need rainscreen advice from anyone who has installed one -using plywood strips
Hi. I wanted to post a (rather long) question with several parts (apologies in advance) on this forum because in my internet research on rain screens this forum keeps coming up. Many great discussions. However, a lot of the discussions are 5 years old or more and I’ve got an odd situation in that I’m re using old windows with no flanges in a totally re built wall . So it’s kind of like old windows in a new construction exterior wall, covered in Tyvek currently.
BTW, I will be using pine novelty siding. (7 1/4 by 3/4) My reasons for wanting the rain screen are as follows-
1) the siding on the back of the house is very close to the ground and I read an article that said rain screens were really good for this type of situation where moisture could wick up from the ground.
2) even though I am in Upstate NY which may not be a notoriously rainy area, the Yearly averages that I looked up are saying something in the neighborhood of 40-44 inches per year which seemed significant. Am I reading these figures wrong? Does that include snowfall? I tried to search for exclusively rain fall figures.
3) the main reason I had to rebuild the wall was due to rot from Moisture and Termites which obviously go hand in hand. We are putting a new roof on this fall which will feature a 12″ overhang over this wall where previously there was only a 3 inch overhang. That will obviously help a lot too.
I’ve read just about everything I can find in the limited time I have to research this regarding rain screens. Here’s what I know and also what I’m thinking I’d do. Please advise. I’m sure I’m wrong about some of it.
1) vertical strips of plywood attached over the Tyvek which I installed a few days ago. This does not need to be Pressure Treated because the design is such that the strips will dry out. Covering the strips with strips of building paper might be a good thing. Does that mean they should be just covered on the face with the edges of the ply still uncovered?
2) Insect screen applied a few inches from the bottom of the sheathing then wrapped up over the face of the plywood strips, creating an insect barrier. This should probably be metal not Fiberglass, correct?
Aluminum ok or should it be galvanized?
3) All aspects of siding and trim would be installed on this new plane created by plywood strips. This would include corner boards, PVC water table (or skirt board), and window and door casings.
4) I imagine the strips under the corner boards would be simple enough. they would just need to be big enough to nail the corner board to as well as the end of the siding that is butting in. So in the case of a 3 1/2 inch wide corner board there could be two ways of doing it- install a plywood strip that is 4 1/2 ” that goes all the way to the edge of the corner and exceeds the width of the corner board by 1 ” creating a nailer for the siding. Or- rip a smaller strip maybe 3″ and just start it 2″ in from the edge of the corner so that 1 1/2″ inches is under the corner board and 1 1/2 is under the siding. I think this might be good seeing as the corner boards will be pre- assembled on the ground so they will be plenty strong even if they are unsupported for the 2″ closest to the edge. This would also allow for drainage behind the corner board.
5) The PVC water table will be attached directly to the strips. There is an integrated drip cap on the product I’m using btw. The question here ( and with the head casings on the window/doors) is usually I would leave the house wrap long, lap it over the water table drip cap and trim it after the siding went on above it. I won’t be able to do that here. Same with a metal flashing piece. even though it’s pvc I believe it’s still recommended to use a metal flashing to keep water from going behind the drip cap. Maybe that isn’t necessary if the water can drain.
6) Windows and Doors. This is where most people’s confusion is….mine too. I have found a lot of new construction detail directions but I’m unclear about my situation. So what do I do here?
Same deal on the side casings as the corner boards….is it one wide strip supporting the entire casing and the end of the siding that butts in? Or is it multiple strips creating a drainage channel? Either way, what happens with the self adhering membrane I plan on using? Is it over the strips? Under? Both?
7) Head casings…..completely confusing. Are these vertical strips again and if so what happens with the house wrap that one would usually be using to overlap the drip cap after he had applied a head flashing?
