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Musings of an Energy Nerd

All About Rainscreens

To lower the risk of wall rot, it often makes sense to provide a ventilated air gap between your siding and your sheathing

Creating an air space with vertical furring strips. One of the most common ways to establish a gap between siding and sheathing is with vertical furring strips.
Image Credit: Image #1: Joel Schuman
View Gallery 7 images
Creating an air space with vertical furring strips. One of the most common ways to establish a gap between siding and sheathing is with vertical furring strips.
Image Credit: Image #1: Joel Schuman
Don’t let your siding touch the housewrap. One way to create an air space between the back of the siding and the wall sheathing is to install siding on vertical furring strips.
Image Credit: Images #2, 3, and 4: Fine Homebuilding
This illustration shows how the top of a rainscreen gap can be vented. The approach shown here is used by builders who don't want the rainscreen gap to vent into the soffit. This illustration shows Mike Guertin's method of venting the top of rainscreen gaps on a gable wall. Home Slicker is a three-dimensional plastic mesh that can be used to create a 1/4-inch-deep rainscreen gap between a home's siding and housewrap.
Image Credit: Image #5: Benjamin Obdyke
To make it easier to attach corner trim, the furring strips need to be extra wide on exterior corners.
Image Credit: Image #6: Joel Schuman
Perforated J-channel can be used at the base of a wall to provide ventilation for a rainscreen gap.

UPDATED on June 24, 2018 with new product information

Twenty years ago, very few residential builders knew what a rainscreen was. These days, however, it’s no longer unusual to see siding being installed on vertical furring strips or a plastic drainage mat. As rainscreens become more common, mainstream builders are beginning to ask, “What’s a rainscreen? How do I know if I need one?”

This article will pull together information to answer the most common questions about rainscreen gaps between siding and sheathing.

What’s a rainscreen?

You can’t really point to a rainscreen, because it isn’t a thing — it’s a system. That’s why I prefer to talk about a “rainscreen installation,” a “rainscreen approach,” or a “rainscreen gap.”

For most residential builders, a rainscreen siding installation is one that includes an air gap between the siding and the water-resistive barrier (the asphalt felt or housewrap).

Some purists insist that you can’t call it a rainscreen gap unless you have ventilation gaps at the top of the wall as well as the bottom of the wall. If it only has drainage gaps at the bottom, but no openings at the top, the purists insist that the system is more accurately called a “cavity wall.”

However, the distinction between a cavity wall and a rainscreen wall isn’t particularly useful — in part because the distinction is based on a faulty understanding of “pressure equalization” principles — so I side with those who use the word “rainscreen” for either type of wall.

Every well-designed rainscreen wall needs:

  • a water-resistive barrier (WRB);
  • an air gap between the WRB and the back of the siding;
  • flashings at all penetrations and vulnerable areas; and (usually)
  • weep holes at the bottom of the wall.

Ventilation openings at the top of the wall are optional.

Why would you want a rainscreen?

Rainscreen gaps help walls manage moisture. A rainscreen gap helps to dry the sheathing, which may accumulate moisture during cold weather. It also helps to dry the siding when it is soaked by rain.

These benefits are due to four functions of a rainscreen gap:

  • The gap provides a capillary break between the back of the siding and the WRB. This limits wicking.
  • The gap allows moisture held in the siding and sheathing to be redistributed to adjacent materials, by evaporation and diffusion, thereby limiting damage due to water that concentrates at leak points.
  • The gap provides a path for liquid water to drain down to the weep holes at the bottom of the wall.
  • If there are ventilation openings at the top of the gap, the rainscreen provides a path for moving ventilation air. This ventilation air rises due to the stack effect, which is strongest when sun shines on the wall. Research shows that this type of ventilation is a powerful drying mechanism.

Does every building need a rainscreen gap?

How do you decide whether your house needs a rainscreen gap? Ultimately, the decision about whether to include a rainscreen gap is a judgment call. Among the factors to consider:

  • In some areas of North America — notably coastal British Columbia — rainscreen installations are required by the local building code.
  • A rainscreen installation is more important in wet climates than in dry climates. Some experts advise rainscreen installations for all houses in areas with an average annual rainfall of 60 inches or more. In areas with lower rainfall amounts — in the range of 20 to 60 inches per year — a rainscreen may not be mandatory, but it is still a good idea.
  • A tall wall protected by a stingy roof overhang is more vulnerable to wind-driven rain — and therefore more likely to need a rainscreen — than a short wall protected by a wide roof overhang. If the wall faces a wide porch, it probably doesn’t need a rainscreen.
  • Some experts (including Joseph Lstiburek) note that a wall sheathed with OSB, which is more vulnerable to rot than plywood or board sheathing, should almost always include a rainscreen gap. Lstiburek has written, “We learned through trial and error (mostly error) that if you use OSB, really good cavity insulation and a housewrap, make sure you have an air gap between the cladding and housewrap/OSB interface.” If your walls are made of SIPs, or if your OSB wall sheathing has closed-cell spray foam on the interior, it’s absolutely essential to include a rainscreen — because the OSB on these walls can’t dry to the interior.
  • Brick veneer installations always require a rainscreen gap.
  • Because it already has an air space behind it, vinyl siding does not require a rainscreen installation unless it is installed on top of rigid foam that is more than 2 inches thick, in which case it should be installed over vertical furring strips.
  • If your walls are sheathed with rigid foam, most types of siding — especially wood sidings — require vertical furring strips. If you’re installing vinyl siding over thin (2 inches or less) rigid foam, you can probably skip the furring strips. (For more information on installing vinyl siding over thick foam sheathing, see Can Vinyl Siding be Applied Over Furring Strips?)
  • Rainscreen gaps are always beneficial. Rainscreen walls are more robust than ordinary walls — because a few minor leaks in a rainscreen wall are less likely to lead to a major rot than the same leaks in a wall without a rainscreen. The only drawbacks to rainscreen walls are the hassle of installing the extra details and the added cost associated with these details.

