Thursday, June 14, 2018

Construction - Interior Framing, Air Sealing and More

The temporary protection for the shell of the building, as described in detail in a prior postgave me the opportunity to proceed with the interior partitioning before the roof was in place.  I was able to get almost all of it done when it was too inclement to work outside -- cold during the winter months and wet during the spring -- and before having finally to postpone inside work in favor of the dirt work and landscaping described in the several posts preceding this one. 

(The original pictures below can be enlarged merely by clicking on them but, unfortunately, the downloaded pictures are not enlargeable.)
T-wall with door opening.  Single 2 x 4
header (blue); blocking to support T-wall
(red); carefully fitted OSB behind T-wall
stud for gluing drywall for a tight air seal.

Interior Partitions
While the exterior walls are anything but typical, the interior walls are more conventional.  What makes them somewhat different is the use of as much advanced framing as possible and building most of them in place in order to fit the slope of the cathedral ceilings more accurately. Advanced framing, as opposed to traditional framing saves material (and its embodied energy) without compromising structural integrity. I was able to use it to support T-walls with horizontal blocking and single 2 x 4s for headers above doors in non-bearing walls.  Anticipating the need for more rigidity to carry heavy 36" solid oak doors, I did however stay with tradition by using jack studs at door openings.  Even at that, the open floor design with its fewer doors required only 12 additional door-height 2 x 4s for the jacks.

Exterior Partitions
By contrast, the truss-built exterior walls required twice as much lumber as traditional 2 x 4 walls but I can easily live with it since all of the lumber was recycled and the energy savings from the thick
walls will soon compensate for any higher inputs.  In retrospect, I would probably have used the advanced framing technique of metal plating to join single top plates rather than using overlapping double top plates, particularly since there are two courses or tandem 2 x 6 double top plates on top of walls that were already so wide as to stand unsupported when raised.  Using half as many top plates not only would have saved lumber but would have made the wall marginally more energy efficient by removing half of the potential for thermal bridging through the top plates.  I say, "marginally" because the tandem top plates will be separated by 4 1/2" of insulation as a thermal break.

Advanced framing for outside corners in 2 x 4 and 2 x 6 exterior walls have just two studs instead of the traditional four (see drawings). The abbreviated design not only saves material but also accomodates more insulation in the corner. 
Our truss walls carry the latter concept 
A single 2 x 4 in an outside corner; also notice the
rough window units to the left before being covered
 over with sheathing and boxed in as described below
to a new level -- there is only one stud in the corner and the cavity for the insulation continues around the corner totally uninterrupted.

Wall-Roof Interface
Without raised heels, the rafters would be resting directly
 on the double top plates; the heels raise the rafters out
 of the way so the insulation can be piled higher
The nearby photo shows raised heel trusses for a conventional ceiling whereby the rafters are a good distance above the top plates in order to provide space for insulation that is only slightly shallower near the exterior sheathing than at the drywall side of the wall.  Raised heels that rest on the inside top plates are especially beneficial for our cathedral ceilings.  The arrangement means that the space for the insulation is at least the same thickness on top of the wall as it is throughout the entire truss bay.  Another advantage is that the wall sheathing continues upward onto the trusses to help brace and anchor them. 
View from within a truss bay.  Raised heels (red) resting
 on double top plates (blue);  blocking to support the
junction between wall and roof sheathing (yellow);  
"L" shaped blocking (between the heels) for gluing the
 wall and ceiling drywall for optimal air sealing

The top cords for our roof trusses did not extend past the plane of the wall and therefore did not help form the soffet (overhang) as shown in the photo above for a traditional roof.  A previous post on the ventilated cathedral ceiling describes how the junction between the wall sheathing and the roof sheathing was blocked on the inside and taped on the outside for maximum  air sealing, then a "mini-attic" or "cool roof" was built on top of the first layer of sheathing,  In the process, the rafters for the secondary roof extended outward to form a soffet with a 2' overhang.

