Best type and location of air barrier
I’m in Zone 4A. I have seen much focus on using the interior drywall as the air barrier layer. This seems impractical, 1) air clearly flows easily in walls of homes here and that would essentially negate the value of the insulation, so stopping the flow (inside/outside) with drywall does little to stop the flow of heat & moisture with that air as it bypasses the insulation. 2) over the life of a home many penetrations to the drywall will occur and these will not likely be consistently sealed, ending your airtightness.
I’ve also seen the application of peel-and-stick membranes over the outside of the structure, with great added complexity of the framing sometimes required.
My feeling is that I should structurally sheath with OSB, add some exterior continuous insulation sheathing, then wrap the entire structure with 6mil Poly sheeting, paying particular attention to transitions between floor and slab, as well as floor to ceiling and floor to floor to maintain the air-barrier.
I’d add furring strips to the walls to attach siding and let the PE act as my water proof barrier also.
For the top floor, i’d run the PE under the truss bottom chord, add 2x below it for wiring chase and DW attachment, blow-in thick cellulose over it in the attic space.
This would put the PE in a durable location on the outside of the enclosure envelope and keep from introducing many punctures for every switch and light. I’d only have a few penetrations to deal with (like plumbing vent stack) and could take care with these.
This would require that all drying would occur to the inside of the house. House wrap is favored in this part of the country (Tyvek and Typar commonly), but while shedding water and passing vapor, these aren’t air barriers, right?
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Polyethylene makes a lousy air barrier, because it is easily damaged. Energy-conscious builders in the 1970s and early 1980s experimented with the use of polyethylene as an air barrier -- it generally belongs on the interior, not the exterior, side of a wall if you want to avoid moisture problems -- and found out that the use of polyethylene as an air barrier is possible but very fussy. (For example, seams must be sealed with acoustical sealant, and this step is quite messy.)
Your preference for an exterior air barrier rather than an interior air barrier makes sense. Most people who follow this path just tape the seams of the OSB or plywood wall sheathing. For more information on this technique, see these two articles:
Airtight Wall and Roof Sheathing
Return to the Backyard Tape Test
If you are a belt-and-suspenders kind of person, and you prefer redundant systems, you can also tape the seams of the rigid foam that you plan to install on the exterior side of your wall sheathing.
If you follow this advice, there won't be any need to experiment with the use of polyethylene as an air barrier.
Finally, it's important to point out that polyethylene is a vapor barrier, and the use of an exterior vapor barrier can lead to moisture problems unless the vapor barrier is associated with an adequately thick layer of foam insulation. You probably realize that, but other GBA readers may not. Exterior polyethylene can easily cause problems on traditional wall assemblies, so don't experiment unless you know what you are doing.
Q. "Housewrap is favored in this part of the country (Tyvek and Typar commonly), but while shedding water and passing vapor, these aren't air barriers, right?"
A. Tyvek and Typar are, in fact, fairly decent air barriers. It's usually not a good idea to depend on a plastic housewrap like Tyvek or Typar as a primary air barrier, however, because these housewraps are easily damaged during construction (for example, by ladders), and the fasteners used to attach the housewraps often lead to leaks.
Plastic housewraps like Tyvek and Typar are examples of water-resistive barriers (WRBs). WRBs are required by code. They are also a good idea. No matter what materials you use as your air barrier, your wall must include a WRB. For more information on WRBs, see this article: All About Water-Resistive Barriers.
OK, the article "Calculating the Minimum Thickness of Rigid Foam Sheathing" indicates that I have no minimum requirement for exterior foam to avoid condensation problems (Zone 4A) so R3 or R4 of XPS seems to be OK. So if I have my insulated (8" blown-in cellulose) and OSB sheathed wall, tape the joints on the outside surface (all joints will be at framing members on the inside), attach the XPS and tape those joints (lapping joints with the OSB), then add house wrap, furring strips and vinyl siding. You agree that this should present a highly air-tight assembly with low thermal bridging, and WRB and rain screen assembly that should be durable?
