ecoRetro

Truly, I am not trying to reinvent the wheel here. I have followed Joe L's recommendations from the Insulating Sheathings research report at http://www.buildingscience.com/document ... archreport On page 12 he describes the assembly I will be using: "Wall Section 1 - Insulating Sheathing and Housewrap over Plywood or OSB: The first strategy involves installing the insulating sheathing over top of a layer of building paper or housewrap and wood sheathing. This is the most durable assembly proposed in this guide as the drainage plane material (building paper or house wrap) is supported by the plywood sheathing, and protected against wind loading and other environmental factors by the insulating sheathing. This type of assembly would be recommended in areas or high exposure and rainfall." Since I am taping all the seams on the polyiso panels, I think of the foam as the first line of defense from exterior water. Moisture coming from the interior of the house is a whole other worry and so I have run dew point calculations. Given the total R-value of the wall and the typical winter temperatures, the wall cavity temperature should remain well above the dew point. Rocon, I'm not a building scientist, but I'm wondering if the problem you mentioned had something to do with the higher perm value of XPS (~1.1) vs. foil-faced polyiso (.03)? Did the building end up with soggy exterior insulation? Do you know if the moisture was coming from inside or outside? Kurt, you have mentioned how poorly the open joint rainscreens wear over time. Could you say more about what were the likely causes of the failures? I have encountered some successful Hardie rainscreens (even in the rainy PNW) and I'm curious what distinguishes them from the crappy ones that you have seen. Thanks everyone for the continued great discussion!

Thanks for all your comments so far. You raise some important issues that I have been researching extensively. My wall assembly is based on a design developed by the folks at Building Science for retrofitting older homes: http://www.buildingscience.com/document ... e-retrofit My wall will have 2 layers of 2" polyiso with staggered seams and everything taped. So this will serve as both an air and moisture barrier (there will be no vapor barrier on the inside of the wall). Taping the polyiso joints to create a WRB seems to be an accepted practice. But just in case the tape should fail, there is a layer of Vortec housewrap behind the polyiso layers that protects the underlying plywood sheathing and 2x4 framing. I have spoken extensively with the technical team at RMax and they indicated that the foil faced product is UV-resistant and didn't see any issue with the open joint cladding. The polyiso foam itself discolors when exposed to UV but doesn't decay. But since I will be taping all edges and seams, the foam won't actually be exposed. I completely agree that I need to get this right the first time. Kurt and others, what issues have you seen with using Hardie in an open joint rainscreen? I know that Tyvek disintegrates when exposed too long, so I'm not using it. I plan to prime/paint all six sides of the Hardie to minimize moisture absorption. What other "gotcha's" do I need to prepare for? Thanks for your input!

P.S. I am insulating the exterior of the building with foil faced polyiso rigid insulation which will have all seams sealed with foil tape. So this will serve as my drainage plane behind the exterior cladding. (So I won't be using a vapor barrier.)

Thanks so much, Brandon, for getting this conversation started (I'm the client who called with the rainscreen question). I found a great blog posting with lots of construction details and pictures of real-world projects at: - blog.buildllc.com/2008/03/rainscreens/ My challenge is that I don't have the giant budget to afford the specialized rainscreen cladding products like Hardie Artisan Matrix panels or Swiss Pearl. However, I have run across some renegades who have used plain old HardiePlank/Panel in an open joint approach and I really like how it looks: - livemodern.com/Members/purekrista/blog/w05blog (scroll down) - aurhaus.blogspot.com/ The homeowners have reported no problems so far with their installations, but I know that looks aren't everything and I am a "belt & suspenders" type! So, I thought it would be good to get the perspective of professional home inspectors. Have you seen this type of open joint rainscreen install using Hardie cement panels and if so, what kind of problems should I be trying to avoid? Thanks so much for your help!