Hardi does not like creative solutions haha.

Yeah they kind of do - if not that's your JOB.

Did I say it wasn't?

Yeah it is my job. That's why on the several ovations where they were not supportive of proposed applications we switched to more flexible product types. It's not a complaint about them, just an observation of the limitations on their product application based on personal experience.

I hot and humid climates, the vapour barrier normally goes on the exterior face of insulation.

This is true but it needs to be really hot and really humid. It's about the dew point location. Also if you're using an insulation with certain types of facing and sealing the joints you don't want a vapor barrier at all. Like I said above - we don't really know enough about the OP's situation to offer absolute solutions.

Also this is a sketch I did for a project I'm working on in western Colorado (temp ranges from 105F to 15F with low humidity). 

I do think the window having a nailing flange is going to create flashing issues at the jamb and sills though . . .

Why is your window so far inset, behind the line of insulation? You have a huge thermal break as shown. HUGE. You also better hope that no water gets behind your insulation. I haven't seen building wrap on top of rigid insulation except when homeowners do a shitty DIY job. What's going on there? Honest question.

The window is inset because you can't install it over the rigid insulation without adding wood blocking - that creates a HUGE thermal bridge.  The window frame is thermally broken. 

The building wrap is typically installed over the surface of polyiso rigid insulation with a 'K" shaped break metal flashing sealed to the building wrap via flexible flashing.  This keeps the drainage plain outside the rigid insulation which is a good thing as polyiso and xps insulation looses a lot of it's r-value when wet and takes a long time to dry out.  

Ok I hadn't considered polyiso... All the same, you can still get insulation over the blocking (or use clips). And if you have a vented rain screen, who cares if the insulation gets a little wet? It'll drain and dry out (which it can't easily do if water gets behind the insulation).

Check out the plywood window box in the "Foam Shrinks..." article I linked further down the page. Also a good discussion of "innies" and "outies" here.

BB - installing wood blocking out to the face of the continuous insulation is more difficult to install and requires a lot of cutting. Also installing continuous insulation equal to the wall r-value around the wood blocking is a pain. Finally a 'little water' in the rigid insulation is a big deal. As I said above it reduces the rigids r-value (around 40%), and takes a long time to dry out (2-3 months). When using a vented rain screen you want to keep the drainage plain outside of the continuous insulation. In this type of system any water has a really, really low chance of water getting behind the insulation where it can't dry out. If you put the building wrap behind the continuous insulation like you're recommending then water is ALWAYS behind the insulation.

All that being said draining continuous insulation wall systems can be a very tricky thing and their is a lot to learn.  They are very dependent on your location and climate.  I do not think I am am an expert on this so I really appreciate this discussion.  

May I ask what type of continuous insulation rain screen wall assemblies are your detailing?

EA - we used to do the 'wood box' around opening in continuous insulated rain screen assemblies. We found it a lot easier to use brake metal flashing around the openings and set the window / door back in the stud wall. Keep in mind this is for commercial construction so we're using thermally broken aluminum frames and don't have any type of trim around the openings.

Chad, noted. It's picky of me but you've still got a nice thermal bridge around the window frame as you drew it. That's why people are wondering why you don't have it further out, more in line with the insulation. Will it cause your building to fall down? No. Will it mean your client spends more in energy over the life of the building? Yes. Is there potential for condensation on really cold winter nights around the window? Yes, but it depends on the temps, and interior RH. Does it have thermal comfort implications for people next to these windows? Yes, but all windows do to some extent. Does any of this matter to your client and you? I don't know.

Oh I agree - however adding a continuous piece of wood blocking around the opening and moving the window out over the insulation like BB recommended creates an even bigger thermal bridge.

That's why I suggested checking out Lstiburek's plywood window box. 3/4" plywood with some intermittent metal straps is less thermal bridging than what you've shown.

I'd have to disagree that the above plywood box has less of a thermal bridge than the original sketch.

