Improving on the basics in Portland
Acting as general contractor for the Live-Work project was a great opportunity to research different wall construction techniques. You can always build (and pay for) more, but we made some major improvements over standard practices.
The Live-Work project web site also has much higher quality photographs than this one and is a necessary visit in conjunction with the notes below.
Almost all of the science and theory behind the build comes from the web sites on the external links list to the right.
The notes and observations below come to mind after completing the structural build and siding.
Foundations
The Live-Work ADU is slab on grade with radiant tubing in the slab. Given the amount of time, money and materials involved in insulating the stem walls, inside and out, it would have made more sense to use ICFs.
We also installed 3″ of xps under the entire slab. Current code calls for R10, or 2″. I did a little math using my own house and its heating bills for data. See the bottom of the spread sheet posted here.
Advanced Framing
- This has to start at the architectural design and engineering phase, otherwise your 24″ oc framing will take a backseat to hold downs, corners, posts, etc.
- Seismic Zone D is not necessarily the advanced framer’s ideal working ground.
- Beauty is in the eye of the beholder, and knowing that your walls were more efficient to build and to own goes a long way towards loving a constrained window and wall layout.
- A 22.5″ window makes more sense than 24″ window, as does a 46.5″ over a 48″.
- Titen HD fastened shearwall hold downs (3×3 plates excepted) use less wood than cast in place threaded rod. I would ask my foundation sub to hold horizontal bar just off center to the outside so that I can drill easily off center to the inside.
- Headers deserve forethought. Specify all of them and be creative with connectors to avoid jack studs. They are rarely necessary at all on gable ends.
- Don’t land a plumbing wall on a parallel joist, but you knew that.
Sheathing to Cladding
This wall is designed to breathe. I believe that it’s close to ideal for the marine climate of the Pacific North West.
This project’s walls, inside to outside:
- latex paint - vapor barrier primer
- drywall (varies in thickness due to fire code and the shop under living space)
- no polyethylene vapor barrier
- unfaced batt insulation
- 2×6 stud wall, 24″ oc
- 1/2″ plywood
- Tyvek® house wrap
- 1″ xps foam insulating sheathing
- fiber cement siding
PDF Drawing of Busse ADU Wall Assembly
Want more? See the video: http://www.portlandbuilt.com/video/video-wall-assembly-design
Not shown on the drawing:
- We used Jamsill Window pans set to the face of the xps. You can find their door pans easily enough, but you will have to order window depth pans direct. They can ship to Portland in one day. We did price galvanized metal pans but they are 3x the cost.
- We also wrapped three sides of each window and two sides of each door with Grace Vycor® Butyl Self-Adhered Flashing draining to the face of the xps.
We settled on this after quite a bit of research. Most of the technical information and analysis traces back to Building Sciences Corporation, even though it is reported from a pretty wide ranges of sources. See the external links list on this page.
Although this assembly would not specifically meet the new in 2010 Oregon R703.1 Code Requirements, we are confident that it is exceptionally robust. It would take only a few tweaks to precisely meet the new wording.
Alternative Wall Systems
We seriously considered a) building a thicker wall with staggered 2×4 studs to accommodate more insulation and b) incorporating a rain screen. After weighing the added costs, our climate, and the technical pros and cons, we made a decision that still seems like the right one. If one were to improve on our construction, I would do so in this order:
- Rain Screen
- Additional exterior insulated sheathing
- Using a self adhering roofing underlayment as the house wrap
- thick wall construction
One of the Building Science Corp. publications talks about building the perfect 100 year wall. It observes that such a wall would also probably take 100 years to pay for itself.
It’s all about the the foam
- Exterior insulating foam raises the temperature and condensation point on the plywood sheathing. This allows one to omit the typical impermeable (polyethylene) vapor barrier between the drywall and insulation, and the wall gets to breathe.
- In Oregon, at this time, code still requires a 1 perm or tighter vapor barrier on the warm side, although 1 perm is not relatively all that tight. We can use latex primers that are rated as vapor barriers to meet the current standard. (We would prefer a less permeable latex, but, unless we can work through an exception in the next few weeks while drywall is going up, code is code. )
- The IRC (Section N1102.5 Vapor Retarders) allows one to omit the typical impermeable (polyethylene) vapor barrier between the drywall and insulation with sufficient exterior foam board insulation. In the 2009 IRC, these provisions can be found in section R601.3; the new designation for the table is Table R601.3.1. (http://www.buildingscience.com/documents/information-sheets/irc-faqs/IRC-FAQ-insulating-sheathing-vapor-retarder-requirements).
