I wanted to do a post and beam/sips house, but cost and convienence considerations are turning me towards conventional framing with spray in foam insulation. I took a class (SIPS) that was very interesting and informative (SIPS School Shepardstown WV) about SIPS, but one thing that I came away with from class was that they were more interested in the low air infiltration of a SIPS house than of the higher "R" value available with SIPS construction. Am I on the right track thinking this way? We'll be building in Maine, and the very knowledgeable instructor/builder at the school was mostly building in the Mid Atlantic reigon, and usually used a 4.5" wall panel and 6" roof panel. I understand that the spray in has a similar R factor to all urethane foam (s) @ about a 7 per inch. do you generally fill the whole wall cavity? (3 1/2 or 5 1/2) Thanks for any insight.

Your conventional lumber framing will introduce thermal bridging and thus reduce your R value. We did a house in Colorado with spray in. It was a nightmare. The studs continued to shrink and twist and it caused very small gaps to open up. Invariably the vapor barrier wasn't perfect so warm humid air entered and started condensing as the dew point was inside. This caused occasional drops of water to fall even from the ceiling! We had to remove the ceiling and scrape out the foam and put in fibreglass and sealed it up.

We're located in Alberta so it's no value for you to obtain our SIPs when there's plenty of SIPs companies in the New England area (I'm assuming!). Good luck.

I'm building an ICF house with conditioned crawl space and attic so the entire house envelope will be sealed tight. In doing the heat load calculation I included the minimum recommended ventilation rate. Even so, the heat load due to ventilation is about 20% of the total heat load. In a conventional framed house there are a lot of air leaks, unless you really work hard to seal every crack and opening, so the heat load due to air infiltration is significant. That's why both SIP and ICF crowds stress the value of their construction in reducing air leaks.

The thermal bridging of wood through the wall, as mentioned above, is a real factor also. It is much less in both SIP and ICF construction.

If you're pretty much stuck going with conventional framing consider doing a "double" wall, with 2 x 4 studs alternating to form a 6" wall. That way you minimize thermal bridging from inside to outside. It also will allow you to have a more continuous foam layer which I believe would go a long way toward mitigating the issues noted above with twisting studs, etc.

I considered using SIPs for my house but chose ICF. The primary reason was ICF allowed me to input a greater proportion of my labor into the total labor requirement. If you're contracting out most everything the higher material cost of SIPs, which includes factory labor, is offset by the lower on-site construction labor cost.

I've never heard of that problem with spray in foam. What kind of foam was it? Foam usually sticks like glue to lumber and has some flex so I would have thought it would stay adhered with the wood even with some movement. Have others on here experienced that problem? 

I understand that closed cell foams are considered a vapor barrier once they are 2" thick but I doubt that open cell forms are considered a vapor barrier at all for ceiling use. Perhaps the foam you used wasn't a vapor barrier at all and a vapor barrier should have been used with it?

Whether the problem was losing the vapor barrier due to separation of the foam from the ceiling joists or from use of open cell foam I would have thought it would make more sense to apply an additional vapor barrier to the bottom of the ceiling rather than ripping the ceiling down and removing the expensive well insulating foam. The fiberglass will certainly not seal better than the foam. That would be like throwing away a Cadillac and buying a new Focus if the tires went flat.

Other than in the labratory, what is the real "living" results, or "feel" from "thermal bridging"?  It seems that the main emphasis of SIPS and ICF construction is the air seal. If (with spray in foam) you stop air infiltration and are able to get a R21 or so you'd be doing pretty good, and hit a decent balance between construction cost and a well insulated structure.

Thermal bridging results when a stud, top or bottom plate, window header, or other framing member, has sheet rock attached on one side and exterior sheathing on the other. You have a heat conductance path directly from inside to outside, or vice versa. In a typical frame construction the area of such framing can be on the order of 20% of the total wall area, not insignificant! Wood has an R-value about 1 per inch so a 3.5" wall has only about R-4 insulating value in the areas of framing. Foam insulation won't break that thermal bridge because it doesn't separate the sheet rock or sheathing from the framing.

For a 2 x 6 wall with R-21 insulation between the studs, and with R-6 at the framing the average R value will be the inverse of (1/21 x .8) + (1/6 x .2) = 1/.0714 = 14. So 20% of the wall being framing reduces the R value by 1/3.

One thought on how to reduce thermal bridging and still use conventional framing is to do 2x6 walls on 24" centers-- fewer framing members thus less thermal bridging. Also, put a layer of foam board on top of your sheathing. These won't eliminate thermal bridging altogether, but they will help mitigate it considerably.