Miscellaneous ramblings
I have been perusing these halls for some time, and have the following questions bouncing around my head… in no particular order… I’d post them to the relavent thread, but that doesn’t seem to work around these parts…
1. Martin has stated that he has not heard of damp applied cellulose in a double stud wall, but I have read it suggested here by one of the “Advisors” (somewhere, can’t find it again) that damp sprayed cellulose on a 2×8 plate;finger jointed 2×3/2×4 double stud wall assembly would best be done with damp applied cellulose, and would create a . Which is it? Or is the 2×8 with 2×3/2×4 double stud not considered Double Stud (big D, big S)?
2. I get it that there are minimum exterior insulation thicknesses to keep sheathing dry… but do these numbers apply in any capacity to rockboard (Roxul) exterior insulation since it is free draining, and free drying?
3. Somewhere (again, I can’t find it) I read Martin’s words that the ventilation cavity for roof sheathing can be *above* the roof… was I dreaming? Can this vent cavity be created using corrugated metal roofing? So I can just put corrugated metal roofing over sheathing (with some type of felt between) and call it a day – no vent channel, no nothing?
4. How terrible would it really be to have roxul boards outboard of sheathing on a roof installation.. won’t it get trampled to death, would a roofer walk off the job in protest?
5. I forget what 5 was.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Ethan T,
I won't get to all of your questions in my first comment. I'll start, though.
Q. "I get it that there are minimum exterior insulation thicknesses to keep sheathing dry... but do these numbers apply in any capacity to rockboard (Roxul) exterior insulation since it is free draining, and free drying?"
A. No. You can install any thickness of mineral wool wool on the exterior side of your wall sheathing, without worrying about "minimum R-value" issues. I addressed the issue in my article, How to Design a Wall, in which I wrote:
"Mineral wool insulation can be substituted for rigid foam insulation on the exterior side of wall sheathing. One advantage of mineral wool over rigid foam: because mineral wool is vapor-permeable, it doesn’t inhibit wall sheathing from drying to the exterior. That means that builders can install mineral wool of any thickness on the exterior side of their walls. You don’t have to worry whether exterior mineral wool meets any minimum R-value requirement. (Of course, thicker insulation always does a better job of resisting heat flow than thinner insulation.)"
Q. "Somewhere (again, I can't find it) I read Martin's words that the ventilation cavity for roof sheathing can be above the roof... was I dreaming?"
A. No. I discussed this approach in my article, How to Build an Insulated Cathedral Ceiling. In that article, I wrote:
"If you prefer, you can locate your ventilation channels on top of the roof sheathing rather than under the roof sheathing. If you decide to do this, make sure that any roofing underlayment that you install above the roof sheathing is vapor-permeable — for example, #15 asphalt felt, VaproShield SlopeShield, or Solitex Mento — and that your local building department accepts this approach to roof venting. If you install ventilation channels on top of a vapor-impermeable synthetic roofing underlayment, the flowing air won't be able to help dry out the roof sheathing.
"If you plan to install ventilation channels above your roof sheathing, it's best to choose a roof sheathing that is vapor-permeable (for example, fiberboard). If you use plywood or OSB, there's a small chance that the sheathing can still accumulate worrisome amounts of moisture over the winter; this is especially true for north-facing roofs.
"You can create 1 1/2-inch-high ventilation channels above the roof sheathing with 2x4s installed on the flat, with the 2x4s located above the rafters, 16 inches or 24 inches on center. Although this approach is less fussy than installing vent baffles underneath the sheathing, it usually costs more, because most types of roofing require a second layer of plywood or OSB on top of the vent channels.
"In some cases, these ventilation channels are installed above a layer or two of rigid foam. It's also possible to purchase nailbase (a type of SIP with OSB on one side instead of two) that includes integrated ventilation channels between the OSB and the rigid foam; one brand of these panels is Cool-Vent from Hunter Panels.
"If you are choosing to build a vented roof assembly, don't forget to include soffit vents and ridge vents."
Q. "Can this vent cavity be created using corrugated metal roofing? So I can just put corrugated metal roofing over sheathing (with some type of felt between) and call it a day - no vent channel, no nothing?"