- Although Oregon lags the IRC , it looks like this will change Summer 2011 when Oregon will likely adopt the IRC standard ( http://www.bcd.oregon.gov/boards/bcsb/board_pack/2010/20100203/BCSB_020310_VIIc.pdf).
- If you still have any doubts, the DOE and its Building America program endorse this assembly as a best practice for our marine climate.
- The continuous layer of insulation mitigates thermal bridging from the stud wall construction and greatly increases the overall r-value of the building envelope. Batt insulation installed in the 2×6 stud bays is rated r-21. 1″ xps foam insulating sheathing is r-5. Admittedly without any data to back this up, it’s hard not to look at the wall and feel that the real world, effective values are higher for the xps and lower for the batts.
- The foam has a tongue and groove on its longer sides so we installed it vertically.
- Instead of taping horizontal seams, we ran the second course of tyvek after the first course of xps. The 9′ Tyvek role works very well with the 8′ xps sheets, allowing us to shiplap between upper and lower xps courses with the bottom edge of each layer of Tyvek.
- One possible problem here: the top and bottom courses of xps come together to form a fairly tight seal, so water may not flow easily from the face of the Tyvek out the shiplap to the face of the plane. However the Tyvek is barrier 3 behind the siding and the xps, and we installed a full depth flashing on the mid level belly band that separates the two stories, using the same approach as for the window headers and the main belly band at ground level. One could gap the the courses of xps, but I think that would have been a waste in this case with the belly band flashing.
- When the foam went up, we ran it continuously over doors and window. It makes for a fantastic temporary weather barrier between finishing the framing and starting windows and siding.
- It’s easy and quick to install. Eliminates the dreaded tyvek flap.
- We used button caps to nail it, 12″oc at the edges and 18″ in the field per instructions of a Fine Home Building article. I am not sure why. It’s going to get covered with siding. Next time I would use the minimum fasteners I need to handle the wind for the brief period that it is exposed until siding.
Flashing
I think we obsessed over this more than any other aspect of the build to date. I am very pleased with the finished product, but it was not an easy install. It helps to have a crew like the one we had from Bronze Construction. They were focused on installing the way we wanted it done, rather than the way it has always been done.
- We flashed every door, window, vent block, lighting block and the lower and mid course belly bands with a piece of sloped z flashing.
- The foam and tyvek was slit horizontally along a line cut approximately 3/8″ above the header trim, block or band.
- A 2″ slot of foam was removed below that cut line, making a real effort not to cut the tyvek at the vertical ends of this slot.
- The flashing was inserted so that upper vertical flange was flush with the plywood and behind the tyvek.
- As the flashing was sloped, it did not sit flat on top on the trim but sloped down to it. This left a small triangular gap at the back of the trim. Copious caulk here.
- I would choose not to slope this flashing in the future. I think it complicates an already challenging install, and I prefer to see drip edge bent on the ends of trim as well as along the face, as opposed to caulked ends described above. The argument for is that with the added 1″ of xps, the flashing runs a greater a distance. If the foam is tight to the flashing and trim, it will hold water at the back of the flashing against the wall. The sloped flashing would prevent water from pooling and then evaporating back up into the wall system.
Miscellaneous
- We went with galvanized fasteners over stainless.
- We used a quality exterior screw to attach the window nailing fin directly over the 1″ xps.
- We did use one metal custom pan from Montavilla Sheet Metal under a second story double door which gets a lot of weather. The gypcrete floor would prevent one from detecting a leak in the sill until a great deal of damage was already done. Conversely, we omitted pans under the first floor doors as they sit on the slab. Water won’t damage the concrete, it would make sense to see the water coming in so one could repair the door.
- Plywood over osb because it is slightly more permeable or breathable, meaning it releases water more readily than osb.
- Tyvek® over other house wraps because the available research appears to rate it over other wraps. We preferred it to 15# felt paper because the wrap laps more easily than felt paper between the xps courses, the 9′ roll works well with the 4′x8′ xps sheets and the 9′ roll installs the quickest.
- 1 layer of 1″ xps over 2 layers of 1/2″ xps. 2 layers would be tighter, but it is close to twice the cost for the same foam thickness.
- The t&g in the xps works well.
- Unfaced xps foam insulating sheathing over faced polyiso for greater permeance/breathability.
- The dirty secret: vinyl siding probably vents the best, is cheapest to buy and install and will last forever. However, no one will ever let you install it, including your wife, on your house…