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Moving from SIPs to Double Stud Walls in Skinny

by Chad Ludeman on February 25, 2010 · 37 comments

in envelope,Skinny Project

In our first few projects at Postgreen, we used EPS SIPs (Structurally Insulated Panels) for our exterior walls and roofs which were very popular with the kids. We have decided to move away from SIPs in the Skinny Project in favor of using Advanced Framing Techniques and a Double Stud Wall design. Many of you have been waiting for an explanation as to why we’ve made this change, so let’s dive into it.

Why do we SIP no more?

First of all, we really have nothing against SIPs in general. We believe they are a great “hybrid prefab” type of product (along with ICF’s) and similar panelized systems. The main reasons we are moving away from SIPs are due to the challenges of our tight infill lots in Philadelphia. Many times the lot lines do not line up as drawn, foundations are out of level and neighboring homes are out of plumb. Since we are sharing party walls in every home, we have no room for error. The panels are ordered well ahead of time and they come perfect with little room for error. In the first two projects there were enough site modifications of the SIPs needed to negate the time and labor savings most SIPs projects are able to capitalize on.

If we were building detached homes, I’m fairly confident we would still be using SIPs as they offer a very tight and well insulated home with virtually no thermal bridging. As we get into larger projects, we may revisit the technology, but for now we are moving on to more traditional framing with some tweaks.

Double Stud Walls

First of all, we are using a double stud wall design on all of our front and rear facades. We are using a simple design that includes two 2×4″ walls on 24″ centers with a 2″ gap between the two walls. The studs are not staggered, but aligned with each other to simplify framing. We will be using CertainTeed Optima Blown-In Fiberglass Insulation in this 9″ thick wall that will result in a minimum of an R-38 wall. The 2″ separation of the studs will remain at the window and door openings, with the windows & doors being installed on the exterior wall. One could argue, and I would, that this reduces thermal bridging levels below SIPs benchmarks as most SIPs still include 2x rough openings that run from exterior to interior layer of OSB.

If you are thinking that we are foolish for not staggering our studs, I have included this lovely diagram for you above. This diagram is taken from a study done by the Consortium for Advanced Residential Buildings or CARB. These bright people had the same question we all have about double stud walls late at night – should we stagger these studs or not? They used state of the art heat tranfer simulation software to develop 10 different THERM models of double stud wall designs. The five with aligned studs are shown above.

The results are that staggering the studs on our 9″ wall would result in an increase in R-value of less that 0.5. Even more interesting is that the 7″ wall with touching studs will only reduce the R-value by 1.0 compared to including a 1″ gap.

Advanced Framing Techniques / OVE

Next, we have employed some typical Advanced Framing Techniques or Optimum Value Engineering (OVE) into our latest envelope design.There are many benefits to using OVE framing techniques, that sadly, most builders have no interest instituting into their buildings simply because it’s not what they’ve done for the past decade or so… Some of the basic advantages include reduce cost in lumber, reduced framing waste and improved energy efficiency due to higher levels of insulation.

There is a beautiful diagram below that illustrates some of the key framing strategies. Here is the list we are using:

  1. All framing is on 24″ centers with floor and roof trusses in alignment with wall studs. This reduces thermal bridging and increases the overall wall R-value as more insulation can fit into the walls with less studs.
  2. Two-stud corners are used to again reduce thermal bridging at critical corner details and pack in more insulation.
  3. Right-sized headers and insulated headers can greatly improve insulation values at leaky window and door sections. We will be using 2×4″ and 2×6″ insulated headers at most windows and doors compared to the standard 2×12″ uninsulated header used by many builders.
  4. Hanging floor joists from top plates – We actually improve on the detail shown below by eliminating the often leaky and poorly insulated rim joist detail completely.

OVE Advanced Framing Techniques
Double Stud Corner Image

That’s it for now on our new envelope design. We’ll get into more detail as we build the homes. If you’re thirsty for more details, the links below are some great sources that were used in the writing of this post.

