R-35 wall assembly for a 4C Marine climate zone
I am about to build on a 85″ rainfall, 6000 HDD and no cooling system (I can withstand a little heat in summer)
I’m thinking of (from outside in)
* 1″ wood siding
* 1″ of air gap with 1×3″ furring
* asphalt paper as WRB and air barrier
* 3″ 2pcf EPS as R10 sheathing,
* A 10″ double frame wall filled with a total * of R-24 polyester insulation
* asphalt paper as air barrier and vapor retarder
* 3-4″ interior wood finish
(see attached image)
The permeability of the 3″ 2pcf density is about 0,6 perms, semi-impermeable. So, I am betting on drying mostly to the interior, although In my heating climate vapor diffusion will mostly travel inside-out…
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A few comments:
At 3" Type IX EPS comes in at about R12.7 - R13, not R10. See: http://www.buildwithplymouth.com/eps_properties_spec_sheet.pdf
It's nearly impossible to detail asphalted felt as a true air barrier. I'm unaware of an asphalted paper product that would qualify as a WRB, but maybe there is a suitable product available in Chile that isn't available near me.
Taping the seams of the EPS with housewrap tape (or 50mm wide aluminum tape) and troweling duct mastic over the tape to guarantee a long term seal can make the EPS layer n air barrier. For long term integrity it's really best to use structural sheathing as a primary air barrier, but if you have no wood sheathing, making the foam an air barrier is still pretty good. If you use two layers of 1.5" EPS tapeing and sealing the seams of each it can be fairly air tight. It works best if the seams of the second layer are at least a foot over from the seams of the first layer.
R12.7-R13 is sufficient for dew point control for wood sheathed assemblies with on R24 of cavity fill in almost all locations with 6000 HDD, which means the interior side asphalted paper would not be necessary unless the 3-4" of wood finish is extremely air-leaky. A 6000 HDD climate is a zone 5 type climate, where R5 of sheathing for ever R15 of cavity fill is enough. That means if the exterior insulation is more than 25% of the total R you're good to go. With R24 of cavity fill that means even R8 on the exterior would get you there, and with ~R13 you would have plenty of margin. If you assume the 4" of interior wood adds another R5 (something of an upper bound) bringing the interior-side up to R29, R9 on the exterior would still be enough.
And, if you are not using wood sheathing it matters even less, since neither the polyester insulation nor the EPS would be susceptible to moisture, and would ride through condensation events just fine.
So, your stack up looks pretty good, with or without wooden structural sheathing. Air tightness to the interior would be far more important than vapor tightness.
Malalcahuello's climate doesn't look like it is even as cold as a 6000HDD climate though. It's mean wintertime temp (averaged hour-by-hour) is about 46F or +8C, which means it doesn't even need even that much exterior R to avoid condensation issues at the EPS/polyester boundary.
I don't understand why you are building a 10-inch thick double stud wall and filling it with only R-24 insulation. Are you assuming that the insulation has an R-value of only R-2.4 per inch? That seems low to me.
Can't you find something that insulates to about R-3.5 per inch in Chile?
You wrote, "I'm unaware of an asphalted paper product that would qualify as a WRB."
In fact, asphalt felt is the only product recognized by the International building codes as a water-resistive barrier (WRB) for walls.
In section R703.2, the International Residential Code requires builders to install a layer of number 15 asphalt felt or paperbacked stucco lath over the wall sheathing or studs of every new home. The requirement includes a qualification: if you don’t want to use number 15 asphalt felt, you can use some “other approved water-resistive barrier.”
Products like housewrap have to undergo an approval process involving lab tests before they can claim to be a product approved as an equivalent to asphalt felt.
For more information, see All About Water-Resistive Barriers.
Thanks Dana and Martin... it's amazing being able to get pro's advice this easily.
EPS R VALUE:
Dana you are right, I just looked into the wrong cell of my spreadsheet, the R-Value for the EPS is 12.58 and not 10. That said, some info might not be exact because chilean product specs differ from US standards.... so, It's not really a 3" EPS, but a 80 mm EPS (3.15"), and the real density is 30 kg/m3 (1.87 pcf). Would it be better to use a 10 kg/m3 (0,64 pcf) one? What about 4 inches thick?
FELT PAPER as WRB:
We don't have Grace, we have Tyvek in Chile, but I like old fashioned felt paper. Tend to distrust on hi-tech barriers... my gore-tex (and similar) shoe membranes, no matter what brand I choose, never last longer than 4 months before they permeate water (in a similar way, Lstiburek made a point mentioning the impact wood surfactancts have on WRB loosing their impermeability to liquid water (http://www.buildingscience.com/documents/reports/rr-0106-problems-with-housewraps)). I also like asphalt paper's ability to increase its permeability (even 12 times!) as its wetness increases. Thanks Martin for your answer and article.
USING EPS SHEATHING AS A WRB:
I am interested, I'll research and maybe finally do it.
