Moving from SIPs to Double Stud Walls in Skinny

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.

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:

• Toolbase Advanced Framing Techniques (check out PDF also)
• Dept of Energy – Advanced Wall Framing (PDF)
• CARB News – Double Stud Walls: Staggered or In-line? (PDF)

Related posts:

1. YouTube Video Shows Us How To Install SIPs
2. Insulation Levels in the Passive Project
3. SIPs Have Been Chosen as the Building Method of Choice