I need to figure this out fast, I have only until tomorrow morning to decide what I will ultimately go with, period.

The application that I will be using the rigid panels for are two, one, is wrapping all of the walls of the house & residing. The other, is use for a roof deck insulation and a unique design.

For the roof deck design, basically the idea is fill the rafter bays with cellulose, sheathe, apply foam panels thick enough to prevent condensation on sheathing (live in Flint MI), put something to protect the panels over that (will be exposed to UV for a short time) put in 2x4 spacers, 2nd layer of sheathing, and roofing material.

My choices are,

2" xps in 2x8 panels.
3" Iso in 4x8 panels.
1.75" Iso in 4x8 panels.

there are pros and cons of each, the Iso is open cell and breatheable, which would be nicer to the sheathing if moisture ever got in, but, if it does pick up moisture, it's R value drops considerably, and if it picks up some dirt, it can harbor mold. XPS is almost too waterproof, as it is quite non-porous, and if water got behind it and the sheathing anywhere, it would not dry easily at all. thus my conundrum.

Thoughts, real world applications, real design information & solutions? help? Thanks!

9 views and no input? Dana if you read this I need your wisdom! :D

Decided to go with the 1.75" Unfaced Iso. My reasoning was that being unfaced, it might let the building breathe a bit more, where xps would not allow any drying to the outside. I plan to put #15 lb. tar paper over the wall sheathing, then 1 layer of the Iso on the walls, and a good house wrap over that, I found one house wrap that is rated to last 12 mo. exposed to sunlight, well beyond any other I found.

For the roof, I was going to basically do the same exact thing, 2 layers is Iso though, I'm not sure what to wrap it in though, if I use a standard breathable housewrap like the same stuff I use on the walls, I'm afraid that while it's exposed the house wrap may not be able to repel the snow and ice as well as something like a regular roof wrap like plytanium or a grace roof wrap product would, however, I would like to again, like the walls, use something that will allow drying to the outside while stopping vapor & water from coming through from the outside. But I don't know what material would work for that purpose well?

Is the iso covered with asphalt felt? If so, you can strap right over that with your 2x4s & plywood.

Lots of talk on the forums about moisture, but remember that mositure travels on air, so air sealing the rafters is critical. Well done it should eliminate most of the moisture coming through the assembly. The Ice & Water should go on the plywood - above the 2x4s; don't use it right above the iso.
Bob Irving
RH Irving Homebuilders

Bob, thanks for the input!

the iso is totally unfaced, there is nothing on it, just bare iso.

my thought was, to use tar paper behind the iso between it and the sheathing/bottom layer roof decking & then use a wrap over top of the iso on the walls / roof.

Where you say "the ice & water should go on the plywood - above the 2x4s" I'm not exactly clear what you mean?

there are multiple layers & points that could be referenced to.

Rafters/tarpaper/iso/tarpaper*/2x4s/plywood/GI&WS/Shingles.

*I don't know how a housewrap will work in that situation; I do know asphalt felt will work.

Thank you bob, I was thinking the same type of thing might work, using tar paper at that point over the iso where there would be the airgap, under the 2x4s that are basically going to be purlins/spacers, I thought that was probably the point you were referring to but I wanted to make sure, and the Grace I didn't know at all since plywood hadn't been specified anywhere.

The biggest reason I had against just using simple tar paper for that portion, was that it will likely be exposed to sunlight for up to 6 months, and I figured tar paper would degrade completely by then, that house wrap I found like I said was rated for 12 months, but that is for walls, not sure how it would perform on a roof, or what else might work for that layer/purpose.


Does plywood hold up much better for a roof deck than osb? I thought about using treated, which would cost an insane mint, but would theoretically last darn near forever even if it got soaked all the time. difference in cost is a lot though, osb can be had for 6.50 a sheet while treated ply would probably be 25ish if I'm not mistaken, big difference that might not justify the cost.

thanks again for the input bob!

OK; look into using 30# felt instead of the 15# normally sold as tarpaper. OSB will work for the roof deck; all the SIPS panels use that because it is more stable, but I have never felt comfortable with it on a "stick framed" roof so I used plywood until a few years ago when I switched to Huber Advantech. It is a little more expensive - about the same $ here as fir plywood - but it is impregnated with some type of wax so it is close to being water proof. Also it has a rough side so it isn't so slippery. (http://www.advantechperforms.com/product-lineup/advantech-sheathing.aspx) If you do use OSB use at least 5/8 at 16" OC.
Bob

good to know, I had no idea Bob, i thought osb was what most people used.

That advantech stuff sounds pretty awesome, is that a plywood that also has wax in it then I'm guessing and not a solid wood product with wax? Still, that would be more environmentally friendly than treated wood, and it would not be as corrosive to fasteners, which is pretty awesome imho. I'll have to check that out, althought it's probably beyond my budget unfortunately, i wish i could have gotten a cool project going around here but I couldn't find any good builders that were interested, really couldn't find any good green builders in the area period actually, which I really need at this point, someone to come in and help me define all the details with real world knowledge, I have so many questions that are slowing the process down that I know a good green pro builder would be able to answer and be worth bringing on board my project. I have to decide exactly what to do for my roof tonight and I have no idea to tell the builder what to do tomorrow (and please no one throw the easy comment of you have the wrong builder obviously etc etc, he's the best I could find in the area that I could afford.) ... so in a bind...