A. Maybe -- if it's old-fashioned corrugated roofing. Most screw-down metal roofing and most standing-seam roofing panels, these days, consist of panels that are mostly flat, with very few ribs -- and for these types of roofing, your plan wouldn't work.
Q. "How terrible would it really be to have Roxul boards outboard of sheathing on a roof installation.. won't it get trampled to death, would a roofer walk off the job in protest?"
A. Not terrible at all. John Straube did it on an outbuilding in his back yard. The biggest problem is convincing roofers to try it.
Ethan,
Q. "Martin has stated that he has not heard of damp applied cellulose in a double stud wall, but I have read it suggested here by one of the 'Advisors' (somewhere, can't find it again) that damp sprayed cellulose on a 2x8 plate; finger jointed 2x3/2x4 double stud wall assembly would best be done with damp applied cellulose."
A. I've never done that; in fact, I don't have much experience with damp-sprayed cellulose. The basic worry with damp-sprayed cellulose is ensuring that the cellulose has a chance to dry before it is sealed up with layers than inhibit fast drying.
Have I heard about walls that were insulated with damp-sprayed cellulose, and then covered with drywall, and then checked with moisture probes by a worried homeowner or builder, only to discover that the cellulose was still damp? Yes, I have.
Some people say, "Don't worry. Even if it takes months to dry, it will dry eventually, as long as the wall can dry in at least one direction."
Other people say, "I have enough worries. I'd rather use the dense-pack method."
Martin, thank you! You answered all my questions!... I'm just trying to figure out if it is worthwhile to double stud rather than thick single stud... At a local Big Box store, the cost of a 2x4x10 is $5, a 2x8x10 is $12, a 2x10x10 is $13. So a double 2x4 wall would put each double stud at $10. So here I am thinking... why not single stud 2x10 with exterior rockwool and forget the hassle of double studs... Won't the rockwool give me my thermal break?
Regarding damp sprayed cellulose, I believe Amando Cobo recently posted that heat leaks due to shrinkage separation leaving thin gaps between the studs and cellulose become apparent in infra-red imaging even with 2x6 framing. That may not be as much of an issue with double studwalls, or it could be even worse due to the larger dimensions (eg: 1% shrinkage on a foot thick insulation wall is smaller than 1% shrinkage on 3.5", and it's a 3-dimensional problem.)
Caveats on #2: From a code point of view the minimum exterior R necessary to be able to use a Class III vapor retarder on the interior does not change with the type of exterior insulation. Even though rock wool allows the sheathing to dry toward the exterior, it doesn't change the low vapor permeance of the sheathing, which would still collect moisture from the conditioned interior if there isn't a Class-II vapor retarder to slow it down.
From a building science point of view there is some relief offered by the ability to dry readily toward the exterior, but whether or not one could get away with Class III vapor retardency would have to be analyzed on a case by case basis. (WUFI, anyone?) But with rock wool on the exterior installing a Class I vapor retarder on the interior would not create a moisture trap, the way it would with most foam.
Similarly, regarding #3, a fundamental difference between venting under the deck vs. above the deck is that with the under-deck venting scheme interior moisture reaches dry outdoor air in the vent cavity before it reaches the moisture susceptible roof deck. With venting above the roof deck interior moisture drives have to pass through the vapor-retardent and moisture susceptible deck first. So with venting above the roof deck the vapor permeance of the interior has to be sufficiently low to not reach high moisture content in the roof deck (even a Class-II vapor retarder might not be tight enough for zone 6, but 6 mil polyethylene probably is.) And as Martin points out, the vapor permeance above the roof deck has to remain high. Yes, it can still dry toward the exterior, but with a Class-III vapor retarder on the interior the total amount of moisture that would have to move through the roof deck goes up by more than an order of magnitude, and the peak moisture content is potentially too high. (Again, WUFI, anyone?)
In response to a previous post, Charlie Sullivan wrote:
Regarding requirements for a Class III vapor retarder... my head's spinning so shouldn't I just use Densglass sheathing and be done with it all? It seems to cost the same amount as CDX.
Then on the roof I'll just fill the joist bays with two layers of R30 Rockwool Bats, leaving a vent space above and call it a day.