We'll agree to disagree then I guess. I don't have any studies or energy modeling (or anything like that) on hand to show it more objectively, but intuitively I believe the 3/4" of plywood around the perimeter is less of a bridge than the full perimeter of 1-1/2" to 2" wide connection between the header/jamb/sill and the front portion of the window frame before the thermal break, and the metal radiator fin with the brake metal flashing you've shown.

For the sake of images ...

I'm not for using a continuous 2x box though. I think option B works better than option A in terms of limiting thermal bridging. I was referring to using a plywood box vs bringing the continuous insulation all the way down the T.O. the window frame (aka the original sketch) 

BB, the detail Chad showed originally is pretty typical in high performance residential construction. The header as shown is R-5.4 so it's not a terrible thermal bridge. Windows perform best, thermally, when located near the center of the wall, and there are a lot of standard water management details for that approach.

@Chad

I hate the UI on this site... 

I was going to say.... With rain screens we often use mineral wool over air/water barrier placed directly on the sheathing. and forgo the vapor barrier (depending on climate) 

For a lot of climates the dewpoint moves from the exterior to the interior when it matters most, and having insulation outside of the water barrier keeps moisture out of the walls and helps avoid mold.... 

We have started doing more and more WUFI simulations for main building assemblies to try to validate in the last couple years. 

What type of climate are you designing for natematt? I ask because I might have omitted a very important bit of info in my posts - the climate I'm designing for. We're high desert with relative humidity around 5% with seasonal temperatures swings from 20 F to 105 F.

a different window type was my solution as well

Neat!

You can use z-channels but you need to thermally isolate them where they attach to the studs otherwise you get a nasty thermal bridge.

The other option is just as you speculated - long screws that attach to the studs with plastic washers at the face of the rigid insulation.

fyi james hardi has tables to show what size and spacing to use for long screws

Lstiburek is so good.

Minutes after posting this, Joe's BSC Newsletter showed up in my inbox.

That's a great article.

Also if you don't already, everyone should follow Joe's daughter on Instagram (building science fight club). She's great.

anyone used these in residential? how spendy are these things?

Where I am, they are about $6/ea supply only. At every 24", even one sheet of plywood gets very expensive. I've actually never seen them used, likely because of cost.

Keep in mind the thickness of the air space in a rain screen assembly is climate dependent. Typically 'wider is better', especially in cold climates.  

Yup. See my responses above. Insanely expensive (about $40/ sheet of sheathing, and then double that for install). So the clip system is something stupid like $2.50/sf alone. The contractors I work with would use these, they're just too expensive. The install isn't any different than brick ties, really.

^^^ yep. This is how it's done in the residential world.

coravent if wood gives you the hives

I'd recommend Benadryl if wood gives you the hives.

Or even still gasket.

this is what we've been doing

Coravent and other products that allow horizontal air flow are cool but don't work well with rigid foam (or rigid wood fiber) because they compress when you install them. At least that's what the builders I work with have told me. We usually use 3/4" strapping. I've tried 1/2" plywood or osb rips and they are too flexible. I've also cross-strapped for a 1 1/2" gap with 3-dimensional air flow--that's the best, but extra work.

you mean the coravent compresses? that's unfortunate. Do you mean steel straps similar to the type used for "blocking" in commercial work?

Pointing out that we've been mostly talking lap siding this whole time. Not sure the mortar droppings would be present, but I'm not sure how they typically install lap siding up in those Canadian cold climates. I suppose setting them in mortar would be possible, but probably would be cause for premature failure of the siding. To each their own I suppose.

My comment refers to "air space in a rain screen assembly"; not necessarily just for lap siding.

Shit, is that what I’ve been doing wrong all these years? Hold my beer, I’m rewriting my wood cladding specs to include mortar. Should I recommend they put mortar on the back side of the planks too like we do for flooring tiles?

Shit, is that what I’ve been doing wrong all these years? Hold my beer, I’m rewriting my wood cladding specs to include mortar. Should I recommend they put mortar on the back side of the planks too like we do for flooring tiles?

What's your point APS?

EA - a masonry cavity wall with continuous insulation is a rain screen system. It's a crappy systems but still a rain screen.

