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Community and Q&A

Wood for Rainscreen Furring

dragonfly63 | Posted in General Questions on

Well after much researching this weekend and Malcom’s comment on Friday about peoples reticence on commenting on my framed chase details with adhered stone veneer I have come to the conclusion……Yep it was a bad idea.  So thank you Malcom for weighing in and getting me to realize the error in my ways!  Now on to plan B.

This side of the house and the chimney chase will get Hardie siding and the chimney shoulders will get asphalt shingles, complete with step, base, and kickout flashing and a drip edge.  The top of the chase is getting a SS chase cover with a drip edge and diagonal creases. I did end up using the Prosoco joint and seam filler and fast flash on the corners and sheathing joints because I had bought the sausage gun and it was pretty straight forward albeit messy to apply. While I would love to hire a siding company to do the siding  most of them will only do a whole house and tell me to just hire a handyman. It’s 200 sq feet so I think we will do it ourselves.  No run on this side of the house is longer than 10 feet so I won’t have any butt joints.

The rest of our house has T1-11 siding with a 4″ OC groove.  Luckily we have great overhangs on this house so most of it is in good condition.  Since this side of the house and the chase will be different than the rest we want something that will look good with the current siding and on the chimney chase.  We just tore of old cedar ship lap siding that had been applied diagonally. Options are:

Hardi plank  – either a 4″ or 5″ reveal (5/16″ thick)
Hardi Artisan siding (shiplap) with lock joint joint system (5/8″ thick)

We plan to use Hydrogap as our WRB.  Both of these products require a drainable housrewrap with a 90% or better drainage efficiency.  The lap siding is not flat to wall but the Artisan siding is.  Is the Hydrogap enough for my Zone 4c climate?  Hardie says it is but I know it’s not a true rain screen.  For the commercial application of the Artisan they require a rain screen with specific information on the type of furring:Wood Furring:

If wood furring is not being used as a nailable substrate, there is no wood species or speci c gravity requirement. Furring should be of suf cient width to assure adequate siding fastener connection; widths from 2 in. to 3.5 in. are recommended. Wall corner intersections may require wider furring to accommodate trim.

If wood furring is being used as a nailable substrate, material must be spruce, pine, fir or any other wood species with a specifc gravity of 0.42 or greater in accordance with the American Forest and Paper Association (AFPA) and American Wood Council National Design Speci cation for Wood Construction (NDS).

Wood furring shall conform to building code for natural decay resistance or treated lumber (2012 IBC §718.2). Typical wood rainscreen furring includes treated 1/2 in., 3/4 in., 3/8 in. thick plywood, or treated nominal 1×4 in. lumber (actual 3/4 in. thick).

I can find nothing on compatibility of house wraps with treated lumber.  I know Tyvek does like cedar next to it but I saw Hydrogap installed with cedar shingles on it but nothing about PT lumber.  Does anyone know?

I know there has been much discussion on this website on whether to use PT or regular plywood for furring.  From Hardies tech docs it appears that PT furring is a IBC requirement but maybe it’s only in relation to commercial or multifamily units.

Any comments appreciated!

Thanks, KJ

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  1. user-2310254 | | #1


    On my last home, I used Tyvek Commercial D and 5/5 pressure treated furring. But I've also read on this site that the higher cost Commercial D isn't necessary and standard lumber is fine since the air gap keeps it dry.

  2. dragonfly63 | | #2

    Thanks Steve for your reply. I've seen nothing about PT lumber and compatibility with any of the house wraps. We will have to talk to the building inspector about wether there is a requirement on types of rain screen furring strips (plywood, Cora a vent, PT lumber).

    I did call both Benjamin Obedyke and Prosoco about compatibility between Fast Flash (on sheathing corners and seams) and HydroGap and they both confirmed there was no compatibility issue. However Tyvek states:
    Dupont Tyvek Commercial wrap and Commercial D wrap must not come in contact with other manufacturers cured or uncured fluid applied waterproofing products due to potential impact on performance properties. Dupont StraightFlash can be used as a transitional membrane.

