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Open joint rain screen questions.


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I had a client call and ask me about open joint rain screens. http://www.vaproshield.com/media/rainsc ... erview.pdf

She is planning on using this system on a project she is working on. Her main concern is that insects can enter, and after looking at the design, I have the same concern. What is done at these 3/8" to 1/2" joints to prevent pest entry?

The horizontal "joints" are beveled so that not much water can pour in, but I still don't see why there needs to be large open joints. Are the open joints to allow for expansion and contraction with large panels, for increased air movement, or both?

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Rainscreen systems get expanded a little with this new product. Not clear what type of fasteners to use with it, how well the cladding holds on the long term, on even what types of cladding are suitable for it. Maybe the cladding should be included into a combined system. I'd be wary of it for now. Maybe keep the project small.

Marc

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Why? There's skyscrapers that use similar systems.

Just design it correctly. And yes, there'll be some wasp nests behind the panels. Hose them out, or kill them with poison.

Tyvek makes a material specifically for this application. It's flat black, and essentially disappears behind the panels.

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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!

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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.)

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Well, the insulation panel is a vapor barrier. You should have another layer of material over the foam, something that's dark, and that's intended for use in this manner.

Don't go building something like an open joint panel rain screen system and cut budget corners that shouldn't be cut. You'll end up with a complete and total mess.

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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.)

So you are relying on the taped joints as your WRB? How well will it perform as an air barrier, and how long will the tape hold? Have you got specific approval from the insulation manufacturer for your application, and for the tape you intend to use? Manufacturers all have (800) numbers with tech people on hand, and I would strongly suggest you talk to them and get their blessing for every detail. Building details have already been developed and tested, it should not be necessary for you to invent anything nor is it wise.

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Hi Lesley,

It's nice to see you on board.

Someone on this forum has the saying that unconventional installations behave in unconventional ways. Just because others have done the installation you speak of with no known problems, does not mean there aren't or won't be any.

For what it's worth, I agree with Kurt and David.

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I've seen enough open joint panel rain screen systems that are improvised out of Hardie panels and Tyvek to know what you end up with.

A total freakshow mess.

Don't mess with this stuff. They're really cool, they work, but you better get it right the first time.

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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!

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"Taping the polyiso joints to create a WRB seems to be an accepted practice".

"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".

Did you receive those statements from a published document from the manufacturer? If not, why even consider trying something that hasn't been proven in the field.

Sorry, but I stick with building methods that have been tested for a minimum of a century.

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Hopefully I'm not disagreeing with Joe Lstiburek here but I don't see putting the WRB behind the insulation. I would probably put it on top of the insulation, cap-staple it to help prevent billowing, and arranging my battens so that it was never going to see a single ray of UV. If you put the foam over the WRB, and the tape joints fail, you will eventually have water working its way into the layers and sitting there on the WRB... which defeats the entire purpose of rain screen walls. And, with the WRB on top you're not relying on the tape nearly as much.

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Anyone here ever seen what it looks like when you leave foil faced polyiso out in the weather for a few years?

I don't honestly care what the mfg's. rep's say in this matter, or Mr. Joe either (oh Lord, did I say that....?)......

WRB on the outside, over the foam, and it should be the flat black stuff. Yes, it costs money. That's what good design means; money, well spent.

Fasteners, standoffs, edging, all of it. I've seen a number of on-the-cheap "developer" designed"open joint rainscreens, and they all look like crap in a couple years.

I love how they look when they're done right.

Done wrong, I'd just as soon see well installed vinyl siding as an improv'ed panel system.

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ECO RETRO,

After reading your comments above I have new concerns. These concerns I do not believe can be answered on this forum. The assembly you mention, in my opinion has several possibilities for problems to develop. In my opinion you must perform a WUFI analysis on the wall assembly to see how the wall will perform in your climate. As you have described your modified assembly, I think you have potential for condensation in the wall. The wall system you have described is very similar to walls that I know to have condensation forming internally. The only way to determine if my suspicion is correct is to model the wall. I would be happy to give you some referrals for companies that can perform the modeling for you. I think it would be a cheap insurance policy for you.

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Agree that modeling the proposed assembly, and/or simply using an existing, proven design would be good.... but if this is in the Portland OR area, and the 4" of foam is roughly R28, are concerns about interior humidity condensing in the wall warranted? Seems like this is plenty to move the dewpoint well into the insulation, and that interior ventilation should keep things under control. Or, are you suggesting something else?

Also agree that this is outside the purview of this forum... but many of you guys are great on topics like this.

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Agree that modeling the proposed assembly, and/or simply using an existing, proven design would be good.... but if this is in the Portland OR area, and the 4" of foam is roughly R28, are concerns about interior humidity condensing in the wall warranted? Seems like this is plenty to move the dewpoint well into the insulation, and that interior ventilation should keep things under control. Or, are you suggesting something else?

Also agree that this is outside the purview of this forum... but many of you guys are great on topics like this.

The particular assembly that the proposed wall reminds me of had exactly that. The dew point occurred between layers of XPS foam, the WRB outboard of the foam did nothing to help drainage. The differences that are pertinent are that the wall in failure had a Titanium panelized Rain screen, and the location is colder than what I believe Oregon might see. That being said I am a firm believer that when you vary from the norm as much as the proposed assembly seems to be the more likely it is to have unexpected problems.

Like someone once said to me “we never have time to do it right but we always seem to make time to do it again when it failsâ€

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If you're talking about flashing / cladding / WRB problems that led exterior moisture into the wall, then yeah, I can imagine a disaster. No matter what your wall detail is you have to keep the rain out. If you're talking about interior humidity finding its way into the foam sandwich then I'm surprised but I guess it's possible.

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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!

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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

1- 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.

2- 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?

"The hardie panels in your case would be the first line of defense. A rain screen wall is not designed to allow water beyond the outer layer. It is designed to manage the water that enters the system. With the closure strips on thie proposed system very little water should ever get in, when we discussed bugs earlier. I believe in the system of foil faced foam as far as this. I do not have a track record of how well the tape at the seams will last. The tape by nature is a reverse lap at horizontal joints. If there is any failure of the tape either due to application or product failure you will have fish eyes at the edges. The fish eyes are conduits for water intrusion. I bet everyone here has seen what happens to peel and stick that has fish eyes. I to am a belt and suspenders guy, last week I sent out a design recomendation, and for no other reason than what if it is inadequate. I put in an additional point for water to get out of a system. So what I would do is over the foil faced foam and tape I would apply a waterproof barrier, probably spray applied with reinforcment over the joints and avoid the tape alltogether."

I do not consider myself a building scientist either hence my recomendation for a WUFI by someone qualified to perform the analysis. It just seems close enough to the assembly that I thought I would mention it. The building I mentioned is a very high profile failure that required over 70M in repairs. The condensation was primarily caused by interior conditions, combined with the wall assembly.

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