Tuesday, December 6, 2011


Here are some updated building sections.  You can see in the overall section cut through the dining space how the eyebrow windows work.  I am still playing with the section and the support of the windows and the roof in this area.  I am showing beams, but I think I will switch and use girder trusses to support the opening since there is very little load involved.  I will probably still have to keep the beams below the window, but these are easy to pick up and are no big deal since they can sit on the exterior walls and will not affect the basement space.

This detailed section shows how the walls are constructed, tied into the floor/foundation and roof structure.  Buildings have 6 sides.  Most people only consider the walls and roof...sometimes.  Basement walls and floors are also important parts to consider.  The building has to work as a whole to keep in the heat or cool, and allow you to control the air in and out.

This starts with the basement slab.
Most people do not like basement floors without carpet or some sort of covering.  The main reason is because it is cold.  That is a result of most basement slabs are uninsulated.  An insulated slab allows you to thermally break the colder ground below and reduce the thermal transfer from the ground to the slab.  By increasing the thermal layer, you can further isolate the slab.  The mass effect of the slab will absorb heat to the exposed temperature.  If it is exposed to the air temperature and it does not have the cold of the earth below it, the slab will start to take on the air temp, approximately 70 degrees.  The slab will still feel cool to walk on since anything below body temp feels cool to the touch, however the slab will not be 50 degrees, or similar to the earths temp.  Some people add radiant heat tubes to the concrete which will produce a warm, even heat out of the slab.  However this sort of install is not necessary for a house of this thermal level, therefore not a wise investment for me.
I do plan on leaving the slab exposed in the family area and going with a darker stained concrete.  This will produce a very durable floor that can not be damaged in the event of water leaks.
The floor sits on 3" of XPS insulation, achieving an r-15 below the slab. Below the foam will be a poly membrane, sealed and taped to prevent moisture penetration from the ground. Below the plastic will be 4-6" of clean, compressed gravel to help prevent moisture from getting to the slab.

Moving to the basement wall, I am using Hobbs vertical ICF forms for at least the below grade walls. Check out their links on the PRODUCTS link to the right.  These straight forms have a higher average r value then most eps forms (around r-27-30) due to the design that also doubles and requires less concrete.  However less concrete does not equal sub par foundation.  It is an engineered system, using concrete and steel right where it needs it. 
With the under slab foam touching the ICF form which goes to the footer, this allows a continuous thermal break between the ground and slab, and also between the slab and the exterior walls.  Thermal breaks like this are important in the overall design of a home like this.

It is still completely undecided, but I will probably go with a peel and stick style water barrier on the exterior of the basement walls.  This typically stops just below grade, but in this case I want to run it all the way up to the top of the form and tape it off to the ZIP sheathing.  This helps continue the air barrier from roof all the way down into the ground, and helps take care of notoriously leaky spots right around the mud sills and floor framing area.

Speaking of this area, since it is very leaky area with awkward areas to insulate, penetrations from electrical and other services, I will seal it from the exterior, but also use 6" of open cell spray foam to seal this area off and expand around the odd shapes of the floor joists.  The mudstill itself will be recessed into the ICF to allow that thermal break in the wood, and sill seal will be applied in a double layer between the mud sill and the concrete prior to being bolted down. 

To continue the thermal break at the floor framing, the 11-7/8" floor joists will rest on the center of the mud sill.  The rim board will then be caulked to the joists prior to being nailed on.  The rim board will also be set in a bed of caulk to the mud sill.  After this is nailed off, a 1" layer of XPS foam will be ripped to the exact with of the rim board and foamed and taped in place to the rim board. The exterior 2x4 wall will be set flush with the outer face of the foam allowing the ZIP sheathing to run continuous past the 2x4 wall, over the foam and nail off.  This achieves a super tight connection of the walls to the foundation, and addressing a very leak prone area of the house often ignored.  As mentioned above, the ZIP will then be taped off to the peel and stick membrane to create an air tight seal. 

