I am in the planning stages of re-siding my house and would like to add rigid insulation to the exterior without reducing its water vapor permeability profile (it's currently quite breathable). I have read a number of places that polyiso sheets with a single side of fiber facing are quite permeable, but I'm having trouble finding such a product. I've found a number of double faced iso sheets that are rated at about 1.5 perms. Are there any known products that can do better than that? Polyiso by itself can do upwards of 6 perms irregardless of thickness, so it's the facing that is the limiting factor. I live in Madison, WI USA. If you know of anything that is better than 1.5 perms, please let me know the product/manufacturer. Thanks!

Can't get there with standard grades of iso, but you can with EPS.

Mind you, 1.5 perms isn't much lower permeance than a couple coats of latex paint, and provides very reasonable drying rates for your climate. If the siding has a ventilate gap between the foam and the siding the drying rate is even better than merely, "reasonable". I've never seen iso that didn't have at least some sort of facer on both sides. Most grades of roofing iso are rated under 1 perm, but will test out well over 0.5 perms which would also provide a decent seasonal drying rate for most wall stackups.

Worry less about making it vapor open, worry more about making it AIR TIGHT, since far more moisture accumulation issues in walls are a result of excessive air leakage than from vapor tightness/openness.

And worry more about getting the flashing around windows and doors perfect than the air tightness, since far more moisture problems in walls are a result of bulk water intrusions than from air-leakage or vapor diffusion. The most vapor-open wall in the world won't save you from rot if you have chronic bulk water penetration.

Thank you Dana. I've been going back and forth between EPS and ISO, and I still can't decide. For anyone living in the Midwest who may be interested, this product is available locally: http://menards.com/main/building-materials/insulation/panels/1-5-felt-face-poly-iso-decking/p-1485631-c-5779.htm I'm starting to price products and will start another post with my planned layup in the next day or so for a little peer review. This forum has been very helpful (and you in particular Dana)--I've been pouring over past posts for a couple weeks and have learned so much.

You guys are a little over my head but I do know to find the fiber faced Iso, check with commercial roofers. they use them all the time.