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PVC shingles and dense-packed insulation

Kevin Camfield | Posted in Green Building Techniques on

We are considering using NuCedar PVC shingles on [the walls of] our new home in the Seattle area.  Our wall assembly is 2×6 with CDX plywood sheathing.  The sheathing will be taped with ZIP tape and has been sealed to the framing around the wall perimeters and window/door rough openings.  The shingles need to be installed on a flat wall surface, so no furring strip type rainscreens.  The sales rep says they believe that a rainscreen, such as cedar breather,  is overkill because of the nature of their product.   My question is will my wall assembly have enough drying capability if I use PVC shingles, 30″ felt WRB (maybe 2 layers) and dense pack cellulose or fiberglass in the wall cavities.  There will be no internal vapor barrier.

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    The rainscreen is to protect the sheathing, not the shingles. At the very least a mesh-type rainscreen underlayment such as 6mm Obydyke RainSlicker would be called for here. (A bit north of you in B.C. local codes would require the 10mm version.)

    The installation instruction sheet spells out:

    "The NuCedar cladding system is not a water-tight barrier. It is
    designed to reduce the amount of water that reaches the underlying weather
    resistant barrier"

    The ZIP sheathing still needs an exterior path to drying, even with the factory applied weather resistant barrier.

  2. Kevin Camfield | | #2

    Dana,

    Thanks for getting back. Yeah, I think in some ways the PVC shingles might actually hinder drying to the exterior. I'll do some more research with NuCedar. I think the reason for the flat surface requirement is to help minimize warping if the shingles get hot. This option was looking pretty expensive anyway despite how good the shingles look and how well they will likely hold up without ongoing maintenance.

    Kevin

  3. GBA Editor
    Martin Holladay | | #3

    While Dana (in Comment #1) mentioned Zip sheathing, the original poster (Kevin) said that the house has CDX plywood sheathing.

    In general, I agree with Dana that a rainscreen gap helps protect the sheathing. For more information, see "All About Rainscreens."

    Most experts say that you don't need a separate rainscreen gap when installing vinyl siding, because vinyl siding is inherently well ventilated. I don't know whether that analysis applies to NuCedar PVC shingles, because their web site doesn't show a cross-sectional illustration (which might reveal whether these shingles have an air gap behind them after they are installed). Maybe you can get a sample and figure out the answer to the mystery.

  4. GBA Editor
    Martin Holladay | | #4

    Kevin,
    This photo from the NuCedar shingle web site appears to show the siding installed over a corrugated housewrap with drainage channels. I'm not sure what brand of housewrap it is.

    That said, the NuCedar installation instructions don't require a drainage-style housewrap or a rainscreen gap. But a rainscreen gap is still a good idea.

  5. Kevin Camfield | | #5

    Martin,
    Thanks for the reply. The house wrap in the photo is GreenGuard Raindrop 3D house wrap. From what I can see online, it's a woven material with vertical cords that provide a dimple or wrinkle for drainage. They also recommend Benjamin Obdyke HydroGap which has 1mm raised bumps for drainage. I talked to the technical person at NuCedar yesterday and they are reluctant to highly recommend a product like cedar breather. I'm not sure why. They tell me I cannot install this product over a 3/8" horizontal rainscreen because of hot spots that can develop and warp the shingles. They have a procedure for gluing the back side of the shingles with sealant in hot areas like where a wall section meets a roof to prevent warping. I believe there are two things that may be driving their reluctance to recommend cedar breather (they say it can be used, but is "overkill"). One I think is cost. Their product is already expensive per square foot. I think the other is that the product is more stable and less prone to warping when installed on a flat surface that has some thermal mass to keep things cool. Perhaps they are trying to avoid causing the customer to be overly concerned by using the term "warping". Product cost is $185/quarter square delivered.

  6. Kevin Camfield | | #6

    I just realized after my last post that in trying to figure out how to use this product I may be painting it in a bad light. The reason I am thinking of using it is that I think it is beautiful. They have a brushing technique that gives each shingle a different tone plus the reflective paint causes additional light variation which to me is a plus. It looks more like natural stained shingles than anything else I have seen. They have a prorated 25 year finish warranty (does not include fading). And the customer service from the sales folks and the supplier, who will ship anywhere in the US, has been outstanding. You can do Boston corners and bend the product in a radius for sloped edges. I looks like a good product. My samples arrive next week. There is a higher initial cost which is somewhat offset by the lower ongoing maintenance cost versus some other products. I am going back and forth between this and pre-finished Hardie shingles. The cost for Hardie is lower but I like the look of NuCedar better. The rainscreen is also easier with Hardie because I can uses 3/8" treated vertical furring strips.