Dana... I get your concerns about damp sheathing... does anyone here who has built a lot of houses have anything to say about framing with 2x10 vs going through the trouble to make a double stud?
Using DensGlass is fine, but it's not structural sheathing. You'd have to install shear panels or let-in bracing to keep the walls from racking. With a double studwall with at least 1.5" of space between courses of studs the bracing may not even need to be let-in, but instead applied to the edges of the structural studwall. (Let an architect or engineer specify the bracing and fasteners.)
A 2x10/R30 wall comes in substantially less than R30 whole-wall, whereas a double studwall with 9.5" of cellulose delivers a true R30. See Case 4:
https://buildingscience.com/sites/default/files/migrate/pdf/BA-0903_High-R_Value_Walls_Case_Study_rev_2014.pdf
Note, Case 2b using 3" of exterior polyiso or 4" of rock wool also delivers a true R30.
Ethan,
If you can, partner with a knowledgeable green builder, designer, or architect. It really helps to have plans with correctly drawn details and a builder that wants to create an efficient, problem-free home.
And a 2x10 wall is a bear to wrestle into place.
Ethan,
Price exterior mineral wool before deciding on the comparative costs of the two wall assemblies.It's not cheap.
Steve K, I'm doing my best to become either 1, 2, or all 3 of the folks you recommend. But, yes, I take your suggestion to heart.
Stephen S, what say you regarding wrestling 2x8 into place?
Malcolm, I have priced mineral wool... Comfortbatt is by far the lowest $/R/SF of the products (non-foam, formaldehyde free) that I am considering - though the only pricing I can get for dense pack cellulose is installed cost, which may be throwing off my numbers.
Ethan: I guess if you have either enough people or the right equipment, any framed wall can be erected. I'm not sure why a 2x10 or 2x8 is called for. If it were me, I'd either put enough foam on the outside or skip the foam and go double stud.
I suspect double stud is easier and cheaper than trying to fasten thick rock wool to the sheathing.
Stephen, thanks for your reply. I guess I am finding myself recognizing that double 2x4 construction is the way to go. I just have to get comfortable with either Solitex Mento or Fiberboard on the exterior.
Dana, in "Wall Sheathing Options," Martin suggests that Fiberglass-faced gypsum panels are structural...
...you suggest that they aren't. Can both be true? Is it a gray area?
Maybe it doesn't matter because a double stud with non-let-in bracing seems like the easiest thing to do.
2x4 conventional framed wall with roxul batts, 1/2" sheathing and 4" comfortboard exterior? cavity R=30 1x4 furring strips with headlok screws (i think 6.5" minimum, maybe 7") and light weight siding.
2" comfortboard 80 is approx $2 retail/sq ft. Perhaps some discount pricing on bulk quantities
Ryan, R=30 isn't enough for me here in zone 5A, but thanks for the idea.
Also, all that rockwool on the exterior sounds great until you try to install it. I think it is one of those things that is easy to draw, and maybe easy to build prefab on a factory floor, but not so much when you have gravity to contend with.
I'm wary of trying to put up one layer of rockwool on the exterior, let alone 2. (2 x 2")
Ethan: I was leery about the logistics of installing exterior foam thick enough to get us where we wanted, about R40. We opted for double stud wall as follows, outside to in:
White cedar shingles
Cedar breather
Typar, joints taped
Advantech, joints taped with 3M 8067 flashing tape
2x4 wall, 24" oc.
5" space.
The 8 1/2" space filled with dense packed cellulose .
Siga Majpel membrane stapled to
2x4 inner stud wall
Inner wall held almost all wiring and plumbing, except sill cocks and exterior receptacles and wiring for outside lights.
Fiberglass batts
Drywall.
Total wall was around R40.
Ceiling was just raised heel trusses and lots of cellulose.
Blower door test results: .59ach50.
Our builder had never done a double stud wall, but found it pretty simple. We're in Whitefield Maine, zone 6.
Stephen, thanks for sharing... a couple of questions...
1. You weren't worried about "cold sheathing" on the Advantech?
2. Was the inner 2x4 wall your structural wall?
3. What was the $/sf on the Siga Majpel (or where did you get it)?
Thanks!