By the way I am planning on using 1/4 CDX plywood for the strips. I’ll use 3/8 if the lumber yard doesn’t have 1/4 CDX. But nothing larger than that. Also this is only for the back wall of the house only. The adjacent walls are keeping their original 150 year old siding. Thanks in advance for any help sorting this out.
Replies
couple of thoughts...
Have you considered using Benjamin Obdyke's home slicker instead of plywood strips for your rain screen venting?
It's 10 mm thick and meets Canadian building codes for rainscreens.This would avoid any moisture issues with wood furring strips and allow air to evenly across the vent plane eliminating dead-end vertical furring strip channels (i.e. under windows). I'd also recommend using bronze insect screen for the longest lasting bug screen. Staple the top edge of a 6" horizontal strip along the bottom of ext. wall, install the home slicker over it, then pull the lower hanging edge up over the homeslicker and secure it with staples. Easy bug screen detail that will last, virtually, forever.
As for your casing and corner boards: you dont have to vent behind these trim elements. You apply them (or their furring) directly to your WRB and flash according. Bulk water should always be shed to the outside with flashings. If you choose to run your vent screen behind windows and corner boards (why would you?), you'd still have to flash your openings and integrate them into your WRB. The details get more involved. I'd have to know why you'd want to do this in order to justify coming up with flashing details that would work for that desired condition.
One thing to keep in mind is maintating a tight air barrier on your exterior insulating envelope. The whole idea of a rain screen is that you facilitate wind wash under the siding. IMO, you want to have low perm WRB/ wall sheathing system in place under the screen in order to reduce the chance of air infiltration negatively effecting your wall cavity insulation.
Thanks for the reply. I am aware of the roll out membranes such as home slicker but was thinking It might be simpler to use the furring strip appraoch. Maybe not simpler, but less expensive and also the home slicker might be a hard to find item at the Lumber yards around here. I don't know that for sure but....
The reason I would run the furring strips behind the casings and corner boards would be to acheive the desired ratio between the siding and the trim. The trim I will be using will be standard 5/4 by 4 which is actually 1 1/8" thick and the siding will be novelty siding at 3/4" thick. This is a difference of 3/8". The original window casings on the adjacent sides of the house where I didn't have to rip off the old siding stands proud of the siding by 5/16". Close eniough to 3/8" for me. So it would seem that having the back of the siding and the back of the trim bourds installed on the same plane would be the way to go.
As far as my envelope is concerned- I sheathed the wall with 7/16 OSB and wrapped it in Tyvek. Not everyone's favorite choices but it's too late to change that now.
So when you say "apply them or their furring dirctly to the WRB and flash accordingly" do you mean (in the case of windows) run a wider furring strip on the verticals that would also catch the end of the siding and then flash with a peel and stick membrane as you would if the furring strip were'nt there? These are old timey flangeless windows by the way so I'm probably going to pre-apply the casings to the jambs then install the windows. So I'll be relying heavily on the casings for weather proofing. Almost as if they were flanges themselves. But without the Vycor over the top. (joke)
I would NOT run furring strips. I would use the homeslicker. The only furring is when you 'pad' the trim out to get the reveals you desire between the finish face of siding and finish face of ext. trim. I probably should have called them pads instead of furring so as not to confuse the issue. Sorry. Anyhow, see rough sketch. You don't necessarily need to run your 'pad outs' past your exterior trim. The home slicker will support your siding nailing. You will tape and flash these edges however. The sill flashing condition would occur also at the sides of window. Only the head detail is different. Hopefully you've allowed for a pitch in your rough of window R.O.'s. If not, I would suggest changing them to include this detail. We always do.
BTW, I have used Vykor in the past. I use Zip tape now. It sticks better and I feel gives a better long term seal and bond. It also layers better over itself. YOu will want to prime your OSB to allow either the Vykor or Zip to stick better to the sheathing. This is one rason we use Zip panals now instead of regular OSB. Another is that zip panels are less permeable than regular OSB. If you wanted to decrease the perm rating of your already installed OSB, then perhaps you could take off your Tyvel and spray an acrylic exterior paint over the entire sheathing area.This would achieve both your priming and less perm goals with only the loss of Tyvek material and labor costs (not much IMO in the scheme of things). You could then use Home Slicker plus which has a built in WRB.