  7. Kevin Camfield | | #7

    Dana, Martin,

    Could part of the reason NuCedar is saying that a rainscreen larger than crinkled house wrap is not needed be that NuCedar shingles is a non-reservoir cladding? I was just looking at this article by Joe Lstiburek and he seems to be saying that with a non-reservoir cladding the inward vapor drive is less and so the drying potential can be less. Would that mean that the 30# felt and the plywood would have enough capacity to temporarily store whatever moisture gets behind the shingles and allow the sheathing to dry to the inside through the dense pack insulation?

    https://buildingscience.com/documents/building-science-insights/bsi-061-inward-drive-outward-drying

    By the way, we actually live in Port Townsend WA which is in that sliver of low rainfall area along the coast. We get 19 inches of rain a year as compared to the coast of British Columbia which gets over 60.

  8. GBA Editor
    Martin Holladay | | #8

    Kevin,
    Wall assemblies sometimes get wet from the exterior (due to rain), and sometimes (more rarely) from the interior (due to exfiltrating air that deposits moisture on cold surfaces). Figuring out the balance of all of the wall components -- several of which may be vapor retarders -- gets tricky.

    The fact that PVC siding is not a so-called reservoir siding does indeed mean that the chance of inward solar vapor drive is reduced. That said, that fact does not affect the rate of wetting from wind-driven rain, nor does it affect the wintertime condensation of interior moisture.

    So there are lots of factors at play here. Suffice it to say that a rainscreen gap is always helpful, and never unhelpful. But a rainscreen gap isn't always necessary.

  9. Kevin Camfield | | #9

    I just found Martin's article on vapor drive too.
    https://www.greenbuildingadvisor.com/article/when-sunshine-drives-moisture-into-walls
    I'm thinking I may be okay without the rainscreen based on what I am reading there and in Joe Lstiburek's article. I would be using a non-reservoir cladding. I have plywood sheathing. I'm planning on using 30# felt. Port Townsend gets ~20" of rain a year, a cut off point in Joe's article. The siding will be light colored with reflective paint. We are close to the water and get wind driven rain which would be an argument for a rainscreen.
    Am I missing anything? Obviously a rainscreen would be better but I am looking for somewhere to save money to help offset the cost of this low maintenance siding.

  10. Expert Member
    Dana Dorsett | | #10

    There's no magic in that 20" rainfall boundary. Most places on earth that only get 20" of rain per year also have higher shoulder season average temperatures, and much higher insolation levels than you get in Port Townsend too. The light reflective paint also lowers the average sheathing temp a bit.

    Yes, you're in the Olympic Mountain rain shadow where it hardly ever rains, but in a more wind-exposed location than Port Angeles or Sequim, a location where some amount of wind-driven rain penetration is a given.

    I believe (but perhaps a local can fill us in on this) directly across the Strait of Juan de Fuca a few miles northwest of you in Victoria (still in the rain shadow, at about 24" per year) a minimum of 10mm rainscreen cavity or comparable capillary break is required for ALL types of siding.

    Most houses in your area would still do fine with just 1/4" (6mm) of gap, but zero is probably optimistic.

  11. Expert Member
    Malcolm Taylor | | #11

    "I believe... in Victoria (still in the rain shadow, at about 24" per year) a minimum of 10mm rainscreen cavity or comparable capillary break is required for ALL types of siding."

    Yes. The only exceptions being siding, like corrugated metal, which creates it's own cavity.

    All houses built before the rain-screen requirement didn't have moisture damage, and it's perfectly reasonable to believe that a well constructed and detailed house would do just fine in this climate, but the resilience adding a rain-screen builds into a house is something I wouldn't forego for a bit less maintenance on my exterior.

  12. Kevin Camfield | | #12

    Guys,
    Thanks for the responses, balanced and informative as always. Martin, I missed seeing your last post before I posted my previous reply. I'll look into the Home Slicker product more. It seems relatively easy to install with shingles. I think the only trick is the thickness of the trim around the windows. I appreciate the help.

