Numerous questions about roof and wall assembly details – GreenBuildingAdvisor

Dear GBA members and contributors, 

We. have this roof and wall assembly and we have a hundred questions about it.  We are hoping that y’all could locate any problems with it and answer questions. Our goals are in alignment with GBA insofar as building ultra high-performance homes that last a very long time are by their nature, green.

I’ve included my ridiculous “stick figure“ drawings to try to communicate the assemblies and connections. This is a 2700 ft.² home in climate zone 3-B, Arizona desert, southeast of Las Vegas, Nevada. It’s a double stabilized rammed earth wall, with 18 inches of blown in dense  packed cellulose insulation.  This is continuous into the 13 inches of blown in dense pack cellulose between the rafters plus the 1.5 inch for the purlins. This is a low slope butterfly roof (.75 in 12) that continues out to be a 10 foot overhang/porch cover. However, this not accomplished with extended rafters and hundreds of holes punched through the control layers; see drawings.  This is for critical water collection, protection of the walls, outdoor space, and aesthetics.

In words ,starting from the ceiling up:

Sheet rock, 12 x 12 rafters, 2 x 4 purlins, zip sheeting fully tape as the primary air and water control layer. Two sheets of 2 inch polyiso, overlapped and offset. Only in the valley of the butterfly roof is a sheet of 10 foot TPO; 5 feet up each side. 1×4 spacers parallel over the rafters, 2 x 4 spacers on top of that parallel over the purlins below, and lastly, the standing seam metal roofing.

SEE DRAWINGS 

What the  drawings should show, is continuity of the roofing materials to down the first 4 feet of the wall. It would be Zip to Zip R3, WRB to WRB, 4 inches of  polyiso to 4 inches polyiso down the wall, and the rain screen gap that’s 4.25 inch on roof and 1.5  inch down wall.

The Metal roof is continuous out 10 feet.

I also have the drawings for: the basic silhouette of the house and details regarding the butterfly roof.

  I’ll include a short explanation about some of the design ideas on The Forever Home below to explain some  material and design choices if you care to read that.

I have a hundred questions, here are a few:

1. Do I need any plywood exterior to the 4 inches of rigid insulation on the roof, or can I, should I, attach rain screen gap materials directly on WRB that would be on top of rigid insulation?  Currently, 1x4s parallel to every rafter on the insulation is drawn. 
2.  Is attaching fasteners for the metal roofing to the structural Zip deck all that code requires?  Or, does the 2×4 “rain screen purlin’’ and metal roofing need to be attached to the rafters?  Or to the real purlins below, on top of 2×12 structural members?
3.  Will using flat face 2×6 purlins over 2×12 rafters work/accomplish what I want them to do? I want a little more insulation.  However, I really want better targets to drive long screws into from the deck. Trying to hit an essentially 1-inch rafter target through all those layers blind……..um, seems uh, difficult… I propose using 2×4 (or 2×6) purlins perpendicular to the rafters and also on top of the rafters in between the purlins to get solid attachment to the rafters ultimately.  Is this a silly idea?
4.  I have Zip to Zip R3 connection from roof to wall.  The Zip R3 will attach to end of rafters, then concrete bond beam, then rammed earth wall.  What I’m hoping for is that that will create a strong air tight connection.  Is there a best practice for creating air tightness to these three different materials?
5.  Big question:  Where exactly is the line, the demarcation point of interior of a wall/roof to the exterior? I thought everything interior of the air, water, and vapor control layers is interior, and everything exterior to it was exterior. Which in a lot of assemblies, that’s the top of the Zip sheathing.  But the layers of exterior insulation also can act as barriers of different sort depending on the type right?  The rain screens have water WRBs immediately before the gap and cladding.  Sometimes Zip is used twice; as the back of the rain screens/ventilation of the metal roofing/cladding.
6.   Should the ceiling be air tight?  That is, does a sheet good type air barrier need to be installed behind Sheetrock or is the Sheetrock enough.
7.  Is plywood an airtight detail (instead of Sheetrock) if it could be taped w/wood strips concealing the tape, or simply butt against each other?   

Why build this particular house?

I am attempting to build what I call The Forever Home.  The last house you ever need to buy. It’s a forever home because it is extremely flexible, all of the load bearing walls are on the exterior and it can (somewhat) easily change from one bedroom to two bedrooms, all the way up to five bedrooms and back to a one bedroom. No more moving every time your life needs change.

And here’s the thing, it is so low in heating and cooling needs and consumption, that the  incremental changes and costs that push you to move, simply aren’t there.  (Don’t be absurd! Of course there thousands of other reasons to move.) This home is off grid in every way. Water is collected, stored in massive built water storage containers. Solar provides all electric needs. A primary concern is how much you want to spend on the battery storage for the solar PV. Batteries are costly.

Here are the design goals in order of importance. However, while one may be a slightly important over another, everything on the list is an important relevant objective.

1. Energy efficiency.
2. Build-ability /ease of building/ fewer steps
3. Cost
4. Security (think castle/bunker)
5. Durability/longevity, 1000 year home
6. Beauty/coolness/aesthetics/less weird
7. Resistance to mother nature and other threats. Basically, the primary threats are earthquakes and sinkholes.  Considering ONLY the walls: they are fire proof, termite proof, high wind resistant, bulletproof, etc., etc. The walls are rock solid because they’re essentially sedimentary rock.

If we wanted the least cost option, I don’t know if there’s anything that tops the 2×4 stick built double walls filled with blown in cellulose. If we wanted an every day average home filled with lame white walls like every other house, we would do a 2 x 4 double wall of some sort.

Instead, we are choosing a double mass prototype home.  Earth walls are a megaton of work, time, money, and complications.  If contractors do all the work, the cost is astronomically prohibitive. The entire construction industry and building codes hinder innovation and almost seem like a rigged game.  However, if the labor is born by the owners, the cost between a double stick frame and a double rammed earth wall may not be as enormous as people think. It’s a large cost to be sure, 15-20%?  Is it worth 5-10 thousand more for something beautiful and long lasting?

We think it is.   

Stabilized rammed earth achieves many goals.  And while there are increased cost in some areas, there are decreased costs in other areas. No drywall on the walls, no mudding and taping, no interior paint, no exterior paint, no repainting the walls, no baseboards….. etc…etc…

The walls have three control layers built into it: the water control layer, air control layer, a vapor retarder.  RE walls fulfill structural requirements and sheer wall requirements.

All the complications and layers of the exterior walls are handled by one step. All the steps that can be seen in the top 4ft of the exterior wall are all the costs and work rammed earth avoids.

The beauty of stabilized rammed earth is like having each wall a work of art.  (whether I can actually achieve that is debatable.).