Moving up to the walls.
After hours upon hours upon hours of research, I have settled on the double stud wall as the wall of choice.  I have explored about everything up there from typical 2x6 with exterior foam and blown, 2x4 walls with all the foam on the exterior, SIPs, full ICF
The 10" thick wall started out as a 12" thick wall.  However after energy modeling and calculations, it was determined that a 10" wall will perform similar but at a higher reduced cost.  Cost savings is not just in 2" of insulation, but it saves floor space.  The downside of double stud is creates a thick wall, so to get similar floor space, you need to expand the foundation which adds cost.  The 2" wall difference will perform almost the same (took only about 500 btu off my load calc) but saved a couple thousand dollars in cost due to savings in foundation and insulation.  It also will have some savings in property taxes since they are measured from the exterior shell. I can still hit an r-40 with blown fiberglass in this wall assembly.  Due to the lack of thermal bridges, the wall will perform pretty close to r-40, very unlike a typical 2x6 wood framed wall.

The 10" thick walls will have the floor plates set in caulk prior to them being tipped up.  This will seal the plates to the subfloor (which is sealed to the floor structure and rim board with the construction adhesive).  ZIP sheathing runs continuous to provide the exterior air barrier.  The proprietary tape seals it off.  Homes can achieve very tight levels using ZIP sheathing.  This system costs about $5 more per sheet, plus the cost of the tape, which adds approx $2 more per sheet.  However it does not require a building wrap, so savings is had there.  Plus the install is faster since you do not need to install the building wrap.  It also eliminates the repairing of the wind ripped wraps.  In the end it works out to only be about a $500-800 upgrade.  Small price for the air sealing and time savings.

Window openings are also taped off continuously from the exterior to the interior.  The window in then set in caulk and an additional layer of tape is applied over the flange.  Low expanding foam fills the interior rough opening gap, and then the window is taped off again to the interior face of the studs.  Opening cracks around windows are very leak prone areas, so attention to sealing these areas are critical. 

1/2" gyp board will be installed on the interior stud wall.  I will double up my air barrier and use air tight drywall techniques.  This requires the gyp be sealed at all exterior locations.  The cleanest way of doing this is installing gaskets to the top and bottom plates, around the windows, and at intersections of interior walls prior to the gpy being installed.  Air tight electrical boxes will be used in the exterior walls and the drywall caulked to the boxes to prevent air leak in these locations.  However I will only use the boxes are required, and will attempt to put as little electrical on the exterior walls as possible.
Some argue this belt and suspender methods are not needed to have two air barriers, but its not much added cost and can only make it a tighter home.

Since my air barrier is on the exterior plane, it needs to continue up my roof.  Since I am using a vented roof assembly with a energy heal truss, it is not possible to maintain an exterior air barrier plane.
Because of this, I need to transfer my air barrier from the exterior to the interior over the top plate.  This will be done using air tight drywall on the ceiling, caulked and gasketed off to the top plate.   The top plates will be caulked together.  A piece of EPDM will be set in caulk and taped across the top plates to seal off the wall and help to transfer the air barrier from the exterior to the interior.  The plan is to leave about 1' hanging to the interior side.  After the trusses are up, the EPDM will be stapled to the underside of the truss.  The gyp will press against this creating a gasketed seal.  There will also be a drywall gasket applied to the top plate further enhancing this seal.  ZIP tape will wrap up and over the plate to seal that crack off.  Fire blocking will be installed in 2' pieces with a 1 1/2" gap in between to set the trusses into.  This will allow us to easily set the trusses without messing with aligning a truss.  Rather we can use a laser and align simple blocks making truss erection much quicker. 
All penetrations in the ceiling plane will be boxed out from above, caulked and taped to the gyp and trusses. 

From this point up, it is pretty typical.  Simple standard wood trusses, sheathing, and roofing choice yet to be determined.  We are considering metal, but we will see how things shake out.

The ceiling plane would then be finished off with r60-70 blown in fiberglass insulation.

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