How big a gap do I need?

A rainscreen gap doesn’t have to be very big. Researchers have learned that even a 1/16 inch gap provides a capillary break, allows drainage of liquid water, and permits “diffusion redistribution.” However, you’ll probably find that (unless you are using a wrinkled housewrap) job-site realities and variations in material thicknesses usually require a rainscreen gap to be at least 1/4 inch deep.

Having read this information, many builders use 1/4-inch wooden lath or rips of 1/4 inch plywood for their furring strips. Other builders use 1/4-inch-thick plastic drainage mats. These 1/4-inch gaps work well.

However, some builders prefer more leeway for installation errors. They may be worried, for example, about puckers in the WRB or bulges in the sheathing that may compromise the 1/4-inch gap. They’re more comfortable with a deeper gap — perhaps 3/8 inch, or 1/2 inch, or 3/4 inch — because it allows for a few minor on-site problems or installation glitches. A 3/8 inch gap is more than enough for drainage, and is also enough to provide useful ventilation drying.

Logic dictates that homes in wet climates benefit from deeper rainscreen gaps — up to about 3/4 inch — because deeper gaps allow faster ventilation drying.

When choosing the depth of your rainscreen gap, there are issues other than water management that come into play. Builders know that 1/4-inch or 3/8-inch rainscreen gaps simplify exterior trim details. It’s much easier to trim out (and flash) a wall with 1/4-inch furring strips than a wall with 3/4-inch furring strips.

Finally, if your furring strips are being installed over thick rigid foam, you don’t really have a choice on furring strip thickness. You’ll need 1x3s or 1x4s that are 3/4-inch thick in order to have something to attach the siding to.

How do I create the gap?

There are three ways to create a rainscreen gap:

  • You can use furring strips. Furring strips are usually installed vertically, directly over the studs (16 inches on center or 24 inches on center).
  • You can use a three-dimensional plastic mesh product.
  • You can use bumpy or wrinkled housewrap.

What can I use for furring strips?

Furring strips don’t have to be pressure-treated, because a rainscreen is designed to stay dry.

For thin furring strips, you can use 1/4-inch lath board or rips of 1/4-inch plywood. If you want a deeper gap, just use thicker plywood.

Although some builders use 1x3s for furring strips, others are frustrated by the fact that 1x3s tend to split easily. If you want a 3/4-inch-deep gap, and are worried about splitting, use 1x4s.

Plastic furring strips

One option for creating a shallow rainscreen gap is to use rips of 3/16-inch-thick Coroplast (a corrugated plastic sold at sign shops). Because this product is corrugated, it allows air movement through the furring strips.

Another option is to use 1/4-inch-thick fanfold insulation (thin foam that is often installed under vinyl siding), cut into 2-inch wide strips on a table saw.

At least seven manufacturers sell plastic furring strips:

  • BattensPlus manufactures BattenUp furring strips. These pieces of polypropylene strapping measure 1 1/2 inch wide by 1/2 inch thick by 48 inches long.
  • Drain-Plane sells polyethylene furring strips.
  • DuPont makes a polypropylene batten called the RainVent batten.
  • El Dorado makes plastic battens that measure 1 5/8 inch wide by 3/8 inch deep by 8 feet long.
  • VaproShield makes a plastic batten called VaproBatten.
  • Cor-a-Vent makes plastic furring strips called Sturdi-Strips.
  • DCI Products makes plastic furring strips called CedarVent strips.

Coroplast battens, BattenUp battens, DuPont RainVent battens, El Dorado battens, Sturdi-Strip battens, and CedarVent strips all have channels that allow water to drain right through the products, even when installed horizontally. VaproBattens aren’t designed for horizontal installation, and won’t drain that way.

Note that none of these plastic battens are good at holding nails and screws, so you’ll need long fasteners that reach to the studs if you go this route.

Three-dimensional plastic mesh and plastic mat products

It’s possible to create a rainscreen gap with a mesh product that look like the plastic wool pads used for scrubbing pots and pans. These three-dimensional mats are especially useful for siding types that can’t be installed over vertical furring strips — for example, cedar shingles or board-and-batten siding.

At least seven manufacturers make three-dimensional plastic mesh or plastic mat products designed to create a rainscreen gap:

  • Benjamin Obdyke Home Slicker mesh is 1/4 inch thick.
  • Stuc-O-Flex WaterWay Rainscreen drainage mat can be ordered in several thicknesses, ranging from “nominal” 1/4 inch to “nominal” 3/4 inch.
  • MTI Sure Cavity creates a gap of about 3/16 inch.
  • Colbond EnkaMat is about 3/8 inch thick.
  • DC 14 Drainage Mat is about 1/4 inch thick. Because it is made of polystyrene and has a low permeance (1 perm), it depends on ventilation drying through air channels rather than diffusion drying.
  • Cosella-Dörken Delta-Dry is about 1/2-inch thick. Delta-Dry is a membrane made of 22-mil high-density polyethylene that has a three-dimensional egg-carton configuration. Like DC 14 Drainage Mat, Delta-Dry is a vapor barrier. The product depends on ventilation drying through air channels rather than diffusion drying.
  • Ventgrid is about 1/2-inch thick. It is a rigid plastic panel that measures 4 feet by 8 feet.

Three-dimensional plastic mesh and mat products are fairly expensive; most cost between 60¢ and 70¢ per square foot.

What about bumpy housewrap?

There are at least eight brands of bumpy housewrap or wrinkled housewrap:

The jury is still out on whether these products provide a significant benefit. In some applications — notably when installed between OSB and rigid foam on walls that have spray foam insulation on the interior side of the OSB — they can lower the risk of an otherwise risky wall assembly. However, these products have very tiny air gaps — StuccoWrap’s wrinkles are less than 20 thousandths of an inch high. Skeptics wonder whether such tiny gaps have much effect.

