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The Vented Rain Screen via Furring Master

by Chad Ludeman on May 14, 2010 · 24 comments

in envelope,facade,materials,Skinny Project

It occurred to me that we haven’t really written a good post dedicated to vented rain screen assemblies. A rain screen is basically a gap created in between the sheathing of a house and the exterior cladding. It is considered by most building experts to be the most durable method of building an exterior wall and almost all commercial cladding details implement some type of a rainscreen. We have decided to use a vented rain screen on all of our Postgreen Homes and will talk about what we have learned to date in more detail below.

Skinny Project - Furring Slice

Benefits of a Vented Rain Screen

There are a couple of key benefits to using a vented rain screen assembly on any building that improve the performance and durability over the life of that building. Let’s put them in list form.

  1. Equalized pressure means less air infiltration inside your building. Since there is an air gap between the exterior sheathing and cladding, the pressure is equalized with the exterior atmosphere. I’m no scientist, but basically this means that air is not going to be fighting hard to enter your building when it hits the cladding because the rain screen has eliminated the large pressure difference that usually exists between the exterior and interior of the home and various layers in between. This helps when you are trying to build homes as air tight as we are in order to achieve maximum energy efficiency.
  2. Water the makes it past the cladding will drain and dry out, keeping the guts of your walls dry. This is where the durability claims come from. Water will eventually make it through any and all cladding assemblies on a building. The vented rain screen makes sure that when water does get through it is able to drain down the face of the Water Resistive Barrier and out the bottom or simply dry out on its own due to the air gap behind the cladding. This not only helps to keep moisture from getting into your walls assembly (which is bad), but it prolongs the life of whatever cladding is used by allowing it to dry out when wet from both sides.
  3. An extra layer of air keeps your building cooler. In most homes, the sun hits your cladding which is in direct contact with the sheathing. The heat from the sun is transferred directly from the cladding into your walls. This makes your building warmer than necessary. The air gap created by a rain screen keeps the face of your sheathing much cooler. This is more important in the South where cooling loads dominate heating loads, but it’s still a nice feature in the north.

How to Build a Vented Rain Screen

There are many other people on the interwebs that I have linked to below that have described the how-to’s of rain screen building, so I’ll keep this brief. Here are the basic elements of any vented rain screen assembly:

  1. Furring strips or a Drainage mat to create the air gap in between your sheathing/WRB and your exterior cladding. We list options for this material below, including our new favorite, The Furring Master!
  2. An insect barrier at the bottom of the rain screen assembly gap with proper mechanical flashing. You want to keep the bugs from nesting in your rain screen from the bottom and water from draining into your basement.
  3. Furring Insect Barrier Detail

  4. A path for air to flow at the top of your cladding assembly. If you seal the top of your cladding to your roof cap or top trim piece, the rain screen is compromised by insufficient air flow from top to bottom.
  5. Top detail of vented rain screen

  6. A bit of extra care and planning at all window and door openings in terms of proper flashing. A good designer or flashing expert can help out here, but there are many details online for this as well that are simple to follow if planned for.
  7. Vented rain screen window details

Material options for implementing Vented Rain Screens

There are many simple material options for creating the vital gap needed to build a vented rain screen system in residential buildings. There are much fancier systems implemented in commercial construction that we are no going to get into here. Some of the choices and their pros and cons are listed below:

  • Pressure treated lumber – This option is one of the most popular as it is a familiar material, readily available and not too expensive. Rips of pressure treated plywood can be used or 1x material. This is what we used on the Passive Project. The main drawbacks are that the wood is not consistently flat and straight for precise cladding applications and water can be trapped between the wooden strips and the WRB with no room for venting where all strips are located.
  • Mesh ventilation product – There are many mesh products available that are thin and easily applied in roll form over your WRB with a slap stapler. These products work great and eliminate the problem of trapped moisture completely that occurs with wooden furring strips. The main cons are the price and the uneven compression that can result in wavy cladding when fasteners are attached with different pressures. This is the product we used on the 100K House.
  • Metal furring strips (The Furring Master) – Metal furring strips are our current favorite option due to a number of factors. The price is much better than the mesh products and not that higher than wood, depending on what type of wood strip you are using. They are perfectly straight and rigid which lends itself to the best finished installation of siding. One of our favorite aspects of it is that it has a hollow channel behind it that allows water and air to flow behind the surface, unlike wood. Lastly, it’s made of galvanized 22 gauge steel that is recyclable and can be left exposed in certain joints if desired without concern for UV degradation, decay or rusting.
  • Skinny Project - Furring Close Up

    Above is a picture of our installed Furring Master strips. We have studs at 24″ on center on the Skinny Project, but we installed the furring strips at 12″ centers. The strips in the middle that are not connected to studs are basically just maintaining the spacing from the wall and keeping the somewhat fragile panels rigid and secure.