David- If you are planning on a thin layer of XPS over OSB, why not use Zip-R instead? It would be much less labor. Just install and tape the joints, skipping the additional foam layer and house wrap.
I agree about not relying on the drywall as an air barrier.
Regarding Zip-R, I'd say 3 reasons but I'm open to the idea. 1) I have not found readily available pricing from local suppliers for the materials. 2) I have no experience with Zip-R and this is a 1-off new house for me and my family that I'll be doing the bulk of the building for. 3) relatively new materials without long track records bring some concerns.
Any good source of pricing and instruction/training that anyone can recommend?
With 8" of Cellulose I don't think that a minimum of continuous XPS is all that is cost effective. The cellulose is so very inexpensive to really over-insulate in my climate. I'm looking to the XPS as more of a supplemental Air/Water layer on top of the primary function of keeping condensation risk further at bay and greatly reducing thermal bridging.
I meant that MORE than a minimum of XPS doesn't seem cost effective.
While rain screens are an excellent idea (read requirement) for many claddings, my understanding is that typical vinyl siding breathes nicely and doesn't require furring. While vinyl siding has had it's detractors, it has some nice benefits.
...Just re-read some of Martin's previous advice, which indicates that vinyl siding doesn't require a rainscreen unless it's installed over rigid foam that's over 2" thick.
+1 for ZIP-R. I was going to use it with our own project. Visit Huber's website to find distributors.
In my area (3A Atlanta) I'm seeing it more and more on new apartments and luxury homes (Luxury homes are also using a lot of foil faced foam too).
Vinyl siding doesn't need a rainscreen- PERIOD. If installing vinyl over 2" foam using furring it's OK to leave the rainscreen gap in place, but it doesn't change the drying rates of the assembly one bit.
David: In zone 4A it's "cost effective" to build out the walls to a whole-wall-R of about R25 (= 2x6/R20 + R10 continuous insulation), since that's roughly the minimum wall performance where you can get to Net Zero Energy with a PV solar array that fits on your house, using current heat pump techonolgy for heating & cooling. PV may seem expensive now, but at the long term learning rate (the rate at which it's costs are falling) it will be by far the cheapest energy you can buy within 15 years or so. It's conceivable that PV efficiencies will improve sufficiently in the next 15 years that you'd be able to do it with an R20 whole wall (2x6/R20 + 1.5" ZIP-R). Anything less than that would be locking yourself out of that potentially cheap energy future for decades.
For a rough guide of what the performance of the other elements of the house needs to be to get there,m see table 2, p10 of this document:
How do you intend to heat & cool this place? (What fuels, and at what are the current costs, locally?
Btw there's a new ZIP-R product with 2.5 inches of foam (R-value of 12.6). Note: It's no longer considered structural sheathing so framing will require additional bracing.
It's not yet on Huber's website but if you go to YouTube and look up Risinger IBS 2016 - Advantech you'll see it a quick discussion about it.
If it's not a structural panel the 2.5" ZIP-R is competing against other nailbase panels commonly used in roofing, with the "special sauce" being the factory applied WRB and ease of air sealing with tapes.
I'm guessing David probably isn't interested in engineering the bracing to buy another R6 in whole-wall-R, and would probably be happier with 1.5" ZIP-R. A 2x6 wall with R23 rock wool and 1.5" ZIP-R would have quite decent wall performance for zone 4A for not a lot of up-charge, and you wouldn't have to specially train crews on how to install it.
Everyone, Thanks for the great analysis and feedback, Also the specific article references are much appreciated.