Moreover, some builders report that many of these products compress easily under certain types of siding, eliminating the wrinkles that provide drainage. According to building scientist John Straube, however, DuPont StuccoWrap works well; Straube says that it’s impossible to fasten siding tight enough to crush StuccoWrap wrinkles enough to prevent drainage.

I remain somewhat skeptical of the value of wrinkled housewraps. My advice: if you want a rainscreen gap, choose a method that gives you a gap of at least 1/4 inch.

Should the gap be vented at the top and bottom?

For some types of siding, it’s possible to omit the openings at the bottom and top of a rainscreen gap. For example, it’s possible to install cedar shingle siding over horizontal 1×3 furring strips. Even though the resulting gaps won’t have drainage, the installation works fine — because not much liquid water gets behind the shingles, and any water that does can evaporate quickly. (If you’re the type of builder who worries about drainage, you can cut kerfs on the back side of your horizontal battens, use corrugated plastic battens with drainage holes, or buy notched battens at a roofing-supply outlet. But I don’t think that level of obsession is necessary.)

Most types of siding — including (especially) brick veneer — require weep holes or drainage openings at the bottom of the rainscreen gaps.

Many builders install a rainscreen without openings at the top, and these installations work fine. However, researchers have shown that including screened vent openings at the top as well as the bottom of your rainscreen encourages ventilation drying (see Image #3, below). Siding installed over a vented rainscreen that is open at the top and the bottom will dry faster than siding installed over a rainscreen that is closed on the top.

Is it permissible to vent the top of the rainscreen gaps into the soffit? Opinions differ. Joe Lstiburek says it’s OK, and so do I. The amount of moisture carried by this air isn’t enough to cause any problems in your attic. However, builders in British Columbia should note that while the building code in that province does not require rainscreen gaps to have openings at the top, the code stipulates that if there are openings at the top, they can’t be connected to a soffit or attic.

Some builders wonder whether they need to come up with a detail to provide openings under each window and at each window head. The short answer: no, you don’t. Just provide a gap at the top and bottom of your furring strips so that air can move sideways around the window.

Detailing the top of a gable wall

Many builders have trouble visualizing the best way to vent the top of a rainscreen on a gable wall.

Mike Guertin, a builder from Rhode Island, does it this way: “The frieze board is padded off the wall to let air flow over the top of the siding, down the back side of the frieze, and out. The rake board needs to be padded off the wall about 3/8 in. to 1/2 in. more than the face of the siding.”

An illustration of Guertin’s method is shown in one of the images below. For a fuller explanation, see “Rainscreen-Siding Details for Gable Ends”.

What about bug screening?

In the early days of rainscreen gaps, many builders stapled a length of horizontal insect screening to the bottom of their walls, with the screen hanging below the bottom of the sheathing, before they installed their furring strips. Once the furring strips were installed (with the bottom 3 inches of the furring strip installed on top of the insect screening), they folded the extra width of the insect screening up on top of the furring strips and stapled it up in place.

These days, most builders use a section of ridge-vent material at this location, or a commercial product like the Cor-A-Vent SV-3 Siding Vent. Another option is perforated J-channel (see Image #6, below).

How do you flash windows in a rainscreen wall?

Window flashing is a big topic and is beyond the focus of this blog. Fortunately, the GBA site has many resources to help you work out your window flashing details.

Your first stop should probably be the GBA detail library. Many of our window flashing details can be found on this page: How to Install Windows Right.

The GBA and Fine Homebuilding websites have many videos and articles on the topic of window flashing. See, for example:

If you ever find yourself scratching your head, looking for a solution to a window flashing problem, remember this mantra: “Flash the rough opening, not the window.” If the rough opening is watertight and able to direct leaks to the exterior, you’re good.

Here are some more flashing principles to keep in mind:

  • You can’t flash properly unless you know which surface is designated as the WRB.
  • All flashings should direct water to the exterior surface of the WRB.
  • Laps and gravity are more dependable than adhesives and tape.

How much do rainscreen details cost?

Every project is different, but many siding contractors report that rainscreen details add about 30% to the cost of a siding project.

What’s going on in Oregon?

Since April 1, 2010, the residential building code in Oregon has required a minimum 1/8-inch-deep gap between siding and the WRB for all new homes.

According to an article in JLC, Oregon authorities have ruled that the use of any one of six brands of bumpy or wrinkled housewrap fulfills the requirements of this code provision. The six approved housewraps are Tyvek DrainWrap, Greenguard RainDrop, Valeron Vortec, HomeGuard HP Plus, Benjamin Obdyke Home Slicker, and HydroTex.

More information on rainscreens

For more information on rainscreens, it’s hard to beat Justin Fink’s article in Fine Homebuilding: “Keep Siding Dry With a Vented Rain Screen.”

A collection of architectural drawings of rainscreen details can be found here: “A Reference Guide of Typical Rainscreen Wall and Window Details.”

Martin Holladay’s previous blog: “Smelly Fiberglass Batts.”

Click here to follow Martin Holladay on Twitter.

49 Comments

  1. Eric Peterson | | #1

    homeslicker and nailing
    I have installed cedar shingles over home slicker and foil faced-rigid foam, and I couldn't help wondering if all of those nails penetrating the foam were creating paths for water and generally ruining the foam as an air barrier.

  2. Jin Kazama | | #2

    Would like to learn about the
    Would like to learn about the energy implications of a ventilated rainscreen
    ( lets assume stack effect one ) with a 0.5"+ gap during heating and cooling seasons.

    Could the upper "vents" be only of partial total length ?
    Let's say 10% opened at top...sufficient to drive air ?

    Any other anti insect bottom flashing/screening devices available ?
    I wonder about durability of coroplast material for fastening purposes... it becomes very brittle with age even when hiden from sun ( 10 years + period )

  3. Gordon Taylor | | #3

    Brilliant article.
    One typo: "However, these product has very tiny air gaps — StuccoWrap’s wrinkles are less than 20 thousands of an inch high." It happens. The article is so good that I hesitate to point out such a minor lapse. This is another keeper, going immediately to my Bookmarks.

