What’s better: 4 inch SIPs with 2 inch XPS or 6 1/2 inch SIPs only?
![](https://secure.gravatar.com/avatar/70aed0d36984880e32e39f8bb92f3c46?s=52&d=https%3A%2F%2Fdwbcgjjbfqbp3.cloudfront.net%2Fapp%2Favatars%2F200x200%2Fa.png&r=g)
This question is just about efficiency, not about cost. The house will be Thermocore SIPS, but we are trying to decide on 4″ SIPS with 2″ XPS on outside, or 6-1/2″ SIPS with no foam.
4″ SIPS = R-24 + R-10 for 2″ XPS, total R-34.
6-1/2″ SIPS = R-40.
once I figure the savings on the SIPS, but add the materials & labor for the foam, plus a couple of other items, the cost difference is almost a wash.
The R-value with the 6-1/2″ is better, but using the 4″ with XPS foam will eliminate the small bit of thermal bridging that will occur at the top & bottom plates, wall intersections, etc.
What would be better, the higher R-value, or less thermal bridging?
Thanks!
Allen Brown
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
![](http://m.etiketa4.com/app/themes/greenbuildingadvisor/dist/img/modules/detail-library/thumb1.png)
![](http://m.etiketa4.com/app/themes/greenbuildingadvisor/dist/img/modules/detail-library/thumb2.png)
Replies
Allen,
Adding rigid foam to the exterior side of SIPs is somewhat controversial. I have defended the practice in the past, but with the increasing availability of mineral wool insulation, I now think that mineral wool is a safer bet than rigid foam for the exterior side of SIPs.
To read a variety of opinions on this topic, you may want to seeHow to Make a SIP Roof Better.
To read about air sealing SIPs, seeHow to Protect Structural Insulated Panels from Decay.