I like rigid foam so much I've decided to put another 1/2" of Dow Super Tuff-R on the outside OSB on my SIPs, but what is the general consensus in regards to the house wrap and foam order? Should I Tyvek the OSB first, then put the rigid foam on top of that, or should I do it the other way around? Also, foil side out or in?

Probly no tyvek needed, read the specs from DOW

Thanks, I had noticed that their site did say that, however the Tyvek came with my "kit" and wanted to know where people thought it would work the best.

Just a thought...

 

Hae you considered not using OSB and instead using Magnum Board (MgO)? With Magnum Board, you wouldn't have to use Tyvek.

 

Something to think about...

I would be pretty nervous about sandwiching osb between two layers of styrofoam. If you ever got moisture in there..... I would put the tyvek on the osb and then the tuff-r. But if the styrofoam is installed with all seams sealed then there is no need for tyvek. But then the tyvek is made to breathe but would not be able to breath under the styrofoam. it would be best to consult with your inspector.

FWIW Inspector here approved a permit to put the "vapor barrier" on the inside of the foam on a remodel - winter heating and no real summer cooling here. I tend to think that the barrier is mostly useless, since the 2" of foam on the outside of it had better be a barrier. I had heard that Tyvek does not hold up well under foam, but that could be a myth. I'm using paper anyway. The original paper on the walls is still in great shape after 50 years - or was until we ripped it off - what more could you ask for.

I do wonder about 1/2 foam over SIPs. In my case, the foam is a retrofit over wood framing prior to a second SIP storey.

If the SIPs don't exist yet, you might just go for a fatter SIP, or a PU SIP of the same width.

If the SIPs are in place already, I doubt that the 1/2 is going to get you much of an ROI and for the cost of the foam, you might want to invest in better windows, more insulation elsewhere, air sealing, more energy efficient appliances, solar, or a host of other areas that should provide a better energy and financial return.

I've got 6 1/2" R-Control SIPs already up. Rolls of Tyvek came with my house kit, but I'd like to add a little more insulation. ROI doesn't concern me as much. I'd go with thicker rigid board if it wouldn't diminish my soffit/overhang area. Windows are Thermotech Fiberglass windows so I'm set on those. Insulation is a mix of closed cell foam and denim so I'm set there as well. I'd just like to make sure that I don't regret once the Nichiha goes up that I didn't do some rigid board. Looks like I'll do Tyvek then rigid foam. Thanks folks!