  • Miscellaneous other products – Although less common, we’ve seen others use creative products like corrugated plastic, rigid insulation and fiber cement strips.

Better links to Vented Rain Screen Instructions

Best post on Rainscreens ever by the BUILDblog – Fantastic diagrams and pictures from guys who really know what they are doing.

Green Home Building’s take on Rain Screens (PDF) – Good info, details and lots of hose wrap bashing. What more could you ask for?

Rain Control in Buildings by Building Science Corp – All sorts of good info about how rain and wind are trying to destroy your building and how to prevent them from succeeding.

If you enjoyed reading this post I can promise you'll love our new writing over at Postgreen Homes. Yeah, we know that's the same thing your favorite band said and their new album is nowhere near as good as their early stuff, but seriously, we are actually still getting better.

There also isn't much conversation to be had here . . . at least not with us. So come on over to the Postgreen Homes Blog and tell us what you think of our new(ish) digs and crazy ideas. We will be sure to tell you what we think of your opinion.

{ 24 comments… read them below or add one }

1 jen May 14, 2010 at 5:37 pm

This article is so incredibly helpful. Thanks very much. I’m now looking into the furring master for our SIPs home.

2 GreenbuildinginDenver.com May 15, 2010 at 12:46 pm

Looks like you guys have “nailed it” again. I have just one last little misgiving about this wall system. The green surface of the sheathing is supposed to be the air and water barrier. Yet you’ve just punctured it thousands of times with the FM and siding fasteners.

I’m guessing the next project will use SIS, which probably handles those penetrations a little better.

3 Chad Ludeman May 15, 2010 at 2:58 pm

Actually, the green material on the ZIP System is self-sealing. We tested nail and screw penetrations during our pre-insulation blower door test and they were not leaking air. The specs and instructions on ZIP’s website back this up as well.

4 Greg Albracht May 15, 2010 at 6:27 pm

This would be true GreenbuilderinDenver.com if you install the cladding direct to the wall without an air gap. Read the new Post Green skinny project post as well as the HUD report, and many other reports that have been available for at least 4 years. An air gap will drain the moisture out quickly and more important the wall will dry out fast. With a rain screen system you can achieve the 4 D’s
Deflection, Drainage, Drying and Durability.

The links below will give you a good education.

Oregon goes to Rain Screen code 2010
http://www.sidingmaster.com/documents/neworegonrainscreencode.pdf

NAHB DEC. 2008
Improving Drainage and Drying Features in Certain Conditions:
Rain Screen Designs for Absorptive Claddings

HUD Report 2006
http://www.sidingmaster.com/documents/moisturehomes.pdf

Canada Report
http://www.sidingmaster.com/documents/NewReportaboutsidingtrappingmoisture.pdf

5 lavardera May 15, 2010 at 10:35 pm

one suggestion – seal/flash the top of your base flashing with the ZIP tape.

6 GreenbuildinginDenverdotcom May 17, 2010 at 11:08 am

OK, now I’m all warm and fuzzy about this system. I would follow up with a caveat to someone trying to save money by using, say, 15lb. felt or Tyvek instead of a self-healing air/water barrier. All those penetrations will leak air and water. That will mess up your HERS score, and even cause some sheathing rot down the road.

7 archaalto May 26, 2010 at 2:12 pm

it is a little disconcerting when looking at an enlarged view of the furring master strips where it shows the metal being “crushed” into the sheathing behind wherever you have a fastener. maybe it’s not a big water infiltration issue with the zip system, but should there be a concern about loss of integrity & usefulness at that point on the furring strip? it doesn’t seem too desirable for trying to install siding as flat & true as possible wherever it crosses those nail spots either.