  4. SULLKA LIMA | | #4

    What about open seam rainscreens?
    I'm deciding between a regular rainscreen and an open seam one (for purely aesthetic reasons)

    Besides the chance of bugs, wasps, birds, etc nesting behind it, is there any major issue with it?

    Is there any proven housewrap or membrane for this purpose (vaproshield has one called revealshield) or plain old 15lb felt would do? (has to be black)

    BTW, with open seam I mean either vertical or horizontal cladding separated at the seams, more as a louvered screen.

  5. Daniel Stewart | | #5

    deeper rainscreen gap
    Is there a problem with making the gap larger than 3/4"? I was thinking the in specific wall sections you could use a 3-1/2" gap (using 2x4 on end, for example) so that you could hide the downspouts behind the siding.

    edit -- the image is not one of my projects, just something found online showing something similar to what I was thinking.

  6. Daniel Stewart | | #6

    re: SULLKA LIMA
    Sullka,

    Our office has a project that used a liquid WRB that is black and (apparently) UV resistant (tremco envirodri, I believe).
    It's not my project so I don't have all the details, but the contractor claimed it was pretty price comparable to vaproshield (our standard product) -- including labor.

    I don't have a picture of this, but they painted the furring strips with the same black liquid for some protection but more for the aesthetic since the finished siding is gapped tiger-wood.

  7. Tim Naugler | | #7

    mat rain screen's
    I have heard 2nd hand complaints from installers with matt rain screens that it get's difficult keeping the siding flat especially when installing shakes or shingles. I believe they were using a product called cedar slicker. Does anybody have any experience like this. or have your experiences all been positive?

    Also Martin i was hoping you might elaborate on why vynal needs a rain-screen when put on top of more than 2'' of foam. Is because of fastener length? or some simple practical reason or is it building science based?

    I'm also curious like Daniel S if using a larger rain-screen has it's downfalls? I've heard that in European construction they are leaning towards larger cavity's 1.5'' or so.

    Good article!

  8. Stephen Carlton | | #8

    Very timely and comprehensive blog.
    Thanks Martin, the information is well detailed and timely as I am specifying these very details on the design of my house at this time. Unfortunately, my builder does not accept the need or value of rainscreens and believes that can actually make some situations worse.

  9. User avater GBA Editor
    Rob Wotzak | | #9

    @Tim Naugler: Yes, cedar slicker can be a challenge
    Tim, It was years ago, but I have had first-hand experience with cedar slicker and its challenges. I installed vertical ship-lap siding over the stuff and had a bear of a time getting it to sit flat and look right. You could use some sort of shims or spacers to deal with low spots as you go, but at that point I imagine it might be just as easy to use furring strips and ditch the mesh.

  10. User avater
    Mike Eliason | | #10

    the rainscreen projects i
    the rainscreen projects i worked on in DE (and that we're utilizing on some of our projects) incorporated a larger airspace - basically 1x vertical battens and 1x horizontal counter battens (so 1.5" gap). the counter battens are strictly an aesthetic determination - we want the cladding to be vertically oriented.

    i've seen commercial projects with deeper rainscreens to incorporate/hide downspouts, but it seems kind of ridiculous/wasted opportunity to address efficiency (e.g. add more exterior mineral wool)

  11. Milan Jurich | | #11

    Hydrogap comment
    Although mentioned in a previous write-up, the Hydrogap product has small bumps on it that actually compress under lap siding if nailed too tightly, thus defeating their purpose. Alternative products, like Pactiv's Raindrop 3D, do not show that same tendency. I re-read the article and did notice that it called out the compression concerns.

  12. John Schlosser | | #12

    Rain screens with sawn red cedar shingle siding
    Would appreciate opinions about the desireability of rain screens when siding is 5" lap sawn red cedar shingles. Located in the rainy State of Washington.
    Thanks!

  13. User avater GBA Editor
    Martin Holladay | | #13

    Response to Eric Peterson (Comment #1)
    Eric,
    Q. "I have installed cedar shingles over Home Slicker and foil-faced rigid foam, and I couldn't help wondering if all of those nails penetrating the foam were creating paths for water and generally ruining the foam as an air barrier."

    A. Most researchers have concluded that, as long as the fasteners stay in place, the fasteners do not significantly reduce the performance of an air barrier.

    Nevertheless, airtightness fanatics often go to great lengths to minimize potential leaks, and your question merits consideration.

    First of all, rigid foam may not be the best air barrier material. If you are concerned about airtightness, it makes more sense to establish your air barrier at the sheathing level. For more information on this approach, see Airtight Wall and Roof Sheathing.

    Second, cedar shingle siding introduces more fastener holes than other types of siding. One approach to minimizing the effect of these fasteners is to install two layers of plywood or OSB wall sheathing: the first layer is fastened to the studs, and the second layer is installed on the exterior side of the rigid foam. This method is sometimes referred to as a "site-built SIP." (It's possible to install the rigid foam and second layer of sheathing all at once if you specify nailbase.) For more information on this method, see my description of the renovation of the home of Claudia King and Lindsey Tweed: More Job Site Visits in Maine.

  14. User avater GBA Editor
    Martin Holladay | | #14

    Response to Jin Kazama (Comment #3)
    Jin,
    Q. "I would like to learn about the energy implications of a ventilated rainscreen (let's assume there is some stack effect) with a 0.5"+ gap during heating and cooling seasons."

    A. If it is properly built, the rainscreen gap is on the exterior side of the insulation. It neither adds any significant R-value to the wall, nor detracts from the wall's R-value. However, if you use foil-faced rigid foam on the interior side of a 3/4-inch-deep air gap, the combination of the 3/4-inch gap and the foil facing will add about R-1 or R-1.5 to the R-value of the wall assembly.

    Q. "Could the upper vents be only of partial total length?"

    A. Yes. As I wrote in the article, "Ventilation openings at the top of the wall are optional."

    Q. "Let's say 10% opened at top...sufficient to drive air?"

    A. The deeper the air gap, and the greater the area of the screened openings at the top and bottom of the gap, the greater the air flow. More air flow translates into faster drying.