    They don't mention any such issue with their home wrap, drainwrap or stucco wrap. I know each house wrap manufactures wants you to use only their product line for continuity. I guess there is another phone call to be made because the fast flash is certainly not coming off..

    My window has 1 3/8" from the flange to the exterior edge of the window. The artisan V groove siding is 5/8" thick so I could almost use 3/8" furring strips and 5/4 trim stock and not have the trim extend past the window but the flange will bump me out (whatever the thickness of the flange is 1/8"?) unless I rabbet that area of the trim. 1/4" furring might be safer and easier for us to detail and use the CoraVent at the top and bottom of wall and top and bottom of window. It only comes in 3/8". I'm going to call Menzies metal tomorrow and see how much their perforated J channel is shipped to me. I would need (6) 10 foot sections but it comes in 1/4", 3/8, 1/2 & 3/4 so it would play nice with 1/4 furring strips.

    I've been studying Hammer and Hands best practices manual on rain screens but they just show 3/4 inch thick furring but the detailing is the same. Hopefully we get it figured out soon cause winter is coming!

  3. Expert Member


    I'm not that familiar with your building codes so I won't comment on the possible PT wood requirement, but will weigh in on a couple other issues.

    - The compatibility issues with Tyvek and the tannins in cedar have been solved for a few years now.
    - I have never heard of compatibility problems with any house-wraps and PT lumber of any formulation.
    - Consider using Tyvek Commercial. It is a lot tougher. It does have a lower perm rating, but that can be beneficial with a reservoir cladding like cement board.
    - Don't run your furring over the window flanges. It creates an uneven substrate, impedes air-movement around the window, and makes any future repairs harder.
    - If you can increase the thickness of your furring try to do so. 1/4" material is finicky to work with and fasten, and a 1/4" cavity is easily bridged by slight wrinkles in the house-wrap defeating the capillary break, and impeding the air-flow.

  4. wranch | | #4

    Not sure if misread your post but Cora-Vent is manufactured in 3/4" thick pieces to match nominal 1x furring strip.

  5. dragonfly63 | | #5

    Not misread Michael. CoraVent comes in 2 different options. Sv-5 is for 3/4" furring strips but they also make a SV-3 which is a 3/8' furring strip. The SV-3 is more applicable to my situation.

    The Hardie Aspire collection groove siding that we want to use is 5/8" thick and with a 5/4" trim would give us a 3/8" shadow line and would work with a max of a 3/8" furring strip (1/4" better as I don't want trim proud of window).

    I contacted JH as their literature for that particular siding suggests that you must use a 1 1/2" trim which is correct for their Artisan Lap siding but they said for the v groove siding (flat to wall) that 5/4 trim would be fine. I'm just trying to find the fine line of product, trim options and do-ability of a DIY rain screen for my situation.

    I have contacted 4 siding companies here in the Portland Metro area today to see if anyone does rain screens to see if their interested in the project. We shall see.

    1. Expert Member
      MALCOLM TAYLOR | | #7


      I may be misunderstanding the problem, but why do you not want the trim to extend past the window? You want the furring and the siding to be covered by exterior portion of the window, but it's preferable to have most of the trim sit proud of the window frame, both practically and I think aesthetically. I've never built a house where that wasn't the relationship between the two.

  6. Stockwell | | #6

    I used 1/2" plywood as furring strips with my Hardi Artisan with SV-3. We mounted the windows on 1/2" ply as well so we wouldn't run into the issues you are talking about with trim and window reveal.

  7. dragonfly63 | | #8

    Noted about using the thicker furring, 1/4 is pretty darn thin and that there have been no issues with PT or cedar with house wraps. 3/8" is about as thick as I would feel comfortable doing on our own. I'm attaching a picture of the window trim as I may not be explaining myself well. Not sure why pics come out sideways but it won't let me rotate it once its attached. This is not my house but you can see the trim does not sit proud of the window which is how it is on my house. It seems if your trim were proud then you would have a ledge that could hold water. We don't have window sills just picture frame trim.