8 Greg Albracht May 27, 2010 at 4:39 pm

The fiber cement boards lay flay on top of the Furring Master. The contractor that installed this job loves my product. The metal does not “Crush” into the sheathing as you state it does. What photos are you looking at? It only slightly indents the face of the Furring Master. The 1/4 inch hemmed legs keep the siding straight. The legs do not “CRUSH” at you stated. The indentations that you see do not effect airflow or drainage, in fact the indentation in the strips prevent pooling of water at the fasteners so it is an actual benefit that the metal indents.

9 Chad Ludeman May 27, 2010 at 5:01 pm

archaalto – There are slight indentations made when nailing some of the strips. The stips are still very straight and did not dig into the sheathing. The install was much more precise and level than either of our previous projects where we used wood and mesh products.

10 archaalto June 2, 2010 at 3:59 pm

maybe the angle of the shot or the light in the photo made it look worse than it actually was [image titled "furring close up"]. I definitely believe that any metal furring strips would perform better than wood or mesh, so it is very encouraging to hear everyone’s satisfaction with the product.
thanks for the replies.

11 Matt June 3, 2010 at 3:08 pm

The metal strips could act as a Faraday cage. It will be interesting to see how the wireless reception is within the house. On the one hand, it could reduce mobile telephone and 3G/4G Internet access performance. On the other, it could reduce WiFi interference from neighbors.

12 Greg Albracht June 3, 2010 at 4:53 pm

I am curious where you are getting this information from and if you have proof that a metal furring strip causes what you are claiming. Metal furring strips are used all the time in commercial construction with fiber cement panels and have been for years. I have never heard of this before. If it was a issue I would think that I would have heard about this because I have been in the exterior cladding industry for 30 plus years. I never say never, but can you please direct me to some links that shows this happening behind fiber cement siding?

13 Matt June 4, 2010 at 2:35 pm

Greg, I said “could.” It depends on the RF frequency, the spacing of the strips, how they’re interconnected, and whether they’re grounded. The reason I pointed this out is that this amount of continuous metal conductor is not normally used in _residential_ construction. I’ll take some dBm measurements after I move in and let you know how much attenuation I see.

14 Greg Albracht June 4, 2010 at 8:46 pm

Please keep me informed. I appreciate it.

15 joseph June 5, 2010 at 8:53 am

Matt is correct,
in theory -
http://www.jeddaniels.com/2007/faraday-cage-part-1/ or
http://online.wsj.com/article/SB126221116097210861.html ; I too lived in a 40s Craftsman house with plaster lath and this was our experience too – TV and radio were fine.
Greg you may not have heard about this over the last 30 years however faraday cages have been around for quite longer than that; my wife doesn’t know about low VOC paint or rain screens either and she had been in new commercial property since the 80′s.
You probably need greater level of mesh instead of just vertical runs and grounding of the metal as the first link talks about.
I wonder if this could be a marketable if you ran fine mesh screen between studs and sheathing ?

16 Greg Albracht June 7, 2010 at 9:11 am

Hello Matt, I talked to a good friend of mine with regards to a Faraday cage: Here is his opinion,

Hey Greg,

Could the strips act as a Faraday cage? It’s possible if they’re all electrically connected.

Other cages currently on houses? What about Aluminum siding? And around here, lots of folks have added Stucco to their houses. That’s done by covering the entire house with chicken wire, which seems like it could be a pretty good Faraday cage too. But I’ve never heard any discussion about it.

I would assume windows, doors and roof area are all large enough gaps that the cage and associated blockage would not be noticeable. And the spacing between strips, even if they were electrically connected, is quite large, isn’t it? Your cell phone works inside your car, even though the body and the roof create a (very poor) Faraday cage.

Creating a truly effective Faraday cage (which is what we have to do with our products at work) is not always easy. Small gaps can have large leakage.

What Matt is saying is true, but pay attention to his 2nd statement. He is very careful to say “could” and “It depends on…”. And that’s all true. The only way to know for sure is to test and see (which is also what Matt is saying).

Don’t worry, it’s extremely unlikely any Faraday cage created would be effective enough to block anything. Let him measure and see what he finds. Likely nothing.

Richard

17 Chuck Weiss June 7, 2010 at 1:21 pm

Well…..I’m sending this message from inside the Skinny Project! If you can read this it means i’m getting good reception on my 3g network (also no problems with the cell phone)!
I should introduce myself… i’m Chuck Weiss and I’m the Project Manager on the Skinny Project. I have nothing but good things to say about the Furring Master. Easy install, allows for plenty of air to move behind our siding panels, provides a even surface to apply siding, reasonable cost, little waste,1/4 inch thickness requires no build out on the windows, and it has a cool name!