Ethan: My architect was on board. The outer wall was structural. I don't remember how much the membrane cost, but we got it from Performance Building Supply in Portland. We also got our Intus windows from them, as well as various other stuff. They were very helpful and knowledgeable. If I were doing it again, I might look into a more permeable membrane, but I'm not worried about moisture, since the wall can dry in both directions and the envelope is pretty airtight. Any condensation forming on the sheathing should dry ok. The Majpel was easy to use. It's pretty tough and hard to puncture by accident.
Ethan,
I think it's fair to say that a consensus seems to have emerged that a well air-sealed house, with a code compliant vapour-retarder, sheathed with plywood or Advantech, and including a rain screen, shouldn't have to worry about the problems of cold sheathing.
Ethan,
1) Roxul does make a roof board called TOPROCK.
2) I was told by a GP rep at a trade show that DensGlass was not structural, which is why you almost always see it steel framed commercial projects. It definitely does not have much nail/screw pull-out strength, so any cladding you would attach over it would need to be secured to the studs. I was recently researching DensDeck for use in a flat roof that I am building and I swear I saw another Dens product that is approved as structural sheathing, but now I can't find it. When you look at the literature on the GP website, they clearly are selling the DensGlass as a vapor-permeable air and water-resistive barrier, when the seams and screw penetrations are flashed with Prosocco Fast Flash, not structural sheathing.
Here is a link to a marketing video showing their products applications:
https://youtu.be/pm1jpTtzb-o
3) The comparable cost for Roxul Combortboard 80 vs 1.5lb locally manufactured foam is $.56/inch/sf vs $.31/inch/sf.
4) I have used both Mento and Majpel. I found the Majpel to be stronger. My local suppliers are Ecosupply and 475.
Jonathan,
Maybe you are thinking of Stedi-R Structural Fiberboard Sheathing.
Ethan, in response to your comment #18. See attached PDF.
From exterior,
Wood siding
1x furring strips
4"roxul comfortboard 80 R16
Water barrier (tyvek)
1/2" sheathing (Primary Air barrier)
2x4 Insulated Stud wall with 3.5 Roxul Comfortbatt R14
Smart Vapour Retarder (Secondary Air Barrier)
2x4 Insulated Horizontal service cavity with 3.5 Roxul Comfortbatt R14
1/2" gypsum
Total cavity R value R44, excellent thermal bridge prevention. Not a "Cheap" wall, but nothing 12" width is. Addresses thermal bridging, keeps sheathing warm, resilient with easy to detail and protected air barrier with very few penetrations. Practically fire resistant as well. Easy to frame, light weight walls easy to lift and place.
Comfortboard is "supported" by the top of the foundation wall. Construction adhesive to hold in place until furring strips are installed
Ryan. Thanks for posting that detail. Were you able to source 4" Comfortboard? My supplier has told me I need to go to a commercial supplier for 4" which is not something I have done before. My intuition has been to try and get down to one course of roxul, though I see an advantage of overlapping joints by doing 2 courses of 2" Which did you do? 4" or two courses of 2" Would you do two courses of 2" again? Was it double the time or did they stack fairly easily? I think I'm imagining stacking batts in my imagination, because I haven't worked with rockwool before, and I understand it is a bit more rigid, especially the comfortboard IS.
Steve,
Yes, that is it.
Thanks.
Malcolm, you write that
There is so much writing here at GBA about cold sheathing on thick walls, that if what you say is true it might need to be posted in big red letters on the front page!
negative, I'm going with two 2", attempting to overlap seams as best as i can. We have not built this yet, going in for permit within the next month.
The comfortboard is pretty firm. Not structural, but firm and easy to work with, low compression compared to the batts. Cuts well with a serraded knife. The plan is to insulate and strap at the same time, working in sections that function well with the scaffolding or boom lift. We have also considered pre mounting a few furring strips. Leaving them +/-5.5" from the sheathing. Once the comfortboard is in place, just tighten the furring strip up against it. That way we have 2 screws started, that we know are centered in the studs. We can just come back after and install the screws at the spacing required.
They also make a 3" R12 board.