    Regarding the furring around the windows. My plan was to use the coravent above and below windows, bottom of wall and top of wall. Looking at Hammer and Hands example they are installing the furring on the flange after WRB, then trim, then slitting the WRB and folding up to attach rigid head flashing, web back down and adding the rest of the furring. Coravents instructions do show their product over the flange but they show a different way of rigid head flashing. I was going to put the rigid head flashing above the trim. If I place furring strips next to flanges I would imagine I would just have to place filler/shims behind my trim in the area of the flange.

    I would use Tyvek Commercial wrap but as I noted above Tyveks documentation specifically says they are not to be laid over another manufacturers cured or uncured fluid applied barrier. The fast flash is already on all corners and sheathing seams. Do I take a chance on the Prosoco deteriorating the TyveK? Hmmm When you look at the H & H rain screen detail the WRB looks a lot like Tyvek but of course their is no branding in the picture.

    Kevin: It's funny I just got through reading your Backyard Building Science post from 2018. When you say you mounted the windows on 1/2" ply do you mean you wrapped your WRB then added 1/2" furring around your rough opening, onto of whatever sheathing you had? Which Artisan product did you use (we're planning on the v groove with lock joint) and what size trim? I plan on using Boral TruExterior trim which they carry at Lakeside Lumber here in Portland. They also make a rabbeted trim that will be helpful at the bottom of the window and at the top of the wall. Not cheap but everyone says its great for wet climates and easier to work with than Hardie trim.


    1. Expert Member
      MALCOLM TAYLOR | | #9


      It is very unusual to have the window frames proud of the trim. Having the trim protrude (say 3/4" ) past them provides a corner for a bead of caulking. The trim at the bottom, whether it has a sill or is picture framed, should have 6 degree slope to shed any water, and be caulked. Protruding trim also adds visual relief to the flat plane of walls.

      I don't want to knock the excellent Hammer and Hand details, but I think they are overly complicated and you can achieve a similarly robust assembly using the method we commonly do here in coastal BC.

      - Don't bother venting above or below windows. The benefit of increased air-flow is offset by the possibility of water intrusion ( if that Cor-a-vent sill detail you posted doesn't leak I'd be amazed!). Instead stagger the furring below and above the window to provide a path for air to flow around the opening - and keep it off the flanges. The flanges are typically only 1 1/4" wide, so there is no need to add shims or blocking to support the trim - especially if it is being installed on top of the cladding.
      - Above the window keep the siding and trim up off the head-flashing 1/8". This allows water in the cavity to drain, and some small amount of air to enter - but is thin enough another layer of Cor-a-vent isn't necessary.
      - The additional benefits of venting a rain-screen at the top are in my mind insufficient except in very cold climates where some condensation at the eaves may occur.

      I'm sorry, I don't have any experience with liquid flashing or its compatibility with house-wraps.

  8. dragonfly63 | | #10

    I admit since I'm not a professional it's easier for me to see things visually along with reading about them. Pictures are worth a lot of words for me :)

    I've seen sloped sills but never sloped flat trim at the bottom but it makes sense. I was worried that protruding trim and a caulked joint would lead to a maintenance issue but I suppose if the sill flashing and WRB are done correctly no big deal if some water leaks behind the trim especially on a furring strip.

    The trim will not go on top of the cladding and I believe Hardie does not allow that. Siding will butt up against the trim with a 1/8 gap. For head flashing: fold down ends over trim or make a end dam?

    To be fair to the Coravent it also acts as a bug screen so below a window or at the bottom of the wall (between the furring strips) seems a lot better than wrapping bug screen. Do you mind sharing what you pay for a 10 foot piece of the perforated j channel from Menzies?

    I've got a call into the Tyvek rep so we'll see what they say about the Prosoco fast flash and I'll let you know. I suppose I could wrap something over the corners to be a transition sheet between them but the horizontal seams are tougher to address.

    We've called six siding contractors trying to get someone to look at our project but only 1 has called back. Hiring contractors in this town is frustrating. You know of a good contractor in the Portland area who would do a one wall siding job do ya?

    I just wanted to say thank you again for taking the time to offer your thoughts and opinions. It is much appreciated as I know I'm being a bit of a pest.