    Q. "Any other anti insect bottom flashing/screening devices available?"

    A. Of course. If you visit a building supply store, you'll find many types of insect screening or ridge vent material that can be adapted to this application.

    Q. "I wonder about durability of Coroplast material for fastening purposes... it becomes very brittle with age even when hiden from sun."

    A. I welcome any comments from readers with experience on this matter.

  15. User avater GBA Editor
    Martin Holladay | | #15

    Response to Gordon Taylor (Comment #4)
    Gordon,
    Thanks for the feedback; I'm glad you liked the article.

    Thanks also for catching the typo; I have corrected the text.

  16. User avater GBA Editor
    Martin Holladay | | #16

    Response to Sullka Lima (Comment #5)
    Sulka Lima,
    Q. "I'm deciding between a regular rainscreen and an open seam one (for purely aesthetic reasons). Besides the chance of bugs, wasps, birds, etc nesting behind it, is there any major issue with it?"

    A. The main disadvantage of open-joint cladding is that this approach allows UV rays to reach the water-resistive barrier (WRB). Since UV light will degrade most types of WRB, you have to be careful to specify the highest quality WRB available. Even then, there will inevitably be some questions concerning the durability of any WRB exposed to UV light.

    Q. "Is there any proven housewrap or membrane for this purpose (Raproshield has one called Revealshield) or plain old 15lb felt would do? (has to be black)."

    A. Using #15 asphalt felt would be very unwise, since it degrades quickly when exposed to UV. One WRB that is advertised as appropriate for open-joint cladding is Delta Fassade S from Cosella-Dörken. It's expensive -- expect to pay between $1.10 and $1.20 a square foot. More information here: New Green Building Products — September 2010.

  17. User avater GBA Editor
    Martin Holladay | | #17

    Response to Daniel Stewart (Comment #6)
    Daniel,
    Q. "Is there a problem with making the gap larger than 3/4 inch?"

    A. No -- at least not from a building science perspective. I hesitate to bring the matter up, because the controversy is complicated, but I should probably mention that some fire-safety experts have questioned whether deep rainscreen gaps could be unwise in areas of the country with a high risk of wildfires. The worry is that dry grass near the bottom of your wall might ignite, and that the fire could be pulled into the bottom of a rainscreen gap. The deeper the rainscreen gap, the more likely it is that the chimney effect might cause a fire risk.

    Building codes are somewhat ambiguous on how fire safety requirements apply to rainscreen gaps. If this issue concerns you, talk to your local building official.

  18. User avater GBA Editor
    Martin Holladay | | #18

    Response to Tim Naugler (Comment #8)
    Tim,
    Q. "I have heard second-hand complaints from installers with mat rain screens that it get's difficult keeping the siding flat especially when installing shakes or shingles. I believe they were using a product called Cedar Slicker. Does anybody have any experience like this? Or have your experiences all been positive?"

    A. I have never used Cedar Slicker, but you're right -- some builders feel that the product is too squishy, and that it can be hard to get siding to look completely co-planar without a lot of fussing and care. Other builders seem to have a knack for using the product, and don't have any problems.

    Q. "Also, Martin: I was hoping you might elaborate on why vinyl needs a rainscreen when put on top of more than 2 inches of foam. Is because of fastener length? Or some simple practical reason or is it building science based?"

    A. Basically, it has to do with fastener length. Vinyl siding can't be fastened tight to the sheathing -- the fasteners need to be just a little bit loose, so that the siding can expand and contract lengthways to accommodate thermal expansion and contraction. This is hard enough to do when you are installing the product directly over OSB; if you are doing it over 2 inches of foam, it becomes even more challenging. Joe Lstiburek says that 2 inches is the limit before you need vertical furring strips. I might even be more conservative, and limit the thickness of the foam to 1 inch before switching to furring strips.

    Q. "I'm also curious (like Daniel S) if using a larger rainscreen has its downfalls? I've heard that in European construction they are leaning towards larger cavities -- 1.5 inches or so."

    A. See my answer to Daniel (above). As long as there is no fire risk, there is nothing wrong with a deeper cavity.

  19. User avater GBA Editor
    Martin Holladay | | #19

    Response to Stephen Carlton (Comment #9)
    Stephen,
    You wrote, "Unfortunately, my builder does not accept the need or value of rainscreens and believes that can actually make some situations worse."

    Perhaps you can urge your builder to read this article.

  20. User avater GBA Editor
    Martin Holladay | | #20

    Response to John Schlosser (Comment #13)
    John,
    Q. "Would appreciate opinions about the desirability of rainscreens when siding is 5-inch lap sawn red cedar shingles. Located in the rainy State of Washington."

    A. The answers to your question can be found in my article. Re-read the section under the heading, "Does every building need a rainscreen gap?" As I wrote, "Ultimately, the decision about whether to include a rainscreen gap is a judgment call."

    Are your walls tall or short? Is your roof overhang wide or stingy? Is your house exposed to wind and the weather? How close are you to the ocean? How many inches of rain to you get a year? How big is your budget?

    Remember, there is no downside to a rainscreen gap. It will help your wall dry faster and limit any repercussions from water entry. If you live in a rainy climate -- and you do -- I think that the cost of a rainscreen gap is money well spent.

  21. Scott Frey | | #21

    Rain screen
    Rain Screens sure are becoming a buzz thanks to you. I wish I would have know about them before I ever installed my first fiber cement job. I have actually first hand seen the effects of condensation behind cement board siding. Last fall I replaced a 3 year old job that the paint was peeling off. When we took off the siding it was completely soaked. Manufacture and paint finisher wouldn't pay a time. Never again will I install cement board in Wisconsin with out a rain screen. I would recommend looking at http://www.innovativeguttersolutions.com/siding-products/furring-master-rain-screen/product-information/ Here you will find all sorts of installation guides. Including a 3/8 furring master with intake and exhaust vents. We can install 2" foam and then use an ET&F nailer to just shoot the siding on.