I know it is, but saying, 'What about masonry rainscreens and the mortar droppings' when we've all been talking lap siding is a bit off topic ... which is why I gave the snark.

1/2" air space even in a cold climate for lap siding is probably good enough. 3/4" to 1" is better, but it wouldn't need 2". Masonry on the other hand, yeah, you'd want 1-1/2" to 2" easily.

I have disagree with you about the thickness of an airspace in rain screen assemblies. I've found that in cold climates where temps get below 0F you want at least 1 1/2" air space.

Chad, I remember 3/8" is the min gap to prevent capillary suction. We have winters that regularly see -40f and always keep the gap to 3/4" to 1".

IIRC, Straube and Lstiburek have a lot of information on the topic.

I've mentioned it before, but I'll say it again. Straube was my building envelop prof in grad school. Class had only 4 students too so it was basically a semester-long private class on this stuff. Final exam (and grade, I believe) was based on an hour long post course one on one interview with Straube.

^ That's why I name-dropped him. I'm just giving you another chance to brag.

I wouldn't add 2x wood blocking around a window with a nailing flange - you're going to create a rather nasty drainage problem.

Chad, I've done that on several occasions and it's really not bad--just wrap peel-and-stick over the built-out bucks. But ThermalBuck does the same thing while also providing a thermal break.

So what you're saying is we should go buck ourselves?

Wood - that's what we've done in the past.  How did you solve the problem of having multiple layers of flexible flashing combined with the protruding drainage flange of the window frame being set behind or in front of the the exterior finish material and acting as a hydrostatic 'scoop' sucking up water?

Chad, I'm not sure what you mean by hydrostatic scoop, but what I did yesterday on a side job was to staple strips of Obdyke's Homeslicker on the back of the casing before I installed it. Very little water should get past the cladding and trim, but I know some will so I provide a drainage plane. I realize that's not clear on the detail I posted.

Here are more articles about that build. You may need to register to read them:
https://www.finehomebuilding.c...
https://www.finehomebuilding.c...
https://www.finehomebuilding.c...

Thanks for the info! That's what we where running into as well - a lot of flexible and metal flashing.

It's import to note the location of your building wrap.  When using EPS foam the drainage plain is behind the insulation unless it has a facing.  When using Polyiso or XPS foam the building wrap is installed over the exterior face of the insulation.  

WG, do you rely on the taped joints of the zip sheathing as moisture barrier? Also, and perhaps more important, I can't stand leader lines that cross cut marks. You had the space, organize those notes, man.

Chad, I generally prefer to place the flange at the sheathing and use exterior extension jambs. Or use flangeless windows. But sometimes options are limited.

On this project we ended up using rigid mineral wool, but regardless of exterior insulation material I locate the water control layer at the sheathing. The minute amount of water that gets through the cladding will not hurt 15 psi EPS. I don't use XPS due to the climate-damaging blowing agent. I usually use recycled polyiso when I have to use exterior insulation, but I typically do double stud walls instead. 

Non, when there is exterior insulation I'm ok with the Zip system as the WRB/WCL but prefer to add a separate membrane over the sheathing. I designed this project almost five years ago and have learned a lot in the meantime.

I agree about the leader lines--I had a drafter/designer help me and we had a ridiculous timeline, so drafting perfection went out the window (no pun intended) while we focused on going from initial contact to construction, including engineering, in four months.

Thanks WG for the clarification. I would personally never rely on horizontal tape for water control. Separate membrane c/w positive laps everywhere of the GC is redoing the work. It's always the first thing I mention at the construction kick off meeting: If they fail to install the membranes correctly, they will redo.

Oh the long tails on those leaders hurt my eyes.

My favorite thing is making a GC rip out window to do proper membrane laps, Although I only got to do it once, it was the first 2 storeys of a 6 storey building. GC hated me after that. I even had to walk out of a site meeting later on because of verbal abuse.

I take it head crown overhang and the head casing are for aesthetics only? A change I would make is that I would take the vapour barrier to the exterior face of the wall around the rough opening, and then take the exterior air barrier over it to the inside face of the opening - it would be a two piece air barrier - one on the inside of the opening, the other on the exterior face of sheathing (tape that goes across zip sheathing joints), in a shingle fashion...dunno if I'm making sense?

