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Community and Q&A

Is a WRB necessary with a rainscreen?

Richard Baumgarten | Posted in Green Building Techniques on

I’m about to enter the WRB stage of my house. I’ve avoided the use of a WRB on my roof, and would like to go without on my walls.

Covering an entire house with a WRB seems excessive in a rainscreen system. Capillary water seems to be the only enemy of a rainscreen wall. Impermeable furring strips, or an impermeable strip at the furring strip layer would appear adequate at preventing capillary moisture from migrating into the wall from wet siding. Asphalt felt, or plastic housewraps do not seem ideal for this task.

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Richard,
    A WRB is required by all building codes. It is also recommended by all building experts.

    If you don't want to use asphalt felt or plastic housewraps, there are other possible choices, including liquid-applied WRBs. But a WRB is not optional. You have to have a WRB so you know where you can tie in the flashing. All flashings must be integrated with your WRB.

    More information here: All About Water-Resistive Barriers.

  2. Richard Baumgarten | | #2

    I know what code, and the experts say... but I'm not convinced.
    WRB's assume bulk water has gotten past the siding, which shouldn't happen. I'm all for flashings, but they can tie into the siding. and lap over the element below.
    I just installed a slate roof. It doesn't need a WRB to be water proof, just proper laps. Why can't siding which isn't nearly as vulnerable as the roof be detailed the same way?
    I'm exempt from building to code, thankfully.

  3. User avatar
    James Morgan | | #3

    "I know what code, and the experts say... but I'm not convinced."
    Richard, I think you are making two mistakes: first, assuming that gravity and capillary action are the only conditions affecting bulk water movement in your wall assembly; second that you are capable of achieving a perfect lapped siding/trim/flashing assembly that will last indefinitely. A WRB is a protection against wind-driven rain or snow working its way past the lapped assembly. It's also an inexpensive insurance against any imperfection in a critical assembly detail which may be compromised by future conditions which you cannot predict. If it never rains and the wind never blows where you live a WRB-free assembly will be fine. Otherwise, proceed at your own risk.

  4. Jeff Nelson | | #4

    I'm hoping, Martin, that you still feel that foil faced polyiso can be used as the WRB. Or would you recommend a wrap-style WRB in addition? My plan is to use polyiso as my WRB, but I'm just now finishing the framing on my build. There's still time to change my mind! Again!

    I'd love to hear from anyone who has had success using foil faced polyiso, seams taped, as their WRB. Oh, and to tie into Richard's original post, yes, I will be using a rainscreen.

  5. John Klingel | | #5

    Richard: What would you be gaining by taking a gamble as big as not including a WRB? Is that rather small gain worth the potentially rather large consequence?

  6. User avatar GBA Editor
    Martin Holladay | | #6

    Jeff,
    As I wrote in my article, Using Rigid Foam As a Water-Resistive Barrier, the code allows builders to use some brands of polyiso as a WRB, as long as the flashing method for the top of the windows is to use tape -- the same type of tape used by the manufacturer during their AC71 testing. The code approval process does not allow builders to deviate from the AC71 flashing method.

    I'm not a fan of that flashing method -- tape over foam. I'm worried about the long-term durability of the tape adhesive. You'll notice that Joe Lstiburek, who is quoted in that story, has an anecdote about tape failures; his solution: tape the tape. That makes me nervous.

    Of course, you can always go outlaw, and ignore the AC71 details, and instead come up with your own (better) flashing methods. After all, very few code officials even understand the AC71 approval process. However, that approach puts you at the mercy of any code official who wakes up one morning and does his homework.

  7. Richard Baumgarten | | #7

    John - I'm trying to minimize the use of manmade products, minimize my toxic imprint, and meet the code. I've done pretty well so far. I'm willing to sacrifice durability for sustainability.

    A perfectly lapped siding system is 100% achievable, but will not last forever. When your building does fail, what are you left with...toxic waste, or inert, recyclable, compostable materials.

    I've seen water soak the backside of different siding applications, but have yet to see water then jump across an airspace. Maybe I'll learn something new from hurricane Sandy.

  8. User avatar GBA Editor
    Martin Holladay | | #8

    Richard,
    Most leaks occur at windows, and that's the part of your house that will be most vulnerable as well.

