Building with Structural Insulated Panels ... or SIPs

We are building the CHouse Modern using Structurally Insulated Panels, or SIPs as they are commonly referenced. SIPs are a high performance building system where the interior structural portion of the wall is assembled like an ice cream sandwich using a 7/16″ layer of oriented strand board (OSB) as the outside of the sandwich, and in our case, a 5 1/2″ interior cavity filled with styrofoam, which would be the ice cream. Sounds good doesn’t it?

There are several advantages to building with a system like this, including:

Excellent Thermal Performance – minimal thermal bridges with the elimination of traditional wood 2×6 framing at 16″ o.c.
Integral Insulation – solid core is expanded polystyrene (EPS) which has an R value of 3.8/inch
Low Waste (Factory Built) – panels are pre-engineered and produced in a controlled environment
Fast Construction Method – erection of the panels is estimated to be as much as 3 times faster than traditional framing
Air-Tightness – fewer joints and OSB sheathing on two sides allow for a much tighter building assembly

Outside air leaking into the home, or air infiltration, is responsible for 40 percent of heat or cooling loss in the average home. Almost any article you read regarding energy-efficient construction, how tight the house is to air leakage is a huge consideration in the final thermal performance. To give you an idea, in a comparative test conducted by Oak Ridge National Laboratories (a multi-program science and technology national laboratory managed for the United States Department of Energy), researchers built two identical 2,600 sq. ft. homes, one made of SIPs and one with conventional wood framing and fiberglass insulation. The SIP research home was five times more airtight than the wood-frame room when measured by a blower door test.

I am also going to confess that I routinely call them “SIP panels” … which is a bit like saying “ATM machine”. I know that the “P” in “SIP” stands for panels so this is like saying structurally insulated panels panels. Just letting you know in advance so you don’t feel the need to correct me next time you and I are talking SIPs over a beer.

Every panel on the project was represented in a series of three-dimensional views that allowed us to quickly scan the project for errors. The image above is just one of several images we received, you can see how the house comes together using several individual panels. [click image to enlarge]

This is section through one area of the house that shows vertical dimensions and is a key map that references other enlarged detailed drawings. I should point out that this isn’t one of our drawings that was simply turned into the shop drawings, MM&I generated this drawing. Maybe some fellow architects who might be reading can appreciate why this is a big deal – too often vendors will take our drawings and simply add notes to them and return them to us as shop drawings … it doesn’t really work that way. [click image to enlarge]

This is an elevation that contains a lot of information – it identifies wall chases (for electrical conduits), hold-down straps, dimension control, headers, overall panel dimensions and rough openings for windows and doors. You can also see that each individual panel has its own letter and number designation. When the SIPs erection crew gets on site, they spread all the panels all around the site and stage them for quick access and assembly. [click image to enlarge]

Did you think I was kidding when I said they spread them “all around”? Despite the fact that this house isn’t ridiculously large (under 4,000 square feet total) there were SIPs literally laying in stacks on almost every available piece of ground.

In the picture above, you can see the 7/16″ OSB laminated on to the styrofoam (expanded polystyrene or EPS). The overhang you see is to allow the panels to nest together and to sit properly on the sill plate. What’s a sill plate? Go to the next picture…

This is a picture of a sill plate – a double sill plate actually – sitting on our post-tensioned concrete slab. These are treated 2x’s that are anchor-bolted down to the slab. The reason there are two is because you don’t want to have your OSB sitting directly on the concrete slab. Water and OSB don’t really like one-another and the first sill plate holds the SIPs off the ground by 1 1/2″ … the second, slightly smaller 2x sits into the channel of the styrofoam we saw in the previous photo. You can also see that the joints between the two sill plates receives caulking to help create a tighter seal (it’s also super sticky).

Once all the sill plates are in place, it’s a fairly simple matter of maneuvering the right SIP into position. While it might take several architects to move these panels into place (see the second to last bullet point), normally just two regular contractors can move them around just fine. [click image to enlarge]

Here’s a finished look at the SIP sitting on the double sill plate … makes sense now doesn’t it?

For the really big panels, a forklift was used to maneuver the SIP into position. You can have individual SIPs as large at 8′ x 24′ although we don’t have any that large on this job … and I can’t fathom how many architects it would take to lift a panel that big [shudders] We’d have to have a convention or something.

At the corners, the SIPs are nailed together using these giant screws – which really pulls the building together and makes it extremely tight. [click image to enlarge]

In the shop drawings above, I mentioned that each panel is labeled and coordinated back against the shop drawings. On site, the method of identifying the panels is definitely old school – a fat magic marker with the panel tag written on the exterior face. This helps the installer not only know which panels go where, it also helps coordinate which side of the panel faces out and which orientation is up and down. And since I have eagle-eye readers, before you start asking where panel E-3 is, it’s the panel above the window and out of frame here.

You know how you can tell these guys are contractors? Because they’re lifting a giant panel and not taking pictures of two guys lifting a giant panel.

One of the things that has made this process run so smoothly has been the vendor we worked with to coordinate the design of the house using SIPs – MM&I Construction & Design, Inc. This was the first project we’ve done that used the SIP system and they were terrific answering questions and providing their expertise to the process.

Since SIPs are pre-built in the factory, you can safely assume that getting everything figured out on paper ahead of time is crucial. What really helps this process along is a quality set of shop drawings, which allows us to makes sure that our design intent is being met, the dimension controls are in place, and that the entire scope of the project is correctly represented. The shop drawings we received from MM&I were very thorough and we were able to coordinate some items while the project was still on paper rather than trying to figure it out once we were in the field.

Since I’ll probably get a question about comparing the insulation values of SIPs, I’ll tell you that they perform better even though the R values are only marginally better. For the wall in this project, a traditionally detailed and sized framed wall with fiberglass batt insulation would have an R value of 23.28, whereas the SIPs came in with an R value of 27.48, an improvement of 4.2. That having been said, there is a lot of thermal bridging that happens with the batt insulation is interrupted with a 2×6 wood stud and as a result of this bridging, the thermal performance of the traditionally framed wall could drop as much as 30%. This means the difference between our SIPs wall and the fiberglass insulated traditionally framed wall could be as high as 9.3 (R value of 27.48 vs. R value of 18.15).

Have I blown your head up with math yet? Hopefully not, we’re almost done.

Finally, I should add that SIPs are more expensive as a product over traditional framing – HOWEVER – the cost becomes more comparable when you take into consideration that the house comes together so much more quickly. With SIPs, you get expensive material cost with lower labor costs, and with traditional framing it’s lower material costs with higher labor costs. Comes pretty close to washing out in the end.

So why aren’t more people choosing SIPs when building their new house? I’m not sure there’s an easy answer anymore. But that’s a different discussion for a different article.

Cheers,