THE INSIDE FELT PAPER
As you say, the 3/4" (I made a typo, my english keyboard key "/" makes a "-" in spanish configuration)wood interior finish will probably be very leaky. That, the idea of retarding at least a little the vapor migration from the inside of the home to the outside (given the fact that I probably will have a semi-impermeable EPS sheathing), and the concern that HBCD (the toxic flame retardant used in EPS) doesn't get inside the house, were the 3 points that made me think on the interior felt paper.
WHY NOT A BETTER R-VALUE MATERIAL?
Now that we are talking about HBCD, that is also the main concern (health and environmental) that makes me avoid a full EPS wall, which would have the same R-Value as the 10" polyester insulated wall in just about 6". Also, Polyester is recycled and cheaper here (yes, even factoring R-values)
Last but not least, please consider that Weatherspark does not have actual data for Malalcahuello, only Temuco, which is kind of different.
In another question, Dana suggested Crescent City as an analogue to Pucon (near my building site)... I googleearthed hard to find a 3000 ft (1000 mts) above sea level in california at about 39° latitude and some 100 miles from the coast. Lots of national parks but no cities from where I could get climate info. In Chile, Malalcahuello is not my exact spot but I think is the closest I can get.Anyway, I also think 6000 HDD might be too much. Probably 5000-5500 would be a better guess. I have no problems in overinsulating, though.
Thanks guys for your answers, after reading Lstiburek guides I kind of turned dazy...
Jose, have you thought of how you will provide shear resistance in your walls without sheathing? Chile is a high seismic area. Just something to think about.
Malcolm, yes, I have thought about seismic resistance. As you say, in Chile it is a must. The thing is that we don't often see plywood sheathing over framing to achieve that purpose. Instead, we use diagonal and horizontal bracing, made of the same wood as the studs (i.e. 2x4"), inserted in between the studs. I attach a picture so you can see. So my plan would be to use the same system and make the framing itself shear resistant, with no need of wood sheathing. A question remains in me, about what is more convenient, plywood sheathing or the chilean system (do you have a word for this chilean system? is it the same as in-let bracing?)
PS. In the attached image you can see a double framing wall, made with outer 2x4"s and inner 2x2"s. In this case we were filling it with a light straw clay mix. But you could use the same framing to insulate with batts, rigid foam or whatever.
Jose, I live in a high seismic area here on the west coast of Canada too. We use plywood for shear, or if there is insufficient wall space without openings we use steel moment frames. Both these systems also incorporate anchoring of the shear plane to the foundation and up to the roof - something that is a lot more difficult with diagonal bracing.
I'm sure your local engineers have developed an equivalent using regional building techniques. Perhaps the clay, straw mix helps? But using the wall assembly you are suggesting, and relying on framing similar to that in the picture you posted isn't going to give you a building that will do well in an earthquake. I do think it might be worth talking to a local engineer.
Ok, I'm hearing your advice Malcolm, and I'm also hearing Martin's.
I changed the design of the wall... Now it goes: (changes in bold)
* 1" wood siding
* 1" of air gap with 1x3" furring
* asphalt paper as WRB and air barrier
* 3" 2pcf EPS as R13 sheathing,
* 3/5" structural plywood sheathing (we do have that thickness, 15mm, in Chile)
* A 6" stud wall filled with 6 inches of either R-24 EPS or R-21 cellulose insulation (I´m waiting for the cellulose guys to give me the quote to decide)
* asphalt paper as air barrier and vapor retarder
* 3/4" interior wood finish
I liked the idea of using plywood instead of diagonals. It's strange to see that in the area where I live. I like it because it seems to give better seismic resistance, less thermal bridges, and be much much easier to build. One bad thing is that it will reduce the vapor permeability of my outer wall even more. Hope it is not something to worry about. If you think it could, please let me know.
Adding up expenses, we finally decided to have a single 6" stud wall, mainly because labor costs raised too much with the additional area needed to allow for the extra thickness of the polyester insulated wall. In regards to R-value:
250 mm polyester = 172 mm cellulose = 150 mm 2pcf EPS.
Anyway, material R-values do not account for the gaps that happen when you EPS insulate within studs (doesn´t happen with polyester or cellulose)
thanks to everybody for your valuable comments.
I'd like to thank everybody for your help. We have built a truly passive solar house that exceeds the Passivhaus standards, in southern Chile mountains. Our house is a strange experience for all visitors and neighbors. Nobody believes that it heats with 90% passive sun energy until they see it by themselves. In our region everybody uses firewood for heating their homes. We are setting a new standard. We hope it goes viral in the close future. Thanks to everybody, Dana, Malcolm and specially Martin for your great help in this and other posts.
Congratulations on your house. It sounds like it is performing very well.
We would be delighted if you submitted a guest blog to GBA about your house! Feel free to contact me by email:
martin [at] greenbuildingadvisor [dot] com.