  13. Expert Member
    Malcolm Taylor | | #13

    Kevin,
    if you do go with the PVC shingles it would be great to hear your experience and impressions of them once they are up.

  14. Kevin Camfield | | #14

    Well, that didn't work! Here is the latest on the shingle install. We decided to go with the NuCedar shingles and the Home Slicker rainscreen. NuCedar said that they didn't recommend this because it was overkill and not necessary but that it had been done several times successfully. We mocked it up on plywood boards a couple of times to make sure we could integrate the shingles with with the trim pieces and the Home Slicker. In each case we had some problems with the shingles cupping vertically when we attached them. We thought it was caused by the transition from the Cor-a-Vent vent piece at the bottom of the wall to the Home Slicker and we thought we had a solution to ease that transition. We are just getting ready to start siding after packing out all of the window openings and we are now finding that the cupping is a function of the Home Slicker and the shingles. No matter how gently we nail the shingles (gave up on stapling) it depresses the Home Slicker enough to cause the shingle to cup and the lower edge to lift off the shingle below it.

    We are struggling with what to do at this point. We used 1/4" AC plywood around the window openings to match the Home Slicker thickness. We made that 1 1/4" wider than the width of the trim we were planning to use to have an edge for the shingles to land on and allow us to caulk there. The window openings are all prepped with liquid flashing. NuCedar doesn't allow a furring strip type rainscreen type installation. The only option we can think of at this point is to go with a thinner, wider trim around the windows and on the corners. So change from a 5/4"x4" to a 1"x6" trim and eliminate the rainscreen. Our architect recommended perhaps going to a double layer of 30# felt to add some protection lost by the rainscreen. Some have suggested that would be equivalent to some of the 1mm dimpled WRB's available today. The 30# felt is on about 30% of the wall now and is integrated into the 1/4" plywood around the windows. We rabbeted the piece at the sill so that we could tuck the 30# felt behind it under the windows.

    Our other options are send back the NuCedar shingles and use a different siding. This would cost us 30% restocking plus freight. Or we could just go with a furring strip type install and hope that worked. This would void the NuCedar warranty.

    Any other thoughts you guys have would be very much appreciated! The NuCedar sales rep is working to find out how the shingles were installed with Home Slicker in the few places it was done that way.

    The shingles are beautiful and the service from the company has been very good as we try to work through this issue.

  15. Patrick OSullivan | | #15

    Hi Kevin,

    What did you ultimately end up doing here and how did it work out?

  16. Kevin Camfield | | #16

    Hi Patrick,

    We ended up going with wider trim and a double layer of 30# felt. We couldn't use thinner trim material because we had already received the windows and they were designed for 5/4" trim. Where one would normally use 5/4"x4" trim, we used 5/4"x6" We are using Azek trim. The wider and seemingly thicker (5/4" trim plus 1/4" backing) gives the house a very cottage / craftsman style look which we like.

    The double layer of 30# felt is not quite a double layer. We didn't want to have many places where we had 3 layers of felt at the vertical overlaps which then results in 4 layers at horizontal overlaps. So we are overlapping 17 3/4" using 36" wide material. We end up with 1/4" that is only one layer thick. We are using 3M spray contact adhesive and ZIP tape at any point were we end up with a reverse lap because of transitions from how the first 30# felt was laid. That's not ideal but these are always over another layer underneath that so if they leak there is still protection for the plywood. The double layer of felt is helping in another way. Because we put the felt on under the windows when we first packed them out (rabbeted out the lower piece so we could tuck the tar paper under) it has been on the wall too long. Even though it has been winter time in the Pacific Northwest which is gloomy for lack of sun, the paper is showing signs of sun exposure. The second layer over the top ensures we have a fresh layer before the shingles go on.

    We are using 13" Nucedar Shingles which have a 5" reveal. We are using a 3/8" stagger so there are some places where the exposure is 5 3/8". Nucedar requires using two small dollops of sealant on the back side of the shingles for the first 4 courses. We are doing that for all of the courses because of the slightly larger exposure and because we have wind driven rain.

    In hindsight I wish the supplier had just told us that we couldn't use the rain slicker product. That would have saved us a lot of time and money but the end result looks great.

  17. Deleted | | #17

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