No vapour barrier? Or is your zip sheathing also a vapour barrier?

I normally take the vb around the rough opening, then cover it with the ab from the outside. Also, I prefer a foam rod and separate sealant/caulking over a single piece of foam sealant. On non-residential projects when we use aluminium frames, we usually do two sets of sealants/foam rods - one on the exterior face of the main frame, and one on the interior face of the same frame.

No wood sill? GWB is ok, but it's too soft to last a long time

Aps, my window details are identical to yours

We used to specify Emseal in non-residential construction, but would never get it. I guess traditional way is cheaper.

I just posted that detail to show Chad how I’ve done exterior wood bucks in the past, but I know the rules here and will defend my detailing…

I have been using the Zip system since it hit our market about ten years ago, with zero issues. There are some tricks to using it. The builders I work with are often nationally known trainers on best practices so I don’t have to worry about things like making sure they roll the tape and allow time for the acrylic adhesive to cure.

When properly installed, the acrylic tape forms a covalent bond with the phenolic resin that coats the face of Zip sheathing. I always include rain screen gaps so even if a tiny amount of water were to get past the tape, it can dry easily to the exterior.

That said, a couple of years ago I decided to make a few changes. I no longer spec Zip (or any OSB) when it will be below the dewpoint for more than a few days per year. In other words, when there’s no (or not enough) exterior insulation to keep the sheathing above the dewpoint.

I now spec CDX or solid wood sheathing, or in one case, no sheathing, just a sarking membrane with diagonal bracing. When there is sufficient exterior insulation to keep the sheathing above the dewpoint, I still like and spec Zip because it’s easy to install, everyone in my market is used to it, it makes a great air control layer, and is a sufficient water control layer. But I recommend adding a separate WRB shingle-lapped, just to be safe.

The project I show was built by and is owned by Mike Guertin, a well-known builder who does a lot of training on best practices. We talked a lot about details and he ended up changing a few during construction. I was annoyed at the time but it’s his house and I know he knows what he’s doing.

The head “crown” (my drafter’s term) and casing is for aesthetics, and it also helps kick water away from the window. I prefer to inset windows to give them a bit of protection, and also to provide shadow lines—modern homes are often boringly planar.

There is no vapor barrier. I never use vapor barriers. I work in climate zones 5 and 6. I often use vapor retarders, when they are needed. With sufficient exterior insulation there is no need for a vapor retarder other than painted drywall. But for added protection, I usually spec a variable permeance interior membrane. This project was sponsored by Owens Corning so we used their Membrain product, not my favorite but it’s ok.

The sheathing is the primary air control layer. Air control transitions to the interior side of the windows, not really shown on this drawing.

We ended up doing drywall returns on all sides of the window interior. The budget and timeline were both extremely tight, and Mike, the owner/builder, planned to rent it out and said that if the interior sills (stools) got damaged he could easily add trim later.

Correction: Certainteed, not Owens-Corning.

Additional note: have you ever tried to get a residential builder to install windows with backer rods? I have tried, it doesn't work. A single bead of low-expansion foam near the interior does a great job.

The sill piece is kinda small and it's unclear how it's fastened. I've had issues like that before. How did you attach it? 

Thank you for sharing your details, it's always rough when the peanut gallery starts picking shit apart, so I appreciate you being willing.

The exterior sill extension? The exterior casing is pre-assembled, with screws up through the bottom of the sill extension into the side casings. The rabbet for siding is only about 1/2" deep and the side casings are 5/4 so there is plenty of "meat" for the screws. I learned to do it that way as a carpenter 25+ years ago and have done it many times since then, most recently last weekend for a window replacement at my mom's house. Pretty standard practice in New England for flanged windows that have to look traditional.

No problem re: sharing details. I should have known what I was getting into--the sharks here are brutal! I'm much more often in the business of correcting others' work, so it's unusual to have the tables turned, but I'm happy to share if it helps others. 