Source Documents:

If you enjoyed reading this post I can promise you'll love our new writing over at Postgreen Homes. Yeah, we know that's the same thing your favorite band said and their new album is nowhere near as good as their early stuff, but seriously, we are actually still getting better.

There also isn't much conversation to be had here . . . at least not with us. So come on over to the Postgreen Homes Blog and tell us what you think of our new(ish) digs and crazy ideas. We will be sure to tell you what we think of your opinion.

{ 4 trackbacks }

Article – Moving from SIPs to Double Stud Walls: (Jenn R.) « Re:Vision Architecture's Newsletter Blog
February 25, 2010 at 10:32 pm
Skinny Project Update: Foundations Complete | 100K House Blog
April 5, 2010 at 11:30 am
A Few Framing Details from the Skinny Project
May 3, 2010 at 3:50 pm
What’s green about it? Super-insulation « >>> farm razed
June 28, 2011 at 1:16 am

{ 33 comments… read them below or add one }

1 Skylar February 25, 2010 at 11:07 am

What is your primary reasoning for utilizing fiberglass over cellulose? In almost every aspect (environmental and performance) fiberglass is an inferior product to cellulose. Why someone would choose a glass rod over a high mass microcellular insulation made of 85% recycled content is beyond me. When I see fiberglass specified in a “green” project it truly makes me cringe.

2 Todd Oskin February 25, 2010 at 11:26 am

Hey Chad,

Was wondering what led you to choose blown-in fiberglass over dense-pack cellulose?

I was planning on dense packing my roof with cellulose… primarily because I thought it had a slightly greater R-value per inch and was less expensive than fiberglass… and possibly easier with a DIY approach.

3 Skylar February 25, 2010 at 11:36 am


What pitch is your roof?
Don’t expect dense packing any fiber based insulation to be “easier”, it is a MAJOR chore to achieve proper densities while limiting how much the netting or scrim bellies.

4 chad February 25, 2010 at 12:06 pm

Todd/Skylar – This is a good question and probably a large enough topic for a separate post. There are definitely pros and cons to both and we actually haven’t set this decision in stone yet.

The main reason we are leaning towards fiberglass right now is that it has a higher R-Value of 4.2 per inch compared to 3.5 per inch for cellulose. Also, fiberglass seems to be a bit more cost competitive in Philly from our research so far as the manufacturers are cutting the price to help installers compete with the price of batts.

We obviously prefer cellulose in terms of the embodied energy of the product and also like the fact that it is a better air retarder than blown fiberglass.

We try to refrain from stating that “a green house can NEVER have X product in it and be considered green” for both our projects and others. These kind of statements miss the whole picture, especially the fact that long-term energy use of a house far outweighs the potentially small negative effects of one specific product that is making that home more efficient. If we tried to eliminate concrete and steel, which others have similar arguments about, from every “green” building in the world, we wouldn’t get very far…

5 meliason February 25, 2010 at 12:20 pm

if you are doing double stud walls, why not use larsen trusses?

and when are you guys going to rock the cross laminated timber?

6 Shawn Busse February 25, 2010 at 2:03 pm

Great article (as always), Chad.

We contemplated the staggered approach on our Live/Work project. However, in the end, we went with external foam to prevent thermal bridging.

In our case, the reason was twofold: first, we didn’t address this issue until late in the game (as in, “we’ve got a foundation”). Second, because of our very small footprint, thicker walls would eat into the usable living space. Since code prohibited going any bigger than 720 sq ft, this was a bit of a deal breaker.

It’s a shame we calculate square footage based on building footprint instead of usable living space. This is an effective “penalty” for those building with thick walls.

Here’s a link to some info on our pink-exterior insulation decision: I’m putting the finishing touches on our video of how we actually detail the windows with the pink exterior (and Tyvek underneath). It’s kinda crazy, but bullet-proof and innovative as heck.