Jonathan and Steve, I've read somewhere here on GBA that Stedi-R isn't made anymore, or is made in limited quantities... Not sure if that's true... but I don't see it anywhere... however, there is also International Bildrite Structural Fiberboard Sheathing 1/2" x 4' x 8'... not sure if it is applicable in your case.
OK, so Malcolm describes emerging consensus that Cold Sheathing (plywood or advantech) is not a worry-inducing assembly...
Martin writes regarding venting above roof sheathing: "Maybe -- if it's old-fashioned corrugated roofing..."
So this leads me to believe that an unvented cathedral ceiling with dense pack cellulose, plywood sheathing, vapor open underlayment, and "old fashioned" corrugated metal roofing providing vent channels to the ridge would be a simple, robust, and cost-effective roof assembly.
Ethan,
I'm sure Martin will weigh-in on the wet sheathing discussion.
I'd really caution you against using corrugated metal as your roofing. Unlike flatter profiles, it holds debris, is much harder to flash effectively at penetrations, gables, sidewalls etc. and (unlike snap-lock) leaves you with a roof of exposed fasteners. It's a great sidewall material, which as you say has a built-in rain screen gap, but for roofing it has been superseded by concealed fastener systems.
At 3/4" fiberboard sheathing is vapor permeable and structural (provided the tight fastener schedule & specs are followed), delivers about R2 (to R0.5 for half-inch plywood or OSB), and is extremely water tolerant. But it's prone to bowing quite a bit if dense packed.
Unfortunately thicker fiberboard isn't widely available in the US, and the European imports are insanely expensive.
Dana... Bildrite Structural Fiberboard (R 1.22, structure) @ $10/sheet (big box) puts it at $0.23/sf. Gutex Multitherm 40 (R 5.8, WRB) from 475 is about $0.31/sf. More expensive? Yes. Insanely so? I'd say not (though not structural...) Steico Protectdry 60mm is only $0.23/sf. Again, different product, but not insanely expensive by my calcs...
Ethan T,
I think I've written three or four articles on the "cold sheathing" issue. I don't think there is a simple one-sentence summary that can be written in big red letters on the GBA home page, just yet.
Briefly, cold sheathing can accumulate moisture during the late winter. To reduce moisture accumulation in cold sheathing, you want a good interior air barrier and an interior vapor retarder. To help damp sheathing dry quickly in March and April, you want sheathing that is at least somewhat vapor-permeable and resistant to rot. You also want a ventilated rainscreen gap between the sheathing and the siding.
I have no problem with Malcolm's assembly.
Ethan,
I don't recommend "an unvented cathedral ceiling with dense-packed cellulose" unless you are willing to install a sufficiently thick layer of rigid foam on the exterior side of the roof sheathing.
You aren't describing an unvented assembly. You are describing a vented assembly, with vent channels on the exterior side of the roof sheathing. Is it a good idea? Only if (a) your roof sheathing is vapor-permeable, (b) your roofing underlayment is vapor-permeable, and (c) you have good ventilation channels between the roofing underlayment and the roofing.
Do the corrugations in old-fashioned corrugated metal roofing constitute "good ventilation channels"? I'm not sure. If the roof got plenty of sun, I wouldn't be too worried.
Martin, thanks for your reply. I think you've finally beat it into this thick skull of mine... I accept that the vented roof assembly makes more sense, that old fashioned corrugated doesn't make sense, and that I can use the 2x4s that make the roof vent to create rafters since that is difficult in an airtight assembly. Thanks everybody for joining me on this fascinating journey...
So what is this mythical vapor open structural roof sheathing that everyone is dreaming of? Or do I move my roof sheathing to the ceiling (and interior wall plane) and use something like Solitex Mento at the top edge of the joists under the vent channels?
In response to a previous post (https://www.greenbuildingadvisor.com/community/forum/plans-review/101444/roofwall-connection-pretty-good-house-zone-5a-cold-6000hdd), Martin did not advise this "European" approach, but it seems to be increasingly common (?) or maybe I just hang out with the wrong crowd.
Ethan,
The mythical vapor-permeable roof sheathing is called boards. Used successfully for at least 200 years.
Oh, diagonal board sheathing! For the roof (?!)
