Posted By adi43d on 03 Mar 2010 03:34 PM
if somehow you'll get moisture between SIP and the extra layer of foam will be very difficult for the SIP to dry. I wouldn't add rigid insulation to a SIP wall (interior or exterior) because the installation and detailing becomes critical. If you really want to add consider some sort of spray foam. so water vapor doesnt represent an issue.

good luck
Adi

Hello SimonD.  Since I am a pilgrim in all this, it is good to hear from you about the cantilevering issue.  I have done quite a bit of research on SIPs and believe that I do understand the load bearing "I-beam" type abilities of SIPs.  My concern about it is with regard to the possibility of using a 4 1/2" poly SIP for the walls.  If I choose to use 2" of XPS on the concrete perimeter, then the 4 1/2" SIP may have to be cantilevered 2" of the 4 1/2 ", or some 44%.  Now, while I am not an engineer, I have a few fundamental questions about this.  For one, would the 44% cantilevering be suitable in the area where I live with the highest high winds in the US?  We are on the eastern side of the Sacramento mountains in south central New Mexico.  The only blemish on our living in paradise out here is that it is nothing for the winds to blow in excess of 70 mph.  Not for just a few minutes but 3 to 5 days without abatement.  We had a period a couple of years ago where the winds blew in excess of 100 mph for over 24 hours!  This is a formidable design issue.  2x6's 24" OC smart framing is totally out of the question, fyi.  Hence, my bottom line is a concern about the structural integrity of such cantilevering in extreme situations.  Otherwise I could use different types of SIPs and/or a different type of foundation.   I sure hope my worries are not justified.  However, I am trying to do my due diligence.  Thanks for taking your time to share your thoughts with me.

Hi Greg.  Yes, we are in Alto, about 1.5 miles from the Spencer Theater on NM 220.  Right now, we are just doing preliminary plans for a build some 2 years out.  Hence, for now, we have not chosen much of anything.  Two of our neighbors out here used Thermocore SIPs on Clydesdale timberframe builds.   It is because of their experience that I am pondering the poly SIPs.  Not 100% sure yet, but, you get the picture.  I have a contractor here who built about half of our current timberframe house working under a GC and then later went out on his own as a GC.  He worked on one of the other houses out here with the SIPs.  So he has experience putting up the SIPs.  Plus he is someone who you can trust.  Not to mention one heckuva a good carpenter. 

We have two things that are free here:  world class sunshine and wood.  If I build a reasonably insulated and tight house, then we could use passive solar for 40 to 50% of the heating, plus a 55000 btu iron+soapstone wood stove (like the Fireview Wood Stove, www.woodstove.com.)  Backup would be baseboard heaters with smart thermostats.  Would put in an HRV.  You mentioned the crawlspace idea.  Yes, we are thinking about that and it depends on just where we build.  If we build on a slope then the crawlspace option moves right into the picture. We have that right now.  On the other hand, we already own 10 acres near here where the building envelope choices are all pretty much flat --- i.e., ideal for either monolithic slab or slab on grade.  As you might already know, a large number of house builds here in Lincoln county are stemwall-crawlspace because of slopes.

As far as PassivHaus is concerned, as much as I would like to go all the way down that road, the math and costs just do not work for me.  But I do intend to borrow what I can from the PH concept for parts of our house plan.  Things like reduction of thermal bridging are good ideas, PH or otherwise.  I suppose that is why I characterize our plan as a "PH Wannabe." 

Posted By SimonD on 08 Mar 2010 01:51 AM
Altovintner,

Your reasoning is sound if you are sticking to a 4 1/2" SIP for your wall. You would need at least a 6 1/2" SIP to do the cantilever. So your choice becomes, do I increase the foam core thickness of my SIP by 2" and do the cantilever, or do I add 2" of foam to the exterior of the SIP and eliminate the cantilever. I guess either way you are needing 2" of foam thickness. Now you have to figure out which is the better choice for your circumstances. If your climate is dry, I wouldn't worry too much about moisture issues, so applying additional foam to the face of your SIPs should be ok. If you think your walls will be subjected to the forces of high winds and need the reinforcement, then additional bearing of the sill plate of the SIP wall on the edge of the slab makes sense. As mentioned above, in a climate with more moisture applying foam to the face of your SIPs could be problematic. The type of slab doesn't factor into the decision. If you went with the cantilever configuration then the top & bottom plates of your SIP wall would be more of a thermal bridge and not be as insulated as applying the additional foam, so that may also influence your choice. You are on the right track.