    Window flashing works best when it is integrated with a WRB. I'm not saying that it's impossible to flash a window directly to the siding -- but to do it right, you would have quite a bit of visible metal flashing at the window perimeter, and the results will depend strongly on the skill of the installer and the quality of the flashing chosen.

    I think you'll have to use soldered copper if you expect your plan to succeed -- and that's expensive.

  9. User avatar
    Mike Eliason | | #9

    if 'trying to meet code' - then meet the code. are you using halogen free wiring? NAUF wood products? is your structure and furniture inundated with PBTs? any heavy metals?

    if overly worried about man-made materials, just use the felt and call it a day.

  10. Dave Cummings | | #10

    Richard,
    How will you be building your walls? Conventional, SIPs, Alternative..?

  11. Richard Baumgarten | | #11

    Martin, I wish I knew what you had in mind with the soldered copper...sounds very interesting. I'm very fond of Erno Ovari's solderless copper flashings over at copper exclusive.

    Mike, don't get sassy. I'm just trying to find out how critical a WRB is in a rainscreen system. If it doesn't provide a big beneift, why build with this stuff IF avoidable. I have no wiring. No urea formeldehyde yet. PBTs, unfortunately yes (I couldn't avoid using polystyrene under my slab). Furniture, hell no. Heavy metals, none. That said I am a hypocrite, but at least I'm trying.

    Dave, my walls are conventional stick frame. Board sheathing on walls and roof...that's all I have done so far. I'm thinking about wool insulation, I think cellulose would spill through the gaps in my board sheathing.

    I don't know which is worse. Polyethylene housewrap as an end product is less toxic than asphalt saturated felt, but I don't know how it's produced. Which one would you consider as the better capillary break?

  12. User avatar GBA Editor
    Martin Holladay | | #12

    Richard,
    Neither housewrap nor asphalt felt is a capillary break.

    A 3/4-inch air space is a capillary break.

    A 4-inch-thick pile of crushed stone is a capillary break.

    Ice & Water Shield is a capillary break.

  13. John Klingel | | #13

    Richard: I think we all understand, admire and empathize w/ your not wanting to use man-made (generally toxic to some degee) materials, but if you or the next cat have to rebuild the house, that has its negative impact, too. Sometimes we just have to lose a battle to win a war. When the house is, eventually, demo'd, the house wrap can go to the recycle center. Put it in and sleep well, imo.

  14. Dick Russell | | #14

    Richard, even if you visualize that any wind-driven rain that gets behind the siding laps will run down the back face of the siding and not contact the board sheathing, I would be concerned about droplets of water tracking down the edges of the furring strips that hold the siding off the sheathing. You can't easily say those droplets will stay on the sheathing side of the strips. Once any of that water gets to the sheathing, it wets the sheathing and gets into the cavity. I'm with the others; add a WRB of any sort to protect the sheathing and insulation.

  15. Richard Baumgarten | | #15

    Martin, I didn't know housewraps and felt are not capillary breaks.

    I'm convinced to use a WRB - thanks for the opinions and suggestions.

    I would still like to have a capillary break. Unsealed cedar siding plus unending miserable rain like this creates a big soggy resevoir cladding. Would it be it be OK to use strips of plastic vapor barrier on or behind my furring strips? It's a possible condensation point, but it's towards the exterior at least. (zone 5b, southern Ontario, heating climate only)

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

    Richard,
    I don't think it's a good idea to install poly strips behind your furring strips. The poly will just interfere with drying.

    Your rainscreen gap will dry quickly. If you want to improve the drying rate, make sure to include screened openings at the bottom and top of the rainscreen gap to improve ventilation (air flow). That will do far more to encourage drying than adding polyethylene strips.

  17. Richard Baumgarten | | #17

    Thanks everyone, you're comments are invaluable.

  18. User avatar
    Michael Maines | | #18

    With all the right details, well-executed, I bet it's possible to build without a WRB. The building code is the minimum standard as agreed upon by a bunch of experts, but that doesn't make it gospel. I would use large siding overlaps, soldered sill pans and head flashings, vertical shiplap sheathing, an insulation that can take some amount of wetting such as dense-pack or mineral wool, airtight drywall on the inside, and good humidity control. Actually I would just use a WRB but appreciate the desire to think outside the box.

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