Rain screen
deerefan
| Posted in Green Building Techniques on
I am building a ranch house in zone 2, our annual rainfall is 31″ or so. The house will have 2 types of 2×6 wall coverings: hardie board and limestone. I plan on using OSB or plywood, housewrap and 1″ polyiso. My questions are:
1. What width of gap do you suggest behind the stone, 1 or 2″. Do I still need a rainscreen on top of the polyiso? What type of rainscreen?
2. Do you suggest a rainscreen behind the hardieboard and if so what type?
Thank you.
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Replies
deerefan,
I assume you are using full-bed stone on this project, or is it a thin adhere'd stone? In either case a rainscreen drainage mat would be appropriate. On full depth stone it is challenging to maintain a predictable air gap behind the stone. You will need weeps as well on the base flashing as well. A rainscreen gap behind Hardieboard is best practice as well, since the Hardieboard can swell if it stays well.
The image is from MTIdry - they offer drainage mats and weeps for natural stone applications like yours. https://www.mtidry.com/stone
Caveat: I am NOT a professional or in the industry, but your details are almost exactly what I did on my house during a rebuild a couple years ago. I'm in zone 3, near Irvine, California.
I opted for a smaller airgap on the rain screen and used Dupont's Rainvent battens behind my Hardieboard, and Keene's Driwall rainscreen behind the limestone fascia. http://www.keenebuilding.com/products/building-envelope/walls-and-siding/driwall-rainscreen-10mm
The layers for the limestone if I recall correctly were sheathing, housewrap, 1" foil-faced polyiso, Driwall, lath, scratchcoat, stone.
By the way, one of the "detail" problems I ran into on this was how to cap the limestone, which we ran about halfway up the walls on the front of the house. (It's a one story house, so about 4' of limestone 'skirt' is present.)
The solution we found was to use similarly colored limestone tiles - 24" long - and had those cut at a 20 degree angle, perhaps 3" wide. With those laid up on top of the limestone facing, it looks much more substantial than you'd expect from a cut stone tile.
Edited to add: if you look to the left of the first image, you'll see the Rainvent battens installed. They're the horizontal strips.
Mike,
Why did you choose the rainscreen options that you did?
Deerefan,
Sorry for my slow response. Being in Southern California and working on this project mostly in 2014/2015, there weren't a lot of contractors familiar with most of these "green" construction techniques I wanted in the house.
My project was an owner/builder job, and in searching for someone to install the rigid foam I fell into a bit of luck and located a gentleman who has a strong background in the industry, Joel Pereda of Enso2.
He ended up helping me in several efforts on the project, one of which was reviewing the building products available in the market. (He called in an industry specialist who had access to many products.)
When I saw the Rainvent battens I thought they looked like a logical product for use behind the Hardieboard. Being plastic they won't be affected by moisture and since they're constructed much like cardboard, they have channels running through them so it's easy to place them horizontally without blocking the downward flow of water.
I believe he directed me to the Keene product too, though I don't recall exactly. Before finding that, I was on the verge of canceling the stone installation after reading about failures resulting from the lack of a rainscreen behind the stone.
Those two products together seemed to work well, given the gap they create, so I made my choice to proceed with them.
Is there a reason you want such a large air gap for your rainscreen? It seems a bit like overkill. (That said, I'm generally a fan of overkill.)
Mike - Was your project adhered stone, or full stone that sits on a ledge? Judging by the photos (with the stone "floating" above the foundation) I assume it is adhered. In this instance a drainage mat is the only way to get a gap behind the stone. Furring strips like the ones you used or Corrugated Lath Strip from MTI are a good option behind siding for a predictable air gap and ventilation.
Deere - if you are going the thin (adhered) stone route then you would expect to see a weep screed at the base of the wall instead of weeps. https://www.mtidry.com/thin-stone If it's the anchored full stone your designs may indicate a 1-2" air gap but the reality of stone is that that gap gets sacraficed to keep the face of the wall in plane and shore up stones with back parging. These issues are reduced if it's dimensional stone that behaves more like brick. In either case a compression resistant rainscreen drainage mat is a good way to design for durability
Correct - adhered (thin - about 1" thick) limestone. Drainage mat was installed beforehand, and a weep screed was detailed at the base. The batts or strips were used behind the Hardieboard.
Tyler,
Correct, I will be using 4" thickness stone, placed on a ledge, as opposed to a veneer. Do you have a diagram where there is an actual air gap behind the stone rather than the cavity being filled with material? How about detailing of the bottom and top drainage areas?
Deerefan,
Here are two images for your reference. There's an article on Masonry Cavities authored by MTI in this month's issue of the Construction Specifier - https://www.constructionspecifier.com/masonry-cavities/
Especially with full stone, the air gap behind the stone really needs to be included in the designs to facilitate the construction of the wall. When it comes to dealing with moisture a rainscreen drainage mat like Sure Cavity is essential.
I couldn't post both attachments in the same post - some error about me posting comments too quickly. Does the system think I'm a bot?
Tyler,
I've received that message a couple of times. I assumed it was Martin telling me to go outside and do some work.