Thanks for all the info Wood Guy. I agree about not using XPS due to the toxic blowing agent. Our firm uses Polyiso a lot. While it's a great product it doesn't do well with water (40% reduction in r-vaule) and takes forever to dry out, even from a light misting of water. Our firm uses it because of the facings that are available on the product - tape the seams and you don't need a vapor retarder. When sealed the facings also keep the water out and allow flashings to be sealed to it. With EPS foam the stuff is super breathable that it dries out quickly and doesn't have a noticeable reduction in r-value when wet. It is rather fragile and often doesn't come with any type of facing so you can't seal flashing to it

Chad, if you find EPS fragile, what type are you using? I spec a lot of type 2 (15 psi) and type 9 (25 psi) and find it almost as cohesive as XPS. I actually can't recall using EPS on exterior walls, but I use it a lot for foundation systems.

I have not found foil-faced polyiso to absorb water when installed on walls, but I have when it's in a pile waiting to be installed. I spec taped seams when using it on walls, and for recycled product to cover exposed foam with tape. I guess a separate WRB over the foam would also work but it's usually pretty clean and tape works fine.

In case anyone is interested, here's an overview of rigid foam I wrote a couple of years ago: https://www.finehomebuilding.com/2018/01/08/get-right-rigid-foam.

Those are all very good points Wood Guy. The EPS we've worked with has been the 15 psi stuff. It's not that the stuff doesn't' have the compressive strength (it's used for geofoam supporting roads and buildings) it's that the corners and edges tend to chip off.

Regarding foil faced and taped polyiso not absorbing much if any water - you're correct.  The issue is that when you have your drainage plain behind the polyiso then all water that get's through the facing will be directed to the unfaced surface.  Over years the small amount of water that gets through will produce wed and moldy insulation.  

For that very reason I wish my firm would move to unsealed EPS foam for CI rain screen assemblies.  

I'll just throw this out there for the people avoiding XPS because of the GWP of the blowing agents ... it's starting to change. I know CA and WA have passed legislation banning certain high-GWP blowing agents and manufacturers are starting to use different agents. Owens Corning for example has been hitting the marketing pretty heavily for their NGX line using different blowing agents. I don't know if that makes them better than other options, or simply "less bad," but it might be worth looking into. I don't know about other jurisdictions, but laws banning the blowing agents go into effect on Jan 1, 2021 in CA and WA. So if you're near the west coast, you can likely get them on your projects.

Edit: I was looking up the link to post and noticed that OC is saying you can get their NGX products throughout the US and Canada beginning Jan 1, 2021. https://www.owenscorning.com/e...

Here is their map showing where the laws and regulations are in effect and being proposed. I think it hits most of us talking in this discussion (West Coast, CO, New England, Canada).

Thanks EA for the info. I'll make changes to product specs in the new year.

While it wasn't brought up in the discussion, I believe this also affects spray polyurethane foam insulations. There are some manufacturers with products promoting lower GWP blowing agents out there too.

I don't know much about the new blowing agents for XPS, other than they have been using CO2 as a blowing agent in Europe for a long time now.

I do know a lot about the blowing agents for closed cell spray foam. The new blowing agents (HFO, hydrofluoroolefin) have roughly 1/1000 the climate impact as conventional HFC-blown foam.

That's great, but it doesn't tell the whole story. Using lifecycle analysis prepared by the spray foam industry, on a 75-year life span, HFO-blown foam is really about 1/4 as bad as conventional foam. That's over 75 years. Over the 10-20 year scale we need to be looking at when it comes to the climate crisis, any foam insulation is best avoided when possible.

Polyiso R-Values actually drop quite dramatically at colder temperatures, so there seems to be some industry trend away from it in applications where this might matter.

They do drop with temperature, but as you track the temperature profile through a wall assembly, it really only affects the outer 10-20% of the insulation, so the overall effect is not usually very large. I typically use an average value of R-5.5/in for polyiso to account for this, and for long-term thermal drift. When the assembly is borderline at risk of moisture accumulation due to dewpoint, I use R-5.0/in for polyiso. (XPS ages to R-4.2/in, something most people don't realize.)