Ethan,
When sheathing a roof, boards are installed parallel to the eaves and ridge -- almost never on a diagonal.
Talk about (to quote D. Dorsett) "insanely expensive"! Unless I'm doing my calcs wrong again, sheathing in 1x6 white pine would cost about $1.65/sf in materials, set aside extra labor. That's compared to $0.31 for Majvest or $0.38 for Mento. It would look really nice though, for a while, before all the layers were added on.
Ethan,
Those of us who use board sheathing usually live in a rural area, where sawmills are common. Rough-cut (unplaned) spruce from a sawmill is inexpensive where I live. The last time I checked, rough boards were cheaper per square foot than plywood, although the labor to install them is more.
Martin, thanks for explaining. I do have access to rough cut boards but unfortunately this project explicitly calls for FSC and I can't really get around that.
Ethan: Funny math on the Gutex Multitherm 40.
"Gutex Multitherm 40 (R 5.8, WRB) from 475 is about $0.31/sf. More expensive? Yes. Insanely so?"
Kinda...
...'cuz it's not really 31 cents.
https://foursevenfive.com/product/gutex-multitherm-40-pallet-54-boards-512-sf/
The way I read it you get 490 square feet per 54 sheet pallet (~512 square feet if you count the tongues of t & g, but 490 net, assembled) , priced at $886.68 /pallet on 475's web-store. That's ($886.68/490= )$1.81 per square foot.
$1.81 is almost 6x the 31 cents indicated in response #34, with and almost 8x the cost of a sheet of half-inch Bildrite, so yes, pretty pricey, at only ~3x the thickness, and lower density, less structural.
In comparison, SONOclimat eco 4 is 38mm thick (which is almost 40mm) , IS STRUCTURAL and reportedly sells for CDN$30-35 in Quebec for a 4' x 9' sheet (that's USD $23-27 at current exchange rates), a bit less for the 4' x 8'. eg:
http://www.canac.ca/fr/product/materiaux/isolation-insonorisation/panneaux-isolants-rigides/panneau-isolant-sonoclimat-eco4br-1-12-po-x-4-pi-x-9-pi_44426.aspx
That's half the per-square foot price of Gutex Multitherm 40 sourced through 475 (and it's structural!), but not available in the US.
http://mslfibre.com/Upload/Documentation/T12670-106_SONOclimat_ECO4_En_08-15.pdf
Looking at it from an insulation perspective it's also only R4, not R5.8, but from a $/R point of view it's still at a real discount.
Dana, I tried to break it out as $/R/SF, not just $/SF... I apologize for using the wrong units in my post above... should I fix it or let it stand?
So yes, Gutex Multitherm is $1.81/SF, but it is 3.712 inches thick, and R-5.8. That works out to $0.31/R/SF. I got a quote for SONOclimatECO4 of $54/board - I need to talk to your supplier. At my prices, that is $1.69/SF and $0.42/R/SF. Bildrite Structural Fiberboard is $10/sheet at M#nards. That's $0.28/SF, but $0.23/R/SF. If I could get your pricing on the MSL fiberboard that would be nice, and would put it closer to the Bildrite...
Sorry that I wasn't clear that I was trying to compare $/R/SF not just $/SF. There is still a premium on the European products, but not as drastic as a pure $/SF comparison makes it seem, especially because the European products are T&G and can serve as a primary or secondary WRB.
Terming a sheathing material "structural" usually means it can withstand the shear requirements for the region where it is be used. But there are other uses for more robust sheathing (like plywood) that are worth considering.
Our code allows rain-screen furring to be fastened to the sheathing, rather than studs, if it is of a certain thickness. This allows a different for the studs and furring, making for a better siding installation. Plywood sheathing also allows you to install trim and flashing to it - something not possible with "structural" fibreboard.
Malcolm, I appreciate you making that distinction. I remember someone around here talking about drilling thousands of holes in plywood to make it permeable. I don't know if that is advisable, but it sounds like an interesting solution to this conundrum.
The discussion about drilling holes in sheathing occurred back in 2012. I was rather blunt, and eventually had to apologize for not phrasing my insights more politely.