  22. Katy Hollbacher | | #22

    Windows flashed into non-rainscreen layer?
    Martin, you noted:

    "Every well-designed rainscreen wall needs:
    • a water-resistive barrier (WRB);
    • an air gap between the WRB and the back of the siding...
    Here are some more flashing principles to keep in mind:
    • You can't flash properly unless you know which surface is designated as the WRB.
    • All flashings should direct water to the exterior surface of the WRB..."

    And from your innie vs. outie windows blog:
    "The WRB (for example, housewrap) can be under the foam or over the foam. A third option is to omit the housewrap and use taped foam as the WRB. In most cases, innie windows require a WRB under the foam, while outie window require a WRB over the foam. (For more information on the question of where to locate the WRB, see Where Does the Housewrap Go?) No matter where the windows are located, flashing details need to tie the perimeter of the window into the WRB."

    So based on all of that, here's an assembly:
    1. Innie windows flashed into the sheathing layer WRB
    2. Cont. insulation (let's assume of a type or thickness that would make it qualify as a vapor barrier) outboard of that, detailed as a second WRB w/ rainscreen

    Why? So the insulation performance won't be reduced because of backventing (wish we had better ideas of the energy implications of that). Also, it can get tricky to try to flash the windows to the outside WRB in this situation, and I like to detail the sheathing as airtight layer. But, it does get redundant. I think this is a solid system w/ EPS, XPS or mineral wool, but would anyone be nervous about this setup if using polyiso (ie flashing windows behind it)

  23. User avater GBA Editor
    Martin Holladay | | #23

    Response to Katy Hollbacher
    Katy,
    If I understand your question correctly, it's this: "If I use housewrap as a WRB, and if I install my WRB between the OSB sheathing and the exterior foam, is it OK to drain my rough window sills to the housewrap layer?"

    A. No. If the housewrap is under your rigid foam, you should direct the water collected by the sill pan under your windows to the exterior surface of the rigid foam -- not to the housewrap.

  24. Katy Hollbacher | | #24

    Window flashing part II
    Thanks for your response, Martin-

    The sheathing WRB would actually be a liquid-applied product ("spray-on drainage plane" as I like to say). Note that if it were possible to flash to the 2nd WRB/rainscreen layer, the only purpose of the 1st (sheathing layer) WRB would be as an air barrier. That aside, I don't understand your answer in the context of your innie-outie post, as the two seem contradictory:

    "The WRB (for example, housewrap) can be under the foam or over the foam. A third option is to omit the housewrap and use taped foam as the WRB. In most cases, innie windows require a WRB under the foam, while outie window require a WRB over the foam. (For more information on the question of where to locate the WRB, see Where Does the Housewrap Go?) No matter where the windows are located, flashing details need to tie the perimeter of the window into the WRB."

    Based on the above, I read that it's OK to flash to the inside of the exterior insulation, but in your response in this thread you noted otherwise. Can you please clarify?

    Reference from your "where does the housewrap go" post:
    https://www.greenbuildingadvisor.com/sites/default/files/Housewrap%20location%20detail%20-%20GBA.jpg

    Another reference, see Figure 7 on page 9:
    http://www.buildingscience.com/documents/digests/bsd-105-understanding-drainage-planes/files/bsd-105_understanding_drainage_planes.pdf

    Thank you again, great topic!

  25. User avater GBA Editor
    Martin Holladay | | #25

    Response to Katy Hollbacher
    Katy,
    Your logic is impeccable, and you are quoting me accurately. Joe Lstiburek and others are satisfied that it's OK to direct water to a WRB behind rigid foam. In theory, it's going to work.

    However, in a few rare cases, more water ends up on the sill pan than can easily evaporate. If your sill pan ever gets lots of liquid water, it's much better to direct it to the exterior of the foam than to the WRB behind the foam. That's just my opinion.

    If I were flashing a rough window sill with your type of WRB, I would extend the flashing to the outside of the foam. Joe Lstiburek might disagree -- I'll try to phone him when I get a chance -- but that's the way I would do it.

  26. Katy Hollbacher | | #26

    Windows part III
    Thank you again, Martin. I'd put my opinion somewhere in the middle of yours & BSC. For now I think I'll stick with my original instincts on this:

    1) whenever possible, flash windows to the outside of the exterior insulation
    2) if that's difficult because of "innie" windows or some other reason...? then:

    - if flashing windows behind exterior insulation, try to use rockwool (or another vapor-open product)
    - second best is EPS or XPS
    - tread carefully w/ polyiso (I don't want water getting into unprotected joints on the backside of polyiso)

  27. Jenny Belman | | #27

    A very detailed discussion on
    A very detailed discussion on rainscreens and its usages. Temperature fluctuations are minimized due the achievement of higher effective R-values therefore creating a much more efficient wall assembly and dramatically reducing the loads on HVAC systems.

  28. User avater GBA Editor
    Martin Holladay | | #28

    Response to Jenny Belman
    Jenny,
    I disagree with your conclusions. Rainscreen gaps do not, and are not intended to, "minimize temperature fluctuations due to higher effective R-values." Rainscreen gaps have very little, if any, affect on a wall assembly's R-value. There is no way that a rainscreen gap will "dramatically reduce loads on an HVAC system."

  29. Brian Rawlinson | | #29

    rainscreens
    Nice comprehensive article Martin. The question I never see addressed is that of fire-spread caused by the chimney effect of an open cavity. Buildings in urban-woodland interface zones cannot have roof soffit vents under some codes, since they would such burning sparks into the attic; there are some special metal screen vents on the market that limit the size of openings and have baffles to prevent spark entrance. Doesn't a rainscreen detail tend to pull sparks from grass fires into the wall?

  30. User avater GBA Editor
    Martin Holladay | | #30

    Response to Brian Rawlinson
    Brian,
    In most U.S. jurisdictions, building inspectors approve of rainscreen siding installations.

