Necessity for rainscreen approach w/ 5700 HDD & Hardiepanel
I’m curious about rainscreens, which seem to be reccomended by building science experts, but not reccomended by manufacturers. A case in point: James Hardie Inc does not reccomend a rainscreen behind its Hardiepanel product, but there seems to be some kind of loose consensus that you will have water and mold problems unless you use a rainscreen behind a product like Hardiepanel.
I have a project with well insulated cellulose walls (12 to 14″) , Exterior OSB that is sealed at the seams , a Water/air Barrier, and Hardiepanel. It’s located in Northern West Virginia in a location with approximately 5700HDD (65deg base). Can someone provide me with clear logic for the need to use a rainscreen product? thanks for your help.
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Replies
Michael,
As you know, your siding will get wet. A rainscreen will allow the siding to dry much faster. This will help the paint job last longer. It will also keep the OSB dryer; trust me, you don't want damp OSB.
If you choose to install your fiber-cement siding without a rainscreen, it will probably last a long time. But the materials (and the paint job) will last even longer with a rainscreen. And your wall will be less likely to have any moisture problems.
MIchael,
The value of a rainscreen depends on many factors, including site exposure, annual rainfall, roof overhang width, the time duration of high relative humidity conditions and the amount of solar radiant exposure.
With "reservoir" claddings, like brick, it's essential to have a drainage space. With wood sidings, its helpful to allow drying and prevent high vapor pressure differentials. With fiber-cement siding its far less functional.
It's always advisable to follow manufacturer's recommendations, particularly if you want them to honor their warranty. Hardie Panel siding is thin (5/16") and would undoubtedly benefit (perhaps require) a solid backing. If you want it to act as a shear panel, then it must have sufficient backing and be fastened according to specs.
Cellulose is a highly hygroscopic insulation that loves to be able to breathe. With a relatively non-breatheable exterior (sealed OSB), you'd better not have any interior vapor barrier or you'll be creating a vapor trap with a large sponge in the walls. If code requires, you can use a 1 perm vapor retarder primer on the drywall and that will allow some minimal drying to the interior.
I don't think Hardie Panel needs the rainscreen to breathe, but Martin is right that OSB is highly susceptible to moisture and may not be very compatible with a low perm panel siding like Hardie.
Martin and Robert, I really appreciate your quick and nuanced answers. I want to make sure you are familiar with Hardiepanel...it is a monolithic 4x8 sheet of 5/16" Hardie fiber cement that gets installed in whole sheets in this case. In your opinion, would using hardie board..(lap siding) make a significant difference in avoiding moisture problems? we will be using modular construction , and the cost benefit of this panel approach is that it can be installed in plant, saving money and time.
Martin and Robert, I also wanted to add that we have 35" of anual rain .
Here is a link to James Hardie's Tech Bulletin on installing their products over rainscreen: http://www.jameshardie.com/pdf/USTB_Attaching-James-Hardie-Products-Over-Rain-Screen.pdf
General tech bulletins: http://www.jameshardie.com/dealer/helpSupport_supplements.shtml
Michael,
Lap siding is always more breatheable than panel siding, until it's covered with a half dozen coats of paint. Its resistance to exterior moisture, however, is entirely dependent on the quality of installation. Hardie now advises against caulked butt joints and requires flashed joints on their factory painted lap siding. That is the preferred method for installation of any lap siding.
My home exterior finish is currently those "Mexican Brick" (some claim it is the Chicago Bricks) - many are very soft and crumble very easily. Matter of fact, squirrels love to claw it away to smitherines, and I hate the old look to home (contrary to other people's tastes). I hope to cover the entire exterior of my home with 4 x 8 sheets of the long-life Hardie Fiber Cement - screwed into the brick mortar work. Would this work or fail?? When installing or painting, what are the pitfalls to watch out for? I run automatic sprinkler systems daily, so what moisture issue should I worry about?? We have lots of phenomenal brains on this medium so I'll standby for advice.
Perhaps that someday not so far away you may not have any left with which to water your lawn, let alone drink and grow food with.