Actually Advantech is an OSB so it is very green. They also have a new product, "the ZIP system" which seems like Advantech with a coating which is matched with a tape, so all the seams are taped for an air seal. you don't need a building wrap & you can leave it like that for some extended period of time.

That sounds like great stuff bob, thank you.

I still have the question of exactly what strategy to use for the attic I wish to make part of the building envelope and how to insulate the entire exterior of the home.


Here is a snippet from another thread that I have going on the subject;

I realized that sealing xps, or putting something like bituthane on the exterior would be like putting a vapor barrier on the outside, in which case the interior could only dry to the inside. What i wondered for that particular scenario, was if the barrier on the outside would stop moisture from getting into the walls by stopping air movement to the outside, and therefore, moisture would not push into the wall cavities and accumulate?

I'm thinking now though, that it might be a safer and wiser bet, to just leave things a bit more breatheable and open, try and emulate the open-ness of an old house somewhat, just slightly improved and more controlled.

I really would like to find a way for the grace membrane over all sheathing to work, it seemed like a great way to protect the sheathing permanently from water damage, but if it would just cause moisture issues on the inside, then it is a negative rather than a positive application. I know this application does work in a 2/3 1/3 scenario, however my insulation values are more of a 50/50 ratio, the 1.75" polyiso should be about R10 and the interior walls filled with cellulose about R13. Now, my heating degree days are not as extreme as they are in Alaska, there they go around 12000 to 14000, where here in michigan, the heating degree days are more around 7000 to 7500, so I thought I might be able to get away with it, but there are times when it goes below zero for a week or more at a time, and so problems could arise during that time even if it worked ok all other times of the year.

Also, there will be a lot of transition areas, such as over my foyer, back room, and garage, where the attic will be standard 1 story blown in openly ventilated space, and I was not sure how that would affect the rest of the envelope that would be theoretically more sealed.

The two best ideas I had at this point were, 1. Do not install the Grace, seal all the seams of the sheathing with caulk and tape, fill voids with spray foam cans, put something simple like tar paper over all of the sheathing and roof deck, put the polyiso up, and wrap the poly iso in a breatheable housewrap for the walls & ? on the roof (tar paper again?) 2. Do basically the same thing, but also do a quick flash on the inside with open cell spray foam, 1/2" or 1" just to tighten up the air seal.

DON'T use the Grace I&WS on the exterior of the sidewalls - there is no way to guarentee that you have sealed all the interior leaks, so it could be dangerous. Idea 1 or idea 1&2 would seem to work.

So, I know that having a double vapor barrier is bad, unless it is 100% intact all the time.

Seems like you are implying that the only way the having the Grace on the outside is to also have an interior vapor barrier?

I guess I'm trying to figure out whether there are any options that would allow for the use of the grace, I like the idea of protecting the sheathing, but only if it's practical.

Ok, so you think 1 & 2 should work too, cool. anyone else have any ideas/comments? All are welcome! Thanks!

Aeridyne,

From your mention of Alaska, you might have been paying attention to Thorsten Chlupp's technique, >2/3 of R to exterior of sheathing, wrap sheathing with Grace or Poly. It seems to be working very well, but part of the technique is to ensure there are no interior leaks with a blower door test before the walls are closed in. As Bob says, if you have any major penetrations in that air barrier, bad things will happen from water coming in through leaks or moisture getting out.

Thorsten has also said the ZIP wall would work well for this technique (as effective as poly or I&WS on the sheathing), the crucial thing is that there is a protected, verified air barrier inside of the thick rigid insulation.

This is also one of the major reasons the Passivhaus system mandates a tightness of less than 0.6 ACH50, it's not just to reduce energy use, it's also the level at which they feel that any inadvertent leaks in the building shell have been dealt with, preventing future structural damage from moisture leaks.

I posted these links on another thread, but I'll post it here again. The CCHRC has been researching these techniques for a while, and have monitored and tested houses to back them up:

Link to the Cold Climate Housing Research Center REMOTE Manual: http://sustainable.cchrc.org/docs/REMOTE_Manual.pdf

JLC article by Thorsten Chlupp: http://www.interioralaskabuilders.com/construct/files/RemoteWall.pdf

Green Building Advisor on these systems: http://www.greenbuildingadvisor.com/blogs/dept/musings/getting-insulation-out-your-walls-and-ceilings

Aeridyne,
You do not want a vapor barrier on the exterior side of the wall insulation. It is an important part of the Passive House construction to make sure that the exterior sheathing is breathable. Even the low infiltration number cannot guarentee that no moisture will end up in the wall, so take no chances. According to the manufacturer of Advantech and the ZIP system, ZIP system sheathings have a perm rating of 3 to 4. NOte that the links in Jesse's post show the VB on the exteiror of the sheathing, but inboard of the majority of the insulation.