Hope this helps.
Cheers!
Wow! Thanks man. I need to look at this more carefully in a few hours after I go demo a portion of the roof. But this sketch is really above and beyond. Much gratitude. More later
So I looked over the sketch more closely. Very, very helpful. Thanks for taking the time. And I found Home Slicker at two of the lumber yards near me so I guess it's not that hard of a product to find after all. I'm trying to figure out if I can justify the cost. Price is probably going to be the deciding factor here.
So I just want to make sure I understand what you mean in your 1st paragraph when you say the "pads" don't need to extend past the ext. trim but I will need to flash them.......
The way I understand that is - In the case of the verticle window casings, I would have a packing strip or pad that is the width of the space between the rough opening and where the outer edge of the casing will sit....it will fill that space ( no drainage channel ) and then will be flashed with an adhesive membrane that goes over it's edge and (I'm assuming) would lap onto the WRB. Do I have that right?
Also, if I'm using an Azek drip cap over the head casing can I skip the metal flashing? It seems like I'll have to custom bend something to conform to that shape. Or maybe I should skip the azek and just go with the metal.....?
As far as removing the WRB and priming the sheathing...I just don't have time for that. I am pretty seriously behind schedule. I could probably cut the WRB for the Rough Openings, fold them back a bit and prime the few areas where the Zip tape is going to be making direct contact with the OSB.
If you've already installed the wrb and haven't flashed around the windows, then you've already seriously compromised the envelope. The wrb has to be installed shingle style with upper sheets overlapping lower sheets. People who are really serious about keeping out wind driven rain use at least two layers of wrb. 100% overlap allows the outermost layer to cover the flashing, vycor, zip tape, or whatever else.
Here's a link to a very good guide on rainscreen details: http://www.mtcc1170.com/images/BCRainScreen.pdf
The WRB is on there but it's just running right over the rough openings. No windows or doors are installed yet. So I still have time to flash windows. I have rolls of protecto wrap for that. The WRB is generously overlapped shingle style. Probaly about 12" overlap. I am still in the process of window and door re-hab so none of those are ready to go in. Really old windows.....no flanges. Oh yeah.....and I'm re framing the roof.
Don't use Protecto wrap if you can possibly help it. It's the worse self stick flashing I've ever used. Use Vycor or zip tape and make sure you roll it after application. It is pressure sensitive.
Plywood in a wet application?
I am only an allegedly humble DIYer, but plywood seems like a real bad idea in a wet application. If it gets wet and swells, it is toast.
Nor have I ever seen plywood used for this application in any magazine like FHB or JCL, or any house porn TV program like TOH.
Good luck.
Thanks for the reply. I've gotten the impression that the ply, like the back of the siding, would have a chance to dry out and therefore not swell and rot. Plus I have seen examples from reputable sources using plywood. Can't remember where but I probably have it bookmarked. I definitely don't want to create a 3/4 " gap with furring strips so if I wanted to do it out of solid wood I guess I would either use lath or rip down SPF strips to the dimension I need.
Honestly I'm trying to figure out if I can afford to use the homeslicker product and not use any strips.
You'd want to use exterior or exposure 1/exposure 2 plywood, not interior stuff.
Thanks for that.How was detailing the windows and doors?
It wasn't a big deal because the old siding was furred out with rough cut lath, so no big change there, and I properly prepared some new windows that were installed during the re-siding. I used peel and stick and copper flashing, but found the Protecto wrap adhesive was not up to the job, so I had to staple it down, Grace gave me no trouble except if it even touched anything, it was going to stay there.
The old windows were furred out enough so the windows were in the proper plane.