Among other things, I wrote, "Drilling holes in your sheathing is silly. Most builders are aiming for airtight exterior sheathing; exterior air barriers are a good idea. ... Drilling holes in perfectly good plywood or OSB is a nutty idea, although I know that some builders have been told by the guys at the lumber yard that it makes sense. ... The practice you suggest -- drilling holes in exterior sheathing -- is fairly common in British Columbia. It started as a fad among builders who were distressed at the massive failures that became known as the Leaky Condo Crisis. However, this method has no basis in building science. It was simply a panicked reaction by builders who were tearing out their hair because of the wet-wall disasters they were seeing."
Eventually, John Straube posted a comment on the thread -- basically, he backed me up.
Here is the link:
https://www.greenbuildingadvisor.com/community/forum/general-questions/23219/am-thinking-following-wall-and-roof-assembly-bc-cost-effecti
Ethan: "Gutex Multitherm is $1.81/SF, but it is 3.712 inches thick, and R-5.8 "
No, it isn't, at least not according to MY tape measure. :-)
Gutex Multitherm 40 is 40 mm thick (the "40" in " Multitherm 40 ") which is just 2mm thicker than SONOclimatECO4. A dimension of 40mm is 1-9/16" less than half the 3.712" indicated.
USD$54 for a sheet of SONOclimat eco4 is still cheaper per square foot, (even for a 4x8 sheet), but such a huge upcharge over street-retail in Quebec that it's almost worth renting a truck to go pick it up. USD$54 is literally twice the Canadian retail price for the 4x9 sheets!
Giving up the (R5.8-R4 =) R1.8 for the benefit of being a structural sheathing seems "worth it", in the over all cost of the assembly.
The 4x8's are priced a ~CDN$30, the 4x9s CDN$34 at CANAC. Ready for a road trip? :-)
http://www.canac.ca/afficherbootstrap.aspx?search=panneau+isolant+eco4&searchtype=2&display=5&langue=fr&unite=001002
OK, I'm sorry I brought up drilling holes in plywood. It was late on Friday... now it's later...
Dana, I really appreciate you getting into the $/SF/R conversation with me because I think that is how most of these materials should be discussed... I mistyped the thickness of the Multitherm, but it is indeed 1 9/16" and R-5.8, so my $/SF/R numbers still stand. That being said, I appreciate what you 're saying about SONOClimatECO4 (that's a mouthful - someone needs to talk to the MSL naming department).
Holy cow! At US$23 a panel, yes the SonoClimatECO4 is the hands down winner... you're not kidding about that roadtrip. But I need about 2500SF, more if I'm doing the roof.
Next I need to understand how to read these structural numbers... MSL says the ECO4 has "robust structural properties (5.84 kN/m) "
I know I need to consult a structural engineer... but are we talking a real alternative to plywood and/or OSB here? And how can I get those Canadian prices here in NY? What ever happened to NAFTA?
"I remember someone around here talking about drilling thousands of holes in plywood to make it permeable."
Ethan, That's a variation of a misapprehension I had a few years ago that Martin, (enlisting the help of John Straub) put me straight on.
Holes in sheathing help make it air-permeable, which is generally not a good thing. I terms of vapour permeability, the holes only affect its ability to allow moisture to pass through in proportion to their size relative to the total area of the sheathing. To significantly alter the perm rating of plywood, you'd need not only thousand of holes, but really large ones. There wouldn't be much sheathing left!
Structural requirements for sheathing vary from location to location. Hurricane zones often require substantially more than other locations, some seismic zones have a firm upper limit as to not make the walls so rigid that they're prone to walking off the foundation during a trembler.. Discuss it with an architect/engineer or your local inspectors. Like thinner fiberboard sheathing I would expect the fastener spacing requirements to be much tighter than those for OSB or CDX, and even at the prescribed fastener schedule the structural capacity will be lower.
Fiberboard sheathing of any type holds a very small market share in the US, and MSL Fibre is just one mill in Quebec. Distribution of SONOCLIMAT eco4 isn't ubiquitous even in Canada, and I don't expect it to show up at US distributors any time soon (at any price), though some may be willing to import it for you on a special order provided they can double their money for their trouble.