    1. Expert Member
      MALCOLM TAYLOR | | #11


      - Definitely end-dams.This is especially important with a rain-screen as water can move horizontally into the cavity. If the end-dam leaves too large a gap between the head-trim and flashing, cut slits in the trim to set it down a bit.
      - I can see maybe using Cor-a-vent over the window, but it seems like a very bad idea to me to leave any gap between the trim and window sill such that you would need it below. We can get any profile of perorated J trim at our lumberyards here so i don't use Menzies Metals. I just paid $19.99 for a ten foot strip of 1" deep stock I used for some roof vents (photo attached).
      - Sorry I don't know any Portland contractors. Things are so busy here right now local contractors are booking jobs well into next summer. A new house can be two years!

  9. dragonfly63 | | #12

    Heard back from Tyvek rep and he said he didn't see a problem with the Tyvek over the Fast Flash on sheathing seams (even though their literature says no other fluid applied barrier except theirs) but said the fast flash would not stick to it which I already knew so rep's statement doesn't match with their install document. Hmmm..

    I asked about compatibility with PT lumber/furring strips and unfortunately our connection was so bad kinda hard to hear him but he suggested Borate treated furring strips, which Parr lumber does carry but only in 1/2 or 3/4 so kinda noncommittal there as well.

    I figured end dams and I have been checking out some new builds around our house and they all incorporate them but all of them have the head flashing above the trim not the window? Wrong or just a different way to do it?

    1. Expert Member
      MALCOLM TAYLOR | | #13


      With rain-screens there are many variations on where the flashing should go based on where the trim is.

      - If the trim is fastened directly to the sheathing, the head-flashing needs to go above the trim.
      - If the trim is mounted on the furring strips and the siding butts to it, the head-flashing needs to go above the trim, and you can provide an optional second one below, but it isn't necessary.
      - If the trim is situated on top of the siding, the head-flashing goes directly over the window.

  10. dragonfly63 | | #14

    Thanks Malcom. We took the rest of our old siding off around the window this weekend. Geez what a mess. This window was obviously replacement window but it did have a nail flange its just that the small amount of tape flashing and LARGE amounts of caulk, the screws holding it at the top flange did not even go into the sheathing/framing. What a mess. I was surprised not to see any rot but like I said this house has good overhangs.

    We'll see if we get the window off in 1 piece there is so much caulk behind it and there is a crack on the top flange. Not sure if that can be repaired or not. Ah the joys of remodeling.

    We have 2 siding contractors coming this week so we'll see what they say and if they are familiar with rain screens.

  11. dragonfly63 | | #15

    Hey Malcolm, Hope all is well with you.

    Well the siding contractors came and it turns out that it would be $7500 to side the one wall so it looks like we will have to conquer this job on our own.

    Thanks for the answer to the rigid head flashing placement. If I install trim on the furring strips would I run the furring strip horizontal above the window? Or would I use short vertical ones the same height as the trim(as shown in hammer and hands manual), then install the rigid head flashing, then fold down the wrb over the flashing?

    Also does the top leg of the rigid flashing get set in silicone or covered with another piece of self adhered flashing?

    Tyvek commercial wrap is only available in a 10 foot height at our local lumber yard which seems like it will be kinda unwieldly although I could order the 5 foot online. HydroGap is readily available here although it would be overkill with a rain screen but the price is right %110 versus $300 for commercial wrap. Thoughts?

    1. Expert Member
      MALCOLM TAYLOR | | #16


      - I don't think it makes any difference whether you use horizontal or small vertical strips behind the head-trim.

      - The head-flashing should be set in a bed of caulking applied to the top of the trim to stop water getting underneath in windy conditions. I don't think the top-leg needs anything as long as the WRB is lapped over it.

      - I've only ever used building paper, Tyvek and Commercial Tyvek as WRBs, so my opinion on other brands isn't very useful. Of those, my preference is for Tyvek Commercial, because it is a lot harder to tear than regular Tyvek. When I want sheet goods in a width other than they come in, I cut the roll in half.

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