    In some areas of the country with high wildfire risk, questions have been raised concerning whether a rainscreen gap poses a risk during a wildfire. The code is somewhat ambiguous on this issue. When the issue was discussed on the GBA website, the controversy generated strong emotions. You can read the previous threads here:

    Rainscreen performance during wildfires

    Rainscreens, Wildfire Hazard and Other Unintended Consequences

    The bottom line: if you are worried about this issue, talk to your local building department.

  31. User avater
    Tyler LeClear Vachta | | #31

    Contractors, Architects, and the Public Need this Information!
    Very interesting and relevant article. It is a sustainability, HSW and energy conservation issue. I would like to recommend a no-cost, AIA approved course (open to all, not just AIA members) entitled "Rainscreen 101 Revisited,"on this very topic. Since it is an AIA approved course, there is no commercial advertising involved. The link to the course is http://bit.ly/chQaCH. A pdf on this topic is also available at http://bit.ly/Y45ZnP

  32. Greg Albracht | | #32

    New FurringMaster 2 Aluminum .032
    Hello Martin , Greg Albracht here. You did an artical in Energy Update magazinel on my Furring master steel strips and Sidingmaster
    I have a new Prepainted .032 aluminum
    FurringMaster 2 system and new vent system.
    As Scott Frey mentioned above my old steel version of FurringMaster
    Is great but I just outdated it with my new FurringMaster 2

    It cost less, less to ship, easier to handle and cut
    and I changed the design that improves fastening capabilities by
    The new Hat channel Design
    It is being used on the new HONDA Smart House
    A live feed of the project will show the system going up on the Honda website.

    https://sites.google.com/site/gorillagutterguard/our-products/furring-master-rain-screen-system

  33. Kyle Grunder | | #33

    Adding On
    What a great article! I really do believe that before long most codes will include this type of system in residential building because of the effective moisture mitigation. Too long we have been focused only on vapor barriers that, when done incorrectly, trap what water does get by the enclosure on the inside of the building. I am a commercial exterior contractor that installs only rainscreen systems and I can tell you that the approaches are a little different, but this is a very effective way to button up any type of building.

    One thing I did notice is that there is not much emphasis on exterior insulation in the article or comments. Without getting technical you want to keep the majority of the wall system (studs/gyp board/etc.) to the warm side of the insulation layer. The main enemies you are trying to fight with moisture is 1) Direct (rain/snow) 2) Indirect or Wicking 3) The hardest one - Condensation. Think of the typical wall makeup that has insulation on the inside of your house wrap typically between studs. Cold air from outside hits the walls and condenses the warm air from inside BEHIND your vapor barrier - exactly what you do not want. It is important that if you want to make your house healthier, and you already plan to fir out your siding, that you consider installing an exterior insulation like rigid foam or mineral fiber outside your air barrier. Keep in mind that you still want to keep the air gap to maintain the wonderful perks of a rainscreen system. All the extra work/materials as stated above will come at a cost, but you can rest easy that you have a healthy and efficient building enclosure.

  34. Claython Mclaw | | #34

    Venting into roof on gable ends?
    The fact that one can vent directly into the soffit will be a welcome simplification of my design, but does this include the gable ends too? I ask because my understanding (if I'm remembering things correctly anyway) is that vented soffits on the gable ends will create an issue with the "face soffit"-to-ridge air flow through a vented roof. Will venting the rainscreen into the (unvented) soffits on the gable ends cause the same issue?

    Thanks!

  35. User avater GBA Editor
    Martin Holladay | | #35

    Response to Claython Mclaw
    Claython,
    While the air flow through rainscreen gaps can make a significant difference in how fast the siding dries, the volume of air we're talking about is small, and the amount of moisture carried by that air is also small. I don't think that terminating rainscreen gaps at the gable-end soffits will negatively affect the ventilation flow through your unconditioned attic.

    There are several reasons why I feel confident that you don't have to worry. In addition to the reasons mentioned above, the main reason is that ventilating attics is more about hocus-pocus and wishful thinking than it is about air flow. For more information on the topic, see All About Attic Venting.

  36. Edward Cambridge | | #36

    Martin. Another product to add
    Martin,
    Looks like a new product from Canada.

    Ventgrid

    4x8 sheets of seemingly fairly rigid plastic grid that should reduce flex on install andreducewavy lines?
    I found online that they priceabout $1/sqft (not confirmed)
    Great article!

  37. User avater GBA Editor
    Martin Holladay | | #37

    Response to Edward Cambridge
    Edward,
    Thanks for letting us know about Ventgrid. If GBA readers are interested in more information about this product, here is the company's contact information:

    Ventgrid
    P.O. Box 959
    4073 Highway #3
    Chester, Nova Scotia B0J 1J0
    Canada
    902-273-3051
    E-mail: info@ventgrid.com

  38. User avater
    Tyler LeClear Vachta | | #38

    Correction on list
    This article is a few years old but still important and often referenced. I wanted to offer a correction to your list of rainscreens:
    MTI Sure Cavity and Gravity Cavity are the rainscreen products, with rigid channels mortar blocking fabric. 1/8, 3/16 and 10mm versions available.https://www.mtidry.com/products/rainscreen-drainage-planes/sure-cavity

    The MTI Perforated Control Cavity is intended for the interior of Single-Wythe Walls.

  39. User avater GBA Editor
    Martin Holladay | | #39

    Response to Tyler LeClear Vachta
    Tyler,
    Thanks for the updated information on the MTI products. I have edited the article to include the new link.

  40. RobzHou | | #40

    Using folded-over asphalt felt as furring strips
    What about your comment I found on your WRB article — is this still a recommended option? It might be a good suggestion for a rainscreen on a budget for fiber-cement lap siding?