18 GreenbuildinginDenver.com June 7, 2010 at 1:39 pm

And lest you think the waves are getting out through the ceiling, I have a steel shingle roof, double chicken wire stucco walls, and all my radio and cell reception is fine.

Greg, it seems to me that in some applications the FM would be easier/better to use with the flat side against the sheathing. Do you allow that?

19 Greg Albracht June 7, 2010 at 4:52 pm

Hello, yes in certain situations you could apply the strips in this manner such as over foam insulation, block wall etc. with certain cladding materials.
Props to Chuck Weiss. He is a guy that really cares about his work and that is hard to find in this day. Post Green as a whole is a class act and a one of a kind company. I wish these guys the best of luck because they are on to something here.

20 BN Consulting September 27, 2010 at 1:40 pm

I do have a few questions myself, and unfortunately from the devils advocate side. What is the primary difference between FM and using small traditional Z-girts or hat channels? What if one were to add insulation to the exterior of the building such as XPS or mineral wool? How does lap siding, or even board siding, fit flat on the FM if one uses a screw to attach the FM to the structure? It seems that the screw head would cause discontinuities on the smooth surface of the FM. Since the FM has a solid face, you must first drill a hole in the FM before one could use a wood screw to attach it to the wood structure, correct (and yes, the same would be for any other metal girts). How does one plumb-up the FM, by use of normal shims?

Thanks….

21 Greg February 25, 2011 at 6:05 am

Hello BN Consulting, I would like to address your questions.
1.You can shoot a nail to attach the furringmaster strips so the screw head that you are describing is not a factor with the how the siding will lay down against the FM. Use shims to plumb. The difference between FM and a z-girt is the FM only builds out the wall 1/4″ which an advantage because it can save money on building out opening’s. It also has a patent pending hollow back design so no moisture is trapped behind the FM unlike a wood furring strip. All types of insulation can be used with rain screen application. I like the mineral wool because of the fire proofing. I hope this answers your questions.

Thanks

Greg

22 Jason September 4, 2011 at 9:04 am

It seems to me that b/c of the small amount of spacing (which I understand is to prevent building out the windows and doors) the siding fasteners will create additional penetrations as well. Penetrations are the enemy of a rain screen. Wraps that close around fasteners have a level of imperfection and are designed for limited penetrations. Also, 1/4″ creates a very small area for ventilation and pressure balance and a some wraps (improperly installed) can push up against the siding in the back bays with will push moisture against the back of the siding (usually unprotected from moisture… i.e. clapboard) causing rot. Even in cases where it doesn’t push against it (it’s impossible to keep a wrap perfectly flat, unless it’s rolled on), it closes in on it making it smaller than the 1/4″. Moisture can build up and still infect the back of the siding. In my opinion, the z girts and build out are essential in a rain screen.

23 Greg Albracht April 2, 2012 at 9:48 am

Jason, I understand your concern and you are correct that fasteners are truly the point of moisture penetration, but you are not taking into consideration the fact that our Furring Master creates an air gap in-between the cladding and the substrate. Moisture that gets Held up if the house warp is improperly installed will drain and dry fast enough to never cause a problem. Look up Martin Holiday on some of these issues. He’s the expert. Any moisture that gets behind the cladding will drain down ward because of the rain screen gap. The moisture that runs down the shaft of the fasteners will drip down before it typically hits the fastener where it is attached to the substrate, so there is no concern with what you described above with our Furring Master. Plus when the fastener penetrates the Furring Master is creates a little indentation that creates a small gap in-between behind the cladding and the face of the Furring Master which allows MOST of the moisture to drain out at that point. We have NEVER ONCE had an ISSUE that you are claiming can happen with our 1/4″ depth. Its because of what I described above. Make Sense?

24 Greg Albracht April 2, 2012 at 9:53 am

Plus Jason, we just launched a 3/4″ depth Furring Strip that you can install vertically or horizontal and it gets air flow both ways. This is the ONLY Steel strip strip on the market that does this. Our Furring Master is also recommended by James Hardie because it performs better then Z-girt or Hat channels.

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