We are working on a LEED Platinum housing project and have been doing a tremendous amount of research on both SIP and ICF construction. For the uninitiated, SIP construction (Structural Insulated Panels) is a construction method where rigid insulation is sandwiched between sheets of OSB sheathing, creating a thermally broken solid wall form. ICF construction (Insulated Concrete Form) is a method where rigid insulation makes up the permanent forms for a poured concrete wall. Both systems are extremely airtight and both are systems that provide thermal breaks in the walls. Very important things for a highly sustainable design.
Our findings so far:
In order to get the full points for LEED credit, we need to either have a mass wall (ICF) with an actual (not performance) R-value of at least R-14. This is easily done with just about any thickness of ICF wall because the foam insulation is where the value is really calculated. You get actual R-17 to R-22 depending on the ICF block – “equivalent” r-values are touted in the 40’s and 50’s, but that includes the thermal mass equivalency and is really an apples-to-bananas comparison anyway, so don’t believe everything you hear.
For SIPs, we wouldn’t consider that a mass wall, so the actual R-value needs to be a minimum of R-21. Again, easy to do in a 6-1/2” SIP panel (which would be R-42 – way over everything else by comparison). If we have poured concrete floors inside the building, then we have plenty of thermal mass inside the home (where we really want it), and not separated by a layer of insulation (which is one of the complaints with ICF).
Both systems are thermally broken systems as far as LEED is concerned. Both systems will be very high performance for airtightness (and will require an HRV). It appears that only SIPs will allow us additional LEED points for pre-built panel assemblies since ICFs are site-built assemblies that are then poured on site. We’re still researching that with LEED though, so I will validate that and amend this post when we have more data on that.
Space is also a fairly large consideration as well. LEED calculates the area from the outside face of the rough assembly, so the additional thickness required by ICFs will add approximately 120-150 sq.ft. of wall thickness for a 2,000 sq.ft. footprint. I know that sounds crazy, but there is quite a bit of square footage tied up in our exterior walls that isn’t useable, and the thicker the walls, the more that hurts our ability to keep the space functional and stay within the LEED guidelines that won’t require us to add additional points to our requirements total. So, to maximize LEED points, it’s not enough for a wall to perform thermally, it also needs to do it with minimum wall thickness – now we really are talking the 21st century modern home here!
Chances are, any sustainably built low-rise Type V building project will have both ICF and SIP components, it really is a question of “how much ICF and how much SIP”. Right now, unless there is some major economic advantage in the initial cost for using ICFs, the information I have is telling me to build the foundation with ICF and use ICF for walkout basement conditions if site topography warrants it. Then run the SIPs from the main level floor up.
This will also allow you to handle common details more easily. For example, you can set the foundation wall to the inside face of the sip so that it can act as a ledge for stone or brick veneer while preserving the thermal break. It’s also worth noting that angles other than 90 degrees at the building corners are easily done with SIPs and won’t pose any problems with designs that are customized. In fact, just about any shape is possible with SIPs – even curved forms. You can do those angles with ICF as well, but it isn’t a lot of fun, so you want to minimize those kinds of design features wherever possible.
It’s also worth noting that there are a lot of good reasons to plan for SIP walls running through the building, separating key areas as they provide great thermal separation and sound attenuation in the same space as a conventional stud wall.
We actually started this project thinking that we would need to go to ICFs in order to maximize our LEED point potential, however, it turns out that SIPs actually have a slight advantage when it comes to LEED because of their reduced thickness. For all intents and purposes, they will perform equally in a LEED thermal analysis though, so unless you can build an ICF wall for less than a SIP wall, the SIP wall would be the best choice.
If you are looking to have a highly sustainable building designed, this information is only one very small part of the comprehensive analysis for the great number and variety of design decisions, and all of these choices must be tailored to your specific situation, location, and site adaptation. With that said, we strongly urge you to engage the services of a design professional that has the knowledge and experience so that the financial investment in your building is validated and actually performs. In fact, in order to get LEED certification, you are required to assemble a design team that has those qualifications. Contact EVstudio if you have any questions or need to discuss your next sustainable project.
For more SIP info read my post Top 10 Important Things to Know About SIPs