7 Rory February 25, 2010 at 2:44 pm

I have been stalking your blog for some time now after a current client turned me on to it.

A question on hanging the floor joists from the top plates: do you use a double top plate in that case? Advanced framing typically calls for single plates and stacked studs, but presumably your joists are tighter than 24″oc.

Also, do you run your lower wall studs a foot long to make up for the lost rim height?

Thanks for being one of the leaders on all this.


8 Katy February 25, 2010 at 2:56 pm

Chad, great to see the Advanced Framing measures; I’m a big AF advocate. Can you explain more about your “hanging floor joists from top plates” detailing–are you attaching to a ledger, and what length studs are you using? Would love to see a detail if possible!

9 chad February 25, 2010 at 3:10 pm

More details. We use a single top plate on top of a roughly 10′ 6″ wall (finished ceiling height is near 9′ 6″). We use a Top Flange Joist Hanger (Simpson JB210) to hang our TJI joists on at 24″ centers.

The subfloor goes over the top plate and terminates at the sheathing. The bottom plate of the next 2×4 wall sits on top of the subfloor and so on…

10 Skylar February 25, 2010 at 3:15 pm


I agree that it is counterproductive to not consider a project “green” based on one product choice, but as you said yourself life cycle performance is most important. It is for this very reason that fiberglass is such an egregious choice. The single most important aspect of a green building is its insulation/envelope performance and it is the one area that you simply cannot skimp. In real world installations, fiberglass performs at its worst when it is extremely cold and you need performance the most. It would be foolish to think that 9 inches of fiberglass at r-4.2 per inch would ever outperform 9 inches of cellulose at r-3.8 per inch; it isn’t happening. Just because the package says your are getting an r-4.2 doesn’t mean this is how the fiberglass will perform in reality.

Cellulose outperforms fiberglass in almost every category. It offers better fire resistance, pest control, sound attenuation, moisture control, thermal performance, and much more mass. The benefits of the increased thermal mass cannot be overlooked. This thermal inertia can be significant enough to reduce peak heating and cooling loads. The folks at Viking House have a good article that describes the benefit of increased thermal inertia with the help of cellulose insulation.

11 chad February 25, 2010 at 6:14 pm

Excellent article Skylar. I’ll keep investigating and get back to you… Thanks for the passionate comments!

12 Skylar February 25, 2010 at 6:20 pm


I live up in the cold mountains of central Idaho, I take my insulation very seriously;) I have really enjoyed following your projects. Keep up the good work!

13 GreenbuildinginDenverdotcom February 25, 2010 at 10:44 pm

One thing my installer pointed out is that cellulose insulation comes from recycled newpapers. Because of the internet, that’s a dwindling resource. I guess the newsprint manufacturers could shred it as soon as it’s made… ?

14 Todd Oskin March 1, 2010 at 10:15 pm

I am not sure of the exact pitch of my roof ( roughly estimated it one time for the insurance policy, to keep my better rate).

On the 3rd floor the lowest part of the roof is about 7.5-8′ and the highest side is about 15′ or so.

I haven’t decided exactly how I am going to go about it, or if I am going to just pay an experienced professional to do it.

I was thinking that maybe it would be easier if I hung the drywall first , instead of using a netting or plastic sheet to hold the insulation… and then cut holes into the drywall or did it in sections (4ft at a time from the lowest part of the roof up to the top).

15 Dave April 7, 2010 at 3:54 pm

I walk by your places on the way to the Berks El stop and am impressed by the work. Given the location to the subway, do you do anything to address noise issues? Are there any plans to use Quietrock, Green Glue, sound clips, etc. or do you think the double stud wall & insulation will offer a high enough STC rating?

16 Kevin R. Karcher April 17, 2010 at 3:41 pm

I am intrigued about your information posted. I am a green builder in my local town. I am interested in understanding what standard materials are needed to build a wall that is comparable to an ICF or a SIP panel?

