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Community and Q&A

Can 2 layers of #30 asphalt felt replace my rainscreen?

Kevin Camfield | Posted in Energy Efficiency and Durability on

We were planning to use Home Slicker Rain screen behind PVC NuCedar shingle siding.  We have packed out the windows and already installed some of the 30# felt on the walls before we discovered that the Home Slicker is not going to work with the shingles.  This is despite working in advance with the manufacturer and us doing mock up installations ahead of time.

We have to install the shingles on a firm surface and the manufacturer does not allow using furring strips underneath the nailing location only.   About 30% of the 30# felt is already on the building and is integrated into the 1/4″ plywood we used to pack out the windows.  That 1/4″ is also already covered with liquid flashing.

My question is, can we get some of the benefit we would have gotten from the Home Slicker product by adding another layer of 30# felt?  It is not a completely smooth product in that it tends to pucker when it gets wet.  Will the second layer act as a dimpled rainscreen and help us with protecting the sheathing?  The PVC shingles are not vapor permeable and so our walls will be drying to the inside.  We have plywood sheathing and plan to use BIBs dense pack insulation in the walls.

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  1. User avatar
    Armando Cobo | | #1

    For clients that can't or don't want to use drainable mats, like Home Slicker, I often specify drainable wraps, like Benjamin's HydroGap, Tamlyn's TamlynWrap or Fortifiber's WeatherSmart, among others. Most drainable wraps have a draining efficiency of +93%. See:

  2. Kevin Camfield | | #2

    Armando, thanks for the reply.

    Would you put that material on top of the 30# felt that is already installed on the wall?

    1. User avatar
      Armando Cobo | | #4

      I specify the 30# flet on top of the drainable wrap.

  3. User avatar
    Jon R | | #3

    > The PVC shingles are not vapor permeable and so our walls will be drying to the inside.

    Strictly speaking, vinyl siding isn't vapor permeable either - but there is enough air movement to provide plenty of outward drying. This may also be true of PVC shingles.

    1. User avatar
      Dana Dorsett | | #5

      That's correct- shingle siding provides at least some amount of back-ventilation, and wall still dries toward the exterior.

  4. Kevin Camfield | | #6

    Dana, are you saying with a PVC shingle that is vapor impermeable there is still enough air movement between the shingles to allow drying to the outside?

  5. User avatar GBA Editor
    Martin Holladay | | #7

    I've never installed PVC shingles, so I have no idea how they snap together, or whether they can form an air barrier. But I suspect they are installed in a loosey-goosey fashion, similar to vinyl siding.

    Vinyl siding allows exterior drying, because (a) there is a ventilated air space between the vinyl and the WRB, due to the shape of the siding profile, and (b) the seams between courses of siding leak enormous amounts of air.

    In other words, vinyl siding isn't an air barrier.

  6. User avatar
    Armando Cobo | | #8

    There is a YouTube video that show the installation of PVC NuCedar shingle siding. Although a PVC shingle is impermeable, unlike Cedar shingles, it is installed with about an 1/8" gap between shingles, making it a bit permeable, but not as loose as vinyl siding, which IMO, is like a flag in the wind. See:
    Installing drainable wraps behind the felt allows for a 1-3mm gap for drainage and venting, using the felt to protect the gap from rips, tears and holes while you install the siding, plus adding protection from felts getting brittle over time.

  7. User avatar
    Peter Engle | | #9

    I agree with Armando that the combination of drainable wrap and felt is a good one. However, the OP has already installed the first layer of felt, and asks if another layer is better? I would answer "yes" to the question. There will be some benefit to the wrinkles and gaps between the layers, as well as simply more water resistance from the two layers of felt.

    I'm not sure the extra protection is worth the small extra cost, though. PVC shingles do act very much like vinyl siding in terms of back-ventilation and drainage. While not as loose as vinyl, it is still not flat on the back and it is still installed slightly loose to allow for thermal movement. that, and the gaps at the joints will allow some drainage and ventilation. I think a single layer of felt would be just fine except in a severe marine or similar windy/wet environment.

  8. Kevin Camfield | | #10

    The shingles are installed like regular cedar shingles with the exception of the nailing which is set only 1/2" from the exposure. The shingles are 13" long with a 5" exposure.

    Yes the felt is already on the wall so we cannot go behind it. We are in an extreme marine environment only 30 feet off of a salt water bay. It is also a windy location. We have sealed everything very well. We liquid flashed all of the window openings, used sloped sills, and put flashing tape along side every place we used 1/4" plywood to pack out the window openings and corners. Then we used Prosoco joint and seam filler behind the edge of the 30# felt to seal it to the flashing tape except at the top of the windows where we want it to drain. We are going to use a wider trim to cover where we packed out the windows and then back caulk the shingles as we install them up against the trim.

    Has anyone ever installed two layers of 30# felt in a wall shingle application? The installers are concerned about being able to get the second layer to lay flat now that the first layer has been on the wall a while and had puckered with the moisture.

    We were looking forward to the benefit of the Home Slicker rain screen and would like to replace some of that benefit. But we are also gun shy at this point with trying anything new and different. We thought we had done all of our homework with Home Slicker ... As a result of this situation, NuCedar is changing their guidance from "not necessary" to "don't do it".

    1. User avatar
      Armando Cobo | | #11

      2 layers of #30 felt is used all the time in the SW stucco homes. You should be fine.

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