Help. I am building a 2500 sq/ft timberframe home in Maine. 90's and humid in the summer and it can be well below zero in the winter. I am looking hard at a hydronic floor for heat and 6 inch polyiso (R-6 to 7) sips for walls and roof. Is that thick enough?

Josh,

Roughly speaking, I would not go less than a 6-1/2" SIP walls (R-24), and a 8-1/4" roof (R-31) with the type of weather you describe, and you are not planning on using any solar or wind power. Going thicker would reduce the cost of heating and cooling, but raise the SIP costs. The next step up would be a 10-1/4" SIP roof (R-40), which would reduce heating and cooling costs even more, and not be that much more expensive. Ask your rep about the price difference of the thicker panels vs the lowered heating and cooling costs.

Steve
GrandCountySIPs.com

A good place to start is the Department of Energy insulation recomendations for your location: http://www1.eere.energy.gov/consumer/tips/insulation.html If you want to increase the insulation beyond DOE recommendations you may want to uses some energy rating program to estimate the additional utility savings versus the cost of additional insulation. From that you can decide if the extra cost is worthwhile in your circumstance. You will also find out there are many factors beyond R-value that effect your energy efficiency that may not even increase cost. A few of these other items are orientation of house, window size/locations, thermal bridges and leaks in building envelope. Two rating programs I've seen are from Passive House USA http://www.passivehouse.us/passiveHouse/DesignTools.html and RESNET/Energy Gauge Home rating system http://www.natresnet.org/ http://www.energygauge.com/usares/

The incremental cost for extra insulation in SIPs is one of the best deals around.
GO with at least R-40, R-60 is better.

My home in Maine is R-65 with home built SIPs and spray foam. The heating load is lower than the DHW load.
That is how it should be!