17 Matt April 21, 2010 at 10:22 pm

Dave, the El noise is something I’ve thought about as well. I walked around the neighborhood yesterday evening and spent a while near the Skinny Project site, which is more than 600 feet, as the crow flies, away from the MFL line. I did not recall hearing the trains from outside the house, so I can say with some confidence that they would not be noticeable from within. I did hear them pretty well as I was walking down Amber St. I think that the train noise propagates farther down streets like Amber and Coral, but on the side streets and on non-through streets like Martha, it doesn’t seem to be there at that distance (more than 1/10 of a mile) from the tracks.

Chad, Courtney, what have you noticed from Amber St.?

18 Jesse May 5, 2010 at 9:23 pm

I really encourage everyone to look at the embodied energy in SIPS panels objectively. Since the Passive House movement is all about reducing energy consumption, we need to consider the amount of energy that is put into the house’s production. According to EPS foam has 4 times the embodied energy of fiberglass and 30 times that of cellulose (this comparison is per insulating unit).
Also, there are environmental implications with the presence styrene and HBCD (a flame retardant) in EPS foam.

19 Jason Napier June 2, 2010 at 9:14 am

I have a concern with 2×4 on 24″ centers. IBC calls for 2×6 and I would think inspectors would stick to that. I am starting a double stud home project this year and current plan is a 2×6 load bearing wall and a 2×4 interior wall. If I could use double 2×4 wall that would help my overall R. Was this not a concern in PA or did you find a way around it? (I am addicted to your blog).

20 Chad Ludeman June 2, 2010 at 11:34 am

Jason – All I can tell you is that this was designed by both our architects and structural engineers. They take this stuff very seriously and both put their stamp on it. The Philadelphia building permit department also approved the plans as described.

We are building attached rowhomes with shared party walls, so that may make a bit of difference if you are comparing to fully detached. Also, I don’t think the IBC is used for residential buildings in Philly, so that could be it also…

21 Greg Clark October 11, 2010 at 10:46 pm

Is anybody using the natural flax fiber insulation that is popular in Europe?

22 Ben Fox December 16, 2010 at 1:30 pm

God morning,
Thanks for posting such a useful site.
Your joist hanger idea is brilliant- it eliminates so many problems. I don’t like having that floppy, rickety,(not really, I’m exaggerating), layer in between the nice rigid walls.
I have one idea you might consider:
In my(one!) house, I used LVLs for all headers. It is only 1 3/4 thick and extremely strong. It comes in really long lengths. I used it to span big sections of windows/doors with one header, and placed it against the top plate.
I placed it on the outside and notched the 2×6 studs to support the LVLs. (Note- they are not all exactly the same thickness…) Where I needed two studs at each end- for longer spans- I used a cut off 2×4 on the outside of the 2×6.
Although the material is more $ than 2x stock, it is all uniform and requires no sorting/crowning or gluing/nailing together.
If I had to do it over again, I would run that stuff all the way around just under the top plate- this would eliminate the need to align all those studs for the second floor. The slight extra cost would save a huge amount of planning and fiddling time.
Also, LVL will not shrink in width like all 2x stock will, causing settling/drywall problems.
Finally, it looks so tidy and strong when you are done.
Hopefully this wasn’t too confusing/useless!

23 Michael McAuley January 6, 2011 at 3:58 am

Great stuff! I’m coming in very, very late on this one but gotta say: I’m concerned about the 50th year of life and beyond in my projects which is why I use so much fiberglass in batts for walls – no sag, moisture survivability, what I buy has recycled content, and so on. If there is evidence that cellulose in wall cavities can perform as well as well installed fiberglass decade after decade then I’ll switch. I liberally go beyond R50 using cellulose in my attics but I’m still not convinced on anything loose holding up in my walls. Would love to be proven wrong! Thanks and great blog!! M

24 Chad Ludeman January 6, 2011 at 6:47 am

Interesting comment. We’re talking about dense packed cellulose in walls, not loose fill like an attic. If installed properly there will be no “sag.” Also, I think you’ll find that modern cellulose is able to handle moisture much better than fiberglass. It’s able to absorb and dry out, where fiberglass can’t. Lastly, I havent looked for case studies on 50+ year cellulose installs, but it might be worth a search. Let us know what you find.

