In design and planning process for a small ( less than 2000 sq ft) timber frame home to be built using SIPs. I live in far north Minnesota, and around here, similar new houses have had 6 inch SIPs installed for the walls and 8 inch for the roof. Does it make any sense to go to 8 inch panels for the walls and 10 inch for the roof? Is it cost-effective or overkill?  Our timber framer, who has built many houses using the SIP technology, seems to think that   6 (in the walls) and 8 (in the roof) are adequate. Approx. how much more (%) would the thicker panels add?   Any thoughts greatly appreciated!

bstahly;

6" is good for walls, but you local code will govern minimum R- Value for roof

When I bought my SIPs the additional cost for 8" vs 6" was not that significant, I think less than 10% of the raw price of the SIPs (there's also a little higher cost in dimensional lumber). I went for the thicker SIPs. I'm not sure that, logically, this is the right choice, but I feel good about it and adding more R to a wall after it's built is not very easy.

If you want to be analytical about it Gary Reysa's heat loss calculator seems to do a fairly decent job of estimating heat loss and energy saving (It approximately matches both our HERS estimate and a Manual J HVAC calc):
http://www.builditsolar.com/References/Calculators/HeatLoss/HeatLoss.htm

You would get more gain from going with polyurethane foam VS styrene than by bumping the thickness. Plus the panels are better structurally from all the data I've seen as the urethane bonds itself extremely well while styrene is glued (usually with urethane!;-).

Just my $0.02.

if 6" has the span capabilities and the minimum R-value requirement for your area then it is good enough. You will need to determine your needs to select the properly sized panels.

While we have PU panels available as well, we don't promote them because of price, poor span capabilities and many other inherent short comings.

We stick with EPS that has superior span capabilities and a better track record, we also have a mineral wool (non-bearing) 1 hour rated panel

As said earlier, going to thicker panels is more expensive, but not significantly so. Compare the prices of heating oil, natural gas and propane 10 years ago with the prices today, add in the increasing demand and dwindling supply, what will the price for heating and cooling your house be 10 years from now? I think it may be smart to look ahead and go for the higher R-value, particularly if you plan on adding solar heat to your home. The higher R-value walls and roof will make a solar system much less expensive.

Steve
GrandCountySIPs.com

We build a lot of SIP structures in Northern MN and as trigem1 stated, the thicker panels don't cost SIGNIFICANTLY more. And as Chris stated, the SIPs utilizing EPS foam have far greater spanning capabilities, which could allow you to cut down on required, structural timbers (if budget is tight). I wouldn't go with anything less than a 10" (we do a lot of 12" SIP roofs) in N. MN do to your snow load requirements.

Good luck,

Joe Quinn
Regional Sales Manager
Enercept Building Systems
(800) 658-3303

Thanks Joe.  Our timber framer I believe has done business with your company previously, as your comany is one of two SIP manufacturers listed on their web site. 
Thanks for the information!