    "I wrote about this approach in an article called ‘Another Rainscreen Tip.’ The article was published in the March 2007 issue of Energy Design Update. In that article, I wrote: ‘The latest tip for creating an airspace between siding and the water-resistive barrier (WRB) is simple and inexpensive: install vertical strips of folded-over asphalt felt every 16 or 24 inches on center. Depending on the choice of asphalt felt (#15 or #30) and the number of folds, a variety of thicknesses can be created; in fact, by varying the strip thickness, the strips can be used as shims to correct minor framing irregularities.’"

    -- Robert Hawke

  41. User avater GBA Editor
    Martin Holladay | | #41

    Response to Robert Hawke
    Robert,
    It's certainly possible to create furring strips out of folded strips of asphalt felt. There are two obvious disadvantages to this approach: (a) Asphalt felt furring strips can't hold siding fasteners, and (b) Cutting up the strips of asphalt felt and folding them over to create a thick enough layer is labor-intensive.

    The disadvantages are balanced by one advantage: asphalt felt is cheap.

  42. Timothy Godshall | | #42

    Why don't wood furring strips need to be pressure treated?
    I appreciate all this information in one place, but I do have a question that I would like better explained. You state that "Furring strips don’t have to be pressure-treated, because a rainscreen is designed to stay dry." If the rainscreen stayed dry, wouldn't it be pointless, since its very existence is to deal with water that gets behind the cladding? I know that the materials in the rain screen will dry out more quickly than they would otherwise, but It seems like untreated 1/4" plywood could start to disintegrate over the years if it was repeatedly getting wet, whether through wall leaks or inward vapor drive through siding.

  43. User avater GBA Editor
    Martin Holladay | | #43

    Response to Timothy Godshall
    Timothy,
    Most types of siding do an excellent job of stopping rain. The amount of liquid water that gets past siding is quite small. Moreover, a rainscreen is designed to encourage fast drying, so when a few drops of water get past the siding, the water evaporates quickly.

    Which is all another way of saying, "Rainscreens work." They create a dry environment between the sheathing and the siding -- exactly what you want.

    Using pressure-treated lumber is not only unnecessary -- it often requires the use of expensive stainless-steel fasteners.

    Lots of researchers and ordinary builders have removed siding and inspected furring strips. The furring strips stay dry.

    If a wall has rotting furring strips, something is seriously wrong with the wall design or installation. Rotting furring strips would usually be a sign of a serious flashing error.

  44. Trevor Lambert | | #44

    re: speed of drying, top vented vs not
    Is there any quantification on how much faster it might dry with top venting? I've been stewing over the decision to vent into the attic, vent below the soffit or not vent the top at all for quite a while. The local building inspector isn't of much help; he says to just build it per the design. Well, that detail is not covered in the design - the designer says to follow the local building code. So it's a circle. By far the simplest and cheapest way for me is to vent into the soffit (or rather, behind the soffit into the attic space). One factor I didn't see mentioned is what about ice damming? With snow on the roof, a sunny day in winter, the air in the rain screen is going to get pretty warm behind the south facing siding. If that's venting into the attic, will that contribute to ice damming? Or is the amount of air too small to be of significance?

  45. User avater GBA Editor
    Martin Holladay | | #45

    Response to Trevor Lambert
    Trevor,
    If you vent into the attic, there is a small increase in the chance that the air flow might contribute to ice damming. I think that the effect is quite minor, however, compared to other ways that ice dams form.

    You'll reduce that risk if you vent the rainscreen gap to the exterior, below the soffit, which is the standard recommendation.

    If you are really worried, you can include a layer of rigid foam behind your soffit -- limiting heat flow from the sun-heated south facade -- while leaving a slot to admit ventilation air to the attic.

  46. Malcolm Taylor | | #46

    Reply to Trevor Lambert
    Trevor,
    Sorry - this is a reply to question you posted some month ago. The NBC includes requirements for fire-stops in concealed spaces. Rainscreen cavities are interpreted as being concealed spaces connecting walls to roofs. The prohibition of venting the top of the cavity into the attic is dealt with explicitly in our code here in BC (because rainscreens are required in some regions), but not in the Ontario code. You should probably run this by your inspector before deciding how to proceed.

  47. Steve Sloan | | #47

    Hi Martin,

    Another excellent article. I have a question regarding the placement of the flashing. I'm adding 2" polyiso to a house that has brick halfway up the wall. I am trying to decide whether to put the flashing against the outer face of the polyiso, which will act as my WRB (option 1) or on the other side of the furring/vent assembly, immediately behind the siding (option 2). It seems that Option 1 allows more breathability in the rainscreen above the brick, but Option 2 would better vent the cavity behind the brick. The top of the wall will vent behind the frieze board. Please see pictures.

    Thanks!
    Steve

  48. User avater GBA Editor
    Martin Holladay | | #48

    Steve,
    The flashing must be integrated with your water-resistive barrier, so your decision hinges on which material you decide to use as your WRB. The decision often hinges on your window location -- because the WRB has to be integrated with the window flashing.

    For more information on this issue, see these two articles:

    "Where Does the Housewrap Go?"

    "Using Rigid Foam As a Water-Resistive Barrier"

  49. Steve Sloan | | #49

    Thanks Martin,

    Sorry for neglecting to mention the windows. The windows will be flashed to the outer surface of the polyiso board, which will act as the WRB, at least for the upper part of the wall.

    Flashing integrated into the WRB makes sense to me. However, image #2 (of 7) for this article, shows and spells out the flashing location: "Flash to back of siding, not to housewrap". This appears to be at the bottom of the wall, which is a similar location as what I'm asking about with my brick transition. This would be closest to my option #2.

    Could you elaborate?

    The difference to me seems to be that in the image, the insect screen behind the 5/4 board allows the rainscreen gap to vent, whereas for my wall, option #2 would allow the upper (above the brick) wall's rainscreen gap air cavity to connect to the wall cavity in the lower wall between the brick and the Celotex, but the resulting overall wall assembly would not have much of an air "intake" at the bottom (behind the brick) as the wall shown in image #2 of this article.

    Thanks again,
    Steve

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