Construction Design-Rain Screen

When I was building full time, one of the common requests homeowners often had for me was no maintenance siding.  Of course, there is no such thing as “no maintenance”, but there are some products that require less.  The siding materials that require the most maintenance are wood sidings that are painted or stained.  The best way to keep wood looking good and reduce how often a finish needs to be re-applied is by keeping it dry or to promote rapid drying if it should become wet.  A way to promote drying is to install a rain screen, which is what this weeks blog is all about.

A rain screen is simply a space behind siding that allows for any moisture that ends up between siding and the housewrap to freely drain.  A vented rain screen is a larger space, recommended to be at least 3/8 inch, behind the siding that allows for both water draining and air movement to promote faster drying.  This larger air space can also help outward drying of the wall assembly should the assembly become wet.  Minnesota’s use of polyethylene sheeting as a vapor/air control layer on the warm in winter side of the wall prevents inward drying of the wall assembly, outward drying is the only path for any moister that should find it’s way inside the wall.  Vinyl and steel lap style siding have a built-in air space because of the product’s profile.  These types of cladding systems do not require a rain screen.

Why have a space, doesn’t the housewrap protect the sheeting if it becomes damp?  Not always.  The right housewrap needs to be chosen and everything needs to be installed perfect, in fact some housewraps will allow water to pass through.  In my opinion, the best housewraps aren’t cheap.  Tyvek is the minimum standard for any of my projects.  The addition of a rain screen, or even better, a vented rain screen, will give a structure a better chance to last a very long time.

The photos below are a couple of products that are manufactured rainscreens.  Both are made by Benjamin Obdyke, an American company that manufactures drainage products for sidings and roofs.  The first product is called Slicker HP.  It is both a rain screen and housewrap.  The yellow mesh material helps to hold the siding slightly away from the housewrap.

The second product is also manufactured by Benjamin Obdyke, called HydroGap, this housewrap has small blue hard plastic bumps that also allow water to move down the housewrap.

Most of the major housewrap manufacturers make some sort of drainable housewrap, Tyvek makes DrainVent Rain screen which is similar to the Benjamin Obdyke Slicker HP.  Tyvek also used to make a product called Wrinkle Wrap, which was simply a wrinkled version of Tyvek’s HomeWrap.  The wrinkles allowed a space for water to move down the wall assembly.

I am helping with the design of a home that will be built this summer.  The home features exterior insulation, (2 inches of rigid rock wool) and a 3/4 inch vented rain screen.  (Simply a 1 x 3 board fastened through the rigid rock wool and into the wall framing.)  The siding will be a wood clad, either LP’s Smart Side or a cedar lap siding and is attached to the 1 x 3 board.  The photo below shows a mock-up I built as an example of how this system would be assembled.

My original plan was to use EPS insulation but changed to rigid rock wool.  The challenges with the vented rain screen are 1, keeping critters from living between the insulation and siding and 2, maintaining the water/vapor barrier around the windows and doors along with all mechanical penetrations through the building envelope.  I will discuss the water/air barrier issues in a future blog posting.

This is a product called Coravent.  It allows air to move through the corrugations in the product but keep bugs and other critters from entering the ventilation space.  Coravent will be place both at the bottom and top of the wall to allow air to move freely through the space.  Another assembly is to simply use a window screen material and cap the bottom and top of the space.

There you have it, some basic information on rain screens.  I will be adding more information about rain screens as the planned high efficiency house build progresses this summer.  Stay tuned.