I despise auto leaders and kill them every time I see them in my CAD. They are fine in revit. I do align with the top line tho but I am very careful on the length and phrasing of the notes to keep things looking good and clean.

You guys really worry about the important things, eh?

Leader at top = if the spacing borks you can still find the beginning and end of note

Leader at center =  ¯\_(ツ)_/¯ 

Also what Wood Guy said.

"...Wood Guy You guys really worry about the important things, eh?..." Presentation matters - sometimes.

careful annotation and quality drawings show that the draftsperson cares about their work. Makes me much more confident in their ability to accurately convey construction details. Sloppy drawings 99% of the time = sloppy details. It takes just as much time to do it right as it does to do it lazy, so why bother with lower standards?

A proper laziness becomes a great efficiency.

Fair enough, Non. I am a pretty careful drafter but the pressure on this one to get it done was intense. I could explain more but I basically agree with you.

Never had a GC successfully use that. It's our name and insurance on the line and if the client goes against this, we have a legal letter on standby. If the client wants my compliance letter for occupancy, then everyone needs to play fairly.

Aps, no offense but if that's your favorite thing than you're an asshole. I work closely with my builders in an integrated process and respect their work, and they respect mine. If something has to be redone it was as likely something I missed including on the drawings as them making changes. I have never had a builder throw me under the bus for the project taking longer than they expected. I wonder why they do that to you?

There are bad apples in every profession, hopefully aps just got one of those and has had better experiences since.

WG, Aps was copy-pasting a response from Ahip above. I've made GCs redo work numerous times and I personally don't care if it hurts their feelings (ie. wallet) or it makes me look like an asshole (I know I am, when it matters). I know my drawings and know my details and unless they (the GC) has a good rational that meets the intent, they will redo the work at their cost.

Seriously this GC was an asshole to the consultants long before I made him rip out the windows. I am not about to let my firm get sued because the GC can't follow the pretty pictures I draw for him. Including the 8 step isometric sketches that spell out how to wrap the rough opening for a window prior to window installation.

Perhaps worthy of it's own discussion, but I'm finding that with every project, we're required to provide more and more details just to curb the default "I've always done it this way, so take this shitty installation" response from mediocre GCs.

Oops, my bad, sorry. I have worked with a few builders I didn't get along with, but I only work with them once. The majority of my projects now are with builders I know and trust. It takes time to build mutual respect. I do all residential work, mostly high-performance, and with a limited number of builders willing to do that work, and limited designers or architect capable of designing them, we have to play nicely. I can turn my asshole factor up to 11 when necessary but find I rarely have to.

I'm entirely private sector and have numerous commercial projects with big landlords. This has never been a situation I've found myself in. Never.

I'm sorry that's been your experience, Aps. I have been designing custom homes and renovations for 15 years, and was a carpentry contractor for 10 years before that. I started designing at a residential design/build firm and have been on my own for six years now. In over 400 design projects I have found the majority of clients to respect me and the services I provide. Many become friends. Same for builders. Occasionally one goes south but I've done enough projects to know that those were either not my fault, or I learn from them.

I'm not an architect, which I know pisses off some architects, but I only do the work I'm legally allowed to do, and the architects I know well tell me that I have equivalent knowledge when it comes to residential work. I wouldn't know where to start on a commercial or institutional project. Builders and architects often ask me for advice on detailing, in fact. I just turned down three separate requests to design swimming pool enclosures--two from architects, one from a builder--because it's outside my comfort zone, as an example.

We've lost clients because they thought we weren't doing our job or were slow. They go shopping to other firms in town for a while, then they come back. "Other guys are worse, so we are back. How would you like to do a building for us?" HA!

I'm talking about major developers here, not mom-and-pop operations.

Aps. I don't have mom and pop clients. My point above still stands. This is not how the real world works here when you have the chops to demonstrate value to your clients.

From the overall temperament of your posts throughout this site it seems like you've worked with assholes your entire career. You know what they say if you keep running into assholes . . .

Aps, I apologized above--I didn't realize you were quoting someone else.