headers in your first floor openings by using the rim joist of the floor above as your header. IRC2021 R602.7.2
Another approach to double framing is to use "ladder framing". I've attached an image. You would use 2x6 studs @ 24" o.c. and then on the inside you would use 2x2 horizontally at 24" o.c.. This nearly eliminates thermal breaks at the studs. You can use an R20 batt between the studs and an R6 batt between the ladder rungs. By my calculation this gives you a whole wall U-value of around 0.0385. Code requires 0.045 max. So this works.
With 2 layers of batts you have the possibility of applying a smart vapor retarder between the 2 layers. You could do blown insulation into the cavities but then wouldn't be able to do the retarder in the middle of the wall. However you could still do one at the innermost face.
Also, for practical reasons relating to attaching trim moldings and gypsum board you may want to go with a 2x4 applied flat against the studs at the bottom and the top of the wall instead of a 2x2.
This is really helpful.
Just to clarify, U-value is 1/R-value. So a U of 0.0385 is equal to R26. And U of .045 is equal to R22.
I think that method of ladder framing with horizontal strapping is very similar to a Mooney wall, if someone is searching for extra information. Can't remember the differences, maybe insulation strategy?
Chris,
One other minor complication of Mooney walls are that the usual framing details don't work where they meet interior partitions, or at inside corners.
I've heard of using the rim joist above as a header and like the idea, thanks. I'll also consider using the 2×2 idea.
Salibonz,
Don't forget that if you move the headers up to the rim-joist, you need to use joist-hangers above the openings.
Yes, as Malcolm says. Also note that I provided the code reference in my earlier comment so that you can get the details right.
Thank you. You guys gave me food for thought.
Are you framing your house yourself, or hiring someone?
You could look at the method used by Stephen Bonfigliloi: https://www.finehomebuilding.com/project-guides/insulation/breaking-the-thermal-bridge
Using polyiso instead of EPS would certainly allow you to hit your effective R-value target of 24 with a 7-1/4" wall cavity.
Have you considered T-studs?
2019 article title:
"Does this new wall truss one-up double-stud walls and continuous exterior insulation?"
excerpt:
"High-performance exterior walls typically handle thermal bridging at framing members in one of two ways—either the building is wrapped in some thickness of continuous rigid insulation outside of the sheathing or the home is framed with staggered, double-stud walls. Each of these two approaches has drawbacks. Window installation is more complicated. There are new details for carpenters to master in terms of managing structural and hygrothermal loads. And these options are also more expensive than conventionally built and insulated walls.
Enter inventor Brian Iverson and the Tstud. Tstuds are an alternative to solid 2×6 framing lumber. Tstuds can be used for pretty much any framing element in exterior walls: studs, jacks, cripples, plates, and headers. They are designed for advanced framing—24-inch on center, with single top plates, and two-stud corners. Tstud are designed to break the thermal bridge within the stud itself."
https://www.greenbuildingadvisor.com/article/petes-products-tstuds