I believe that I agree that R25 is a great target for wall insulation when accompanied by high levels of air-tightness which should reduce the cost of heating and cooling. I plan to use Solar Thermal water heating augmented / pre-heating with heat-pump water heater. I'll use a heat-pump for active heating & cooling of the house. A pair of HRVs with fresh air intake bypass activated when useful. I have pretty much settled on this wall profile: (drawing attached) Structurally 2x6 @ 24", Interior Drywall, Exterior OSB taped joints, NO insulation in the wall cavity (open for plumbing and electrical). On the exterior attached 'Larson' trusses 8" deep all sides (10" on north side) loose filled with cellulose (re blown after settle and designed with 'head room' to over fill to combat settling long term) Larson trusses covered with 1/2" XPS (vertical joints taped, horizontal Z-flashed), covered with Tyvek house wrap and clad with vinyl siding. I'll use elements of OVE (stack framing, California Corners etc.) coupled with some better elements of Swedish Platform framing (let-in ribbon joist/truss supports etc.) to reduce thermal bridging as much as I can. I'll fabricate 1/2" plywood box frames for each window/door inserting these so that sealing and flashing can be continuous over the structure.
Anyone have any other ideas I should keep in mind or any concerns with that approach that I may be missing? Remember I'm in Zone 4A (Mixed Humid) and we generally have more days with cooling than heating in a year, most months will involve BOTH heating and cooling loads.
Here's the drawing
Half inch XPS isn't sufficiently structural to hold in even loose fill insulation on the exterior of a Larson Truss- it will bow out and deform over time. Better choices would be exterior grade gypsum board (eg DensGlass) or asphalted fiberboard (eg Celotex), either of which would have better structural capacity and much higher drying capacity than half-inch XPS (and lower environmental impact too.)
The Larson Trusses have to be anchored on the structural studs, not (as shown) on the sheathing. You might be able to design it such that it will work hanging on the bottom & top plates, but why make it complicated?
This is an extremely thick and complicated R25-ish wall. It would be easier & cheaper to do it with a 2.5-3" nailbase panels on a INSULATED 2x6 wall with let-in bracing.
If the stud bays are to remain empty, there's no point to being 2x6- you might as well go with 2x4 16" o.c., which is about the same board-feet of lumber, and could be aligned with your 16" o.c. Larson Trusses.
An empty studwall cavity becomes a fire spread hazard. Blowing it full of cellulose or fiberglass after air-sealing the sheathing and running the wiring adds significant fire resistance and significant thermal benefits (more than half the design target R25.)
A much simpler "Mooney Wall" of 2x6 24" o.c. with 2x4 interior girts full of cellulose would run about R25 whole-wall using standard sheathing & siding. http://www.builditsolar.com/Projects/Conservation/MooneyWall/M01.png
Download and read this document:
Case 3 is a Mooney Wall using crummier low-density insulation, and only 2x3s, coming in at R21+ whole-wall. With blown cellulose or 1.8lb fiberglass and a 2x4 horizontal elements it would come in at about R26-ish, maybe R27+ depending on actual framing fractions. If you installed a membrane air barrier over the interior of the 2x6 wall before installing the 2x4s for air tightness it would be fairly well protected from deterioration over time. Rather than 6-mil polyethylene, 2-mil nylon (eg Certainteed MemBrain) would allow the assembly to dry in either direction when needed.
I'm in 4a and I'm finishing up my home with Zipp-R. My architect put me on to it and I think it was one of the best decisions we made for a few reasons.
1. It was just another zipp panel to the framers. They did not flinch a bit. One thing I've learned in this process is anytime you throw anything "unconventional" at a sub mistakes happen. And if you are looking for advice on this site, you are almost by definition unconventional.
2. It's nice knowing I have an insulated "skin". No, there should not be many double studs or uninsulated corners, but again, I'm using the labor that's available in my market. So I have my share of framing mistakes. It's nice to know I have R-6 everywhere despite these mistakes. Plus it should be a tight air barrier. I'll be getting a blower door test soon.
3. It was not that much of a premium when considered as a whole. Per sheet it is expensive. Around $40 I think. I bought it through my local Zipp dealer. They can special order it. But you can use r-15 batt insulation (good luck finding a quality installer though) or blown to provide the rest of the insulation without worrying about wind washing, thermal bridging, etc. You should have a legit R-20 wall at the end of the day using conventional labor and close to conventional materials. This just strikes me as the best "bang for your buck" vs other wall assemblies, especially if you are using conventional subcontractors.