25 Seth Hassinger January 17, 2011 at 2:33 pm

I’m curious about your choice to hang the joists. If you use an air barrier on the interior, how do you maintain continuity from floor one to floor two? The rim board usually provides an easy method of connecting the air barrier from top plate of lower wall to bottom plate of top wall.

26 Chad Ludeman January 17, 2011 at 5:15 pm

Our air barrier is on the outside. The ZIP system sheathing. I would argue though, that our top plate being connected directly to the subfloor, which is directly connected to the bottom place of the next floor is a better air barrier than a rim joist detail, but we don’t need it…

27 Robert Riversong January 18, 2011 at 3:37 pm

I question the conclusions of the CARB simulation comparing aligned with staggered studs: “The results are that staggering the studs on our 9? wall would result in an increase in R-value of less that 0.5.”

I also question the statement that “in this 9? thick wall that will result in a minimum of an R-38 wall”, even assuming the claimed R-4.2/inch.

While one-dimensional modelling isn’t quite as accurate, a UA analysis will show that your wall system, assuming a 16% framing factor, would result in a whole-wall R-value of 31.7. Staggering the studs by 12″ would yield a whole-wall R-value of 35.7.

And, a secondary but significant advantage of staggered studs (offset by the thickness of the wall) is that both exterior and interior framing are laid out from their respective corners, minimizing the amount of cutting and waste of sheathing and drywall (and making it easier to locate framing at later stages).

28 Robert Riversong January 18, 2011 at 3:42 pm

I will also agree with those who criticize both the real world effectiveness and the “greenness” of fiberglass (as well as other manufactured materials such as Zip Wall and TJIs), compared to simple sawn lumber (air dried preferred over kiln-dried) and cellulose (which is by every measure superior to fiberglass).

29 January 18, 2011 at 4:58 pm


You’re right that one-dimensional UA analysis isn’t as accurate. In this case it’s off by about a factor of 5.

The heat transfer is greatly diminished compared to an averaged value that a one dimensional model would predict.

It’s definitely a two dimensional problem: The main reason is that the stud is losing heat to the surrounding insulation as the heat travels along the stud, an effect that the one dimensional model ignores. That’s why the colors in the diagram show the stud isotherms very close to the insulation isotherms. A one dimensional model would predict the interior stud tip to be four times warmer, just based on the U value.

Kevin Dickson, PE, MSME (heat transfer and solar)

PS, Your anti-foam stance is definitely sinking in with us, see the comments at:

30 Robert Riversong January 18, 2011 at 5:36 pm


Sorry, but I don’t accept that the error is factor 5. And that isotherm illustration above is not a thermal image but a computer simulation.

31 Andy July 14, 2011 at 6:59 pm

Hi all,

I’m conducting a survey for a grad school research project on advanced framing. If you’d like to participate in my survey it’d be greatly appreciated. Thanks! – Andy

32 Drap February 29, 2012 at 1:52 am

I have been doing a little research on double wall framing for my future house I have never seen this question asked.

I understand 2×4 or 2×6 exterior wood wall with a gap then a 2×4 interior wood wall.

My Question is: Why have I not seen anyone use 2×4 metal studs on the interior wall with a 2×4 or 2×6 wood exterior wall?

Thermal bridging being the problem with the wood and the wood it self filling up valuable space insulation could fill.

Does this not seem like a good idea?

33 Mike August 13, 2012 at 12:15 pm

Hey Drap…..I, like many here, are fans of constant education. I wonder about ur question using steel studs. Thermally it seems interesting. From an embodied energy standpoint vs. Wood I think it goes backwards. I would love some smarty :-) science types to weigh in!

Mike M

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