Blocking for Air Sealing
The junction between the wall drywall and the ceiling drywall throughout the house is backed up by blocking so that the edges of the drywall can be glued or at least caulked (presently I can't decide which is best) to give a better air seal than is typically possible with conventional construction.  Also, as shown in the top photo, OSB blocking behind the T-wall studs makes sure that the vertical edges of the drywall on the exterior walls are also
Example of "L" shaped blocking
that was cut from a 2 x 4
(depicted by the green arrow in
 the photo above and the left
red arrow below)
 sealable.  The goal for air sealing is to have the edges of all drywall lining the envelope of the building backed up so they are not only positively fastened with screws but are either glued or caulked as well.

Window and Door "Framing"
I am using this section to describe the unique construction of the window openings in the walls.  But, since the windows will be deeply recessed, I am also veering away from construction long enough to discuss the role of the recessed window design in enhancing thermal performance. 

The "framing" for the doors and windows in the thick exterior walls was a time-consuming challenge.  (The quotes around framing acknowledge 
that the process was more about using OSB to create a box within the wall to house a window or door than using dimension lumber to "frame" the opening in a typical fashion.)

As covered in a prior post, the dimension lumber part of the framing the for windows was done in jigs many months ahead of time then the pre-built units were set into the walls as they were raised.  Each pre-made window unit had horizontal supports under the window, spanning the width of the 
Continuation of air seal blocking around the corner of the
ceiling (red arrows); wall trusses (yellow); roof trusses
(blue); tandem double top plates (green)
wall, that positioned the window 3 1/4" closer to the inside surface of the wall than to the outside surface.  The outside sheathing and the inside drywall plus the decorative framing around the window on both the sides of the wall will make the total wall thickness around the window about 18".

With the window offset 3 1/4" towards the inside, the jam on the outside of the window will be about 8' wide plus another 1 1/2" for the frame.  Consequently, the surface of the glass will be recessed by almost 10" from exterior plane of the wall which, from an energy standpoint, has two advantages .  

First, the shadow box effect will block the summer sun in the early morning and in the late afternoon when it is low enough to shine under the overhang above the windows.  This configuration will be especially advantageous in September and early October when the sun's trajectory moves sufficiently southward to shine under the overhang a little longer each day. Blocking sunshine at this time of year is a plus because it is not yet needed for passive solar heating and could make the temperature in the house uncomfortable.  
Boxing in the second floor clerestory window openings. 
The raw pre-built window units are in the foreground and
 the boxed-in units appear in the distance.  The mullion for
 the partially boxed middle pair of windows has been 
insulated with EPS around the two horizontal 2 x 4s that 
support the sides of the windows 

Wind Washing
Another advantage to the shadow box configuration for the windows is that it ameliorates "wind washing". In one of its iterations, the term describes the movement of heat through window glass in cold weather.  The story goes like this.  Heat seeks cold so it conducts through the glass.  The conducted heat 
Windows installed with nailing flanges are nearly flush
with the plane of the wall
warms the air next to the glass such that the heated air lingers on the surface of the glass and slows the transfer of more heat through the glass.  The ability of the warm air to linger and "insulate" the glass depends on wind action.  On windy days, the heated layer lingers less.  The faster it "washes" away, the faster heat is lost through the glass.  New construction windows with nailing flanges have the thermal advantage of excellent air sealing between the rough opening and the installed window but are more susceptible to wind washing because the plane of the glass is virtually flush with the plane of the wall.  O
ur shadow box windows will not only reduce wind washing per se but, by facing south, will also be in better position to shield the glass from the north and west winter winds.

Mullion Insulation 
In most instances, our windows are to be paired but, instead of ordering the mullions between the windows factory-installed by Pella, I requested individual windows so they would be easier to handle while working alone.  Doing so meant that a mullion the width of a 2 x 4 had to be built into the window units initially.  Then, when boxing in the window openings, I added 3 horizontal 2 x 4s to the mullions for secure fastening of the sides of the windows.  As shown in the photo above, this arrangement created compartments in the mullion that would be inaccessible when the rest of the wall was insulated with rice hulls that I filled with rigid Styrofoam board.  As a result, the mullions should be warmer than would have been the case with factory mullions.


  1. Replies
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As a do-it-selfer-in-training, I welcome your comments.