Yes Jesse you are correct, I was referring to Thorsten's technique along with the REMOTE manual and other things that I found on the cchrc, and also, that exact blog, all three of those are exactly what I had read actually. (haha! damn good guess)

You said "It seems to be working very well, but part of the technique is to ensure there are no interior leaks with a blower door test before the walls are closed in. As Bob says, if you have any major penetrations in that air barrier, bad things will happen from water coming in through leaks or moisture getting out."

I'm not clear on what you mean by this, no interior leaks before the walls are closed in, you mean after he puts up the grace they check to make sure that there are no leaks? Otherwise that leakage point would accumulate moisture I'm guessing on the interior side of the sheathing?

Well, I'm thinking that the grace on the sheathing technique might be a bit much, so I was going to go with the more breathable routes. The foam was delivered today, it goes from one side of the house to the other stacked on pallets to the ceiling! lol. 22 cents a board foot is what it came out to, not too shabby for iso even if it is used, most of it is in pretty good shape, and I kept it out of the landfill :)  anyway, it actually does have a facing on both sides, its a white fiberglass type looking paper of some sort, not sure what it is, but likely porous, actually it must be open cell now that I think about it, my dad said he threw 3 sheets aside that were soaked. So it must be open cell type, with this paper fiber type stuff on both sides of each sheet.

with those properties in mind, i'm thinking of what the best way to use it is. since fiberglass can harbor mold, i thought it might be a good idea to put some tar paper up over the sheathing, then the foam. Or, also I thought maybe use some low expansion foam, put a bead of that around the perimiter of the sheathing where I place each panel to sort of stick it on, then tap in some plastic cap nails. Go over the whole thing with either more tar paper, or a good house wrap. Again, the roof is the same dilemma only even more intricate.

FWIW, most fiber-faced iso has a permeance well over 30, so it'll dry outward just fine as long as you don't place a vapor retarder between it and the siding. Standard 15# felts & or housewraps would be good enough (both are over 30perms), but set it up as a rainscreen & drain-plane for best results. Do NOT use ice & snow barrier (very low permeanace), and maybe not even 30# felt (~20 perms) on the exterior of your highly permeable iso, to maximize it's outward drying capacity.

If the fiberglass never gets cold enough to condense and has a good path to drying it would be very unlikely to grow mold. The more foam you have on the outside, the warmer/drier the inner fiberglass & studs are.

Building in a rainscreen gap between the siding & iso is always a good idea- it'll let the housewrap or felt function better as a drain-plane, and allow wind-driven rain that gets behind the siding to dry. If the siding traps rain leaks agains the housewrap & iso it can, in some cases allow the iso to absorb bulk moisture in those spots. (It's a very slow process with low-density iso, but it can happen. Stick a chunk in a bucket of water- see how many days/weeks it takes.) If you used vertical 1x furring strips long-screwed through the iso at every stud to mechanically secure it to the siding, that can be your 3/4" rainscreen/drain-plane gap- hang the siding on the furring.

Foam seal all seams & edges of the iso and it makes good air-barrier. There's little to no advantage to adding felt between the sheathing and the iso, especially if you're taping the seams of the sheathing.

Thank you Everyone, and thank you Dana!

the only reason I was going to put tar paper between the sheathing and the foam was if there ever was mold growing on the inside face of the iso, it could xfer to the board and I thought the paper might slow/barrier that some and protect the sheathing. However, if you don't think it is a concern, I will skip it and adhere the foam directly to the sheathing.

I have a lot of areas where the sheathing is pretty messed up, one example is the southeast corner on the front of the house, we did not move the walls on the foundation to square them up, we did though make it more square above for the 2nd level, and it makes the wall plates on that corner stick out about an inch past the old sheathing on the 1st level.

I have been thinking what the best way to address this when I apply the foam would be without a huge amount of hassle, I figured that factors affected by this would be potentially air circulation behind the board, which I didn't want, or irregularities to the surface when I go to side it, which would look terrible when finished. It's also about 15 degrees outside consistently here already. I thought maybe use a ton of can foam, but thought that might not work very well or be practical, or work in such cold temperature for the foam curing, and at that point while staring at the corner there I got pretty discouraged and went back to doing something else......

I think that as much as I wanted to do stucco, it's going to end up costing more than I can afford, which sucks, because if we did it ourselves I could probably have done it pretty cheap, and I loathe vinyl, this is my first house and although I've never had vinyl on a house, I know I would not be happy and waiting until I could change it, but I also want something that is very low maintenance, vinyl or stucco seemed to be about my only two choices. I had just figured out a way I thought the stucco could work too over the foam, I was going to just use multiple layers of #15 felt, 2 or 3 between the foam and stucco finish, but my sheathing would have to be very strong, and in some places it's just asphalt fiberboard, so I figured I would have to fur it out and use the rain screen method, but, would then lose it's thermal benefits, and the stucco would be very hard to make level and not come out wavy with the spaces  behind the furring strips.

With all that in mind, what does everyone think? I need to make a decision on what I'm going to do, I just want to make sure I have all the facts straight and what I do makes sense, thanks again everyone.